ANSI-HI-9-6-1

ANSI/HI 9.6.1-1998 American National Standard for Centrifugal and Vertical Pumps ANSI/HI 9.6.1-1998 for NPSH Margin 9 Sylvan Way Parsippany, New Jersey 07054-3802 www.pumps.org This page intentionally blank. Copyright © 2000 By Hydraulic Institute, All Rights Reserved. org Approved March 3.ANSI/HI 9. Inc.pumps. .1-1998 American National Standard for Centrifugal and Vertical Pumps for NPSH Margin Secretariat Hydraulic Institute www.6. Recycled paper Copyright © 2000 By Hydraulic Institute. All Rights Reserved. 1998 American National Standards Institute. Published By Hydraulic Institute 9 Sylvan Way. NJ 07054-3802 www. The use of American National Standards is completely voluntary. or procedures not conforming to the standards. in an electronic retrieval system or otherwise. processes. CAUTION NOTICE: This American National Standard may be revised or withdrawn at any time. whether he has approved the standards or not.org Copyright © 1998 by Hydraulic Institute All rights reserved. in the judgement of the ANSI Board of Standards Review. or withdraw this standard.pumps. marketing. Requests for interpretations should be addressed to the secretariat or sponsor whose name appears on the title page of this standard. revise. No part of this publication may be reproduced in any form. and that a concerted effort be made toward their resolution. The American National Standards Institute does not develop standards and will in no circumstances give an interpretation of any American National Standard. purchasing. Substantial agreement means much more than a simple majority. Consensus requires that all views and objections be considered. All Rights Reserved. . Parsippany. from manufacturing. The procedures of the American National Standards Institute require that action be taken periodically to reaffirm. but not necessarily unanimity. Consensus is established when. Printed in the United States of America ISBN 1-880952-25-4 Copyright © 2000 By Hydraulic Institute. no person shall have the right or authority to issue an interpretation of an American National Standard in the name of the American National Standards Institute. consensus and other criteria for approval have been met by the standards developer. without prior written permission of the publisher. Moreover. or using products. Purchasers of American National Standards may receive current information on all standards by calling or writing the American National Standards Institute. their existence does not in any respect preclude anyone.American National Standard Approval of an American National Standard requires verification by ANSI that the requirements for due process. substantial agreement has been reached by directly and materially affected interests. . . . . . . . . . .6. . . . . . . .1. . . . . . . . . . . . . . .1. . . .2 Suction energy level . . . . . . . .4 Nuclear power/cooling tower . . . . . . . . . . . . . 10 Appendix A Index . . . . . . .6. . . . . . . .1 Petroleum process pumps. . . . . . . . . . .2 Suction energy determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 9. . . . . . . . . . . . . .5. . . . .2 Chemical process pumps . . . .1. . . . . . . . . . . . . . . . . 7 9. . .6. . . .6. . . . . . . . . . . . . . . . .5 Application considerations . . . . . . . . . . .10 Pipeline . . .5. . . . . .3 Electric power pumps . . . . .6.1. . .1. . 6 9. . . . . . . . . . . . . . . . . . . 9 9. . . . . . .1. 8 9. . . . . . . . . . . . . . . . . . . . . . . .1. . . . . . . . .9 Slurry . . 7 9. . . . . .5. . . . . . . . . . . . . . . . . . . . . . . . . . .6. . . . . . 6 9. . .5. . . . .5. . . . .6. . . . . . . . . .1. . . .5. . .1 Suction energy factors. . . . . . . . . . . . . . . . .1. . . .6. . 10 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6. .1. .7 Pulp and paper . . . . . . . . . 2 9. . . . . . . . 9 9. .1. . . . . . . . . .6. . . . . . . . . . . . .6. . 9 9. .5 Water/wastewater . . . . 13 iii Copyright © 2000 By Hydraulic Institute.6. . . . . . . . . . . . . . . . .5. . . 9 9. . . . . . . 4 9. .Contents Page Foreword . . . . . .6. . . . . . . . .1. . . . . . . .1 Introduction . .2. . . . . . . . .1. . . . .3 Cavitation damage factors . . . . . .2. . . . . . . . . .6 Summary. . . . . . 1 9. . . . . . . . . . . . . . . . .1. . . . . . . . . . . . .5. . . .1.1 Pump NPSH margin . . .5. . . . . . . . . . . . . . . . . . . . . . . . . . . 1 9. . . .6. . . .6 General industrial . . .6. . . . . . . . . . . . . . . . .6. 6 9. . . . . . . . .1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1. . . . . . . . . . . . . . . . . . . . . .6. . v 9. . . . . . . . All Rights Reserved. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 9. . . .6. .4 NPSH margin ratio recommendations . . . . . . . . . . .1. . . . . . . . .11 Waterflood (injection) pumps. . . . . . . . . . . . . . . . . . . . .5. . . . . .8 Building services . . . . . . . .6. . . . . . .6. . . . . . 1 9. . . . . .5. . . . . . . . . . . . . . . . . . .1. All Rights Reserved. .This page intentionally blank. Copyright © 2000 By Hydraulic Institute. rating. d) To increase the amount and to improve the quality of pump service to the public. If a dispute arises regarding the contents of an Institute publication or an answer provided by the Institute to a question such as indicated above. the point in question shall be referred to the Executive Committee of the Hydraulic Institute. All Rights Reserved. safety. operating characteristics. 2) Use of Hydraulic Institute Standards is completely voluntary. c) To appear for its members before governmental departments and agencies and other bodies in regard to matters affecting the industry. Standards. among other things: a) To develop and publish standards for pumps. v Copyright © 2000 By Hydraulic Institute. transportation and other problems of the industry. dimensions. Questions arising from the content of this Standard may be directed to the Hydraulic Institute. Section B: “An Institute Standard defines the product. of the By-Laws of the Institute. construction. process or procedure with reference to one or more of the following: nomenclature. e) To support educational and research activities. b) To collect and disseminate information of value to its members and to the public. Definition of a Standard of the Hydraulic Institute Quoting from Article XV. f) To promote the business interests of its members but not to engage in business of the kind ordinarily carried on for profit or to perform particular services for its members or individual persons as distinguished from activities to improve the business conditions