GP 44-25 Guidance on Practice for Depressurisation

Document No.GP 44-25 Applicability Group Date DRAFT 3 January 2007 Guidance on Practice for Depressurisation GP 44-25 BP GROUP ENGINEERING TECHNICAL PRACTICES DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation Foreword This is the first issue of Engineering Technical Practice (ETP) BP GP 44-25. This Guidance on Practice (GP) is based on parts of heritage documents from the merged BP companies as follows: BP RP 44-4 CP 37 Guide to Depressurisation. BP Engineering Code of Practice CP 37 – Guide to Depressurisation. Copyright  2007, BP Group. All rights reserved. The information contained in this document is subject to the terms and conditions of the agreement or contract under which the document was supplied to the recipient’s organization. None of the information contained in this document shall be disclosed outside the recipient’s own organization without the prior written permission of BP Group, unless the terms of such agreement or contract expressly allow. Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 2 of 38 DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation Table of Contents Page Foreword ........................................................................................................................................ 2 Introduction ..................................................................................................................................... 6 1. Scope .................................................................................................................................... 7 1.1. General....................................................................................................................... 7 1.2. Objective..................................................................................................................... 7 1.3. General requirements ................................................................................................. 7 1.4. Calculation methods ................................................................................................... 8 2. Normative references............................................................................................................. 8 3. Terms and definitions............................................................................................................. 9 3.1. Terms ......................................................................................................................... 9 3.2. Definitions................................................................................................................... 9 4. Symbols and abbreviations .................................................................................................. 10 5. General................................................................................................................................ 10 6. Operational vapour depressurisation ................................................................................... 11 6.1. General..................................................................................................................... 11 6.2. Plant and equipment ................................................................................................. 11 6.3. Pipelines ................................................................................................................... 11 7. Emergency vapour depressurisation.................................................................................... 12 7.1. General..................................................................................................................... 12 7.2. Vessels, aboveground pipework, and valves............................................................. 12 7.3. Compressors ............................................................................................................ 12 8. Depressurisation requirements ............................................................................................ 13 8.1. General..................................................................................................................... 13 8.2. Emergency shutdown system ................................................................................... 13 8.3. Emergency shutdown valves .................................................................................... 14 8.4. Emergency depressuring (EDP) system ................................................................... 15 8.5. Emergency depressuring valves ............................................................................... 16 9. Application of depressurisation systems .............................................................................. 16 9.1. General..................................................................................................................... 16 9.2. Manned production platforms and floating production facilities.................................. 17 9.3. Unmanned production platforms ............................................................................... 17 9.4. Onshore gas/condensate plants................................................................................ 17 9.5. Hydrotreating/hydrocracking reactors........................................................................ 18 10. Time for depressurisation..................................................................................................... 18 10.1. Fire case................................................................................................................... 18 10.2. Non-fire case ............................................................................................................ 19 10.3. Stopping depressurisation......................................................................................... 19 Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 3 of 38 ................................................................................... 30 Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 4 of 38 .....................2 General assumptions ....... 19 11.................................................... 27 Annex B (Normative) Methods for estimating the minimum wall temperature of depressurised vessels and pipework............................ Depressurisation by zone............................................................................................................ 33 B................................................... 19 11..................5..................................................5 Method 2.............................................................................................. Page 16)......... Page 20)................................2..............3........................4.... grade 70) rupture stress versus time to rupture (bibliographical reference [1].................................................................. 32 List of Figures Figure A1 ................................................... 33 B.......................................... Methods of depressurisation ..................... 33 B.................................... 21 12.........Typical carbon steel (SA-515..................................................................... 33 B........... 28 Figure A2 .........................................................................DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation 11.......... Depressurisation systems designed for pool fire exposure.7 Method 4. Effects of depressurisation....................... 20 12...1....2........3...............................................................1........................ 33 B...................................................................5...................................................................... Depressurisation purpose ........................................................................................................................................... Controlled depressurisation ........................................ 19 11....High temperature tensile properties for 18-8 stainless steel (1) ..................................................................... 30 Table A2 ........................................................................................ Uncontrolled depressurisation......................High temperature tensile properties for typical carbon steel (1) ............................................................................................................................................................ Draindown...................................................... Repressurisation................................API RP 521 figure on average rate of heating steel plates exposed to open gasoline fire on one side .................................................................. 33 Bibliography .................... 26 A.... 33 List of Tables Table A1 . Depressurisation systems designed to minimize leak size .......... Depressurisation flow rates... 29 Figure A4 .......................................... 26 A........................ Hydrates and ice ........................ 22 13.................................................. 22 Annex A (Normative) Background to the selected depressurisation time .................1........................................................ Calculation of depressurisation mass flow rates........................... 20 11...................................................................2.....6 Method 3........................................ 20 11................................. 23 A............................................................................................. API 521 guidelines ................................................ grade 70) tensile strength and yield stress versus temperature (bibliographical reference [1]....Typical carbon steel (SA-515................................... 23 A.................4 Method 1.................................................................................................... 33 B.......1 Proposed methods.. Auto-refrigeration ................................................ 21 12.................... 23 A.API RP 521 figure on effect of overheating steel (ASTM A515 grade 70) ... 33 B..3 Vessel contents .................................................................................4........................... 29 Figure A3 ........................... . grade 70) internal pressure versus pool fire exposure time to minimize potential for vessel rupture .............Effect of depressurisation on reduction of distance to overpressure effects (e..................................Typical carbon steel (SA-515.............................. 304L) rupture stress versus time to rupture (bibliographical reference [1]... 31 Figure A6 – 18-8 grade stainless steel (304.......................................................... 304L) internal pressure versus pool fire exposure time to minimize potential for vessel rupture............................................................. Page 20)........................... Page 140)............ 304L) tensile strength and yield stress versus temperature (bibliographical reference [1]............................... 33 Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 5 of 38 ...........................................................g............DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation Figure A5 ................................................ 33 Figure A9 ............. 32 Figure A8 – 18-8 stainless steel (304............................... sideon overpressure)... 