Chemical Plant spacing

Factory Mutual Property Loss Prevention Data Sheets7-44 17-3 September 1975 Revised December 1998 Page 1 of 6 SPACING OF FACILITIES IN OUTDOOR CHEMICAL PLANTS Table of Contents Page FLAMMABLE VAPOR OR GAS EXPLOSIONS IN THE OPEN .................................................................. 2 FLAMMABLE VAPOR OR GAS EXPLOSIONS IN PROCESS EQUIPMENT ............................................ 2 OTHER VESSEL FAILURES ........................................................................................................................ 2 DETONATIONS ............................................................................................................................................. 3 DAMAGE FROM BLAST WAVES ................................................................................................................ 3 SPACING OF FACILITIES ............................................................................................................................ 4 SEPARATION GUIDELINES ......................................................................................................................... 5 List of Figures Fig. 1A. Spacing distances for outdoor chemical processing equipment. (English units) ............................ 3 Fig. 1B. Spacing distances for outdoor chemical processing equipment. (Metric units) .............................. 4 ©1975 Factory Mutual Engineering Corp. All rights reserved. No part of this document may be reproduced, stored in a retrieval system, or transmitted, in whole or in part, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without written permission of Factory Mutual Engineering Corp. The severity and probability of potential fires or explosions. Factory Mutual recommended practices for flammable liquid storage tanks. but it is now vented. The shock wave moves at sonic velocity and does damage quite similar to that done by detonation of TNT or other explosive. further chemical reaction or combustion of gases does not add to the shock wave. These are usually larger than those produced by a detonation and do not travel as far. The reaction progresses at a rate below sonic velocity. The spacing needed is based on many factors. vapor or gas explosions in process equipment. There were 108 known accidental unconfined vapor-cloud explosions in a recent 42-year period. a deflagration reaction can occur. storage facilities. If this pressure is above the failure pressure of the vessel. For additional details see Data Sheet 7-42. a large proportion of the liquid instantaneously vaporizes. the deflagration reaction may continue. and the primary effect on the surroundings is intense heat. it usually does not add to the explosion damage. 2. generating pressure from boiling vapors or decomposition products. except that the liquid is usually above its normal boiling point. If the vapors enter an enclosure before ignition and the hot gases are confined. All rights reserved. Missiles also are produced by pieces of the vessel. The confined energy is then released all at once. The susceptability of building contents and equipment contents to direct and subsequent damage. When the pressure is released by vessel failure. exposure from buildings. ©1975 Factory Mutual Engineering Corp. The susceptibility of building components and outdoor equipment to direct damage. the reaction may accelerate. or exposure from outdoor storage of other commodities give distances which will furnish acceptably low levels of damage from heat and prevent fire spread due to the applicable fire exposure. A shock wave similar to that produced by a detonation can result. A deflagration is a propagating reaction where the energy transfer from the reaction zone to the unreacted zone is accomplished through ordinary heat or mass transfer. The energy release then is similar to that from a confined vapor explosion. the energy generated in the form of compressed gases is dissipated by expansion as fast as it is generated. OTHER VESSEL FAILURES When an exothermic reaction occurs in a vessel without sufficient cooling or venting. until the vessel reaches its failure pressure. Unless a chemical subject to detonation is present. adding to the shock wave resulting from the expansion of the confined gases already present. . After the vessel failure. the vessel may fail catastrophically. The relative value and importance of facilities. and detonations. The different types of explosions which may be expected in chemical plants may be classified as vapor or gas explosions outside of equipment. Since the deflagration wave is slower than the shock wave and behind it. a pressure buildup can result until vented by failure of the enclosure. 