Control Valve Actuator Selection 2

[mborges]

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

Mike

 

[atad]

Your selection does not really start at the Actuator. It starts at the Valve Duty with you specifying the type of control requirements you have available i.e.,

1. Electric / Pneumatic / Hydraulic - Actuation

2. Electric / Hydraulic / Pneumatic - Control Signal

3. Speed of Operation

4. Control Flow Rates

5. Media Type

6. Pressure in the Media Line

7. Temperature of the Media

Once you have answered most of this, the Valve vendor will advise you on the Actuator selection from a usually extensive range of alternatives and Prices!

 

[skills]

I'm a little confused, are you asking about end actuators or control valves as they are different animals. In pneumatic actuators, the decision to go diaphraim is usually to eliminate piston sealing ring stiction, 'or static friction of sealing components'.

 

[Techtalk]

An excellent question !

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.

When 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.

Hope this helps.

 

[209larry]

mborges,

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.

209larry

 

[ilan7]

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.

 

[mborges]

Dear Ilan7,

you are saying that it is not advisable to use piston actuator in applications other than on-off?

 

[ilan7]

I prefer to use Diaphragm for Control applications.
I do use piston actuators for control when:

1. Need fast action (using Pneumatic control
valve for turbine governor and bypass with
less then 1 sec. strok time)

2. Large valve size when price of the diaphragm
is much higher (more then double) then a piston
actuator no spring but emergency air volune tank.

3. I never use diaphragm for On/Off application
but with positioner and analog input signal.

 

[mborges]

Ilan7,

is it possible to have a stroke time less than 1 sec only using a piston actuator? A volume booster will not be necessary? I am considering a bypass valve for an anti-surge control system for a centrifugal compressor.

 

[hacksaw]

surge control is a different beast. it is not just a matter of stroke speed.

you need to decide if you intend to perform "surge control" in a strick sense (depends on your compressor and system curves or just surge avoidance.

in high pressure multi-case machines there can be a cost benefit for using active surge controls to push the envelop, this is not always the case in low pressure single case centrifugals. there surge avoidance is more beneficial.

the valve speed requirements are reduced in surge avoidance.

we've used both types of actuators. it depends on the valve size, pressures, flow rates, speed, cost, etc...

 

[ilan7]

mborges
It has been some years since I worked on anti-surge control system for a centrifugal compressor, so I do not think I'm good source for those problems.
About fast action pneumatic control valves I have two or three good sources:
1. Masonailan (It was some years ago and I do not recall the valve type any more)
2. Fisher Controls (H.P. Butterfly valve 8500 sizes 8" to 20" with piston actuator (1061), good positioner, two air volum boosters (3/8" to 3/4&quot, Air volume tank and large pipe Dia. (1/2" to 3/4" ) between tank - Boosters - actuator. Only 1/4" tubing between positioner and boosters signal.
3. Tyco: Vanessa Type 30000 valve, Morin actuator, Electronic (Smart) Positioner, two large (3/4&quot volum boosters and a Volum tank. Have to connect all of them by using local workshop as they not do it at tyco.

Fisher Controls has a "flow scanner" system which can help you tune the valve to the optimum performance. You can also get the data from the positioner (You need a smart one) directly into a data logger (Computer) for calibrating and checking the valve performances.
Regards ilan

 

[mborges]

Ilan,

as far as I know, Vanessa valves are mainly used as zero leakage block valves. An anti-surge valve has to modulate the bypass flow. Besides, as a control valve it can be class VI, at most.

Michael

 

[ilan7]

As 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 VI or Class IV as needed.
Valves are Tripple Offsets. The problem with them is that They do not make actuator, positioners and other aux. The Sister company, Fibi make very expensive actuators mainly Hydraulic and Electrical.

 

[mtmurthy]

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. Apart 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&quot. Piston actutors have an edge over spr& dia in higher sizes, higher pr. requirements & better stroke time requirements.

 

[antisurge]

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, for 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.

By the way, i only recomend Fisher 2625 volume boosters, with 125 psi rating so no regulator is required, as i have seen nothing else that comes close.

For sizing the recycle valve, rule of thumb is to have valve about 50% open at the surge point.

I could go on for some time about speeding up antisurge valves.
reid
515-253-3219

 

[sridharJalei]

I have workded on valves with shut off time less than 1 second. It is mandatory for the gas expansion turbines we use and this quick shutoff is generally tested for less than 1 second by giving a trip command and checking the feedback time in the PLC / DCS. I use a piston operated valve with air volume boosted and quick exhaust valve. The volume boosted is more critical for closing the valve whereas the quick exhaust valve is very critical for quick shutoff. In your case of anti surge valve, even though the requirement is for quick opening, since the valve is a fail open valve, you will benefit by using a quick exhaust valve. Please give me your feedback on this later after you have tried this out.
Sridhar

 

[NGiLuzzu]

About VANESSA and BIFFI mentioned above, you can also see the relevant web-sites:
*

http://www.vanessavalves.it/

*

http://195.103.75.99/

*

http://www.tycovalves-eu.com/

and the following other posts:
* Thread404-72775
* Thread404-79216
* Thread404-76820

Hope this helps, 'NGL