and lawful interests of all of its members. and to this end. distribution. tolerances. to help the Hydraulic Institute prepare even more useful future editions. the purchaser and/or the user and to assist the purchaser in selecting and obtaining the proper product for a particular need. Existence of Hydraulic Institute Standards does not in any respect preclude a member from manufacturing or selling products not conforming to the Standards. quality. engineering.” Comments from users Comments from users of this Standard will be appreciated. composition. It will direct all such questions to the appropriate technical committee for provision of a suitable answer. performance.Foreword (Not part of Standard) Purpose and aims of the Hydraulic Institute The purpose and aims of the Institute are to promote the continued growth and well-being of pump manufacturers and further the interests of the public in such matters as are involved in manufacturing. safety. Purpose of Standards 1) Hydraulic Institute Standards are adopted in the public interest and are designed to help eliminate misunderstandings between the manufacturer. material. which then shall act as a Board of Appeals. . testing and service for which designed. . If no revisions are made for five years. graphs and sample calculations are also shown in both metric and US units. A. Kellogg Co. Marine Machinery Association National Pump Co. Inc. LLC Alden Research Lab Bechtel Corporation Black & Veatch Brown & Caldwell Camp Dresser & McKee CH2M Hill Chas S.. and revisions are undertaken whenever it is found necessary because of new developments and progress in the art. Montana State University Montgomery Watson MWI. All Rights Reserved. It describes the benefit to pump life when the NPSH available is greater than the NPSH required by a suitable margin. Messina Pump & Hydraulics Consultant John Crane. Monsanto Co. Johnston Pump Co. Crane Pump & Systems DeWanti & Stowell Dow Chemical DuPont Engineering Electric Power Research Institute Engineering Devices Resource Group ENSR Consulting & Engineering Essco Pump Division Fairbanks Morse Pump Florida Power Corporation Floway Pumps Flowserve Corp. Lewis & Co. Ahlstrom Pumps. Inc. Raytheon Engineering & Constructors Robert Bein. and suggests margins for various applications. Inc. it is important that the selected units of measure to be applied be stated in reference to this standard. Grundfos Pumps Corp. W. Inclusion in this list does not necessarily imply that the organization concurred with the submittal of the proposed standard to ANSI. PC Garvin & Associates Price Pump Co. Fluor Daniel. Consensus for this standard was achieved by use of the Canvass Method The following organizations. Lawrence Pumps. corresponding US units appear in brackets. Iwaki Walchem Corp. M.Revisions The Standards of the Hydraulic Institute are subject to constant review. If no such statement is provided. William Frost & Associates vi Copyright © 2000 By Hydraulic Institute. PACO Pumps Patterson Pump Co. Ingersoll-Dresser Pump ITT Industrial Pump Group ITT Flygt Corp. Chesterton Company Agrico Chemical Corp.P.W. Units of Measurement Metric units of measurement are used. Since values given in metric units are not exact equivalents to values given in US units. recognized as having an interest in the standardization of centrifugal pumps were contacted prior to the approval of this revision of the standard. Malcom Pirnie. Moving Water Industries Oxy Chem National Pump Co. Charts. Inc. metric units shall govern. Inc. J. Scope This standard applies to centrifugal and vertical pump types. the standards are reaffirmed using the ANSI canvass procedure. All Rights Reserved. Inc. a working committee met many times to faciliate the development of this standard. Ingersoll-Dresser Pump Co. Price Pump R. Summers Engineering. Systecon. The Process Group. Sundstrand Fluid Handling Robert Stanbury. ITT Industrial Pump Group OTHER MEMBERS Ronald Brundage. the committee had the following members:” CHAIRMAN .Allan Budris. ITT Industrial Pump Group Herman Greutink. At the time it was developed. Johnston Pump Al Iseppon. Inc. Barry Erickson.Sewage & Water Board of New Orleans Skidmore South Florida Water Management Southern Company Services. Sta-Rite Industries Ray Perriman. US Bureau of Reclamation US Army Corp of Engineers “Although this standard was processed and approved for submittal to ANSI by the Canvass Method. Sta-Rite Industries Stone & Webster Engineering Sulzer Bingham Pumps. LLC Union Pump Co. Flowserve Corporation vii Copyright © 2000 By Hydraulic Institute. Greg Case. . ITT Flygt Fred Buse. Inc. Inc. Copyright © 2000 By Hydraulic Institute.This page intentionally blank. All Rights Reserved. . the rate of flow of the pump. The full published pump head will not. The NPSH at incipient cavitation can be from 2 to 20 times the 3% NPSHR value. the NPSHR of a pump is the NPSH that will cause the total head (first stage head of multistage pumps) to be reduced by 3%. the vibration and possibly the reliability of a centrifugal or vertical pump and mechanical seal may be significantly affected if an appropriate Net Positive Suction Head (NPSH) margin is not provided by the system above the published Net Positive Suction Head Required (NPSHR) by the pump. as the suction energy of a centrifugal pump increases. Unless advised otherwise. the pump speed. NPSHA = hatm + hgs + hvs + Zs – hvp Where: hatm = atmospheric pressure head hgs = suction gage head hvs = suction velocity head zs = hvp = Figure 9.HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 9. and that it is based solely on the 3% head drop criteria. however. minus the NPSH Required by the pump.1.5 times the NPSHR value just to achieve the 100 percent head value. and measured at the inlet to the pump. Most standard low suction energy pumps can operate with little or no margin above the NPSHR value. The head will be 3% less than the fully developed head value (see Figure 9. vibration.6.1. . The NPSH Margin Ratio is the NPSHA divided by the NPSHR. A few manufacturers do include the NPSH Margin in their pump NPSHR curves which then supersede the guidelines spelled out in this standard. It is also recognized that. NPSHR is by no means the point at which cavitation starts. increases the suction energy of the pump.2 Suction energy level The suction energy level of a pump increases with the casing suction nozzle size. the service life of the pump. the user must assume that there is no margin in the published NPSHR. The higher ratios are normally associated with high suction energy pumps or pumps with large impeller inlet areas. The rpm ties directly to the inlet tip speed of the impeller and relative inlet 1 Copyright © 2000 By Hydraulic Institute.6.6 Centrifugal Pump Tests for further details on the definitions of NPSHA and NPSHR. without seriously affecting Most pump manufacturers use the industry standard 3% head drop for NPSHR values and provide the NPSH Margin recommendations separately. be achieved (by definition) when the NPSHA equals the NPSHR of the pump. The 3% head drop criteria was selected for the NPSHR value based on the ease of determining the exact head drop off point. All Rights Reserved. the suction specific speed and the specific gravity of the pumped liquid. 