Air Consumption _on Off Valve

Sole Solenoid Va V alve Sizi Sizing ng for Shutdown V alve A ctuator : Inst I nstrum rumentation tion and and Proce Process ss Control Page Page 1 of 7

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Soleno Sol noiid Va V alve Si Sizi zing ng for Shu Shutdo tdown wn V alve Actuator I n most most app appllicati cation on unl unlike control control val valve tha that trigg trigge ered red by I/P I /P Positi Positione oner to modul modula ate, te, theshutdown shutdown valve is triggered by a solenoid valves. Since the shutdown valve is a ball valve that simply open or close (on/off) the solenoid valve act as a gate of instrument air supply to the actuator. For example, if the plant is in normal condition, then the shutdown valve is open or close by energizing a solenoid valve. The energized solenoid valve (usually 3/2 way valve) will make an instrument air supply goes to the shutdown valve actuator while when the solenoid valve is de-energized, the instrument air supply supply wil will close close and and ma make the the actuator loss loss ai air suppl supply. L oss of air suppl supply at actua actuator wil will make the the actuator goes to its fail position (failure close, failure open, or failure in last position). See below picture picture to get get a clea clear understandi understanding. ng.

I n the the proje project specif specifiication or datas datashe hee et, usually thesolen solenoi oid d val valve is spe specif cified as as ¼” in i n size. Why Why it it should should be ¼”? Why Why it it doesn’ sn’t 3/8”, ½”, ½”, 1” etc? etc? Actua A ctually, when the project specification mentioned that the solenoid valve shall be ¼”, its ok, but the shutdown

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Solenoid Valve Sizing for Shutdown Valve Actuator : Instrumentation and Process Control Page 2 of 7

valve vendor shall confirm. Why? Cause the solenoid valve has a certain Cv. This Cv will determine how much the instrument air can be delivered to the actuator with a certain pressure (usually minimum instrument air pressure goes to valve is 80 psig). So the actuator air consumption is the key factor to determine the solenoid valve size. The bigger the actuator, it may lead to a bigger solenoid valve too. So let’s derived the actuator air consumption first and then calculatethe required Cv to select the right solenoid valves size. Calculation of Shutdown Valves Actuator Air Consumption Vs =Area (in2) x Stroke (in) Vs =…. (in3) Divide by stroking time to get the volumetric flow rate, Q =Vs (in3) / t (second) Q =Vs (in3) / (t / 60) minute Q =…. (in3/minute) Convert to cubic feet per minute, Q =( (Vs (in3) * 0.000578704 (ft3/in3) ) / (t/60) minute Q =…..(ft3 / minute) Q =……CFM (cubic feet per minute) Convert to standard condition, Q =CFM * (P actual / P standard) * (T standard / T actual) Q =CFM * ((14.7 +Ps)/14.7) * (520 / (460+T)) Q =……SCFM (Standard Cubic Feet per Minute) Where, 14.7 =absolute atmospheric pressure (in psia) 520 =standard conditions in Rankine (60 Fahrenheit =520 R) Ps =gauge pressure of instrument air (psig) T =temperature of instrument air (F) Now let’s determine the required Cv with the following formula provided by solenoid valve vendors. Cv =Q / ( (Differential pressure*Po/((460 +T)*SG))^0.5 * 22.5) Where,



SG =specific gravity of instrument air =1 T =100 F (assumption of typical instrument air temperatures) Po =Outlet pressure (psia) Then we can calculate the solenoid valve Cv as follows: Cv =Q / ( (Differential pressure*Po/560)^0.5 * 22.5) Case example, A shutdown valve 16” size ANSI Class 600, with the following selected actuator data: Vs =Area (in2) x Stroke (in) =1318 in3 (taken from vendor catalogue) Required Stroking time =16 second Q =1318 in3 / (16/60 minute) Q =4942.5 in3/minute Q =4942.5 in3/minute * 0.000578704 ft3/in3 Q =2.860245 ft3/minute Q =2.860245 * ((14.7 +80)/14.7) * (520 / (460+100)) Q =17.11 SCFM Cv =17.11 / ( (5*94.7/560)^0.5 * 22.5) Cv =0.82699239 So the required Cv is 0.827. Select the solenoid valve size based on the vendor published catalogue below (Cv data from Versa). Solenoid Valve Cv size 1/4

1.8

½

4.0

1

11.1

1¼

14.9

From the table above we could select the solenoid valve size required for this application is ¼ inch only. Conclusion,



For most application, ¼” solenoid valve is enough to chatter the actuator instrument air consumption. So as default we could specify the solenoid valve size as ¼” in the shutdown valve datasheet. But the vendor of shutdown valve shall confirm this size per their actuator selection.

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Instrument Air Consumption Calculation Instrument air consumption shall be calculated during detail design to determi by pneumatic operated instrument as well as package which requires air for its purpose. Instrument air consumption calculation will be the basis to size instru consists of air compressor, air drier, and air receiver. Actuated valve such as shutdown valve, blowdown valve and control valve are i instrument air for their operation. Instrument air requirement of each instrum size and its operation. Shutdown valve and blowdown valve may be considered control valve working continuously. It should be noted that the instrument air steady condition is much lesser than during its transient condition. ssumption To determine the instrument air consumption, some assumptions should be m assumptions are for example only, not as standard reference, and may differ on Normal air consumption demand ■

Control Valve From total number of control valve, 90% of control valve operates in stable state air consumption only, while 10% could be in unstable condition hence consumption.

■

Shutdown valve / Blowdown valve

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Shutdown valve and Blowdown valve only require instrument air when they predominantly during start up after shutdown, so it is considered intermitte assume that only some number of valve are working simultaneously. It coul will operate simultaneously for normal air demand calculation. Peak air demand From total number of control valve, 70% of control valve operates in stable con air consumption only, while 30% is in unstable condition hence requires transi Shutdown valve / Blowdown Valve Let’s say 100% of the valve will operate simultaneously for normal air demand *Note that the number of percentage state here is assumption only and may dif endor data The next step is to gather data from vendor / manufacturer catalog which provi consumption. The following is data taken from catalog: ■

Control valve in stable condition 0.3 scfm

■

Control valve in unstable condition 7 scfm

For actuated valve, the instrument air consumption depends on size of actuator obtained from vendor catalog. The required opening or closing time of the mai consumption rate. Calculation For instrument air consumption calculation, simply multiply the number of val consumption data, then sum for all instruments. Do the calculation tabulation f condition. In addition, other package which requires instrument air shall also b

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Air Consumption _on Off Valve

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