31 Figure A7 – 18-8 stainless steel (304.... technical result. users are urged to inform BP of their experiences in all aspects of its application using the “shared learning” folder on the ETP website. specific depressuring systems. Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 6 of 38 . In any case. statutory and local regulations must be complied with. For this reason. and design of. This GP refers to National and International Standards that are widely accepted.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation Introduction This Guidance on Practice (GP) provides guidance on depressurising systems that are within its stated scope and is for use in determining the need for. The value of this GP to its users is significantly enhanced by their regular participation in its improvement and updating. Codes and Standards of the country where the equipment is manufactured and/or operated should be considered and may be accepted if they can be used to achieve an equivalent. safe. Coupled with a fire and gas detection system. Early detection and isolation of hazardous releases and reduction of certain hazardous inventories can substantially limit the consequences from an emergency situation such as a major release of flammable materials. and the final depressurisation system design shall be subject to general discussion with and written approval by BP.DRAFT 3 January 200 1. above ground plant and facilities and may also be used to assess possible hazards in existing systems. typically consisting of one or more pressure vessels. Gas. calculation methodologies. Fluid flow may pass through any or all equipment and pipework in the system being considered. and are supplemental to plant pressure relief protection systems. 3.2.1. This GP specifies BP general requirements for vapour depressurising systems as applied to all sectors: Exploration and Production. GP 44-25 Guidance on Practice for Depressurisation 1. hydrocarbons or a fire. The EDP and ESD systems are integral parts of the overall operations and safety systems provided for a facility. Scope General a. services. to shut off secondary fuel sources that could feed a fire or vapour cloud. and facilities covered by this GP require some form of operational depressurisation. systems. During depressurisation of these systems: c. This GP provides guidance on depressurisation as it relates to system design and selection of process equipment and piping. Refining and Marketing. An emergency Shutdown (ESD) and emergency vapour space depressurisation system shall be provided in situations where rapid isolation of uncontrolled releases is desirable. c. This document describes and provides guidelines for the Emergency Depressuring (EDP) system and the Emergency Shutdown (ESD) system as it relates to the EDP along with briefly discussing depressurisation as it relates to maintenance. However. Clause 9 provides the information necessary to select a depressuring system and means of disposal applicable to a particular facility. Renewables. This GP is applicable to new. additional sector factors may have to be taken into account and reference should be made to any sector specific GP(s) in the ETP library. Contractors are to develop designs and apply their services in accordance with the principles of this GP as amplified or modified by any accompanying supplementary specification(s). Supply and Trading. b. 1. Fluid may reverse flow direction to its expected flow path during either normal or abnormal operation. Proposed depressurisation designs. General requirements a.3. Depressurisation can also be required for emergency situations. Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 7 of 38 . Plants. Objective a. b. heat exchangers. and to minimize releases through the use of rapid depressurisation. b. 1. 1. This GP generally covers the depressurisation of connected systems. Depressurising flow through centrifugal compressors and other rotating equipment may result in “windmilling” of the equipment. 2. strategically located and properly designed ESD and EDP valves can significantly reduce exposure from fire and vapour clouds. rotating equipment and other equipment items interconnected by pipework as described herein. These systems do not replace any requirement for providing pressure safety valves (PSV) as required by regulation. which in turn could impact the temperature reached by the fluid (and could cause damage to the rotating equipment). c. constitute requirements of this technical practice. American Society of Mechanical Engineers (ASME) ASME VIII Boiler and Pressure Vessel Code. Guidance on Practice for Valves. Guidance on Practice for Human Machine Interface for Process Control. Depressurisation loads. Guidance on Practice for SIS – Implementation of the Process Requirements Specification. Section VIII. for Intermediate and Higher-Temperature Service. Guidance on Practice for Fire and Explosion Hazard Management of Offshore Facilities. Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 8 of 38 . BP Refining Process Safety Standard No.DRAFT 3 January 200 1. Guidance on Practice for Relief Disposal Systems. BP GP 24-03 GP 24-10 GP 24-20 GP 24-24 GP 30-35 GP 30-45 GP 30-76 GP 30-80 GP 30-85 GP 43-54 GP 44-70 GP 44-80 GP 62-01 PSS 10 gHSEr Guidance on Practice for Concept Selection for Inherently Safer Design.4. Pressure Vessels. Guidance on Practice for Control Valves and Pressure Regulators. a. through reference in this text. b. GP 44-25 Guidance on Practice for Depressurisation Calculation methods This GP provides guidance on calculation methods to be used for determining: 2. American Society for Testing and Materials (ASTM) ASTM A515 Specification for Pressure Vessel Plates. British Standards Institute (BSI) BSI PD 5500 Unfired Fusion Welded Pressure Vessels. Guidance on Practice for Depressurisation of Pipelines. Guidance on Practice for Overpressure Protection Systems. Guidance on Practice for Fire Protection – Onshore. 10. American Petroleum Institute (API) API RP 521 Guide for Pressure-Relieving and Depressuring Systems. Offshore Passive Fire Protection. Carbon Steel. Guidance on Practice for SIS – Development of the Process Requirement Specification. Guidance on Practice for Fire and Gas Detection. The effects of depressurisation on the temperature of plant piping. Normative references The following normative documents contain requirements that. BP’s Getting HSE Right.0 Hydrotreating/Hydrocracking. The effects of depressurisation on the minimum design temperature of vessels. The latest edition of the following normative documents apply. 3. Stephen Richardson.c. b. ‘shall’ and ‘must’.is used if a provision is preferred.2.l. as applied to BP. 3. 3.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation Energy Institute (Previously known at the Institute of Petroleum) IP 9 Model Code of Safe Practice in the Petroleum Industry. and Handling of Liquid Natural Gas. Controlled Depressurisation An instrument controlled depressurisation generally giving a lower maximum flow rate over a longer period of time than uncontrolled depressurisation. Imperial College. For the purposes of this GP. Part 9 . 2.used if a provision is mandatory.is used only if a provision is a statutory requirement. “Should” . “Must” . Draindown Draining of liquid from vessels or storage to remove the source of flashing liquid that may feed a fire. BP BP p. the words ‘should’. and their associates. a plant or part of a plant in an emergency. the following terms and definitions apply: 1. Terms and definitions Terms a. Definitions Blowdown Can be used interchangeably with ‘depressurisation’ but more commonly taken to mean a rapid relieving of all system pressure down to atmospheric or low pressure levels. when used in the context of actions by BP or others. Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 9 of 38 .Liquefied Petroleum Gas. Throughout this document. by release of gas. “Shall” . Other BLOWDOWN™ Software developed by Imperial College in London. London United Kingdom SW7 2BY. Department of Chemical Engineering. have specific meanings. Note The contact for information on this program is Dr. is used if BP does not wish a design to proceed unless certain features have been agreed in writing with a contractor or supplier. National Fire Protection Association (NFPA) NFPA 59A Standard for the Production.1. In this GP the term ‘approve’. Depressurise or Depressurisation To reduce the internal pressure of process equipment. Emergency Depressurisation The ability to rapidly depressurise. This does not imply that all details in a written document have been considered by BP and does not affect the design responsibilities of the contractor or supplier. 3. Storage. Disposal is normally to a flare. a. to permit decommissioning and maintenance operations. This type of shutdown normally includes controlled depressurisation of the production facilities. To minimise the uncontrolled release of hazardous gases. Yellow Shutdown On an offshore production facility. It is normal to be able to utilise part or all of the relief system for depressurisation.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation Operational Depressurisation The ability to depressurise equipment. by release of gas. not ESD) SI Systeme International d' Unites SIL Safety Integrity Level SIS Safety Instrumented System General A depressurisation system is a means of reducing the pressure in a process plant or pipeline below the normal operating pressure. size and/or duration) of a jet fire. Like pressure relief. EDP Emergency Depressuring EDPV Emergency Depressuring Valve ESD Emergency shut-down ESDV Emergency shut-down valve LPG Liquefied petroleum gas MCR Main Control Room PAS Process Automation System PSD Process shut-down (i. Symbols and abbreviations For the purpose of this GP. To reduce the failure potential of pressure containment equipment for scenarios involving over-temperature from a fire or exothermic/runaway process reactions.e. Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 10 of 38 .e. 4. The main reasons for this are: • • • • • For maintenance and inspection. the following symbols and abbreviations apply: 5. depressurisation is generally to a flare or remote vent. but an atmospheric vent may be pursued for emergency discharge in rare instances when the project can demonstrate flaring is not an option and GP 44-80 conditions for venting with the required management approvals are obtained. Other systems remain in operation. To mitigate the effect (i. the production process from wellhead valves to export lines is shutdown and isolated. To minimise release of fuel that may be feeding a fire or could be ignited. although a dedicated operational depressurisation valve is often used. if installed.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation Vents must also follow applicable local regulations.1.g. b. Sections of plant or individual plant items containing hydrocarbon gas and/or liquid should normally be isolated. The maximum rate of depressurisation is influenced by: • • • The reduction in temperature due to auto-refrigeration (see clause 12. Section 5). minimise leakage to flare. b.1). and provide a means of testing the valve. Occasionally depressurisation of pipelines may be required to allow for inspection or repair. pipeline depressurising times are extensive. making the pipeline unavailable for transportation. which may be particularly important when more than a single operator is reliant on the pipeline availability. 