9. however. due to flow blockage from cavitation vapor in the impeller vanes. that level is referred to as incipient cavitation. It can take up to 2.1 Pump NPSH margin 9. so does the need for a larger NPSH margin above the 3% NPSHR of the pump. or the specific gravity.1 suction elevation head liquid vapor pressure head See the ANSI/HI 1.1. The NPSH Margin is defined as the NPSH Available (NPSHA) at the pump inlet.1 Introduction HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin The noise.6. By Hydraulic Institute definition. The suction nozzle size is used for simplicity because it approximates the impeller eye diameter and ties to the rate of flow of the pump. depending on pump design. to avoid excessive noise.1). The Net Positive Suction Head Available (NPSHA) is the total suction head available. does not mean that providing that much NPSHA will necessarily give satisfactory pump life.6.1.6. Anything that increases the velocity in the pump impeller eye. over the vapor pressure of the liquid pumped corrected to the center line of the impeller (or impeller inlet vane tip datum if vertically mounted). and possible cavitation erosion and seal damage. Just because the definition uses the word Required. Higher or lower rates of flow cause a mismatch between the angle of the approaching liquid and the impeller vane inlet tips. IMPELLER VANE Figure 9. • Operation away from the best efficiency point (BEP) of the pump. . and resulting NPSH margin requirements of a pump. and suction recirculation adds to the suction energy level. as found in split case pumps have higher suction energy levels due to the right angle turn in front of the impeller.1.HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 velocities.000 U.000 U.1.D.1. (120 ft/sec) are considered high suction energy. such as found on two or three vane impellers (see Figure 2). can allow the high discharge pressure (energy) to recirculate into the impeller suction at low rates of flow. The higher the value the higher the suction energy. for more information. radial inlets. The greater the variation in velocity across the impeller inlet and the higher the magnitude of velocities. Suction specific speed values below approximately 8.3 for suction specific speeds between these values.2 • The incidence angle between the inlet impeller vanes and the approaching liquid.6.000 metric (20. the higher the energy level. All Rights Reserved. See ANSI/HI 9.) • The specific gravity of the liquid pumped. The greater the incidence the greater the turbulence and suction energy. NPSHR is based on 3% head drop at BEP. Overlap values less than approximately 15 degrees.6.3-1997.6. use one half total rate of flow. In double suction pumps. • The geometry of the inlet piping to the pump. units) generally represent low suction energy. The NPSHR in the suction specific speed is appropriate as a measure of suction energy in that larger impeller eye diameters are normally required for lower NPSHR values. Overlap is defined as the angular amount that the trailing edge of one vane (low pressure side) overlaps the inlet leading edge of the following adjacent vane (at the outer diameter). Values below approximately 15 m/sec. Those used in the above definition are factors which are typically available from standard pump manufacturer’s technical literature. • 9.1 Suction energy factors The overlap of the impeller vanes. (Note: Q is the BEP rate of flow entering the impeller eye. more than used in the above definition. See Figure 9. while above approximately 23. For this reason. At reduced rates of flow the pump may operate in its suction recirculation region. and the suction specific speed is also dependant on rpm and rate of flow. which increases the impeller tip speed. For general information a list of suction energy factors is provided below: • • 15° VANE OVERLAP The peripheral velocity at the O. Centrifugal and Vertical Pumps for Allowable Operating Region. Operation off BEP rate of flow also increases the incidence angle to the impeller vanes.S. • Thermodynamic properties of the liquid. while values above approximately 35 m/ sec. The suction specific speed of the pump (S = n × Q½/(NPSHR)¾). of the impeller eye. 2 Copyright © 2000 By Hydraulic Institute. The turbulence (added suction energy) that is generated at the pump inlet from piping turns and large changes in pipe diameter adds to the suction energy at the pump inlet. Cold water has one of the highest energy levels for imploding cavitation bubbles. • ROTATION The geometry of the pump inlet. See section on Electric Power pumps for more details.000 metric (7. Typically an impeller is designed to have a “zero” incidence angle at design rate of flow. (50 ft/sec) are generally considered low suction energy. units) are considered high suction energy.S.2.6. Manufacturers of custom engineered pumps may use alternate evaluation methods to establish NPSH margin requirements and these would supersede the guidelines spelled out in this standard. Many factors are known to contribute to the suction energy level. 1. reducing the suction nozzle size.HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 9.6. If this pump were to be operated at 3.1.6. will not reduce the true suction energy of a pump. decrease nozzle size by one size. and reduced mechanical seal life from cavitation. and the NPSH margin level required to avoid these undesirable effects. 