Instrumentation & ControlChulalongkofll UniversityChapter 1: INSTRUMENTATION L. Module 8: Control Valves and AccessoriesCHAPTER 1: INSTRUMENTATION EQUIPMENTMODULE 8: Control Valves and AccessoriesMODULE OBJECTIVES: At the end of this module, you will be able to:1. State the combinations of pneumatic actuator and globe valve body required to provide:a) air to open action; b) air to close action.2. State the three flow characteristicsusually available for a globe valve. Sketch the flow characteristiccurve for each kind.3. State a typical application for each of the three flow characteristics and explain why it is used in thatapplication.4. State the effect of the unbalanced force on valve position and explain how, by giving an example,bench setting would correct this problem.5. Briefly explain at least five advantages a positioner gives when it is used in conjunction with acontrol valve.6. Explain the situations where the by-pass switch of the positioner should not be operated.7. Describe the failure mode of a control valve, when the valve configuration and positioner action aregiven. ,8. State the advantages of the following control valves: a) Y-ball valve; b) a rotating plug valve;c) a diaphragm valve. .9. Explain, with a diagram, how a three-way solenoid valve can be used for ON/OFF control of apneumatic valve.10. Explain the type and function of typical electric motor operated valves and state a typical applicationfor a Rotork, Limitorque and Hopkinson valve.11. State why it is often advisable to use different sized valves in parallel when the process variable hasa very large range.page1-0-1Instrumentation& ControlChulalongkornUniversityControl Valves and Accessories 1: INSTRUMENTATION Module 8: Control Valves and AccessoriesThe final controlelement in a system is used to vary the flow of a manipulated variable in an attempt tomaintain a process parameter at a desired setpoint. Final control elements include control valves,meteringpumps, dampers and louvers, variable pitch fanblades, and electrically driven controldevices.Thecontrol vah&.isthemost widelyusedtypeof final control elementanditmustperformsatisfactorilywithaminimumamount of attention, eveninsevereconditionsoftemperature, pressure, corrosionandcontamination.A control valve functions as variable resistance in a pipeline. It provides a pressure drop, called throttling,whichlimits the flow through a pipeline.There are many different kinds of control valvesin commonuse: globe valves,butterfly valves, ball valves,eccentric disc valves and diaphragm valves are some of the more popular types encountered.page1-0-2Instrumentation& ControlChulalongkornUniversityGlobe ValveChapter 1: INSTRUMENTAnONEl./JIPMENTModule8: Control Valvesand AccessoriesGlobevalvesarethemostfrequentlyencounteredcontrol valvesinprocessplants. Figure1showstheside view of a typical globe valve, which is named after its globular shaped valve body.The globe valve consists of two main components:1. the valvebody;whichcontainsandregulates the fluidflow, and2. the valve actuator, which converts electrical orpressureenergyintostemmovementwhichchangesthe regulating effect of the valve body.-+--PNEUMATICVALVEACTUATORFigure 1: A Typical Globe Valve.page1-0-3VALVE BODYASSEMBLYInstrumentation& ControlChulalongkom UniversityChapter 1: INSTRUMENTATION EQv,?MENTModule 8: Control Valves and Accessoriesr-----=-.,.....- PLUGSTEMVALVEPLUGPACKING BOX___.JlALVE BONNET:&NUTSVALVESEATPACKlNG.- _SPRINGTEFLON R1NGS-Globe Valve BodyThe globe valve body itself is made up of thecomponents shown in Figure 2. The bonnet assembly is the part of thevalvebody through which the valve plug stem moves. Thebonnet provides a means of sealing againstleakage alongthe stembyusingpackinginthepackingbox. Forceisexertedbythestudandnuts on the packing material to squeeze itagainst thewalls of the packing box and thevalve stem. This acts as a effective seal. Thevalvestemextendsthroughthebonnettopermit positioning of the plug, andtherefore provide avariable restriction to thefluid flow.Figure 2: Cross-Sectional View of a Globe Valve Body.page1-0-4Instrumentation& ControlChulalongkorn UniversityChapter 1: INSTRUMENTATlON E.....JiPMENTModule 8: Control Valves and AccessoriesGlobe valve bodies canbe classified as either direct or reverse acting, as shown in Figure 3VAlVE PLUG STEM ...I I 2 (VALVE PLUGa2VALVE SEAT:,: ,: IF......,,."