1、Considerations for EvaluatingControl Valve CavitationApproved 2 June 1995ISARP75.231995RECOMMENDED PRACTICEISA The Instrumentation,Systems, andAutomation Society TMCopyright 1995 by the Instrument Society of America. All rights reserved. Printed in the UnitedStates of America. No part of this public
2、ation may be reproduced, stored in a retrieval system, ortransmitted in any form or by any means (electronic, mechanical, photocopying, recording, orotherwise), without the prior written permission of the publisher.ISA67 Alexander DriveP.O. Box 12277Research Triangle Park, North Carolina 27709ISARP7
3、5.231995, Considerations for Evaluating Control Valve CavitationISBN: 1-55617-572-8ISA-RP75.23-1995 3PrefaceThis preface, as well as all footnotes and annexes, is included for informational purposes only and is not part of ISA-RP75.23-1995.This recommended practice has been prepared as part of the s
4、ervice of ISA, the international society for measurement and control, toward a goal of uniformity in the field of instrumentation. To be of real value, this document should not be static but should be subject to periodic review. Toward this end, the Society welcomes all comments and criticisms and a
5、sks that they be addressed to the Secretary, Standards and Practices Board; ISA; 67 Alexander Drive; P. O. Box 12277; Research Triangle Park, NC 27709; Telephone: (919) 990-9227; Fax: (919) 549-8288; e-mail: standardsisa.org.The ISA Standards and Practices Department is aware of the growing need for
6、 attention to the metric system of units in general, and the International System of Units (SI) in particular, in the preparation of instrumentation standards, recommended practices, and technical reports. The Department is further aware of the benefits to USA users of ISA standards of incorporating
7、 suitable references to the SI (and the metric system) in their business and professional dealings with other countries. Toward this end, this Department will endeavor to introduce SI-acceptable metric units in all new and revised standards to the greatest extent possible. The Metric Practice Guide,
8、 which has been published by the Institute of Electrical and Electronics Engineers as ANSI/IEEE Std. 268-1992, and future revisions, will be the reference guide for definitions, symbols, abbreviations, and conversion factors.CAUTION: The information presented within this ISA Recommended Practice is
9、believed to be accurate and reflects the current state of knowledge within the field. The information is an interpretation and condensation of a large volume of literature and experience, some of which is contradictory and speculative. Therefore, application of the information to particular situatio
10、ns requires the exercise of the independent professional judgement of the user. ISA is not responsible for any results from such use of the information and shall not be liable for any damages caused by such use.It is the policy of ISA to encourage and welcome the participation of all concerned indiv
11、iduals and interests in the development of ISA standards, recommended practices, and technical reports. Participation in the ISA standards-making process by an individual in no way constitutes endorsement by the employer of that individual, of ISA, or of any of the standards that ISA develops.The fo
12、llowing people served as members of ISA Subcommittee SP75.16:NAME COMPANY*F. Cain, Chairman Valtek InternationalW. Weidman, Managing Director ConsultantG. Barb Consultant*R. Barnes Valtek InternationalS. Boyle Neles-Jamesbury, Inc.*One vote per company4 ISA-RP75.23-1995NAME COMPANYD. Buchanan Union
13、Carbide CorporationL. Driskell ConsultantL. Griffith Retired/ConsultantJ. Harkins ConsultantH. Illing DeZurik Valve CompanyC. Koloboff Chevron Research more rigorous definitions are found in the references.Noise measurement and prediction methods are beyond the current scope of this document. Method
14、s of liquid flow noise measurement and prediction may be found in standards of the International Electrotechnical Commission, CEI/IEC documents 534-8-2 and 534-8-4. The relationship between cavitation parameters used in this recommended practice and those of the IEC documents is discussed in Annex B
15、.2 PurposeCavitation as an applied science has not evolved to the highly refined level of that supporting the more traditional control valve sizing calculations. However, there is a great need by users and manufacturers alike for practical information in this area. The purpose of this document is to
16、 supply that information, and to that end it is necessarily broad in scope. It embodies several objectives:a) to provide educational material in a background section that condenses the literature and educates the reader in state-of-the-art valve cavitation knowledge and practice;b) to establish a ba
17、sis for communication by defining cavitation parameters and nomenclature;c) to propose methods for evaluating the cavitation characteristics of individual control valves through testing procedures and application experience; andd) to offer guidelines for selecting control valves for given applicatio