3 Copyright © 2000 By Hydraulic Institute. with the higher values applying to high and very high suction energy pumps.6. are considered high suction energy pumps. which will accurately tie all of these factors together to predict pump noise.2) increase suction nozzle size by one or two sizes before using Figure 9.3) is a simplified method for identifying high suction energy pumps.3. an end suction pump with a 10" suction nozzle size and 9.1. All Rights Reserved.6. and continuous operation outside the preferred operating region of the pump.3. vibration. As an example. and normally has a close relationship to the impeller eye.1.6. • For axial split case (side Suction) pumps.5 to 2.6.6. (see Figure 9. .2.1. Therefore. It could even increase cavitation. The attached graph (Figure 9.1. should have the suction nozzle decreased by at least one size before using Figure 9. or higher.3B (US units) Notes for Figure 9. erosion.3 because it is more often available to the pump user.0 times the values shown in Figure 9.6. high suction energy pumps are susceptible to noise and increased vibration. without a corresponding reduction in the impeller eye diameter.3A (metric) It must be stressed that the impeller eye diameter is actually a better factor for identifying the suction energy level of a pump than the suction nozzle diameter. Generally speaking.600 RPM (2 times 1. Pumps above the appropriate suction specific speed curve as shown in Figure 9. • Inducers.3: • For two vane impellers and impeller trims with less than 15 degrees vane overlap.1.3.6. The nozzle size was chosen for Figure 9. Recommended margin ratios can typically range from one to five times the NPSHR value of the pump. Very high suction energy pumps can be defined as pumps whose actual impeller operating speeds are in the range of 1.6.1.500 suction specific speed is shown to start high suction energy at 1. which are generally beyond the scope of this document. Figure 9.2 Suction energy determination This is a complex situation and a single equation or relationship has not been developed.800) the pump would be considered to have very high suction energy.1.1.1.3. Figure 9.1. before using Figure 9. but will not suffer significant erosion damage (especially with more erosion resistant impeller materials) when sufficient NPSH Margin is not provided.3.6.6. Very high suction energy pumps will more likely experience erosion damage from cavitation under inadequate NPSH margin conditions.800 RPM. 1 will normally have what is considered “acceptable” seal and bearing life. .6.6. Warmer liquids tend to release less dissolved gas. The gas content of the liquid. • The corrosive properties of the liquid. but with slightly reduced discharge head. with very high suction energy pumps. and enter the graph at 3600 rpm. In the case of low to high suction energy levels. High and very high suction energy pumps that operate with only the minimum NPSH margin values recommended in Table 9. The table is based on the experience of the many pump manufacturers with many different pump applications. It will typically take an NPSHA of 4 to 5 times the 3% NPSHR of the pump to totally eliminate cavitation. All Rights Reserved. cooling tower water treatment agents. the Impeller Inlet eye diameter should be obtained from the supplier and used as the suction nozzle size. the suction nozzle sizes should be increased.1.1. thus increasing the inlet velocity of the liquid and creating even more cavitation. 9. while stainless steel.HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 • For pump speeds higher than 3600 rpm. This can accelerate the damage.3. which run intermittently. which increases the noise level of a pump. They may still be susceptible to elevated noise levels and erosion damage to the impeller. • Solids/abrasives in the liquid.4 • Multistage pumps. which distorts the relationship between the impeller eye diameter and the suction nozzle size. On the other hand gas can collect in the inlet of a pump which will block portions of the flow area. and a low of 2 for some pumps with low suction energy levels.1. within the allowable operating region of the pump (with standard materials of construction). will affect the degree of cavitation erosion damage (and sometimes noise) within a pump when sufficient NPSH margin is not provided above the NPSHR of the pump. For example. This increases the apparent NPSHR of the pump. There are other factors which. • 9. Pump size. and can reduce the resulting noise.1 offers suggested minimum NPSH margin ratio guidelines (NPSHA/ NPSHR). Fire pumps. Adding abrasives to the high implosive velocities from the collapsing vapor bubbles increases the wear rate. The lack of any entrained gas can have the opposite effect.1. rarely have a problem with cavitation damage for this reason.6. This can require more frequent impeller replacement than otherwise would be experienced had the cavitation been totally eliminated. There are studies that show that the maximum cavitation damage can actually occur at 4 Copyright © 2000 By Hydraulic Institute. • For vertical turbine (line shaft diffuser) type pumps. increase the nozzle size by 2 times if the speed is doubled. These nonsuction energy factors are: • The duty cycle of the pump. under the same cavitation conditions. Rigid plastics and composites are normally the least cavitation resistant materials. titanium and nickel aluminum bronze will have much less damage. when using Figure 9. The longer a pump runs under cavitation conditions. Additives in the liquid which increase vapor pressure can increase cavitation damage. • Additives in the liquid. the greater the extent of damage.6. Cast iron and brass will experience the most damage of commonly used metals. Small amounts of entrained gas (1 to 2%) cushion the forces from the collapsing cavitation bubbles. the force of the collapsing cavitation bubbles may be too great for any real cushioning. vibration and erosion damage. Table 9. although not affecting the suction energy of the pump. This ratio can reach 20 for very high suction energy pumps.6. Vertical turbine pumps often operate without NPSH margin without damage. The net result of these two counter effects of gas content on pump noise and vibration will vary based on the suction energy level of the pump. gas may be to quiet the pump.1.3 • • Cavitation damage factors The impeller material. so the noise and damage will increase with increasing gas content. proportional to the increase in speed. Large pumps (impeller inlets over 450 mm (18 in) in diameter can be more prone to cavitation damage than smaller pumps. since the cushioning may more than offset the added cavitation. For example. are excluded from this figure due to the typically large shaft diameters in the impeller eye. However. the net effect of NPSH margin ratio recommendations Field experience is the most accurate predictor of future performance. such as used for boiler feed and pipeline services. Cavitation damage is time related. c) Or 1.6.0c Water/waste water 1. the NPSHA of the system may be lower than