What are the guidelines for selecting the type of actuator for a control valve. That is, how do I choose between diaphragm and piston?Oil pumps

There are two aspects to this question.If you are not bothered about performance of the assembly,select a diaphragm actuator as first preference ( if it is cheaper ) and when there is no size available to meet the required thrust or torque select a piston actuator.Further, as you approach the higher sizes, piston actuators are more economical as these have high thrust capability.

fire pumpsWhen performance is an issue, always go for diaphragm actuator as first choise due to following reason.

The piston typically has a smaller area requiring higher pressure and larger volume of air supply resulting in higher dynamic dead time.Greater no of guiding surfaces tend to give higher friction due to inherent difficulties in alignment as well as friction from O ring.The friction problem tends to increase over time due to degradation of the o ring material and wear on guiding surface.Increased valve response time and dead band will be the result.

Depends on the manufacturer you want. Fisher and Masoneillan will offer you diaphragm actuators while Valtek, Bettis, rotork, hytork will offer you piston actuators.

Manufacturers have the right models for you. What matters are cost, delivery and previous experience with the control valves.

Basically the piston is for ON/OFF aplication while the Diaphradm is for Control aplications.
Diaphragm can have higher torque (With spring return). We are using piston actuator without spring but with emergency air volum tank for control when we need higher speed and more accurate valve moving.  When the actuator does not need spring return the size is much smaller and the air volume reduced.

Split case pumpsAs a valve,  Vanessa is the best I worked with.  Both for ON/OFF aplication and control aplications.  In both cases you can calibrate it to have zero leak or to limit the actutor to get class

Piston actuators have more thrust o/p comparing with spring & dia actrs for a given size.
Force= Pressure*Area. More air pressure can be used for piston actuators since diaphragm has limitation on the max. pressure that can be used.  For valve sizes 8" and higher, using piston actuators will be of very compact size comparing with using spring,dia actuators.

Piston actuators are also available in inherent fail safe mode, i.e. double acting spring return piston actuators are available, in which springs can be used to achieve fail action. These are actuators are also used for throttling applications and it has  proven track record.The main advantage of piston actuators is they have more stiffness than spring & dia actuators. These actuators are more suitable in low plug throttling conditions. Vacuum pumpsApart from this,better stroke time is achievable with piston actuators.

Generally stroketime less than 1 sec, requires the use of flow boosters/ Quick exhaust valve depending the nature of control, size of the valve  & actuators.

With the advent of smart positioners with tuning facilities the problem of deadband, friction etc., are rectified.

spring & dia actrs have an edge over piston actuators at lower valve sizes ( upto 4" and also 6"). Piston actutors have an edge over spr& dia in higher sizes, higher pr. requirements & better stroke time requirements.

Antisurge control valves should stroke as fast as possible with general guideline of less than 2 seconds closed to open and say less than 4 seconds open to closed. generally either piston or diaphram will stoke this fast if volume boosters are added (2 for piston, 1 for diaphram). I discourage quick exhaust because of the one sided stroke speed increase. it is possible to obtain stroking speeds of less than 1 second but the ability to get air in/out becomes critical, In-line pumpsfor example the diaphram will have a .25 inch threaded tap and installing a nipple to a female tubine fitting will be faster than a male tubing fitting alone and you can order Fisher diaphrams with .75 inch taps. another example is with piston actuators from Valtek, they have two size spools in the positioner which directs air to each side of piston, Valtek has two sizes of spools and installing the larger spool increases speed even if volume boosters are correctly installed.

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