Pneumatic valve operation

[remp]

Hi

I have been given a project in which I have to reuse up to 20 existing open/close pneumatic control valves on cooling towers and chillers and I need to put in a new pneumatic control panel to open/close them with pneumatic solinoid valves.

The thing is the existing contol panels up to 30 years old and are a complete mess...no drawings or labels on the solinoids...

Some of the actuators have a single tube going into the actuator. I presume these are drive closed (with compressed air) and spring return.
Other have 2 tubes going into the actuators head ( i dont know that the head is for) and from the head 2 tubes going into the actuator, I presume they are drive open/drive colsed...(see the attached photo.)

The problem is there are others that have 2 tubes going into the actuators head like in the photo and from there only one tube (say the white tube) going into the acturator.... what could be going on there? why only one tube to the actuator given they ran 2 tubes to the head? Anybody with experience on this sort of stuff???

 

[DRWeig]

Hi remp,

That's a little too much for me to try to explain in the forum -- my best suggestion is for you to find a local (older) pneumatics guy to help you out.

Typically, modulating actuators with only one air connection are receiving branch air only from the controller, they stroke through their spring range. Two connections means there's probably a positive positioner that needs main air also.

Or, you may find a tutorial on TAC, Siemens, or Honeywell web sites.

Let us know how it works out!!

Good on ya,

Goober Dave

 

[willard3]

I second finding an old pneumatic guy.

I still do pneumatic controls when I can because they are inexpensive and easy to mainbtain.

 

[BM4BRANES]

I doubt the valve is spring return, one signal opens and the other drives the valve closed. 2 positive positions, right?

 

[DRWeig]

BM4BRANES,

That's a good intuit, but not quite how a positive positioner works. It's actually a position controller that takes in main air (say 20 or 100 PSIG) and branch air position signal (maybe 3-15 PSIG), then outputs to the valve diaphragm head whatever pressure is needed to make the valve go to its intended position (open, closed, or stopping somewher in between). The positioner, since it can apply main air if needed, overcomes the varying force that a one-pipe actuator has.

There's a spring against the actual diaphragm, or else the valve would have an unknown default position -- where does it go on loss of air pressure? These could actually be modulating valves if they're on cooling towers -- part of the control scheme to maintiain a (possibly variable) return water temperature to the chillers, in addition to taking tower cells in and out of service.

My company still sells lots of replacements for this kind of stuff. High pressure pneumatics still get spec'd in industrial and oil field applications a lot.

REMP -- don't forget to post us when you have a solution!

Good on y'all,

Goober Dave

 

[Compositepro]

Your "head" is a quarter turn valve positioner. The actuator looks to be a dual opposed piston design with rack and pinion gearing. It could be air-to-close and air-to-open, or single acting with spring return. Knowing this it should be easy to figure out how it all works. A positioner will have an air supply line to power the actuator and a signal line to tell it what position to be in. It will have one or two lines to the actuator.

 

[KiwiMace]

The last two posts were on point. That looks like a double acting (no spring) Fisher 1035 actuator...

http://www.documentation.emersonpro...documents/instruction_manuals/d500243x012.pdf

Not sure what the actuator is from this angle.

But back to your original post; this is a modulating control valve not a two-position. In CT and CH control, you usually only need to motor open/closed.

You could chuck out the positioners which will be a maintenance headache (if you have no-one to identify or service them) and replace them with solenoid operated, spring return, five port/two position pneumatic switching valves.

The other thing to consider is that the status switches for open/close indication/confirmation are inside the positioner (that what the wires are for). If you want this function, you might have to fit new switches or keep the positioners and switch a 15psi signal into the positioner with the solenoid valve above.

Lastly, you'll need to service the actuator & valve, which is pretty easy, or at least test for bleed past the o-rings by pressurising on each side in turn. Check the valve seat for degradation.

 

[steamdog]

Being the Fisher rep here in NY Metro, I passed the picture around and indeed this is a double acting actuator (no spring) and it could be modulating or on/off depending on the internals of the positioner. For your system, each valve has to be dealt with individually, looking at exactly the control signal being sent, standard valve position, and feedback. Fisher may be a good place to start, they do a lot of pneumatics. I would look up your rep and give them a call.

 

[CH5OH]

i see two tubes going to the actuator, 1 tube going to the controller (on the bottom?) and then 2 cables
so one tube provides air to a solenoid valve inside the controller, and one cable is providing the power supply for the solenoid.power on = air to 1, 2 to vent and power off = air to 2, 1 to vent (or vice versa).then one cable provides feedback (wired to end switches inside the controller)

 

[steamdog]

I was thinking that the two tubes that go TO the positioner: one is a pneumatic control signal and the other supply air?? so it could be a pneumatic modulating positioner, then the electric would just be for limit switch feedback?

 

[KiwiMace]

I can't help but comment that not all old pneumatics guys are old you know... I am still on the fresh side of 40 and was trade indentured in pneumatic instruments. Of course, I was also obsolete at 21!

 

[willard3]

Old in the sense that you've been doing pneumatics for a while........20 years seems like a while.....