_RP_550-1-6-1985_scan.pdf

着IIManual on Installation of RefineryInstruments and Control SystemsuIPart I-Process Instrumentation and ControlSection 6-Control Valves and AccessoriesRefining DepartmentIAPI RECOMMENDED PRACTICE 550FOURTH EDITION,MARCH 19851I】IAmericanPetroleumInstitute】Part I-Process Instrumentation and ControlSECTION 6-CONTROL VALVES AND ACCESSORIES6.1 Scopetice reduces the possibility of such fluids contacting hot linesThis section presents recommended practices for the in-or equipment if the control valve manifold is drained andstallation and maintenance of control valves and related ac-the valve is removed,or if leaks developcessories such as positioners,boosters,and transducers.ItCertain rotary-motion control valve types utilize low-also outlines control valve design considerations,discussesfriction plastic lined bearings and as a result are susceptiblecontrol valve sizing and noise,and defines types of com-to static electricity and should be grounded.Follow themonly used control valves and their actuators.manufacturers recommendations.A number of instruments and accessories can be mountedUsers should be aware that long bolts used with flangelesson and used in conjunction with control valves.Other sec-valves can expand when exposed to fire and cause leakage.tions of this manual will be referred to for installation,pip-A fire deflection shield and/or insulation is recommended.ing.and electrical practices covering those devices.In addition.high-tensile-strength bolting is required.Similarly,control valves used in process lines or fuel lines6.2Generalto fired heaters should be located outside the firewall around6.2.1 ACCESSIBILITYthe heater.If no firewall is provided,the control valvesshould be located on the sides of the heater away from theAll control valves should be installed so that they are read-burners or at a sufficient distance from the heater so that theily accessible for maintenance purposes and for operationline may be drained and the control valves removed withoutof a handwheel,if one is provided.They should generallydanger of a flashback.An alternate method is to pipe thebe located at grade unless pressure head or other design con-drain or bleed connection a safe distance from the heater.ditions make such an arrangement impractical.When locatedBecause high temperatures can cause premature failure ofabove grade,control valves should be installed so that theydiaphragms and electrical or electronic components,controlare readily accessible from a permanent platform or walkwayvalves should be located so that topworks are not adjacentwith ample clearances for maintenance purposes.to hot lines or equipment or in an area where temperaturemay be excessive.Consult the manufacturers literature for6.2.2 LOCATIONmaximum permissible ambient temperature.Electrically operated items-motor actuators.solenoidWhere there is a choice of location,it is desirable to havethe control valve installed near the piece of operating equip-valves,transducers-should be approved for use under theapplicable hazardous area classification.ment that must be observed while on local manual control.Control valves are normally protected with covers forIt is also desirable to have indication of the controlled variablereadable from the control valve.flanged openings and threaded plugs for threaded openingsduring shipment and storage prior to installation.6.2.3 INSTALLATION CLEARANCESDuring startup of any new facilities,care should be takento keep scale,welding rods,and other foreign material fromIf a control valve is to be maintained in place,sufficientplugging or damaging control valves.One method is toclearance should be provided above.below,or on the sideremove the valve and substitute a spool piece during flushingso that the valve trim and the valve actuator may be removedoperations.from the valve body.Extra clearance is required where extension bonnets or ac-cessories are used.Clearance should always be provided on6.3Control Valve Design Considerationsthe side of the valve for maintenance of positioners and other6.3.1 GENERALdevicesA control valve,as shown in Figure 6-1,consists of two6.2.4 PRECAUTIONSmajor subassemblies:a valve body and an actuator.The valveControl valves that handle combustible fluids should bebody is the portion that actually contains the process fluid.kept away from hot pumps,lines,or equipment.This prac-It consists of a body,internal trim,bonnet,and sometimesa bottom flange.This subassembly must meet all of the ap-SECTION 6-CONTROL VALVES AND ACCESSORIESand its good lubricating properties.is one of the most popular5EMvalve packing materials.It may be used in solid molded.%C:hGg。44braided.or turned form(chevron rings)or as a lubricant forPSCKING FOLLOWER:3asbestos packing.Its temperature limit with standard pack-ing box construction is 450 F(232 C).Another popular packing type is braided asbestos withnickel-chromium-iron alloys wire employing additives suchLTas mica or graphite for lubrication.This packing may be used打tSOLATING VA.vEin high-temperature steam and petroleum service,ap-proaching 1000 F(538 C).Manufacturers should be con-NOTE:Many packing box assemblies available today do not require lubrica-sulted to ensure proper valve construction.tion.In such cases the lubricator and isolating valve shown in this figureare not required.A recent addition is an all-graphites product that is essen-tially chemically inert except when strong oxidizers areFigure 6-2-Packing Box Assembly(Bolted)Withhandled.This type of packing can be used for temperatureLubricator and Isolating Valveapplications approaching 