阀门选择手册 第五版 Valve Selection Handbook 5th Edition.pdf

VALVE S E L E C T I O N HANDBOOK F I F T HE D I T I O N TLFeBOOK TLFeBOOK VALVE S E L E C T I O N HANDBOOK F I F T HE D I T I O N Engineering fundamentals for selecting the right valvedesignforeveryindustrialfl owapplication By P E T E RS M I T H Editor and Contributor and R.W.Z A P P E AMSTERDAM BOSTON LONDON NEWYORK OXFORD PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO Gulf Professional Publishing is an imprint of Elsevier Inc. TLFeBOOK Gulf Professional Publishing is an imprint of Elsevier 200 Wheeler Road, Burlington, MA01803, USA Linacre House, Jordan Hill, Oxford OX2 8DP, UK Copyright 2004, Elsevier, Inc.All rights reserved. No part of this publication may be reproduced, stored in a retri system, or transmitted in any or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher. Permissions may be sought directly from Elsevier’s Science and automatically operated valves for preventing back fl ow and relieving pressure. The manually operated valves are referred to as manual valves, while valves for the prevention of back fl ow and the relief of pressure are referred to as check valves and pressure relief valves, respectively. 1 TLFeBOOK Valve Selection Handbook2 Rupture discs are non-reclosing pressure-relieving devices which fulfi ll a duty similar to pressure relief valves. Fundamentals Sealing perance and fl ow characteristics are important aspects in valve selection. An understanding of these aspects is helpful and often essential in the selection of the correct valve. Chapter 2 deals with the fundamentals of valve seals and fl ow through valves. Thediscussiononvalvesealsbeginswiththedefi nitionoffl uidtightness, followed by a description of the sealing mechanism and the design of seat seals, gasketed seals, and stem seals. The subject of fl ow through valves covers pressure loss, cavitation, waterhammer, and attenuation of valve noise. Manual Valves Stopper type closureglobe, needle Vertical slidegate Rotary typeball, plug, butterfl y Flexible bodydiaphragm Manual valves are divided into four groups according to the way the closure member moves onto the seat. Each valve group consists of a number of distinct types of valves that, in turn, are made in numerous variations. The way the closure member moves onto the seat gives a particular group or type of valve a typical fl ow-control characteristic. This fl ow- control characteristic has been used to establish a preliminary chart for the selection of valves. The fi nal valve selection may be made from the description of the various types of valves and their variations that follow that chart. Note For literature on control valves, refer to footnote on page 5 of this book. Check Valves Lift check Swing check single and double plate TLFeBOOK Introduction3 Tilting disc Diaphragm The many types of check valves are also divided into four groups according to the way the closure member moves onto the seat. The basic duty of these valves is to prevent back fl ow. However, the valves should also close fast enough to prevent the ation of a sig- nifi cant reverse-fl ow velocity, which on sudden shut-off, may introduce an undesirably high surge pressure and/or cause heavy slamming of the closure member against the seat. In addition, the closure member should remain stable in the open valve position. Chapter 4, on check valves, describes the design and operating charac- teristics of these valves and discusses the criteria upon which check valves should be selected. Pressure Relief Valves Direct-loaded pressure relief valves Piloted pressure relief valves Pressure relief valves are divided into two major groups direct-acting pressurereliefvalvesthatareactuateddirectlybythepressureofthesystem fl uid, and pilot-operated pressure relief valves in which a pilot controls the opening and closing of the main valve in response to the system pressure. Direct-acting pressure may be provided with an auxiliary actuator that assists valve lift on valve opening and/or introduces a supplementary clos- ing force on valve reseating. Lift assistance is intended to prevent valve chatterwhilesupplementaryvalveloadingisintendedtoreducvesim- mer. The auxiliary actuator is actuated by a foreign power source. Should the foreign power source fail, the valve will operate as a direct-acting pressure relief valve. Pilot-operated pressure relief valves may be provided with a pilot that controls the opening and closing of the main valve directly by means of an internal mechanism. In an alternative type of pilot-operated pressure relief valve, the pilot controls the opening or closing of the main valve indirectly by means of the fl uid being discharged from the pilot. Athirdtypeofpressurereliefvalveisthepoweredpressurereliefvalvein whichthepilotisoperatedbyaforeignpowersource.Thistypeofpressure relief valve is restricted to applications only that are required by