3. There have been a few cases where massive vapor or gas release has covered many acres and where the reaction increased in velocity to the point where a damaging shock wave was produced. including: 1. other vessel failures. The hot gases are confined so pressure can build up to about six to ten times the original absolute pressure. FLAMMABLE VAPOR OR GAS EXPLOSIONS IN PROCESS EQUIPMENT Where the vapor space of a process vessel is in the explosive range. Where the only hazard to be considered is fire.7-44 17-3 Page 2 Chemical Plants Factory Mutual Property Loss Prevention Data Sheets The extent of a single fire or explosion loss in an outdoor chemical plant can be minimized by providing separation between process units. FLAMMABLE VAPOR OR GAS EXPLOSIONS IN THE OPEN Flammable vapor or gas explosions are normally deflagrations. Where a small explosion takes place in the open. 4. and important buildings. DAMAGE FROM BLAST WAVES Studies indicate that the characteristics of the blast waves from vessel ruptures and from detonations are not identical.5 m3) from Fig. 1. These curves are Fig.000-gallon (7.000 gallons (3. If the liquid is heated more than 50°F (28°C) above its atmospheric boiling point so that a substantial amount of the liquid will turn to vapor on vessel failure. Sometimes detonable materials such as acetylides or organic peroxides are accidently produced or accumulated. The shock wave produced is somewhat similar to that produced by commercial explosives or by catastrophic rupture of a vessel. However. and loss experience. For example. . use only the volume of the vapor phase under the worst credible condition.8 m3) of liquid and is subject to a thermal explosion with failure expected at 150 psig (1000 kPa) (10 bars). a 2. 1A. 1B (for metric units). and Fig.000 gallons (7. the failure pressure should be increased by a percentage equal to the percent of liquid in the vessel. If a deflagration is confined to the vapor phase and the liquid in the vessel is below its atmospheric boiling point. the effective pressure at failure will be 150 psig × 150% = 225 psig (1500 kPa) (15 bars). the energy released by expanding gas liberated by catastrophic rupture of a vessel can be approximately related to the equivalent weight of TNT. If these can be anticipated and predicted. 1A (for English units). the TNT equivalent will be about 15 lb (6 kg).5-m3) reactor contains 1. they are so similar that a common set of curves can be derived which agrees reasonably well with theoretical calculations. All rights reserved. and the full vessel volume should be used. ©1975 Factory Mutual Engineering Corp. If the energy can be predicted in terms of TNT equivalent. As an estimate the vessel failure pressure may be taken as four times the design pressure. the extent of damage to structures or equipment of varying types of construction can be predicted with reasonable accuracy.Chemical Plants Factory Mutual Property Loss Prevention Data Sheets 7-44 17-3 Page 3 DETONATIONS A detonation is a propagating reaction which progresses at or above sonic velocity in the reactant. Since the reactor will be 50% full of an overheated liquid. This is done by using the volume of the vessel and the failure pressure of the vessel. it may be possible to provide safe isolation for such hazardous processes. (English units) Using the curves. Fig. With a reactor volume of 2. Spacing distances for outdoor chemical processing equipment. experimental tests. If the quantity and type of material present is known and the TNT equivalent can be obtained. tank farms. important buildings. load-bearing masonry construction. Spacing in accordance with that for light wall sheathing is recommended to minimize missile damage. and ensuing fires is expected within 100 ft (30 m). Spacing distances for outdoor chemical processing equipment. 1A or Fig. Sufficient spacing between major process blocks.7-44 17-3 Page 4 Chemical Plants Factory Mutual Property Loss Prevention Data Sheets Fig. Severe blast damage normally occurs over a greater distance than missile damage. SPACING OF FACILITIES Separation or other protection is normally provided between an exposed facility which must be protected and a hazard which may constitute a fire or blast exposure. Fig. and major plant utilities is normally needed to confine fire and explosion damage to the major unit of origin. 1B may be used to determine the extent of expected damage. . Such window damage may be acceptable unless vital facilities inside such as computer or control equipment could be damaged by glass fragments or the blast wave entering the room. or both. Greater spacing for fire exposure only may be needed as outlined in applicable Factory Mutual recommended practices. acetylenic compounds. It is seldom practical to isolate such individual facilities except important control houses and unusually hazardous process equipment. commercial explosives. a spacing of about 110 ft (33 m) is specified for light sheathing and framing. Beyond 220 ft (66 m) some windows may be broken. it is possible to estimate the approximate radius of severe damage using the same curves. damage from missiles. Extensive damage to large windows may be expected for about 220 ft (66 m). ©1975 Factory Mutual Engineering Corp. Some materials used in chemical processes may be subject to detonation. vapor cloud explosions. (Metric units) Once the TNT equivalent is obtained. 