2000 F(1093 C).Manufacturersshould be consulted to ensure proper valve constructionFemale National Pipe Thread connections are common in6.3.6 SEAT LEAKAGEvalve sizes I inch and smaller and are sometimes used forcontrol valves up to 2 inches.The control valve plug is the moving component of theWelded ends may be used where high pressure,highvalve that throttles flow by positioning itself within the seattemperature,or highly toxic fluids are encountered.Careorifice and shuts off flow by contacting the seatshould be taken that the valve body material specified is com-The degree of sealing(shutoff)accomplished is dependentpatible with the adjoining pipe material.upon the valve construction,the materials and condition ofFlanged-end globe bodies generally conform to the stan-the seating surfaces,and the actuator power availabledardized face-to-face dimensions listed in the InstrumentDouble-seated globe valves,due to machining tolerances,Society of America(ISA)standard S75.03-1984,Face-to-do not seat tightly.Manufacturers therefore publish a leakageFace Dimensions for Flanged Globe-Style Control Valverate of 0.5 percent of the rated valve capacity coefficientBodies,with the exception of Saunders types,angle bodies.(C).Single-seated valves with metal-to-metal seating sur-and rotary valves.The flange rating is determined by thefaces provide less leakage,and the normal published leakagetype of service,required material,maximum pressure,andrate is 0.01 percent of rated Cv.maximum fluid temperature.Most control valve manufacturers follow the voluntary testFlangeless valves have no flange connections as part ofand leakage standards established by ANSI B16.104-1976.the valve body and are simply bolted or clamped betweenWith additional considerations,such as careful lapping ofthe adjoining line flanges.Flangeless valve face-to-facethe seating surfaces and increased actuator power.single-dimensions generally conform to ISA S75.04-1984.Note thatseated globe valves with metal-to-metal seating surfaces canbutterfly valves are not covered by a standard.meet even more stringent seat tightness requirements.For specialized applications,control valves can be pro-6.3.5 PACKINGvided with a resilient composition insert(soft-seat construc-The packing box assembly,Figure 6-2,provides a meanstion)in either the plug or seat ring that will allow the valvefor preventing the leakage of process fluid past along the sur-to obtain bubble-tight sealing with relatively small actuatorface of the valve stem.Packing boxes should be easily ac-force.Common insert materials are nitrile rubber7 orcessible for periodic adjustment and should permit the addi-polytetrafluoroethylene.8 However,before an insert materialtion of at least one more split packing ring withoutis selected,it should be determined that the insert is com-disassembly.The packing material should(1)be elastic andpatible with the process.easily deformable,(2)be as chemically inert as possible,(3)be able to withstand applicable process conditions.(4)pro-vide a degree of fire resistance,and(5)minimize friction.Polytetrafluoroethylene4,because of its excellent inertnesss For example.InconelFor example.Grafoil7 For example.Buna NFor example.Teflon.For example.Teflon.API AP 550-PART I6.3.7 CONTROL VALVE CHARACTERISTICSControl valve flow characteristics are determined princi-pally by the design of the valve trim.The three inherentcharacteristics available are quick opening,linear.and equalpercentage.These are shown in Figure 6-3.A modifiedpercentage characteristic generally falling between the linearand equal percentage characteristics is also available.The three inherent characteristics can be described asfollows:1.Quick Opening.As the name implies,this characteristicprovides a large opening as the plug is first lifted from theseat,with lesser flow increase as the plug opens further.Thistype is most commonly used where the valve will be eitheropen or closed with no throttling of flow required.2.Linear.Linear trim provides equal increases in C forequal increases in stem travel.Thus the C.,increase is linearwith plug position throughout its travel3.Equal Percentage.Equal percentage trim provides equalpercentage increases in Cy for equal increments of stemtravel.This is accomplished by providing a very small open-ing for plug travel near the seat and very large increasestoward the more open position.As a result,a wide rangeabil-ity of Cy is achievedThe pressure difference across the valve often varies withflow.This results in an installed characteristic,which willdiffer from the inherent characteristic.Figure 6-4-Globe Body Valve,Cage Guided6.4 Control Valve Types100906.4.1 GENERAL80Todays control valves operate by one of two primary mo-QUICK OPENINGtions:reciprocating(sliding stem)motion,or rotary motion.70The selection of a valve for a particular application is primar-WnWIXYW60ily a function of the process requirements,and no attemptLINEARwill be made here to cover this subject.Some of the more50common types of control valve bodies are discussed in 6.4.240through 6.4.8.306.4.2GLOBE BODY VALVE20EQUALThe globe valve design illustrated in Figure 6-4 is knownPERCENTAGEas cage guided.This design uses a piston or plug movinginside a cylindrical guide(cage)having characterized ports.0102030405060708090100In this design two options are available:PERCENT OF VALVE OPENING1.A single-seat construction for minimum leakage in theclosed position.Figure 6-3-Representative Inherent Flow2.A balanced construction requiring less actuator force,butCharacteristic Curvespossibly allowing more leakage in the closed position.The