18、ns.ISA Subcommittee SP75.16 recognizes that the science of cavitation is in its infancy in terms of defining the behavior of cavitation in complex valve geometry. The final objective of this recommended practice is to promote additional research and testing. Subsequently, this practice can serve as
19、a starting point for those seeking to advance the state of the art.10 ISA-RP75.23-19953 Definition of termsTerms used are per ISA-S75.05 and additional terms as follows:3.1 cavitation: A two-stage process associated with the flow of liquids. The first stage involves the formation of vapor cavities o
20、r bubbles in the flow stream as a result of the local static pressure in the flow stream dropping below the liquid vapor pressure. The second stage of the process is the subsequent collapse or implosion of the vapor cavities back to the liquid state when the local static pressure again becomes great
21、er than the fluid vapor pressure. (The evaluation of “gaseous“ cavitation, i.e. the sudden dissolution of dissolved gases in a liquid, is not currently within the scope of this document.)3.2 cavitation coefficient: A characteristic number for (e.g., i, c, mv, id, ch), determined for a given valve, v
22、alve opening, and pressure conditions, which corresponds to the numerical value of the cavitation index at which the levels of incipient cavitation, constant cavitation, maximum vibration cavitation, incipient damage, and choking cavitation occur.3.3 cavitation index: The value for the operating ser
23、vice conditions of a valve, expressed as and numerically equal to (P1- Pv)/(P1- P2).3.4 cavitation level: The degree to which cavitation is occurring, i.e., incipient, constant, in-cipient damage, choking, or maximum vibration. Levels can be determined by testing for vibration, pitting or metal loss
24、, and changes in valve capacity (Cv).3.5 choking cavitation: A limiting flow condition in which vapor formation is enough to limit the rate of flow through the valve to some maximum value. Further increases in flow rate through the valve are only possible by increasing the valve inlet pressure, beca
25、use reducing downstream pressure will no longer increase flow rate.3.6 constant cavitation: An early level of cavitation characterized by mild, steady popping or crackling sounds that may be audible or detected by vibration measurements. It is the next higher inflection point on the cavitation profi
26、le above the point of incipient cavitation. (See Figure 1.) This level is represented by the constant cavitation coefficient c.3.7 duty cycle: The ratio of the amount of time a valve spends performing one particular function to the valves total installed time period. It may be expressed as a percent
27、age of total time (service time vs. installed time).3.8 flashing: A flow condition in which vapor pockets formed inside a valve persist downstream of the valve because the valve outlet pressure is at or below the fluid vapor pressure.3.9 flow separation: A flow condition in which the fluid boundary
28、layer flows away from the boundary wall instead of flowing along the wall. A turbulent wake exists downstream of the point of flow separation that is characterized by the presence of vortices. These vortices contain regions of high local fluid velocities and hence low, local pressures. The areas of
29、low pressure are potential sites for vapor formation.ISA-RP75.23-1995 11Figure 1 Cavitation parameter plot NOTE This is a classical curve illustrating acceleration versus sigma. Tested valves may not result in this specific configuration or exhibit all the inflection points or coefficients shown abo
30、ve. Test data are subject to expert interpretation.12 ISA-RP75.23-19953.10 incipient cavitation: The onset of cavitation, where only small vapor bubbles are formed in the flow stream. (See Figure 1.) This level is represented by the incipient cavitation coefficient ior 1/xFz.3.11 incipient damage: A
31、 cavitation level sufficient to begin minor, observable indications of pitting damage. (This is not to be confused with incipient cavitation. See 3.10.)3.12 influences: Factors or effects that change the damage rate or extent of damage but do not change the numerical value of cavitation coefficients
32、. See Figure B.2.3.13 manufacturers recommended cavitation limit: An operational limit expressed as a cav-itation coefficient mrsupplied by the valve manufacturer for a given valve type, size, opening, and reference upstream pressure. Application of the limit may require scale effect and influence f
33、actors if the service conditions and valve size are different than for the reference pressure and size.3.14 maximum vibration cavitation: The level of cavitation associated with peak vibration measurements and determined from a cavitation level plot at the peak separating Regime III and Regime IV. T