6. depressurised. GP 44-70 and GP 44-80 contain further guidance on both system design and required process components for depressurisation systems. c. plugging of the depressurisation system due to solids being formed (freezing) or the need for more expensive materials to avoid brittle fracture issues. Plant and equipment a.2. and purged to allow access for inspection and maintenance.. Generally. discharge to a safe location (refer to API 521. Operational vapour depressurisation General Operational depressurisation is often required for the shutdown of machinery and as preparation for plant inspection and maintenance.3. In onshore applications. For manual depressurising the valve auto position should be overridden by local key lock to prevent the depressurisation logic becoming over complex. the system may be the same as offshore. depressurisation can cause low temperatures (auto-refrigeration) which can form large volumes of liquid condensate. 6. Offshore. Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 11 of 38 . drained. Dependent upon the nature of the contained fluid. operational depressurisation should use the same valve as emergency depressurisation. to minimise weight. Pipelines Pipeline depressurisation is not covered within this GP. The addition of supplemental fuel gas to the flared gas may be required to maintain the minimum heating value required for proper flare operation when depressuring vessels containing inert gas or other gas with a low heating value. Depressurisation systems may be installed to cover operational and/or emergency situations and may be automatically or manually initiated. and The pressure rating of the depressuring system and connected equipment. and have appropriate dispersion analysis conducted. measured in days rather than minutes. 6. refer to GP 43-54 for pipeline depressurisation guidelines. 6. flare) capacity. d. c. The disposal system (e. Depressurisation systems shall be developed in accordance with the strategy and associated performance standards for managing major hazard events in GP 24-03 and either GP 24-10 (for onshore facilities) or GP 24-20 (for offshore facilities) as appropriate. and valves Vessels and aboveground pipework are normally protected from overpressure in the fire case by relief valves. 7. GP 44-25 Guidance on Practice for Depressurisation Emergency vapour depressurisation General Emergency vapour depressurisation is generally used to avoid incident escalation. Deluge systems are designed to minimise the temperature rise of equipment in and surrounding a fire. Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 12 of 38 .e. including the pig launcher/receiver and downstream pipework to the pipeline ESDV at the riser/topsides interface. c. and/or duration) of a jet fire. Depressurisation is required within the hold-up time of the seal oil overhead tank to prevent gas escape via the seals. Minimising the potential uncontrolled release of hydrocarbons or hazardous gas. size. d. or apply external water for cooling. the compressor may be considered as part of the depressurisation system.DRAFT 3 January 200 7. When dealing with vapour. 7. Detection of the gas followed by depressurisation minimises the possibility of ignition. exothermic or runaway process reaction. Vessels.1. depressurisation on fire detection limits fuel supply to the fire. Blocking in and depressurisation of centrifugal compressors is required on seal oil failure. or to depressurise the vessel by means of a vapour depressurising system. Examples are: 7. Reducing the failure potential of pressure containment equipment for scenarios involving over temperature from a fire. The platform topsides section of a subsea pipeline. For compressors with seals not relying on a seal oil system to contain the gas. high pressure vessels or systems can heat-up rapidly and rupture at pressures below vessel design pressure or relief valve set pressure.2. b. Compressors a. Provision may be made to insulate the vessel vapour space. Mitigating the effect (i. Minimising the release of fuel that may be feeding a fire or could be ignited. a. the objective of a depressurising system should be to keep the internal pressure of the exposed vessels and piping below the rupture pressure as the yield stress of the wall reduces due to overheating. thereby reducing risk to personnel and limiting property or environmental damage. b. If ignition has already occurred.3. impact. in addition to or instead of deluge systems. Refer to fire protection GP category 24 for more specific information and GP 30-85 on fire and gas detection. c. a. However during a fire. Passive fire protection may be considered for vessels particularly at risk in a fire.. The rate of temperature increase and hence the decrease of vessel strength is more rapid for gas filled systems since the rate of heat transfer and thermal capacity for a gas are less than a liquid. The platform design shall ensure that the ESDV and downstream pipeline are fire protected. d. Depressurisation may also be used to minimise uncontrolled release from a vessel or pipework resulting from a leak such as a blown gasket or leaking valve. The same applies to the gas space of vessels that could be subject to flame impingement. so the pipeline inventory can be maintained. should be depressurised through the normal platform system. b. aboveground pipework. . The SIS shall be independent of the PAS though operators may view the ESD/EDP status through a common human machine interface. Depressurisation requirements General a.1) from depressurisation. 5. Noise induced by depressurisation. The ESD and EDP systems shall have a simple interface with plant operators to allow a safe shutdown and. a localized geographic plant area [often referred to as a zone]. and lighting system are not stopped or tripped. individual plant equipment. and trip associated pumps. The following factors shall be considered in determining the maximum rate of depressurisation: c. failsafe. 6. 3. and steam to reboiler. Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 13 of 38 . 2. 4. if required. Process flare capacity requirements.g. 5. but not be limited to. Impact on pipe supports. Emergency shutdown system The PSD/ESD system normally comprises a hierarchy of shutdown levels (i. Reduction in temperature due to auto-refrigeration (see 12.DRAFT 3 January 200 8. potential for extinguished pilots due to high depressurisation rate).1. the ESD and EDP systems should perform. GP 44-25 Guidance on Practice for Depressurisation 1. manual ESD valve activation shall override any automatic process shutdown. Note that ESD valves may be shared by other shutdown systems. a process skid or module.e. as a minimum. Automatic activation of a shared ESD valve by other systems is allowed. 8. turbines. Some factors influencing the extent of an emergency shutdown include. Enable opening of dedicated EDPVs. the following functions: 1. but are not limited to. The extent of a process or plant shutdown and the respective initiating conditions are generally defined as part of the initial design. and the entire facility). furnaces. flare restrictions. Impact on flare pilots (e. operating philosophy. Pressure rating of the relief/ vent system and connected systems. Some facilities such as lube oil and seal oil system for compressors. b. depressuring without plant operators having to consider many alternatives. 4. and based on proven design concepts utilizing a SIS designed in accordance with GP 30-76 and GP 30-80.2. Manual ESD activation is at the discretion of an operator and is typically accomplished via hard-wired manual switches located in the MCR and at strategic positions throughout the facility. 2. however. Stop selected drivers on pumps and compressors. 3. equipment operating pressure rating. Stop outlet liquid hydrocarbon streams by closing ESDVs on vessels requiring inventory containment. if any). Stop flow of incoming thermal energy or heat sources within ESD zone (Such as fuel sources. ESD and EDP systems shall be reliable. In case of an emergency. The SIS for an ESD/EDP system shall comply with the SIL determined during Front-End Engineering Design and detailed design. 8. and available fire fighting facilities. See GP 30-45. Stop selected inlet and outlet hydrocarbon streams by closing dedicated ESDVs. rail. be determined by regulations and consequence analysis. 2. For applications not subject to NFPA 59A. air/pneumatic operated block valve. g. The GP 24 series referenced in clause 2 provides further guidance. Other equipment such as compressors that can impede or affect depressurisation. Storage tanks containing highly corrosive or toxic materials.3. e. expansion joints. In determining ESDV locations. ESDVs should not be located at long distances from the process unit just to avoid installation in the fire zone without considering the consequences of having a potentially larger hazardous material release. 3. ESDVs on accumulator and process vessels should be placed as close as possible to the vessel outlet flange. d. Refrigeration systems using a flammable medium. Failures of rotating equipment. c. Check valves or other flow restrictions that can impede depressurisation. An ESDV and its accessories shall be of fire safe design if located inside a fire zone. an ESDV shall be a tight shut-off. It is important that the location and number of not only ESDVs but also depressuring valves consider: 1. LNG service ESDVs should be located in accordance with NFPA 59A. Packed vessels where packing can be entrained by depressuring. Note in some cases the depressurisation path may be opposite to the normal flow path and/or at a significantly higher flow rate than normal. Discharge lines on charge pumps. Process pumps. and fuel supply to the affected area. Vessels containing large hydrocarbon inventories. The minimum hydrocarbon liquid level contained in vessels and tanks shall: be specific to the type of fluid. hazardous utility streams. Pumps and manifolds for ship. “fail close” (on loss of signal or power source). h. ESDV location(s) and use shall be optimised based on the individual system configuration. ESDVs for process area isolation shall be located at the edge or boundary of the process area being isolated. GP 44-25 Guidance on Practice for Depressurisation Emergency shutdown valves a. or toxic exposures as well as operational requirements associated with the process. Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 14 of 38 . Feed gas lines. Hydrocarbon loading lines.DRAFT 3 January 200 8. and require that an ESDV be installed. Generally. b. Some typical ESDV locations are potentially located at: • • • • • • • • • • • Pressurised and refrigerated storage tanks. Fired heater process lines and fuel pumping systems. or truck loading and unloading. consideration should be given to equipment prone to failure or containing inventories of materials that present significant fire. Turbine fuel systems. and loading operations are recognized as high frequency and in some cases having high consequence loss exposures. f. vapour cloud. Final ESDV locations should be reviewed by process hazard analyses and design hazard reviews that address the impact of isolation on operations. the ESDV shall stop the flow of inlet and outlet process streams. Upon activation. fired heaters. c. Activation of EDP is typically accomplished via hard-wired manual switches that are located in the MCR and. an air bottle or hydraulic accumulators sized to provide an independent air or hydraulic supply for at least three (3) cycles. to provide motive energy to move the valve to its fail safe position shall be provided. including wiring. increasing the failure potential due to overheating. Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 15 of 38 . Further guidance on ESD valve selection is provided in GP 62-01. Control valves shall not be used as ESDVs. The scope and requirements to provide online testing of the ESDVs shall be developed during the design to meet the required integrity levels. If double acting air cylinder or hydraulic valves are used as ESDVs.DRAFT 3 January 200 8. The depressuring system shall consider. shall be investigated with consideration of constructability and maintainability. for large size valves.e. the application of double acting air cylinder type or hydraulic type. which require large torque. automatically on confirmed detection of fire or gas release. etc. The type of ESDV shall be defined dependent on the service requirements and size of the valve. k. In general. or the total plant. Additionally. if applicable. Significantly longer depressurisation times are likely required compared to cases where the vessels are only filled with gas. The scope of details of EDP shall be developed during design. these shall be equipped with a secured supply of actuating fluid and back-up system and be protected as necessary against potential hazards. automated “liquid blowdown” capabilities. GP 44-25 Guidance on Practice for Depressurisation i. In general. etc. the use of spring return air operated valves shall be selected for the ESDVs at first from reliability and maintainability points of view. n. Removing liquid from a vessel exposed to a fire eliminates the cooling effect of the wetted surface area and the equipment heats up quicker. process area. etc. shall be enabled only after activation of the ESD system. Vapours released during depressuring shall be vented to the flare system or other BP approved location. i. for some offshore installations. Emergency depressuring (EDP) system a. The advantages and disadvantages of liquid blowdown shall consider that the depressurisation rate depends on the boiling rate of the liquid. If hydraulic valves are selected. EDP for each skid. In addition. should be protected against potential hazardous exposures. unless specifically justified and failure of the control loop cannot cause a demand on the ESD function. d. Inherently failsafe actuation is preferred. on a case-by-case basis. The design of piping and equipment shall consider the temperature reached during autorefrigeration. e. EDP system (once activated) shall be able to reduce the pressure of the system to the pressure and maximum duration provided in Annex A. valve and actuator components. However. l. j. air supply. it should be designed to enable minimization of fuel inventory that might otherwise aggravate a fire and to minimize the uncontrolled release of flammable or toxic gases. ESDVs should be located outside of buildings housing hazardous processes or hazardous utility equipment. b. If necessary.4. ESD valve shall be equipped with open/close position limit switch and the open/close indication shall be displayed on the PAS. Close-open-close-open. The EDP system shall have adequate venting capacity to achieve reduction of stress in selected equipment affected by fire to a level at which stress rupture is not an immediate concern. a hydraulic actuating system in lieu of air may be used. m.. ESD valves shall not be provided with handwheels. During this assessment the following points should be considered: 1. Water deluge. The EDPV is typically designed to “fail close” or fail in the last position. Note the inventory of a gas plant is generally significantly less than an oil installation and therefore oil fires could last appreciably longer. See GP 62-01. or insulation. If a depressurising system is not required by regulation. Should the EDPVs fail open. Emergency depressuring valves a. reducing the possibility of rupture. Application of depressurisation systems General Plants and process systems covered by this GP require some form of operational depressurisation. The potential risk of personnel injury shall be adequately assessed. This section summarizes where emergency depressurisation systems are required. Vessel design pressure may be substantially higher than the maximum operating pressure. This common usage also needs review during the design hazard review to ensure this design does not result in a common mode failure that could contribute to the cause and thus defeat a protective system. then fully evaluating personnel risks and plant replacement costs with and without a depressurising system installed.5. 5. may provide sufficient protection to minimise wall temperatures over the duration of the fire. d. 7. 6. This entails developing a fire damage assessment. assessing the viability of depressurising to mitigate undesirable leaks and spills. b. operational upsets due to malfunctioning valves or inadvertent opening of the valves should be considered as-well-as any single mode failure resulting in the EDPVs simultaneously opening and potentially exceeding flare capacity. Use of a single control valve to serve as both an EDP and a letdown-to-flare should follow GP 30-35. 9. c. a. Heavy flaring caused by activation of depressuring system (refer to BP gHSEr). the failure position of each valve shall be reviewed during the design hazard review. 8. The use of a depressurising system maintains the integrity of equipment by reducing the possibility of vessel rupture.1.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation In a non-fire case. The scope and requirement to provide online testing of the depressuring valves shall be developed during design to meet required integrity levels. 4. the risk and effectiveness of the system should be considered. Inadvertent activation of depressuring system can cause significant production loss and possible downtime. 2. 8. or both. Upgrading material selections for equipment subjected to low temperatures as a result of depressurisation and auto-refrigeration can substantially increase investment costs. Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 16 of 38 . 9. in general. Depressuring valves shall. be tight shut-off (to avoid loss of hydrocarbons during normal operation) air operated block valves. The probability of a fire and/or significant leakage of flammable material. 3. However. Stress rupture may not occur as the fire may die out or be extinguished. liquid blowdown frequently reduces auto-refrigeration temperature effects. GP 44-25 Guidance on Practice for Depressurisation Manned production platforms and floating production facilities a.DRAFT 3 January 200 9. Fire protection in association with paving and drainage systems design to limit the spread of fire should be considered. However if they are used they should initiate yellow shutdown and depressurisation.2. Fire protection with water deluge and/or passive protection should be used with drain down if appropriate.3. by operators at the controlling platform or terminal. Pressured LPG storage The protection and safety of pressurised LPG storage systems is covered by IP 9. Unmanned production platforms a. either initiated by operators visiting the platform or if there are no personnel on the platform. These detectors should produce a pre-alarm signal to allow possible operator intervention before the plant trips and depressurises. c. 9. as appropriate. b. shall be used for fire protection. depressurisation is unlikely to be necessary after an ESD has taken place. gas detectors are unlikely to detect anything but very large leaks.1. 9. a fire case.4. Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 17 of 38 . 9. Onshore gas/condensate plants 9. the shutdown and depressurisation should be the result of gas detection by two detectors in a voting system. To minimise spurious trips. Use should be made of fire protection for systems most at risk from a delayed depressurisation. b. On open deck platforms. Floating production facilities are invariably manned and are therefore covered by the same. Initiation of depressurisation should consider helicopter operation in the platform vicinity. gas detection should initiate yellow shutdown followed by automatic depressurisation. Fire protection.2. both passive and with water deluge. On platforms where process plant is totally enclosed or enclosed by louvers.4.4. and no vapour depressurisation is required. On open deck platforms a vapour cloud is much less likely to form than in an enclosed platform and if the leak is large. above criteria. Manned platforms shall have emergency depressurisation systems to safeguard personnel and protect the investment. Refer to API RP 521. Unmanned platforms should be fitted with emergency depressurisation systems if the production facilities contain more than 30 tonnes (30 tons) of hydrocarbons either stored at or capable of exceeding API 521 criteria (50% of vessel design pressure) during normal operations. Gas terminals If a terminal contains pipework and tankage but no process plant. c. Gas or fire detection on unmanned platforms shall result in a process shutdown. or a process excursion. depressurisation is unlikely to be appropriate in emergency situations.3. Stabilisation or fractionation plants and gas terminals with processing plant Emergency depressurisation should be considered for high pressure plants (significant hydrocarbon stream(s) above 70 barg or 1 000 psig) and those with a large inventory (greater than 10 tonnes [10 tons]) of pressured hydrocarbon. 9.4. d. The depressurisation should be manual. General 9. BSI PD 5500 or equivalent which are 25 mm (1 in) wall thickness or greater. High reactor temperature is defined as 30°C (54°F) above normal operating temperature or reactor mechanical design temperature. Hydrotreating/hydrocracking reactors 9. b. Fire case a. Time for depressurisation 10. 10. The depressuring valve/s shall be dedicated shutdown valve/s separate from normal unit controls. Fail safe means that the valve(s) used for depressuring the unit fails open on loss of signal or de-energisation. whichever is the lower pressure. Existing hydroprocessing reactors with cracking catalyst a.3. b. but in no cases shall be less than 14 bar/min (200 psi/min). 9. b.