expected and the NPSHR for the pump will be higher.1.3c Electric power 1. whichever is greater.5c 2.2b Pulp and paper 1. (See ANSI/HI 9. NPSH Margins are not normally a consideration for most standard vertical turbine pumps. whichever is greater. to ensure a steady uniform flow to the pump suction at the required suction head. Optimum pump performance also requires that proper suction/inlet piping practices are followed.1a 1. thus giving a smaller (or possibly negative) NPSH Margin.0c Nuclear power 1. b) Or 0. and cavitation noise is normally not an issue. including at low water level.1 Minimum NPSH margin ratio guidelines (NPSHA/NPSHR) Suction energy level Application Low High Very high Petroleum 1. extra margin may be required to account for changes in the pump geometry which can increase NPSHR.1. which decreases the NPSHA to the pump and causes added cavitation. . Pump Intake Design). be equal to or larger than the NPSHR over the allowable operating region of the pump. The determination of the minimum submergence required to avoid the formation of sump vortices Table 9. NPSHA must.1. In addition to the minimum NPSH Margins recommended in Table 9. All Rights Reserved. All pumping systems must be designed to have a positive margin throughout the full range of operation.6. 5 Copyright © 2000 By Hydraulic Institute. The NPSHR may also be affected by the gas content of the liquid pumped. however. Centrifugal and Vertical Pumps for Allowable Operating Region).8-1998.5c 2.1a 1.3c Slurry 1.6.3c Building services 1.1a — Pipeline 1.3c 2.5c 2. a factory NPSHR test should be requested. If the application is critical.3c Chemical 1.1a 1.0c Water flood 1.5c Cooling towers 1.2b 1.3-1997.5m (5 feet).0c 2.7c 2.1.1a 1. NPSHA Margins of two to five feet are normally required (above those shown in Table 9. according to the Hydraulic Institute Standards (see ANSI/HI 9. Added NPSH Margin may be needed to cover uncertainties in the NPSH available or the actual operating rate of flow. such as wear that can open impeller wearing ring clearances and increase the internal flow through the impeller eye.1a 1. and this added margin requirement could be even greater depending upon the severity of the conditions.3b 1. since they generally have Low Suction Energy. whichever is greater.0c a) Or 0.0c General industry 1. Poor suction piping can result in separation and turbulence at the pump inlet.1a 1.5b 2.6m (2 feet).HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 NPSHA values twice the NPSHR or more for very high suction energy pumps.6.1a 1.1) to account for these uncertainties in the actual NPSHR and NPSHA values.3b 1. If a pump runs further out on the curve than expected.9m (3 feet). Vertical Pumps for Nomenclature and Definitions. Application considerations 9. All Rights Reserved.1. whichever is greater.HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 around the pump inlet must be considered independently from NPSHA. the margin should be 10% of the NPSHR or 0. • For high suction energy pumps the margin should be 30% of the NPSHR or 1.6. 2) Process requirements are typically well known and demands can be planned and predicted.5.1.1 Petroleum process pumps Pumps used for petroleum (hydrocarbon) services can usually survive with relatively small NPSHA margins for several reasons: 1) Processes are typically steady. Taking these issues into consideration.5.8-1998.3.2 Chemical process pumps Pumps for these applications frequently share the following characteristics: 1) Operation frequently occurs at a wide variety of rates of flow. The margins can be lower than for other applications. NPSH tests are recommended if the 6 Copyright © 2000 By Hydraulic Institute. (See ANSI/HI 9. These factors emphasize the need to apply large NPSH margins when selecting pumps. The majority of vertical turbine pumps in the petrochemical industry are normally installed in a barrel or can as shown in Figure 2.” This “correction factor” is applied to the water NPSHR values to “correct” for the fact that the vapor volume of “flashed” hydrocarbon liquid is substantially less than that of “flashed” water and. 9.1.1. as might be the case with other liquids. thus.6. the following NPSH Margin guidelines are proposed for Chemical Process pumps to account for the many uncertainties: • For low suction energy pumps. The manufacturer then determines the length of the pump required to achieve sufficient NPSHA at the first stage impeller inlet to account for the NPSHR. 4) Less energy is released when hydrocarbon vapor bubbles collapse (velocity from implosion is less). Hydrocarbon liquids. pump inlet losses (inlet to eye of first impeller) and margin. since they are a separate phenomena. whichever is greater. are sometimes associated with a “hydrocarbon correction factor.1-2. Single Stage Double Suction Multistage Pumps: For all hydrocarbon liquids use a NPSH Margin Ratio of 1. 3) They may operate with relatively low NPSHA. The NPSHA is normally given at ground level or pump inlet level.5m (5 feet). Typical NPSH Margins for pumps on hydrocarbon services are as follows: 4) Operators are frequently located remotely from the pumps. 9. has the effect of reducing the amount of NPSH required by the pump at a given rate of flow before cavitation results in a 3% drop in the developed head (first stage head) of the pump.6. the volume of the resulting vapor does not choke the impeller inlet passages as severely as does water vapor during cavitation. and this means less damage occurs as a result of cavitation. This results in a smaller drop in developed head for the same NPSH margin. Normally. 2) Materials of construction are often stainless steel impellers. 