..,..a) Direct Acting (b)Reverse ActingFigure 3: A Simplified Diagram Showing Control Valve Body Action. In a direct acting valve body, a downward movement of the valve plug stem results in the valve closing Inareverseactingvalve body, adownwardmovement of thevalve plugstemresults inthevalveopeningpage1-0-5Instrumentation& ControlChulalongkornUniversityValve ActuatorsChapter 1: INSTRUMENTATION El.._,PMENTModule 8: Control Valves and AccessoriesAcontrol valveactuator isadevicewhich is usedtodrivethevalveplugstemandthereforesetstheposition of the plug withrespect to the valve seat.Themost commonvalveactuatoristhepneumaticdiaphragmactuator. It issimpleinconstructionandveryreliable. It operates by the injection of a single,low pressure air signal into the diaphragm housing.ACTUATOR The diaphragm housing is made up of twosections. Between the two sectionsaflexiblediaphragm is installed. The air pressure applied on the diaphragmdevelops a working force. This force istransmitted to the actuator stem via thediaphragm plate, which is a supportive metaldisk attached to the diaphragm. The actuator spring provides a restoring forcewhich positions and returns the actuator stem. Thetravel indicator(apointer attachednearthestemconnector)indicatesthe valve travel ontheindicator scale. The actuator is supported rigidly on the valvebonnet assembly by the yoke. The actuator stem is connected by the stemconnector to the valve plug stem.DIAPHRAGMDIAPHRAGM foL6.TEACTUATOR SPRIN'---TRAVEL INDICATOF\--1:! ACTUATOR STEMSTEM CONNECTORTRAVEL SCALEINDICATOR--YOKEFigure 4: A Typical Pneumatic Diaphragm Actuator.page1-0-6Instrumentation & ControlChula/ongkorn UniversityChapter 1: INSTRUMENTATION EG/uIPMENTModule 8: Control Valves and AccessoriesThediameterof thediaphragmplatedeterminestheforcethatwill beappliedtotheactuatorstem. Forexample, if the maximum input signal pressure is 100 kPa and the plate diameter is 30 cm, then:Force applied to stem =Pressure x Plate Area= 100 kPa x 3.14 x (0.15)2 m2=7.07 KN (1590 Ib)1LRetum Spring ---'Diaphragm ----Stem -------l1+1....-----Yoke -----.....Althoughthe signal pressure of 100 kPa seems tobe fairlylow, a substantialforcecan still begenerated ifthe diaphragm diameter is large.Diaphragmactuators, as in the case ofvalve bodies, can be classified as eitherdirect or reverse acting. Adirect actingactuator will causetheactuatorstemtobepusheddownwardsasaresult ofapplyingsignal airtothetop of the diaphragm (Figure 5(a)). The reverse acting actuator will pushtheactuator stemupwardsassignal airis applied to the bottom of thediaphragm (Figure 5(b)).(a) Direct Acting (b) Reverse ActingFigure 5: Direct and Reverse Acting Actuators.page1-0-7Instrumentation& ControlChulalongkornUniversityChapter1. INSTRUMENTATION E(.,vIPMENTModule 8: ControlValves and AccessoriesValve ActionControl valvescanbe built fromvariouscombinations of valve actuator andvalvebody. Thecombinationof actuator andvalvebodyisusuallychosentoprovideaparticular failuremodeshouldthecompressedinstrument air supply fail for any reason.Retum Spring --"Diaphragm ----Stem ------1.(l ~ Yoke ~ = =f._--"'a..---..Any failure mode can be obtained with acombination of direct or reverse actingactuator and direct or reverse acting valvebody. An air to close (A/C), and therefore failopen valve, can be obtained with thecombination of a reverse acting actu,atoranda reverse actingvalvebodyor a directacting actuator and a direct acting valvebody. (Figure 6(a)). Similarly, an air to open (A/O), andthereforefail c l ~ valve, can beobtainedwithacombinationof direct actuator andreversebody or reverse actuator andgirectbody (Figure 6(b)).ValveFigure 6(a)Air to Close (A/C) ValveFigure 6(b)Air toOpen (A/O)page1-0-8Instrumentation & ControlChulalongkorn University ~ a p t e r 1: INSTRUMENTATION E