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation 9. Vessels designed to ASME VIII and pipework of wall thickness less than 25 mm (1 in) should be depressurised proportionately faster (see Annex A).1. Initial depressuring rate shall be at least 14 bar/min (200 psi/min).5. vessels designed to BSI PD 5500 should be depressurised to 6. The main purpose of this depressuring system is for high pressure inventory disposal under unit emergency conditions such as fire. Depressuring shall not be able to be interrupted until temperatures are 25°C (45°F) below trip setting. New build hydroprocessing reactors (post January 2000) containing catalyst with cracking function and above 70 barg (1 000 psig) separator operating pressure shall be provided with a fail safe depressuring system that can be activated from the control room and in the field local to the plant.1.5. The depressuring valve/s shall be dedicated shutdown valve/s separate from normal unit controls.5. should be depressurised to 50% of their design pressure in 15 minutes if the vessel is uninsulated carbon steel construction (see Annex A). a. Vessels designed to ASME VIII. New build hydroprocessing reactors with cracking catalyst a. whichever occurs first. The depressuring system shall have automatic initiation by high reactor temperature.9 barg (100 psig) or 50% of the design pressure. High reactor temperature is defined as 30°C (54°F) above normal operating temperature or reactor mechanical design temperature. Fail safe means that the valve(s) used for depressuring the unit fail open on loss of signal or de-energisation.2. Hydroprocessing reactors above 34 barg (500 psig) separator operating pressure shall be provided with a fail safe depressuring system that can be activated from the control room. in 15 minutes. Fail safe means that the valve/s used for depressuring the unit fail open on loss of signal or de-energisation. If depressurisation is not the controlling rate for design of the flare. b. The depressuring system shall have manual or automatic initiation by high reactor temperature. The protection and safety of the pressurized reactor section of a Hydroprocessing unit shall comply with BP Refining PSS 10. whichever occurs first. The depressuring valve can also serve as the normal pressure control valve.5. Depressuring shall not be able to be interrupted until all temperatures are 25°C (45°F) below trip setting. Initial depressuring rate shall be maximized up to the industry practice of 21 bar/min (300 psi/min) subject to reactor bed support and flare system constraints. Existing hydroprocessing reactors containing catalyst with cracking function and above 70 barg (1000 psig) separator operating pressure shall be provided with a fail safe depressuring system that can be activated from the control room and in the field local to the plant. Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 18 of 38 . Refer to formal risk analysis. It is more applicable to onshore plants. This creates an initial peak flow rate that decays over the major portion of the depressurising period. Non-fire case The 15 minute depressurisation guideline does not apply to non-fire case. If necessary to reduce the flow rate of gas to flare or vent. Stopping the depressurising process should be evaluated on a case-by-case basis if it is deemed discontinuing depressurisation can be conducted safely. and headers. The depressurisation rate selected and method of calculation shall be subject to BP approval. Annex A. 10. For equivalent wall thicknesses. Methods of depressurisation Initial depressurisation flow rates may be substantial and create difficulty in the design of the flare or vent systems. i. areas separated by firewalls within which a fire can be contained. Vessels containing liquid or having external insulation take longer to heat up than non-wetted or bare vessels. restriction orifices. and consequently more gas is routed to flare increasing the total gas depressuring rate. process plant often occupies a number of fire zones. Consideration should be given to depressurising only those vessels and equipment within the fire zone in which fire or gas is detected. In addition to the load from the system being depressurised there is a possibility of continued in-flow into the system being depressured. If depressurisation governs flare design capacity. fire in one area can be prevented from reaching other areas by distance and bunding or kerbing.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation c. Stopping depressurisation Depressurisation should not be stopped on reaching the target pressure (See Annex A). 11. See clause 13.g.2. Depressurisation of vessels and equipment in Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 19 of 38 . Onshore.1. stainless steel vessels generally allow lower depressurisation rates than carbon steel (see Annex A) because stainless steel begins to lose strength at a higher temperatures than carbon steel. 11. There are ways to reduce this high initial flow as discussed below. 10. This division into fire zones is an important means to reduce the maximum flare load in cases of emergency depressurisation.3) are of great importance when determining the load to flare or vent. the rate of vessel(s) and pipework depressurisation should be related to the reducing strength of the steel exposed to fire. Also vessels of wall thickness greater than 25 mm (1 in) take longer to heat up than the 25 mm (1 in) thick vessels used as a basis in API RP 521. Pressure normally continues to reduce until atmospheric pressure or the flare or vent backpressure is reached. e. longer depressurisation times may be proposed for BP approval for vessels containing liquid. a. Uncontrolled depressurisation In a normal uncontrolled depressurisation the total system is shutdown and isolated and the pressure in the process equipment is discharged to flare or vent through the emergency blowdown valves. The depressuring time and the end pressure (clause 10. and API RP 521 guidelines.3. but can be applied offshore if good fire zone segregation is achieved.e. d. having external insulation.2. 11. or having wall thickness greater than 25 mm (1 in). Depressurisation by zone Offshore. from wells that have not shut-in. g. Maximum depressurisation flow rates tied to a flare system shall not exceed the flare system capacity.4. but in all cases reliability analysis shall be required for a controlled depressurisation system.5. In some instances it may be inappropriate to depressurise to flare. the peak flow rate can be maintained for a substantially longer period before the decaying phenomenon sets in. e. In exceptional circumstances. b. The use of controlled depressurisation is subject to BP approval. b. c. Depressurisation flow rates a. 11. Systems containing fail open or fail shut valves rather than control valves are generally favoured. The use of any such reduction shall require specific. Alternative calculation methods that take this into account shall be considered for this case. Flow restriction orifices are used in conjunction with snap open valves to set the maximum rate of depressurisation to flare or vent. 11. the depressurisation flow rate is reduced in magnitude compared to the peak flow rate of an uncontrolled depressurisation. breakdown of a distributed control system in one area shall not affect any other process areas. Draindown a. written approval of the BP responsible Engineering Authority (EA). but in any case should be delayed to minimise the size of the flare or vent system. This may be possible using the normal product withdrawal system to remove liquid from the vessel being protected Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 20 of 38 . For controlled depressurisation. but still depressurise in the designated time. Depressurisation flow rates and the resulting system temperatures for gas and gas/liquid systems shall be calculated in accordance with one of the methods given in Annex B. b. since controlled depressurisation instrumentation can be less reliable than that used for uncontrolled depressurisation. Since peak flow is maintained over a longer time. 11. In a fire. This increases the flow to flare after the peak flow has passed and the system pressure has reduced. c. BP may approve a reduction in the peak discharge rate due to the distributive nature of the relieving flow and the requirement for pressure build-up to discharge the system inventory. the flow may not be totally through single fixed orifice(s). The designer should be aware of and prevent common cause failures. vessels containing volatile hydrocarbon should be protected by passive fire protection and/or a water deluge system. simultaneous loss of instrument air to all depressurisation valves). It is possible to limit the total flow rate. or be at the discretion of the operator. To limit vapour generation and possible spread of fire.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation adjacent fire zones should be timed to follow. An example of this would be if a vessel has a considerable inventory of highly volatile hydrocarbon liquid such as LPG. Alternatively. facilities may allow for removal of liquid from the system. Controlled depressurisation In a controlled depressurisation. Depressurisation valves may also be designed to fail closed or in the last position to eliminate common mode failures provided that reliable valve actuation to the appropriate SIL is assured.g.3. by either controlling the flow rate bypassing the orifice with a control valve or snap open valve(s) initiated by a timing sequence. depressurisation valves may be sized to ensure the disposal system capacity is not exceeded when all valves go to the full open position. Flow from each section being depressurised is normally initiated by the opening of a valve with the flow controlled by a restriction orifice. A reduction in flare size by zoned depressurisation shall only be accepted if total depressurisation cannot occur from a common cause (e. the availability of separate liquid storage and the fire resistance of the system shall be considered for each case. but could require special consideration of the electrical power and utility system status. 12. the rate of heat transfer from the vessel to vapour is negligible. Auto-refrigeration 12. an isothermal process may often suggest that plain carbon steel construction would be appropriate. The heat capacity of the vessel is also high compared to that of the vapour and the reduction in wall temperature is low. General If the contents of a vessel. Draindown can also be used to avoid low temperatures resulting from boil-off of highly volatile liquids and hence reduce the requirement for expensive alloy steel. or other mitigations to accommodate these low temperature blowdown effects.1. and sweating up to ice layer. In many cases the advantages of retaining the liquid in the vessel to minimise the rate at which the vessel heats up outweighs the benefits from removing the flammable liquid from the fire zone. The rate of heat transfer from the container to the liquid is significant. sweating (re-condense water from humidity). Technical studies have yet to firmly establish the type of process within the vessel being depressured (isentropic. For example. resulting in the pipework generally approaching isenthalpic flash temperature. potentially suggesting a need for low temperature materials.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation during fire exposure. Available information should be carefully considered before selecting the thermodynamic process for vessel depressurisation. An exception to this can be exit nozzles where significant forced convection takes place during depressurisation requiring the use of stainless steel nozzles. In contrast. if the vessel fluid is all vapour. since this will probably only be used when a fire has reached a high severity.1. Downstream of the orifice where fluids are usually at their coolest and the flow very turbulent. isothermal or somewhere in between). it would be reasonable to go to a more complex Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 21 of 38 . d. pipework. the rate of heat transfer is high. Note that flow up to the orifice throat is an isentropic process while flow in the piping downstream of the orifice is generally considered to be an isenthalpic process. or a pipeline are depressurised from a high pressure. even though fluid kinetic energy gains downstream of an orifice can limit available fluid thermal energy and push fluid temperatures below those predicted by an isenthalpic flash calculation. Vessels and pipework In general. if a simple flash (Method 1) shows stainless steel is required for a large vessel by a small margin. gas expansion and liquid evaporation normally cause cooling. The isentropic assumption neglects heat gain from the surrounding environment and can predict low temperatures upon depressurisation.1. Effects of depressurisation 12. 