Typical control valve actuator design - help with basics

[scott1971]

I am confused by all of the features normally found on a "typical" control valve actuator. To keep this post simple, I am referring to the traditional diaphragm actuator type represented in the link. The purposes of the diphragm, spring, and actuator shaft are obvious. However, if all I'm doing is regulating pressure with a simple feedback loop, the remainder of the components—the yoke, spring adjuster, travel indicator and scale—seem to be added features only and not necessities.

I normally only work with diaphragm block valves on bioprocessing equipment. These valves typically have spring-loaded diaphragm or piston actuators. Aside from the fact that this type of valve is not best-suited for throttling, is there a good reason why it couldn't be throttled if the air pressure sent to the actuator is controlled via an I/P controller? We are using a Gemü 650 (click here for internal view), and it can supposedly be used in the just the way I've described (direct-acting only, pics show reverse-acting).

Can somebody please explain to me the benefits of the design of ubiquitous actuator desribed in my first paragraph over a simple direct-acting actuator? I don't understand why a control valve actuator needs to be so sophisticated.


Scott
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[Joesteam]

Actually, the two actuators are very similar. The Gemu has a spring, diaphragm, and plates etc. to make everything work. The main difference is that the components for the 'typical' actuator are MUCH bigger, probably used for a larger valve or for higher pressures that need greater forces to open/close the valve. These forces require the beefier yoke than the Gemu, but if you look at the attached Gemu internals, you can probably match up all of the parts from each type of actuator - just smaller.
As for the adjustability, this enables the same actuator to be used on a number of different valves, or the same type of valve with different pressure conditions, so that you can develop a 'bench set' that will allow, for example, a fail closed valve to shut off with the spring force, and completely open under a certain air pressure.

Joe Lambert

http://www.control-associates.com/

 

[JLSeagull]

The example appeared to be about 1 NPS in a diaphragm valve pattern for on/off service.

Most of my valve applications are for valves larger than 1 NPS and for higher flange classes. The mix includes on/off and throttling applications. For shutdown or vent applications we typically apply piston type actuators with rack and pinion for valves up to about 3 NPS and scotch yoke actuators for larger applications. Most of my diaphragm type actuators are for throttling. These require position indication. Normally we use digital valve positioners instead of the simple current to pneumatic positioners. As the control valves become large we use the scotch yoke style actuator with a digital positioner for throttling. Valve position indication is a requirement for both throttling and on/off valves.

 

[roydm]

The "Typical Actuator" has a large diaphragm and a spring that opposes it. By varying the air pressure you can vary the stroke (Valve opening) for throttling.
The Gemu has a relativly small diaphragm and could also be used for throttling but the diaphragm type valve it's fitted to is normally used for simple On/Off service with a fixed air pressure and 3 way solenoid valve.
Roy

 

[scott1971]

Thank you everyone for the responses thus far. To be clear, I am trying to understand the design origins of the typical control valve actuator (or "top works"). For a diaphragm actuated assembly, the diaphrgam is usually 8 to 12 inches in diameter and sits on the very "top." It is either connected to or integrated with a housing for a large spring. Between this upper assembly and the valve we have the yoke, which inlcudes a shaft with a travel indicator. This is finally connected to the bonnet of the actual valve.

My thoughts are that one of the reasons for the complexity I've described above is to allow bench calibration (as Joesteam alludes to), meaning one can adjust the spring tension and travel to match a specific control valve. The purpose of a large diaphgram seems to be to allow a fairly small pressure signal for the input (3-15 psi), which in turn also reduces the compressed air utility consumption. The purpose of the yoke seems to be to allow a valve stem travel indicator, but also a universal mounting point for a positioner.

So basically what I am asking is: If I don't need a positioner, and if I don't need the ability to adjust the performance characteristics of the actuator, and if I don't need to be able to visually inspect the position of the valve stem, is there any reason why a simpler pneumatic actuator without the yoke, without a "large" diaphragm, wihtout a spring adjuster, and without a valve travel indicator can't be used? It seems that Fisher came up with the commonly seen design for control valve actuators decades ago and just about everyone has emulated their design ever since. The pneumatic actuator in my original post is very common for simple open/closed service, but it seems that it can also be used in an analog fashion if an I/P transducer is used—just as if the same valve had a handwheel on it and someone manually throttled the valve (again, I understand that the valve itself my not be most appropriate for this function, but is there a reason it can't be done?).

Scott
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[roydm]

There's probably nothing to stop you connecting the diaphragm direct to the valve without a yoke, I'm sure you will find some. The yoke design has another advantage, it somewhat isolates the process from the air system. If the stem seal leaks it leaks to the outside, not to the air side which would be hard to detect. A few makes to check out are Singer, Clayton, Rollseal they make a different style of control valve.
Roy