3) Most hydrocarbon liquids have relatively low vapor volume to liquid volume ratios. if the liquid should vaporize at or near the pump suction (impeller inlet). • Low Suction Energy Single Stage Overhung.5 • High and Very High Suction Energy Single Stage Overhung. The NPSHA must exceed the NPSHR over the expected range of operation. Pump Intake Design). . Vertical and Multistage Pumps: For all hydrocarbon liquids use an NPSH Margin Ratio of 1. the customers will give a margin value which will vary from 0 to approximately 1. therefore.6 of the Hydraulic Institute standard ANSI/HI 2.2. This favorable vapor bubble size situation with hydrocarbons should be taken into account when determining the NPSHA Margin requirements for petroleum pumps.6m (2 ft). This means that. not as critical that cavitation be avoided. It is.5m (5 ft). with few system upsets (transients) or quick flow change demands. because of their relatively low vapor volume. especially mixtures of hydrocarbon liquids. system upsets. careful consideration should be given to ensuring that. severe changes in pump suction pressure. For such an application. If a pump is applied to the right of BEP. and the materials of construction must be capable of withstanding the erosion associated with cavitation. with a certain amount of cavitation present. units). is often incorporated in the NPSH Required curve by the manufacturer.2. If the above criteria cannot be met and there is no prior experience with the specific pump in the application. the NPSHA is in excess of the NPSHR of the pump. have special demands or operating requirements which impact on NPSH and NPSH Margin requirements.” i. They too. this volume ratio is one-half to one-tenth that of water. The quantity of flow through the pump. Hot water. system upsets may occur which result in rapid changes in pump flow demands and.4 Nuclear power/cooling tower Pumps in nuclear power plants share the following characteristics and requirements: a) Nuclear Reactor Duty: In addition to possible severe vaporization effects. Since they are typically required to operate with very low NPSHA. Vertical Pumps for Nomenclature and Definitions. on the other hand.5×10–4 cubic meters (0. at the maximum flow rate permitted by the system.1-2. When water is heated to 250-300° F. One pound of water at room temperature which occupies 4.5. Some systems operate on what is termed “cavitation control.016 cubic feet). can act similar to hydrocarbon liquids. Other pumps in the power plant are not usually exposed to such severe transients as those in the boiler water system. All Rights Reserved. It is not unusual. and system. Tests should also be conducted at four additional rates of flow at approximately even intervals from the minimum to maximum anticipated rates of flow to fully define the NPSHR (3%) characteristic curve. One test should be conducted at the rated conditions and must demonstrate that the NPSHR (3%) is equal to or less than the rated NPSHR. over NPSHR (3%). 7 Copyright © 2000 By Hydraulic Institute. during such 1) Users are more frequently requesting NPSHR curves based on a 1% head drop. Condensate pumps and heater drain pumps are usually isolated from severe system upsets. causing loss of suction flow and allowing the pump to “run dry”. 2) The NPSHA Margin. and the pump must be designed to withstand constant cavitation. the opportunities for system transients increase significantly with temperature. Vertical turbine type pumps used as condensate pumps are normally installed in a barrel or can as shown in Figure 2. In such a system.e. when it vaporizes (flashes). the vapor volume characteristics become similar to that of a typical hydrocarbon. Unlike hydrocarbon liquids handled by petroleum pumps. sometimes catastrophic failure. . the pumps operate with cavitation at all times. general seal deterioration and premature.6 of ANSI/HI 2.000 to 1. This is a volume ratio of 75. for flashing to occur in the suction line to the pump. typical power plant operating cycles are not stable. there is no NPSH margin. For typical hydrocarbon liquids. NPSH tests should be conducted on the pump. they are designed to function.1.5. This results in higher impact velocities when the vapor bubbles implode. This is especially true for pumps in the boiler water systems such as boiler feed pumps and boiler feed booster pumps. 9.HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 pump specific speed is above 2. many times.S.300 metric (2. are of low to high suction energy levels. thus higher suction energy. 9. 3) High horsepower reactor cooling pumps. expands tremendously. A common side effect of a pump running dry is rapid mechanical seal face wear. This means that the effects of flashing are diminished. is “controlled” by the intersection of the pump reduced head–rate-of-flow curve and the system curve. water. however.000 U.1. The pump flow demands vary widely with power demands. will flash to over 34 cubic meters (1200 cubic feet) of vapor. Cold water is one of the most difficult liquids to pump in that cavitation can cause severe damage. however. also called primary heat transport pumps. and survive.3 Electric power pumps Power plant pumps are water pumps. the pump is constantly under some degree of cavitation which results in a reduced pump developed head. This means it must be of rugged construction to offset the detrimental effects of cavitation related vibration.6. Because of varying power demands. Most pumps in these services do not remain at constant flow rates for extended periods of time. and its controls.6. or transients. guaranteeing that the pump would run far from the point of cavitation. Increase the suction nozzle sizes by one or two sizes for pumps with one to three vane impellers before using Figure 9. The change of the water level in the sump will also change the pump’s total head. A flow duration diagram can be used to determine where the pump will operate most frequently. two and three vane impeller designs are common in wastewater applications. putting margin on top of margin would add to the cost of the pump stations. These materials are preferred for water/wastewater. It is. however. In the on/off mode of operation. it is possible to measure the head at a number of points to develop the system head curve. but they do not stand up well under heavy cavitation. 7) Vertical Turbine barrel or can type pumps on water booster services are generally applied with little or no NPSH Margin. It is advisable to change to tougher materials such as stainless steel or aluminum bronze alloys if the pump must withstand destructive cavitation levels. although the exact number must be experimentally determined. but the sump level will vary between a maximum water level and a minimum water level. It seems as though the simple answer would be to over-compensate by adding margin on top of margin.5 Water/wastewater The following considerations apply to pumps for this application: 1) During variable speed operation. The protective layer that is built up under normal operation is destroyed by cavitation. and the malfunction of a pump must be avoided. pump speeds. 6) Single.8 meters (6.1.0 feet).6. very important to ensure that the calculated system head curves be as close as possible to the actual. All Rights Reserved.3. and rates of flow exist. This reduction can be as high as 1. c) Cooling Tower Duty: Cooling tower water typically has modified chemistry due to water treating agents. The above items are listed to illustrate the uncertainies related to the NPSHA calculations. causing abnormal material removal rates. A failed pump station processing water or wastewater will cause considerable inconvenience to the public. 