12. Transfer of heat in from the surroundings often is not negligible in the timescale of the depressurisation except for fire. The time and effort expended on this calculation could depend on the likely equipment cost savings. c.1. vapour expansion and evaporation from the liquid cools the liquid. The reliability of the deluge system. Annex B gives four methods of increasing complexity and accuracy when calculating minimum wall temperatures resulting from depressurisation.2. In a vessel containing liquid and vapour. internal sleeves. Consideration should be given to providing a nitrogen pressurised methanol injection system upstream of the valve. Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 22 of 38 . e. If depressurisation results in temperatures lower than the design minimum temperature of a system. c. If a severe hydrate or ice formation problem either upstream or downstream of the depressurisation valve is possible. Whether immediate repressurisation after EDP activation is required shall be determined by the project in conjunction with operating staff. This may require a process system to be substantially above the minimum design temperature to allow for temperature reduction from gas expansion or high rates of liquid evaporation during the process restart. the system shall not be repressured until all temperatures throughout the depressurisation system have risen to a calculated safe value above the design minimum temperature. d. In such systems. Depressurisation systems immediately downstream of a re-pressurisation valve may be much colder than expected during the early re-pressurisation stages due to JouleThompson expansion across this valve coupled with the thermal energy transferred to the downstream fluid from its kinetic energy gain. 12. so calculated. Less severe problems may occur when depressurising downstream of dehydration plants. Repressuring a process at temperatures below a known. If not.2.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation analysis to establish if the minimum wall temperature. or other methods used to inhibit hydrate formation. If immediate repressurisation is not possible due to low metal temperatures. other BP approved chemical injection. a lower design minimum temperature should be employed. temperature monitoring and warm up facilities shall be provided in order to permit restart/repressurisation within the time period defined by the project and by operational requirements. would allow use of a less expensive material. Hydrates and ice a. provision should be made for methanol injection. process consideration should be taken for units with a cracking catalyst function to minimize the possibility of reaction runaway. If depressurisation is interrupted. b. consideration should be given to using Method 2 in Annex B to establish if a supporting temperature and pressure can exist at the same time. turbulence around the valve coupled with the relatively short time needed for plant depressurisation will probably prevent formation of a mass large enough to cause a blockage or other damage. b. Under certain conditions it is possible for hydrates or ice to form in a hydrocarbon system containing free water. In this case although hydrate or ice formation may be predicted. 13. Repressurisation a. re-pressuring shall be adequately controlled such that an adequate warm-up period exists. if the temperatures calculated on emergency depressurisations using a simple adiabatic flash are low enough to support hydrates or ice. If below minimum design temperatures can occur. safe restart temperature may result in brittle fracture. type of failure to be mitigated (vessel yield or vessel rupture). If available. initial temperature.. The plate would reach an “equilibrium” temperature of about 760°C (1400°F) in 20 minutes. unwetted plate that is exposed to fire on one side. such as ASME Section VIII and Section IID. There can be significant variability in the heat up rate and ultimate temperature attained in a pool fire due to the influence of total heat capacity of the vapour within the vessel. fire intensity. the API RP 521 criteria is specifically based on a relatively large and/or high pressure carbon steel vessel (i. This would reduce or mitigate the consequences from the leak or rupture.. assumed to be a metal. allowable stress Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 23 of 38 . jet fire heat flux data should be utilized that is representative of jet fires one might expect in a plant as opposed to theoretical analyses or data obtained in strictly controlled tests. type of fire (i. has a maximum allowable design stress of 138 MPa (20 ksi or 20 000 psi) per ASME Section IID at up to 260°C (500°F) temperature. Note While fabrication stress concentration factors consume a portion of this design stress differential. This is needed because the strength or load carrying ability of a metal decreases with temperature. Although the methodology can be applied to any metal vessel. pool fire or jet fire). b.g. presence of external insulation. no liquid so all non-wetted surfaces) and exposed to a pool fire. etc. Higher fire heat fluxes require higher depressurisation rates than predicted below. Figure A1 of API RP 521 provides data on average heat-up rates of steel plates exposed on one side to an open gasoline pool fire. The normal starting point in specifying depressurisation criteria for a specific vessel would be determining the vessel wall heat up time from pool fire exposure.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation Annex A (Normative) Background to the selected depressurisation time A. fire duration. material of construction. A typical carbon steel pressure vessel is designed on a conservative basis using a recognized “code”.2.e. Depressurisation purpose The two primary purposes for an emergency depressurisation system are: A. wall thickness of 25. exposed to a pool fire in order to extend the time before failure occurs. Remove inventory from a vessel(s) to minimize the amount of material that would be released through a leak or rupture. localized heating rates that significantly exceed that of a standard hydrocarbon pool fire. API RP 521 suggests depressurisation of equipment to 50% of the design pressure in 15 minutes. b.e. and presence of fireproofing or other fire protection means (e.. water spray systems). Note that the following approach is applicable to standard hydrocarbon pool fires. d. c.e.4 mm (1 in) thick steel plate to heat from ambient temperature to 650°C (1200°F) in the pool fire..4 mm (1 in)) that is gas-filled (i. Depressurisation systems designed for pool fire exposure a. The desired depressurisation rate can vary significantly based on vessel wall thickness. a vessel fabricated with SA-516 Grade 70.1. This Figure indicates it takes about 15 minutes for a 25. The equilibrium temperature is the temperature at which heat gain from a typical pool fire is balanced by heat loss to the environment and assumes a bare. Exposure to jet fires can result in intense. Remove internal pressure from a gas-filled vessel(s) to reduce the applied stress on the pressure boundary material. For example. a. Note that the API curve shown in Figure A2 is extrapolated from the source data shown in Figure A3 for exposure times less than 0.. However. near design conditions).e. predicts the vessel would rupture in about 1 hour at 593°C (1 100°F) and 0.. it can be easier to use the short-time tensile strength as a limiting stress that. Maintaining a constant pressure near the design allowable stress would be comparable to having the vessel protected by a pressure relief valve. This data can be used to determine depressurisation versus time for vessels using carbon steel with about 483 MPa (70 ksi) tensile strength when coupled with heat-up rates. as the metal wall temperature gets quite high. would cause vessel failure. Figure A2 (or Figure A3). the extrapolations in Figure A1 at exposure times less than 0. yielded) at stresses significantly lower than the rupture stress. an increase in the unwetted wall temperature above the “code” allowable temperature causes a decrease in material strength. Typically the allowable stress is the lower of 2/3 the yield strength or 1/3. For a pressure vessel exposed to a pool fire. This data was obtained from bibliographical reference [1] (see Figure A3 for the original source data) and is characteristic of all grades of carbon steel.g. including residual and applied.. vessel rupture can occur in a relatively short time period once the metal reaches a high temperature as long as the internal pressure remains high (e.1 hours should not be used because the vessel integrity cannot be maintained with an internal pressure that results in total stresses. Figure A2 (taken from API RP 521) shows the effect of temperature increase on rupture strength of typical carbon steel pressure vessel plate (i. The allowable stress value is based on time independent properties. Above this temperature. the vessel should not rupture (unless there are material defects/ flaws/ previous damage or if one of the components such as a nozzle or flange is a weaker link). the material undergoes small metallurgical changes with time. exceeding these cause failure).g.1 hour data. For a carbon steel vessel with a 138 MPa (20 ksi) design stress. e. The data in Figure A2 or A3 can be used to determine the maximum stresses to avoid failure. It is also important to note that the vessel can be damaged (e. load or stress on the pressure boundary material required to rupture a vessel) versus the time to rupture for several elevated temperatures.1 hours. Figure A3 also plots the Short Time Tensile Strengths at or near the 0.5 of tensile strength at the design temperature. h. Hence. The “code” refers to these as time-dependent properties. These are typically reported as creep or stress rupture strengths. One way to extend the time before failure occurs is to reduce the internal pressure within a vessel (i.e.. depressure it). but this requires a fairly comprehensive analysis. If the vessel was Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 24 of 38 . any equipment exposed to fire would need a follow-up inspection and assessment to determine fitness-forservice even if out-right failure did not occur. g. The scenario involves pool fire exposure whereby the internal pressure increases to that equivalent to the design allowable stress in the pressure boundary wall and then stays constant as the vessel continues to be heated.e. As long as the internal pressure results in stresses below the rupture stress. short term failures can occur as predicted based on creep or stress rupture data. that exceed the vessel’s tensile strength. Immediate failure may not be predicted because of conservative “code” design. Hence. The Short Time Tensile Strengths should be considered the limiting stresses for a given temperature (i. These figures plot rupture stress (i. Hence. This means that the material properties do not change with time up to a “code” allowable temperature that is related to a specific material. the effect of depressurisation on the time to rupture is required for a large (say 25. For example. f.e.1 hour (6 minutes) at 650°C (1 200°F).DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation values from ASME Section VIII provide a significant safety factor well below the ultimate tensile strength of the material.. if exceeded even briefly. Tensile and yield strength data for typical carbon steel are given in Table A1 and illustrated in Figure A4.. grade SA-515). during the initial heatup phase.4 mm (1 in) wall thickness) vessel fabricated from SA-516 Grade 70 carbon steel with a design allowable stress of 138MPa (20 ksi). However. 