code. TLFeBOOK Valve Selection Handbook4 Rupture Discs Rupture discs are non-reclosing pressure relief devices that may be used alone or in conjunction with pressure relief valves. The principal types of rupture discs are forward domed types, which fail in tension, and reverse bucklingtypes, whichfailincompression. Ofthesetypes, reversebuckling discs can be manufactured to close burst tolerances. On the debit side, not all reverse buckling discs are suitable for relieving incompressible fl uids. While the application of pressure relief valves is restricted to relieving nonviolentpressureexcursions,rupturediscsmaybeusedalsoforrelieving violent pressure excursions resulting from the defl agration of fl ammable gases and dust. Rupture discs for defl agration venting of atmospheric pressure containers or buildings are referred to as vent panels. Units of Measurement Measurements are given in SI and imperial units. Equations for solving in customary but incoherent units are presented separately for solution in SI and imperial units as presented customarily by U.S. manufacturers. Equations presented in coherent units are valid for solving in either SI or imperial units. Identifi cation of Valve Size and Pressure Class Theidentifi cationofvalvesizesandpressureclassesinthisbookfollows therecommendationscontainedinMSSStandardPracticeSP-86. Nominal valve sizes and pressure classes are expressed without the addition of units of measure; e.g., NPS 2, DN 50 and Class I 50, PN 20. NPS 2 stands for nominal pipe size 2 in. and DN 50 for diameter nominal 50 mm. Class 150 stands for class 150 lb. and PN 20 for pressure nominal 20 bar. Standards AppendixCcontainsthemoreimportantU.S.,British,andISOstandards pertaining to valves. The standards are grouped according to valve type or group. TLFeBOOK Introduction5 Additional Chapters There are three additional chapters in the fi fth edition of the Valve Selection Handbook that have not been included previously Chapter 8Actuators Chapter 9Double Block and Bleed Ball Valves Chapter 10Mechanical Locking Devices for Valves A comprehensive glossary has also been included in Appendix E to assist the reader. This book does not deal with control valves. Readers interested in this fi eld should consult the following publications of the ISA 1. Control Valve Primer, A User’s Guide 3rd edition, 1998, by H. D. Baumann. This book contains new material on valve sizing, smart digital valve positioners, fi eld-based architecture, network system technology, and control loop perance uation. 2. Control Valves, Practical Guides for Measuring and Control 1st edition, 1998, edited by Guy Borden. This volume is part of the Practical Guide Series, which has beendevelopedbytheISA.Thelastchapterofthebookdealsalsowithregulatorsand comparestheirperanceagainstcontrolvalves.WithinthePracticalGuideSeries, separate volumes address each of the important topics and give them comprehensive treatment. Address ISA, 67 Alexander Drive, Research Triangle Park, NC 27709, USA. Email http//www.isa.org TLFeBOOK TLFeBOOK 2 FUNDAMENTALS The fundamentals of a particular type of valve relate to its sealing characteristics, which include in-line seat sealing when closed and where applicablestemsealingwhichshouldpreventpotentialleaksintotheatmo- sphere. In the case of process systems handling hazardous fl uids, harmful to both the atmosphere and personnel, stem sealing is considered to be of more importance. FLUID TIGHTNESS OF VALVES Valve Seals One of the duties of most valves is to provide a fl uid seal between the seat and the closure member. If the closure member is moved by a stem that penetrates into the pressure system from the outside, another fl uid seal mustbeprovidedaroundthestem. Sealsmustalsobeprovidedbetweenthe pressure-retaining valve components. If the escape of fl uid into the atmo- sphere cannot be tolerated, the latter seals can assume a higher importance than the seat seal. Thus, the construction of the valve seals can greatly infl uence the selection of valves. 7 TLFeBOOK Valve Selection Handbook8 Leakage Criterion A seal is fl uid-tight if the leakage is not noticed or if the amount of noticed leakage is permissible. The maximum permissible leakage for the application is known as the leakage criterion. The fl uid tightness may be expressed either as the time taken for a given mass or volume of fl uid to pass through the leakage capillaries or as the timetakenforagivenpressurechangeinthefl uidsystem. Fluidtightnessis usually expressed in terms of its reciprocal, that is, leakage rate or pressure change. Four broad classes of fl uid tightness for valves can be distinguished nominal-leakage class, low-leakage class, steam class, and atom class. The nominal- and low-leakage classes apply only to the seats of valves that are not required to shut off tightly, as commonly in the case for the control of fl ow rate. Steam-class fl uid tightness is