34、he test conditions at this point define the conditions for calculating the valve cavitation coefficient mv. See Figure 1.3.15 pressure recovery: The increase in fluid static pressure that occurs as fluid moves through a valve from the vena contracta to the valves outlet and downstream piping. The re
35、covery, which may be expressed as the difference P2- Pvc, is caused by the velocity-reducing, diffusing action of the downstream geometry.3.16 scale effects: Differences in cavitation coefficients occurring between the flow test condi-tions and actual valve operating conditions. These scale effects
36、result from differences in valve size and operating pressures. Scaling equations are used to modify the reference values of cavitation coefficients supplied by valve manufacturers in order to evaluate equipment at other than reference conditions. See Figure B.1.3.17 vapor pressure: The pressure, for
37、 a specified fluid temperature, at which both the liquid and vapor phases of a fluid exist in equilibrium. The vapor pressure is more commonly thought of as the thermodynamic saturation pressure. 3.18 vena contracta: The minimum area of a flow stream. It is smaller than the area causing the flow con
38、striction, because the streamlines continue to converge for a short distance beyond the constriction. Average flow velocity is highest and mean static pressure is lowest in the vena contracta. However, local vortex pressures in separation regions and turbulent boundary layers can be lower than the v
39、ena contracta pressure.4 NomenclatureNomenclature used is per ANSI/ISA S75.01 and additionally as follows:a Empirical characteristic exponent for calculating PSEb A characteristic exponent for calculating SSE; determined from reference valve data for geometrically similar valves.ISA-RP75.23-1995 13C
40、vValve flow coefficient*, Cv= q(Gf/P)1/2CvRValve flow coefficient of a reference valved Valve inlet inside diameter, inches (mm)dRValve inlet inside diameter of tested reference valve, inches (mm)D1Internal diameter of upstream pipe, inches (mm)D2Internal diameter of downstream pipe, inches (mm)e Na
41、pierian base, e = 2.71828. (for natural logarithms)FDCDuty cycle factor for modifying the intensity indexFFLiquid critical pressure ratio factor*FLLiquid pressure recovery factor*FpPiping factor for ISA valve sizing*FTTemperature factor for modifying the intensity index FUVelocity factor for modifyi
42、ng the intensity index GfSpecific gravity of the liquid at inlet flowing conditionsI Intensity indexKB1Bernoulli coefficient for upstream pipe reducer*KB2Bernoulli coefficient for downstream pipe expansion*K1Head loss coefficient for upstream pipe reducer*K2Head loss coefficient for downstream pipe
43、increaser*N1-4Numerical constants for units of measure used in equations. See Table 1.PaAtmospheric pressure, psia (kPa)P1Valve inlet static pressure, psia (kPa)P2Valve outlet static pressure, psia (kPa)PSE Pressure scale effectPvAbsolute fluid vapor pressure of liquid at inlet temperature, psia (kP
44、a)PvcFluid static pressure in valve vena contracta*, psia (kPa)q Volumetric flow rate, gpm (m3/h)SSE Size scale effectSPL Sound Pressure Level referenced to 20 x 10-6Pascal (2.0 x 10-5N/m2)T Fluid Temperature, F (C)TaveAverage temperature between a liquids freezing and boiling temperatures for a spe
45、cified pressure, Taveis equal to (TF+ TB)/2, F (C)TBBoiling temperature of a liquid for specified pressure, F (C)TFFreezing temperature of a liquid, F (C)U Average velocity at the valve inlet, ft/s (m/s)*More completely defined in ANSI/ISA S75.01 and ANSI/ISA S75.0214 ISA-RP75.23-1995UoPitting thres
46、hold velocity determined at the valve inlet, ft/s (m/s)xFzCoefficient of incipient cavitation per IEC-534-8-2; xFz 1/iP Measured valve differential pressure, psi (kPa)PchPressure drop at choking, psi (kPa) Cavitation index equal to (P1- Pv)/(P1- P2) at service conditions, i.e., (service)2Alternate c
47、avitation index equal to (P2- Pv)/(P1- P2) at service conditions. See B.5.6.cCoefficient for constant cavitation; cis equal to (P1-Pv)/P at the conditions causing mild, steady cavitationchCoefficient for choking cavitation; chis equal to (P1-Pv)/FL2(P1-FFPv) at the point associated with choking in t
48、he valveiCoefficient of incipient cavitation; iis equal to (P1-Pv)/P at the point where incipient cavitation begins to occuridCoefficient of incipient damage for cavitation; idis equal to (P1-Pv)/P at the conditions causing onset of damage by cavitationmrCoefficient of manufacturers recommended mini
49、mum limit of the cavitation index for a specified valve; mris equal to minimum recommended value of (P1-Pv)/(P1-P2)mvCoefficient of cavitation causing maximum vibration as measured on a cavitation parameter plot (See Figure 1.)pCavitation coefficient vthat has been adjusted for the effects of installing a smaller-than-line-size valve with reducers in the pipeline.RA reference value of a cavitation coefficient.ssCavitation index scaled for service pressure and size effects for use in intensity index calculations; ssis equal