6 MPa (4 ksi) or 27. It is important to note that the vessel wall temperature can continue to increase per Figure A1. Because of the faster heat-up rate (see Figure A1).5/138 (5/20) = 25% of the MAWP to extend the time to rupture to about one hour and reduced to about 27. the vessel will be damaged.5 MPa (5 ksi) or 34. including low alloy steels. Alloy materials. side-on overpressures) due to vessel failure is illustrated in Figure A9. the tensile strength would be the limiting factor (about 117 MPa (17 ksi) at 670°C (1 240°F) from Figure A4). thinner walled vessels require faster depressurisation. a fire could simultaneously expose multiple vessels of varying wall thicknesses.. Judgement needs to be taken as to which vessels are included in the depressurisation system design. The general effect of depressurisation on distance to overpressure effects (e. In addition. then the vessel will rupture when the wall temperature reaches about 650°C (1 200°F). In such cases. i. In addition to reducing the likelihood for rupture. The material data presented above applies to typical carbon steel. This figure illustrates the depressurisation path to minimize failure potential due to fire exposure as a function of percent of MAWP. j. l. Figure A1 indicates the average wall temperature 15 minutes into the pool fire would be about 670°C (1 240°F). These varying rates of depressurisation become important when designing a common depressurisation system for interconnected vessels of different sizes and/or design pressures. the mechanical energy (gas stored under pressure) is converted to a blast wave that can damage adjacent equipment due to overpressure. Upon rupture. further depressurisation would be required to avoid rupture. depressurisation would reduce the consequences in the event of rupture due to fire exposure. Rupture can occur at lower temperatures if there are material defects or a weaker component of the vessel construction than the base metal. Based on the data in Table A1 and Figure A4. typically in the form of bulging. Possible impacts of the failure should be analysed on a case-by-case basis. Figure A4 indicates that at 670°C (1 240°F) the 69 MPa (10 ksi) stress on the carbon steel pressure boundary would exceed the yield point (about 62 MPa (9 ksi)). Figure A1 indicates it will take slightly over 10 minutes for the average vessel wall temperature to reach 538°C (1 000°F). The depressurisation path as a function of time for carbon steel of several thicknesses is illustrated in Figure A5. At the other extreme would be the case where the vessel wall temperature does not appreciably increase until the end of the 15 minute exposure in which case the metal wall sees 670°C (1 240°F) but only for a very short time. For our vessel with the 138 MPa (20 ksi) allowable design stress (thus applied stress of only 69 MPa (10 ksi) once depressured to 50% internal pressure).6/138 (4/20) = 20% of the MAWP to extend the time to rupture to about 4 hours. but less dramatic: a sample of rupture strength versus Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 25 of 38 . the vessel starts to yield at this temperature because the applied stress (138 MPa (20 ksi)) would become equal to the yield stress. In other words. The actual internal pressure where rupture occurs will likely be in between the rupture stress and tensile strength given the pool fire heat-up rate is not instantaneous. Note that this rupture stress is based on the vessel seeing a wall temperature of 670°C (1 240°F) for the full 15 minutes of pool fire exposure (see Figure A2 or A3). about 14 minutes into the fire (see Figure A1). it ignores heat-up time and assumes the wall reaches 670°C (1 240°F) at the onset of fire exposure. Thus. Rapid depressurisation required for small vessels must be balanced not only with the available disposal system capacity but also with the failure consequences of small versus large vessels. show similar strength loss with temperature as carbon steel. fragments can be produced upon failure. The bottom curve in Figure A2 (or Figure A3) indicates the internal stress would need to be reduced to about 34. In this case. Based on a maximum wall temperature of 760°C (1 400°F). m. but not the rupture stress (about 90 MPa (13 ksi)). stainless steels. API RP 521 suggests the internal pressure be reduced by 50% within 15 minutes of pool fire exposure.g. If the applied stress stays constant at 138 MPa (20 ksi) and further wall heating occurs. and nickel base alloys.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation initially at ambient temperature. k. Location and configuration of adjacent equipment (particularly important regarding mitigation of jet fire impacts). Inventory of fluid requiring depressurisation including liquid that can flash into vapour upon depressurisation. extent) can further complicate depressurisation requirements. flammability hazard (vapour cloud explosion and/or flash fire extent and potential).. 3. the designer should be aware that the values provided by all such references are not necessarily consistent with each other. reduces not only the leak rate but also the leak duration through reduction in vessel inventory. and 4). 2. Nature and extent of consequence (toxicity hazard.e.. Calculation of depressurisation mass flow rates a. The second use of a depressurisation system is to mitigate the effects of a leak. The design of a depressurisation system to mitigate consequence of leaks is dependent upon the following: 1.3. jet fires. n. maximum equilibrium temperature. the size and effects are attenuated. d. it is recommended the designs be handled on a case-by-case basis.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation temperature for type 304 stainless steel (i. the jet fire would in essence be “turned off” as the source of fuel would be depleted. environmental aspects. In many circumstances. view factors. A. duration. always Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 26 of 38 . 2. plant personnel. Manual calculations are only approximate. Given the variability in assumptions. Location of potentially vulnerable public. 5. However. environmental release). flare). equipment.7 MPa (100 psig) is commonly used as the design basis. the designer should be careful not to overanalyse the depressurisation system requirements but to select an appropriate criteria for their specific facility requirements. 4. 3. Depressurisation is one of the few mitigation measures for a jet fire whereby if the source of the jet fire were depressurised. Because of the variability in processes and reasons for depressurisation in non-fire situations. b. Consequence modelling should be performed to determine specific depressurisation goals. A. size. such as a flare. A reduction of vessel pressure through depressurisation to a “safe” location. Variability in the pool fire (heat-up rate. It is important to recognize that the tensile and rupture strength data given in this Annex are used as an example of how depressurisation calculations for pool fire exposure can be made. Once depressurised to near atmospheric pressure. Capacity of the disposal system where the depressurisation system discharges. Depressurisation systems designed to minimize leak size a. manual calculation of blowdown times by step calculations at various reducing pressure increments is no longer adequate. 18-8 grade) is shown in Table A2 and Figures A6 through A8. The sources for this data are typical industry data (1. It is critical to stay within the disposal system capacity even during utility failures such as loss of instrument air that may cause all the depressurisation valves to open simultaneously (if so designed to fail in the open position upon air failure and without a backup air supply).4.g. a starting point in the depressurisation system design would be using a total instantaneous depressurisation rate equal to no more than the capacity of the disposal system (e. a depressuring level to no more than 0. Evacuation considerations. Generally. c. With depressurisation calculations now embedded in process simulation software. 6. and can provide very misleading results depending on the assumptions made and coefficients being used.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation require subsequent verification from a more rigorous analysis. As one example. Process simulation users should recognise there often can be limitations to these built-in depressurisation solutions. A. API 521 guidelines Refer also to API 521 Section 3.19 for further guidance on depressurisation flow rates. Some packaged software calculations fail to take proper account of phase changes and heat transfer (or heat loss). Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 27 of 38 .5. high purity liquids stored near their bubble point are not handled well. b. DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation Figure A1 .API RP 521 figure on average rate of heating steel plates exposed to open gasoline fire on one side Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 28 of 38 . API RP 521 figure on effect of overheating steel (ASTM A515 grade 70) Figure A3 . grade 70) rupture stress versus time to rupture (bibliographical reference [1].Typical carbon steel (SA-515.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation Figure A2 . Page 20) Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 29 of 38 . grade 70) tensile strength and yield stress versus temperature (bibliographical reference [1].2% Set.High temperature tensile properties for typical carbon steel (1) Temperature. Grade 70. MPa Yield Stress 0.2% Set.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation Table A1 . Figure A4 . psi Yield Stress 0. Page 16) 40000 Strength or Stress PSI 35000 30000 25000 20000 15000 10000 5000 0 1000 1100 1200 1300 1400 Degrees F Tensile Strength Yield Stress Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 30 of 38 . °F Temperature. psi Tensile Strength. Reference: bibliographical reference [1]. °C Tensile Strength.Typical carbon steel (SA-515. Page 16. MPa 750 399 58 000 400 24 600 170 900 482 45 500 314 23 500 162 1 000 538 36 500 252 20 100 139 1 100 593 27 200 188 14 250 98 1 200 649 20 000 138 10 200 70 1 300 704 13 500 93 7 375 51 1 400 760 9 025 62 3 750 26 (1) Applies to SA-515 and SA-516 carbon steels. grade 70) internal pressure versus pool fire exposure time to minimize potential for vessel rupture 100% Maximum Pressure to Avoid Failure (% of MAWP) 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 0 5 10 15 20 25 30 35 40 45 50 55 60 Time from Fire Start (minutes) 1/8 Inch Thick 1/4 Inch Thick 1/2 Inch Thick 1 Inch Thick Figure A6 – 18-8 grade stainless steel (304.Typical carbon steel (SA-515.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation Figure A5 . Page 20) Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 31 of 38 . 304L) rupture stress versus time to rupture (bibliographical reference [1]. psi Yield Stress 0.High temperature tensile properties for 18-8 stainless steel (1) Temperature. °F Temperature. °C Tensile Strength.2% Set. Figure A7 – 18-8 stainless steel (304. psi Tensile Strength. Page 140. Reference: bibliographical reference [1]. Page 140) 60000 Strength or Stress PSI 50000 40000 30000 20000 10000 0 1000 1100 1200 1300 1400 Degrees F Tensile Strength Yield Stress Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 32 of 38 . MPa Yield Stress 0.