2) Actual system head curves often differ from the calculated values. 5) Pump stations often operate unattended. 4) Materials of construction are typically cast iron (wastewater) or cast iron / bronze fitted (water). b) Boiler Feed Duty: 1) NPSHR based on a 3% head drop is specified. therefore. When suction elbows are necessary.6. 3) Many pumps are installed in wastewater applications with elbows mounted in front of the impeller eye. and each one may add a margin of their 8 Copyright © 2000 By Hydraulic Institute. These additives can increase the vapor pressure. 9.5. since they are mostly low suction energy applications. with no or minimal vane overlap. This will cause the NPSH Margin calculation to be incorrect. which results in a lower NPSHA than calculated for pure water. and should be designed to be as trouble-free as possible. It is also important to note that there are a number of people involved in the supply chain from the specifier to the end user. Two system curves should be calculated for new installations: one for the system as it will be installed. 2) Suction energy levels are between low to very high. Even though an excessive amount of NPSHA is often not detrimental to the pump. the speed and rate of flow will be relatively constant.HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 4) Reactor cooling pumps normally operate at high temperatures and suction pressures. For existing systems. they will operate at ambient temperature and low suction pressures during transients and commissioning. also slightly changing the speed and rate of flow of the pump.1. all possible wet well levels. but the duty point will be nearly constant compared to variable speed operation. it is best to use reducing or long radius elbows. and a second to represent the condition of the system after some increase in pipe roughness has occurred. and at the same time demonstrate the importance of accuracy when determining the required NPSHA. It is important that the pump can function properly over the full operating range of the system curve. this will not help the seals or bearings. . All Rights Reserved. the specifier must carefully calculate the NPSHA in the system. pumps.1.9 Slurry Pumps used in slurry service are frequently constructed of either hard metals or elastic materials.5. they normally fall into the Low Suction Energy category. NPSH margin is a very important consideration.1. 9. as well as the physical expansion of process systems to meet higher production rates. 9.6. or pumps having Specific Speeds greater than 2300 metric (2000 US units). Due to the low suction energy of most general industrial pumps. it can usually be remedied by increasing the fill pressure.3 or a margin of 1.5m (5 ft). making them relatively insensitive to the mechanical effects of cavitation. In general an NPSHR versus rate of flow curve has a parabolic shape. slurry pumps are of an extremely rugged design. . Also. The use of hose connections and the associated piping bends must be accounted for. These pumps are often sold as standard catalog. 9. the NPSHA for open systems should exceed the pump manufacturer’s stated NPSH-required (NPSHR) by a minimum of 0. use an NPSHA Margin Ratio of 1.1 or a margin of 0. or a minimum of 1.1.6 General industrial Pumps for this application are used to pump a great variety of liquids. 9. Another consideration in the NPSH Margin of catalog type pumps is the common changes in flow rates experienced during process changes. and have NPSHR values below 6m (20 ft). ranging from water to concentrated chemicals.6. whichever is greater. Increasing the NPSH margin will improve pump operation and reliability. Pumps operating at these established minimum NPSH margins may experience some degree of impeller erosion and/or noise but these effects should be minimal. They are generally low suction energy designs. 9 Copyright © 2000 By Hydraulic Institute. If everyone was to add a margin. For High Suction Energy pumps the margin ratio should be increased to at least 1. it is normal to add sufficient NPSH Margin to account for the uncertainties in the actual NPSHR and NPSHA from poor suction piping and entrained air. to minimize erosive effects. Because of this. shaft and bearings.5.6. NPSH Margins are suggested for stock consistencies up to 6%: • For Low Suction Energy pumps use an NPSHA Margin Ratio (NPSHA/NPSHR) of 1. As a result of this.6m. taking into account the vapor pressure of the liquid at the extreme operating temperature. whichever is greater. If an inadequate NPSH available (NPSHA) condition should occur. when in fully developed cavitation. slurry pumps often operate at low speeds (less than 1200 RPM). NPSHA on tank draining applications should be calculated for the lowest possible level of the liquid in the tank during the pumping process. • For High Suction Energy pumps. the result of this excess margin would increase the cost of the pump stations dramatically. System construction may contribute to the problem of noise. The following minimum Fluid systems for the building trades or HVAC Industry are comprised of both closed and open pumping systems. and cavitation. It is also common for the slurry concentration and flow rates to change rapidly. NPSH is generally not a concern when designing closed pumping systems.1 times the NPSHR for Low Suction Energy Pumps.6m (2 ft). This may cause large changes in NPSHR especially if the pump is being run to the right of the best efficiency point. The use of hose or tubing connections with internal diameters smaller than the pump suction inlet should not be used on the suction side of the pump.6. (2 ft) or 1.3. Some pump manufacturers include a margin in their published NPSHR curves.7 Pulp and paper For horizontal end suction stock process pumps situated close to the suction chest. and the erosive nature of many slurries. For open systems.1.5m (5 ft). The typical closed system is filled and then pressurized to a “fill” pressure of 4 to 10 psig.HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 own. and operating in the continuous allowable operating region.5. Typical problems are frequent replacement of the mechanical seal as well as the front motor bearings (on close coupled pumps) due to the intense vibration caused by the collapsing bubbles. imposing significant loads on the impeller.8 Building services Due to the variety of liquids pumped through an extreme range of temperatures. operation of the pump without any NPSH margin does not normally cause substantial damage to the internal components of the pump.5. As a guideline. For sizing of the pumps initially.1 or a margin of 0. NPSH Margin requirements. The NPSH Requirement based on 40.6m (2 ft) whichever is greater. and a second based on the NPSH required to guarantee a 40. • Low Suction Energy pumps can normally operate at or near their NPSHR with little or no problems from cavitation.6 Summary In summary. 