relevant to the seat, stem, and body-joint seals of valves that are used for steam and most otherindustrialapplications.Atom-classfl uidtightnessappliestosituations in which an extremely high degree of fl uid tightness is required, as in spacecraft and atomic power plant installations. Lok1 introduced the terms steam class and atom class for the fl uid tightness of gasketed seals, and proposed the following leakage criteria. Steam Class Gas leakage rate 10 to 100 g/s per meter seal length. Liquid leakage rate 0.1 to 1.0 g/s per meter seal length. Atom Class Gas leakage rate 10−3to 10−5g/s per meter seal length. In the United States, atom-class leakage is commonly referred to as zero leakage. A technical report of the Jet Propulsion Laboratory, California Institute ofTechnology, defi nes zero leakage for spacecraft requirements.2 According to the report, zero leakage exists if surface tension prevents the entry of liquid into leakage capillaries. Zero gas leakage as such does not exist. Figure 2-1 shows an arbitrary curve constructed for the use as a specifi cation standard for zero gas leakage. Proving Fluid Tightness Most valves are intended for duties for which steam-class fl uid tightness is satisfactory. Tests for proving this degree of fl uid tightness are normally TLFeBOOK Fundamentals9 Figure 2-1. Proposed Zero Gas Leakage Criterion. Courtesy of Jet Propulsion Laboratory, California Institute of Technology. Reproduced from JPL Technical Report No. 32-926. carried out with water, air, or inert gas. The tests are applied to the valve body and the seat, and depending on the construction of the valve, also to the stuffi ng box back seat, but they frequently exclude the stuffi ng box seal itself. When testing with water, the leakage rate is metered in terms of either volume-per-time unit or liquid droplets per time unit. Gas leakage may be metered by conducting the leakage gas through either water or a bubble-ing liquid leak-detector agent, and then counting the leakage gasbubblespertimeunit.Usingthebubble-ingleakage-detectoragent permits metering very low leakage rates, down to 110−2or 110−4sccs standard cubic centimeters per second, depending on the skill of the operator.3 Lower leakage rates in the atom class may be detected by using a search gas in conjunction with a search-gas detector. TLFeBOOK Valve Selection Handbook10 Specifi cationsforprovingleakagetightnessmaybefoundinvalvestand- ards or in the separate standards listed in Appendix C. A description of leakage testing s for the atom class may be found in BS 3636. SEALING MECHANISM Sealability Against Liquids The sealability against liquids is determined by the surface tension and the viscosity of the liquid. When the leakage capillary is fi lled with gas, surface tension can either drawtheliquidintothecapillaryorrepeltheliquid, dependingontheangle of contact ed by the liquid with the capillary wall. The value of the contactangleisameasureofthedegreeofwettingofthesolidbytheliquid and is indicated by the relative strength of the attractive forces rted by the capillary wall on the liquid molecules, compared with the attractive forces between the liquid molecules themselves. Figure 2-2 illustrates the forces acting on the liquid in the capillary. The opposing forces are in equilibrium if πr2P 2πrTcosθor P 2Tcosθ r 2–1 where r radius of capillary P capillary pressure T surface tension θ contact angle between the solid and liquid Thus, if the contact angle ed between the solid and liquid is greater than 90◦ , surface tension can prevent leakage fl ow. Conversely, if the contact angle is less than 90◦, the liquid will draw into the capillaries and leakage fl ow will start at low pressures. The tendency of metal surfaces to a contact angle with the liquid of greater than 90◦depends on the presence of a layer of oily, greasy, or waxy substances that normally cover metal surfaces. When this layer is removed by a solvent, the surface properties alter, and a liquid that previously was repelledmaynowwetthesurface.Forexample,kerosenedissolvesagreasy surface fi lm, and a valve that originally was fl uid-tight against water may TLFeBOOK Fundamentals11 Figure 2-2. Effect of Surface Tension on Leakage Flow through Capillary. leak badly after the seatings have been washed with kerosene. Wiping the seating surfaces with an ordinary cloth may be suffi cient to restore the greasy fi lm and, thus, the original seat tightness of the valve against water. Once the leakage capillaries are fl ooded, the capillary pressure becomes zero, unless gas bubbles carried by the fl uid break the liquid column. If the diameter of the leakage capillary is large, and the Reynolds number of the leakage fl ow is higher than critical, the leakage fl ow is turbulent. As the diameter of the capillary decreases and the Reynolds number decreases below its critical value, the leakage fl ow becomes laminar. This leakage fl owwi