Diaphram vs. Piston-Type Control Valve

Toomuchfun here, good morning RAJ KUMAR,

As for the black box the answer is "NO", there will always be TOO many unknowns! If you need a variable resistor for controlling mimi-amps of energy and cannot afford a new one, salvage one from an old computer monitor, TV, or even a old boom box, likely they will have value ratings that are sufficient to provide for some overload protection.

Now regarding your control valve! What substance are you passing through the restricted area. The diaphragm may be attacked by the chemical composition of what ever you are passing past it. The same can be said for the piston type. Look at the material that you are working with first, and determine it's effects on the equipment you are using to control it. You may be limited by this action to the kind of controller that can be used by the cost factor vs expected usable life span of your choices. Before anyone could offer any better info on this subject, you need to provide a more complete quantity of information in your request!

TMF

why?? cant we apply Ohms law on such loops?? if we can, then we should treat transmitters as a variable resister as V=I*R and if V is constant and I is varying then R should also be varying? what do uyoy think.

and my quuestion regarding control valve is not dependent on process fluid. if we take same conditions for both types of valves then which valve can provide more actuating power with same area???

regards.

Regarding your points about ohms law. You started off with a black box. You may have some idea as to what parts exist inside. I do not. Neither do I have any idea as to its limitations, Therefore I say "NO" black boxes. Precisely what are you trying to transmit or control, frequencies or electrons, etc.? After all, we do not place a directional controlling diode, where we need to simply reduce or slow down the flow of energy.

Now as to your issue concerning the control of pressure with the two entirely different valves. I find that a diaphram type of pressure regulator works well and is cost effective when we wish to control the pressure of gases. Piston type control valves seem to be better suited for the control of { for example } materials that are pumped at high pressure in a liquid state. I am sure that cost to purchase and maintain such valves must be considered.

TMF

Hi there,

Firstly, yes you can use a variable resistor to vary the current in the loop. As you have stated in your post V=IR(Ohms law). So if you voltage stays constant, then by changing the resistance you will change the current.

Now as to your question about diaphragm vs piston on power. The diaphragm actuator uses varying air against the diaphragm to overcome the spring force. The piston uses varying air to overcome the process medium force.

If you were to use the same area and pressure as well as the same opposing force - both valves would exert the same power.

If your valve is battling - then you would normally use a piston actuator as it can handle higher air pressures than the diaphragm. This in turn meanse more power to overcome the opposing force from the process medium. You could also try a stiffer spring in the diaphragm type actuator to achieve the same thing.

If you need even more power you can use a double acting piston, and for those really high power requirements you could use hydraulic or elctro-mechanical.

As stated above the major difference is how the valves work. Why the piston appears to give more power is because you can use a higher air pressure. You also get a longer valve stroke with the piston type actuator.

You must bear in mind that the signal pressure from the I/P (20-100KPa) is not the same as the air pressure that is sent to the valve.

I hope this makes things a little bit clearer for you.

Regards,

Craig