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation Table A2 .2% Set. 304L) tensile strength and yield stress versus temperature (bibliographical reference [1]. MPa 1 000 538 53 000 365 14 000 97 1 100 593 48 500 334 12 000 83 1 200 649 43 000 296 11 000 76 1 300 704 35 000 241 11 000 76 1 400 760 27 000 186 10 500 72 1 500 816 20 500 141 10 000 69 1 600 871 17 650 122 --- --- 1 800 982 9 600 66 --- --- 2 000 1 093 4 900 34 --- --- (1) Applies to 304 and 304L stainless steels. DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation Figure A8 – 18-8 stainless steel (304.Effect of depressurisation on reduction of distance to overpressure effects (e.g. side-on overpressure) 100 90 80 70 60 50 40 30 20 10 0 0 20 40 60 80 100 % Reduction in Distance to an Overpressure Effect Relative to Failure @ the MAWP Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 33 of 38 .. 304L) internal pressure versus pool fire exposure time to minimize potential for vessel rupture 100% 90% Maximum Pressure (% of MAWP) 80% 70% 60% 50% 40% 30% 20% 10% 0% 0 10 20 30 40 50 60 Time from Fire Start (minutes) 1/8 Inch Thick 1/4 Inch Thick 1/2 Inch Thick 1 Inch Thick Failure Pressure (% of MAWP) Figure A9 . The vessel is warmed by the air surrounding it. It may be possible depending upon the rate of cooling for the vessel wall temperature to fall below 0°C (32°F) at which time the vessel could get an insulating covering of ‘snow’ or ice. Staged flash. may often be required. The heat transfer between vapour and vessel walls is usually much poorer than between a liquid and the vessel walls. Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 34 of 38 . Calculations are usually performed by depressurising from maximum design pressure and minimum ambient temperature to atmospheric pressure. with vessel heat conduction calculation at each stage. Heat transfer downstream of the depressurisation valve can be sufficiently high that an internal sleeve. it is possible for water in the air (i. Staged flash. providing a convective separation gap between the depressurised fluid and outer pipe wall. 3. 4. while the fourth is the most complex and time consuming but most accurate/ least conservative. depending upon the fluid inside the vessel.1 Proposed methods a. The first method is the most simple.2 A theoretical (reversible) adiabatic. least accurate. Staged flash. and most conservative. However. the second additionally requires vessel design data and the third and fourth additionally require details of the depressurising procedure. depressurisation rate. Additionally the thermal capacity of all but the highest pressure gas is much lower than that of the vessel walls and temperature depressurising effects on vessel walls containing only vapour are generally small and only of concern if the pressure is very high (greater than 100 bar (1. General assumptions a. There are four methods of increasing complexity and accuracy for estimating minimum vessel and pipework temperatures during depressurisation: 1. humidity) to condense on the vessel wall maintaining the wall temperature no lower than the water dew point.e. b. Specialist software such as “BLOWDOWN™”0 shall be used to account for different fluid strata within a vessel. More reasonable starting conditions might be maximum design pressure and minimum fluid flowing temperature. Further cooling of the wall temperature could then occur to relatively low temperature. 2. The first method requires only process data. B.450 psi)). instantaneous flash. d. if the contents of the vessel would not be shut-in long enough for the temperature to drop to minimum ambient temperature. infinite heat transfer between vessel and liquid. or the vessel is normally operated at subambient temperatures. but experience has shown that it may not have a significant effect during the rapid depressurising timescale in certain cases (particularly non-fire scenarios). Process simulation programs generally assume the fluid is perfectly mixed in the vessel. b.. c. finite heat transfer between vessel and liquid.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation Annex B (Normative) Methods for estimating the minimum wall temperature of depressurised vessels and pipework B. and so on. c. The temperature drop for the next stage is decided upon. Vessel fluid cooling comes from expansion of the contained gas and the liquid standard enthalpy change of vaporisation (or heat of vaporisation). If the vessel liquid boils (or vapourises) significantly as pressure is reduced. calculations should be done at both the reasonably highest and lowest liquid levels that may occur in the vessel. To perform this type of calculation.3 B. The height of liquid before each depressurising stage is calculated from the liquid fraction. such as in the pressurised storage of pure gases as liquids. In processes where the liquid phase is relatively non-volatile. The vessel depressurises instantaneously. 1. The method is similar Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 35 of 38 . and the heat lost from the vessel walls over the fall in temperature is added to the internal energy of the vessel contents. 5. Method 2 a. B. Return to stage 1 until the vessel pressure is down to the required figure. The heat transfer from the vessel wall in contact with the liquid is assumed to be infinite. The ensuing fluid temperature is taken to be the minimum wall temperature. 4. several oversimplifying assumptions are made: 1. or it could be insensitive to liquid level. vessel contents are depressurised isenthalpically using a process simulator such as HYSYS. b. then the minimum possible temperature could be experienced at maximum liquid levels. b. so that the liquid contents of the vessel and the wetted wall are at the same temperature. Vessel contents are well mixed. 3. 2. 3.DRAFT 3 January 200 B. The thermal capacity of the wetted vessel walls is calculated. This method is used for vapour filled vessels and also for vessels partially filled with liquid where the liquid is removed before significant depressurising occurs.4 Vessel contents a. Therefore. 7. the lowest reasonable liquid level (with maximum vapour volume) should be considered to accurately calculate the expected minimum temperature. If significant liquid vaporisation occurs. b. when using Methods 2 to 4 below. Method 3 a. 2.6 GP 44-25 Guidance on Practice for Depressurisation Using Method 1.5 B. 6. In this method. A reversible. the minimum temperature is experienced when vapour volume is greater. the thermal mass of the vessel is taken into account. The vapour and liquid streams are re-mixed. The amount of vapour considered in the model is reduced so that the volume of fluid corresponds to the vessel volume. Method 1 a. The vessel is depressurised until the fluid temperature drop reaches the required figure. adiabatic depressurisation to the final system pressure is achieved. The contents of the vessel are to be depressurised in steps corresponding to given small temperature drops. the vessel is divided up into sections small enough that the properties of the section can be assumed to be constant throughout the section. 2. the heat capacity of liquids are much higher than vapour and heat transfer into the gas is generally much less than the liquid temperature changes. Heat flows from the vessel to the liquid and vapour are calculated. The vessel is depressurised until the fluid temperature drops to the required value. assigning heat capacities to the points. If the model is too simple. The amount of vapour considered in the model is reduced so that the volume of fluid corresponds to the vessel volume.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation in theory to Method 2. The finite difference model differs from the finite element model in that it represents the vessel by a network of points. if liquid is still present in the vessel. 7. The number of sections or points depends upon the resolution of the temperature profile required by the mechanical engineer. and it may prove necessary to recalculate the heat transfer coefficient as the depressurising continues. In practical terms. and upon the needs of the mathematical method.7 1. and from this. Warming of the fluid by the vessel walls is now dependent upon the time taken for depressurising. Therefore the time/ pressure relationship shall be calculated. Method 4 a. Liquid height before each depressurising stage is calculated from the liquid fraction. The most important properties in this case are obviously the heat flows and temperature gradients. The differences between the two representations are more of mathematical and computing concern than of concern to the engineer. 3. but entails a detailed analysis of the temperature profiles within the vessel walls at each de-pressuring stage. Return to stage 1 until the vessel pressure is down to the required figure. The temperature profile throughout the vessel walls is calculated using a dynamic finite element or finite difference model of the system. The vapour and liquid streams are re-mixed. 2. the assumption that the properties can be assumed constant across the section is no longer valid. 3. b. There are different heat transfer coefficients for thermal flux into the liquid and vapour. In a finite element model. 6. or smaller sections. and Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 36 of 38 . the new temperature profile is determined. Each point corresponds to the centre of the sections in the finite element model. This greater detail can allow a lower minimum temperature to be withstood by a metal than would otherwise be permissible. except for step 5 which is replaced as follows: 1. 4. but heat transfer between the fluid and the vessel is finite and either calculated or estimated before depressurising starts. 4. B. The temperature drop for the next stage is decided upon. This method is based on the previous method. The stages are per Method 3 above. The usual way to determine whether the model is fine enough is to rerun the model with a finer grid. usually by assuming choked flow through the venting orifice. The depressurising shall be done in small time steps. and thermal conductivities to the connections between points. b. 5. This allows the calculation of vessel wall stresses from the thermal gradients. The choice of method probably depends upon the software available to the engineer. The time required for that pressure drop is calculated. Heat conduction between adjacent sections is calculated by a computer program. 8. Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 37 of 38 . then the model shall be rerun with a yet smaller grid. If there is. until two grids give consistent results.DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation see whether there is any significant change in the results. DRAFT 3 January 200 GP 44-25 Guidance on Practice for Depressurisation Bibliography th [1] “Digest of Steels for High Temperature Service”. Simmons and H. Canton. 1970. ASTM Data Series DS 11S1. C.F. W. Downloaded Date: 6/17/2008 11:12:31 PM The latest update of this document is located in the BP ETP and Projects Library Page 38 of 38 . Supplement to Publication DS 11. Simmons and H. Cross. 5 Edition. formerly ASTM STP 180. Ohio. ASTM Special Technical Publication 151. ASTM Special Technical Publication 124.F. [2] “An Evaluation of the Elevated Tensile and Creep-Rupture Properties of Wrought Carbon Steel”. published by the Timken Roller Bearing Company. Cross. C. January. January 1952. October 1953. 1946. [4] “Report on the Elevated-Temperature Properties of Stainless Steels”. ISBN 8031-2004-4. [3] “Report on the Elevated-Temperature Properties of Chromium-Molybdenum Steels”. W.