Proportional Valves

[Weld_Man 0]

Hello, After spending weeks with no progress, I thought I’d ask you guys. I have never worked with proportional valves before, or even specked one out for that matter. I am having trouble finding a proportional valve that fits my needs. I’ve looked everywhere from Moog, Parker, to Emerson. Most I’ve found deal with hydraulics or pneumatics. Keep in mind I don’t know what really to look for either though. It might even be called a Control Valve (same thing?) A little background on what our process is now: We have a constant speed pump (5 h.p), that pumps water, and to change pressure we have manual valves we adjust by hand to achieve our set psi. Our set psi will vary anywhere from 0-120 psi. The psi varies because it is a test setup that requires multiple pressures to take data from. What I am trying to do: Spec out a proportional valve that will accept a 4-20 ma signal to change pressure thus eliminating adjusting valves by hand. This valve will be adjusted according to the pressure transducer upstream of the valve. The problems I am having: It’s pretty bad when you already have problems and not even the valve in your hand huh? Maybe I am looking in the wrong spot. But I cannot find a proportional valve with around 1” NPT fittings and a pressure rating of 150 psi. 2% accuracy would be fine. Valve needs to fully close and fully open. 4-20 ma signal (preferred) I have done my research and so far this looks like what I should be looking for in a proportional valve: -Zero overlap -Linear flow -Recommended to use a valve with an amplifier that is set/matched from factory (I guess this is to eliminate any calibration issues) Omega seems to have the perfect one here: http://www.omega.com/ppt/pptsc.asp?ref=PV_Valves&Nav=grej10 but unfortunately the max psi for a ¾” inlet is 70psi. Even uses a stepper motor for control, which would be nice. I’ve also looked at www.emersonprocess.com and it seems like I’m getting closer. Before I make this post too long, can anyone point me in the right direction or have any tips/pointers on choosing a proportional valve? Any problems I could run in to? Thanks for your time

[newpageboba 7]

Emerson (Fisher or Baumann) is a good place to start. What you need is a final control device or control valve. Find an Emerson rep/salesman. Tell him the application and they should be able to size the valve for you. Good Luck

[TConnolly 13]

Is this a centrifugal pump? Pressure and flow will be related. Why not put a VFD on the pump motor? You can do this for around $400

[DanW 41]

Yes, you want a modulating control valve and an actuator with either a positioner or an I/P. Normally, best control is achieved with a valve body size that one or even two line sizes smaller than the line, because the purose of a valve is to drop pressure. So a restriction in line size with a 3/4" or 1/2" valve body in a 1" line helps with a pressure drop to start. The supplier can "size" the valve from the pressure and flow rates you give him. The thing driving the valve itself is the actuator. The typical actuator is a pheumatic actuator, but there are electric actuators as well. Electric acuators use 4-20mA as an input signal, the pneumatic control standard is 3-15 psi. Your control system provides a 4-20mA which can be converted to a 3-15psi control signal from an I/P (Eye to Pee) or current to pneumatic transducer. The actuator assembly includes a spring that is loaded against the actuator diaphragm. The spring is there so that if the air signal dies, the actuator is driven by the spring to either open or close. It could be either fail closed or fail open. The same concept using the applied pneumatic ressure is called air-to-open (fail closed) or air-to-close (fail open). In some cases, one can get by with a spring sized so that it provides valve full valve stroke for the 3-15 psi input to the pneumatic actuator. However, most industrial duty control valves will use a positioner, that is actually a proportional controller mounted right on the valve bonnet. The positioner receives the control signal (different models for those that take 3-15 vs those that take 4-20mA) and uses supply air (maybe 60psi) to feed the actuator to position the valve stem. The positioner ensures that the valve stem goes to where it should by using mechanical feedback from the stem. If you need a positioner, then don't get one that requires an expensive handheld communicator or software to set up. There are "smart" positioners that don't need expensive setup accessories. There are literally dozens of control valve manufacturers and you might even get by with a light duty valve designed for HVAC applications, since your medium is water. Other big control valve names are Masoneilan (Dresser), Almost all commodity valves in this size are not sold directly from the manufacturer, but through a rep or distributor. "Valve houses" stock components and assemble what you need for quick delivery. Enjoy. Dan

[Steve Bailey 118]

I think you should be looking for a variable displacement pump with a pressure compensator that can be controlled by a 4 - 20 mA signal. Parker, Vickers, Continental, and many others all have hydraulic pumps that will fill the bill. Ask your local rep about operating them with water instead of hydraulic fluid.

[Ken Moore 23]

You can save a lot of money by going with a non-linear valve. You can purchase a "standard" 90 degree valve with a positioner this will work on most non-critical applications. It makes the PID tuning a little more difficult, but not that much.

[BobLfoot 302]

We're using Flowserve Valves from Worcester in Lynchburg, VA for a lot of our fluid control loops.

[Weld_Man 0]

Thanks guys! You all have given me great and very valuable information so far. Yes, it is a centrifugal pump, and unfortunately the pump setup/plumbing was installed a couple years ago when I was not involved, A VFD is not an option. This would have saved us a lot of hassle if we did implement this but I don't see it happening any time soon. A pneumatic actuator is also out. Do pneumatic actuators require more space, and are they noisy? Actually the purpose of this valve will be to increase pressure, as the test(s) will start out at 0 psi, and gradually work the way up to a final test pressure of 120 psi. We do have an exhaust pipe leading back to the tank to dump the extra water. And as far as I know, plan to put the control valve on the exhaust side to regulate pressure at our outlet. (this is how we do it now, just with manual valves). VFD does seem like the way to go. I've never installed one, or controlled one for that matter, it is something I'll look into. Just can’t see it happening here any time soon though. Thanks again for the suggestions!

[GerryM 2]

Why is it not an option? A VFD will change the pump output by the affinity laws: N1/N2 =Flow1/Flow2 ; (N1/N2)^2=PSI1/PSI2 Depending on your hydraulic requirements it may or may not work. Pnematic actuators are usually a little larger depending on what you need. They are generally faster, fullstroke in 3-6 seconds depending on the size of it. Electric actuators at least the ones I have take a long time to open, 30-60 sec.