9.5. 2) The NPSHA Margin. the NPSH Margins required by the pumps are relatively small in order to ensure satisfactory. giving NPSHA/NPSHR ratios in excess of 1. Suction flow is usually gravity fed. should be reviewed to assure satisfactory performance.000 hours minimum impeller life is being requested more frequently in this market. Pumps used for pipeline service normally share the following application criteria: 1) Customers more often request the NPSH “Required” values to be based on a 1% head drop. characteristics of the slurry. Consequently the NPSHA is normally in excess of 9m (30 ft). . and the NPSHR performance of the pump.1.6. 10 Copyright © 2000 By Hydraulic Institute. ii) Material of impeller. For applications where the margin is less. Typical NPSH Margins for injection pumps are set based on the following criteria. All Rights Reserved.1. considering variations which could occur during the life of the injection project: 1) Pump NPSHR at maximum expected flow rate. over NPSHR (3%).000 hrs) over the full Allowable Operating Region for the pump (Minimum to Maximum Flow). 2) Minimum NPSHA expected at this maximum flow rate. pipelines are defined as hundreds of miles in length for the transport of hydrocarbons or water. except for the 3% head drop.5. iii) Acidity of pumpage (pH). the following key points should be understood about cavitation in a centrifugal pump. 9. 9. The system requirements vary with time. iv) Temperature of pumpage. Waterflood (injection) pumps Water injection pumps for flooding of oil wells typically operate against relatively constant systems. but normally these variations are gradual and do not impact on operating NPSH Margins.10 Pipeline For this paper. however it is a function of: i) Suction Energy Level. 4) Specifications frequently require that the NPSHA exceed the NPSH “Required” (40. is often incorporated in the NPSH “Required” curve by the manufacturer. One being the conventional NPSHR curve based on a 3% head drop. The NPSH “Required” (0%) vs NPSHR (3%) ratio throughout the Allowable Operating Region flow range. Assuming that any changes in the nature of the suction source would also be gradual.6. 5) There is no standard method for determining the NPSH “Required” for 40. • The Suction Energy level of a pump (as installed in a system) determines if the cavitation that frequently exists in a pump will cause noise.11 The recommended NPSH Margin Ratio for slurry pumps is 1. 3) Some pipeline designers and operators request two NPSH Required curves. consistent pump performance.5.HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 vii) Slurries are typically water based and at ambient temperatures. v) Suction Specific Speed.000 hour impeller life. vibration and/or damage to the pump.1. vi) Operating rate of flow vs pump best efficiency point.6. NPSHR considerations are based on a) expected flow rate requirements (changes) over the planned life of the injection project and b) the nature of the suction source for the pumps. and how they are affected by the Suction Energy level of the pump: • Cavitation exists when NPSHA is at and substantially above the NPSHR of a pump.000 hours impeller life. will have high vibration and are likely to experience reduced pump life if sufficient NPSH Margin is not provided. or dissolved air which comes out of solution in the impeller eye. 11 Copyright © 2000 By Hydraulic Institute. . Very High Suction Energy pumps are very susceptible to problems from poor suction inlet piping.HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998 • High Suction Energy pumps are likely to be noisy with higher vibration and will possibly experience less than optimum pump life. All Rights Reserved. • Entrained air. can quiet the noise and vibration of High Suction Energy pumps at low NPSH Margins. • Very High Suction Energy pumps will be noisy. if sufficient NPSH Margin is not provided. • High Suction Energy pumps are more susceptible to problems from poor suction inlet piping. This page intentionally blank. All Rights Reserved. Copyright © 2000 By Hydraulic Institute. . 2 Suction energy level. but is presented to help the user in considering factors beyond this standard. 10 determination. 6. 7 Gas content. 10 13 Copyright © 2000 By Hydraulic Institute. NPSH margin. 4 and NPSH margin. 1. 4 Impeller eye diameter. 3. Additives in liquid. 1f. 10 damage factors. 6 Pipeline pumps. 4 Impeller vanes incidence angle. 2 Building services pumping systems. 2 Multistage pumps. 7 general industrial pumps. 1 Suction specific speed. 4 Impeller material. 2 Suction energy. 9 chemical process pumps. 9 Solids/abrasives in liquid.. 6 pipeline pumps. 2f. indicates a table. 4 Net positive suction head available. 4 Pump size. Note: an f. 1 slurry service pumps. 2 Vertical turbine pumps. 8 Waterflood (injection) pumps. 2 Inlet piping geometry. 2 Industrial pumps. 7 defined electric power pumps. 8 waterflood (injection) pumps. 6 cooling towers. 9 ratio. 10 pulp and paper pumps. 3f. 10 NPSHA See also Net positive suction head available NPSHR See Net positive suction head required Nuclear power pumps. 9 Cavitation. 5t. 3. 2 overlap. 7 Corrosive properties of liquid. 1f. 10 building services pumping systems. 4 Chemical process pumps. 9 and vertical turbine pumps. 7 petroleum process pumps. 6 Cooling towers. 6 water/wastewater pumps. 9 Pump duty cycle. 4 BEP See Best efficiency point Best efficiency point. 9 guidelines. 2 Petroleum process pumps.HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin Index — 1998 Appendix A Index This appendix is not part of this standard. nuclear power pumps. 1 Thermodynamic properties. factors. 1. 10 Pulp and paper applications. 4 Slurry service pumps. 3. All Rights Reserved. 4. and a t. Net positive suction head margin See NPSH margin Net positive suction head required. . indicates a figure. 9 Inlet geometry. 6 and inlet eye diameter. 4 Electric power pumps. 6 Water/wastewater pumps. 7 Peripheral velocity. 1. 4 Specific gravity. All Rights Reserved. .M118 Copyright © 2000 By Hydraulic Institute.