1B. and compounds of similar instability. but interior damage should be light. All rights reserved. In the above example where a yield of 15 lb (6 kg) of TNT is expected. and process vessels. Equipment and structures within a process block are usually spaced in accordance with production requirements. Wherever there is an explosion hazard. These include some organic peroxides. Chemical process equipment can normally withstand substantial blast pressures but is more subject to missile damage. Many facilities expose one another because they constitute both high values and a severe hazard. cooling towers. process area. Large quantities of ordinary combustibles in a building or in yard storage. based on the most severe combination. Separation between a high value facility and a vapor cloud exposure: ©1975 Factory Mutual Engineering Corp. until adequate isolation can be provided. yard storage. 2. Processes or storage containing unstable or explosive materials. tank farm. and the separation or other protection needed. Vapor cloud explosions in the open can cause damage in excess of practical separation distances. Processes at pressures over 5000 psi (34. Process equipment of high value or importance to production. The following can be sources of blast or missile damage due to rupture of process vessels or detonations. It may not be possible to rearrange existing plants to provide the needed spacing. etc. processing building. or the evaluation of existing plants. Many facilities will fall into more than one group. 3. warehouses. 1. additions to existing plants. or yard storage. or nature of the reaction itself. Process reactions of an exothermic nature where. 1. Included would be some nitration or polymerization reactions. For flammable liquids. Large quantities of flammable liquids or gases in a tank. water and electrical services. 3. 4. flammable liquid storage tanks. which can explode in the open or enter other buildings: processes or storage facilities at such temperatures and pressures that release of large quantities of flammable vapors or gases can be expected upon failure of equipment. 6. Important control rooms and administrative buildings. compressors. within buildings or outside. Unprotected process or storage tanks under pressure where weakening of the pressurized shell can result from an external exposure fire. High value storage facilities. drainage area. pump houses. Separation between a high value facility and fire exposure: Distances for fire exposure only should be based on applicable Factory Mutual recommended practices for exposure protection. Processes using equipment of large volume which must be taken through the explosive range at periodic intervals for maintenance purposes. The following can be the source of a flammable gas or vapor cloud. . Judgement must be used to determine the extent of damage which can be expected.Chemical Plants Factory Mutual Property Loss Prevention Data Sheets 7-44 17-3 Page 5 Separation or other physical protection is normally needed to protect the following from an exposing fire or explosion: 1. such as large tanks. 2. Utility buildings and equipment vital to production or fire protection.5 MPa) (345 bars). and maintenance shops. such as major boilers. The following can be sources of fire exposure and need separation from high value processes. drainage facilities. dikes. 2. All rights reserved. 5. a strong possibility of the reaction getting out of control exists. and need separation from high value processes: 1. and the slope of land should be taken into consideration. but changes made over the years should progress toward a satisfactory level of safety. 2. flare stacks. because of newness. Unusually severe hazards should be minimized through extraordinary preventive procedures such as blast resistant barricades. SEPARATION GUIDELINES The following guidelines are for fire separation and predictable explosion separation for the planning of new plants. lack of complete investigation. tank farms. or burning pits. Separation needed is the greatest distance. 4. Processes which have a significant history of explosions. For materials such as LP gas. hydrogen peroxide. commercial explosives. 1 or blast exposure in guideline No. organic peroxides. the exposing hazard should be barricaded by blast and missile resistant construction or the exposed facility should be of blast and missile resistant construction. more. Departments of the Army. 3 is seldom practical. FMELPC June 1975 ©1975 Factory Mutual Engineering Corp. the distance should be not less than 100 ft (30 m). use spacing guide lines in Figures 1 and 2. use the specific spacing recommendations contained therein. with a minimum of 100 ft (30 m).) There is no NFPA standard on this subject. if ground slopes so that flow is toward the building. a separation distance beyond that required for fire exposure in guideline No. Separation between a high value facility and a blast or missile exposure: a. 3. For other blast or missile hazards. ethylene oxide. NAVFAC P-397. ammonium nitrate. All rights reserved.. Where needed distances cannot be provided. etc. Navy and Air Force Manual (TM 5-1300. (Design data is available in Structures to Resist the Effects of Accidental Explosions. AFM 88-22) November 1990 Edition. . b. for which Factory Mutual loss prevention practices are provided. or if discharge of vapors is very likely as from flare stacks or normally venting relief valves.7-44 17-3 Page 6 Chemical Plants Factory Mutual Property Loss Prevention Data Sheets Because of the unpredictibility of the area which may be affected by a release of flammable vapors or gases in the open. To minimize the possibility of entry of vapors into buildings.