Pneumatic control systems trouble shooting

[blackbird307]

I have a pneumatic pressure source which goes to a pneumatic thermostat which opens up to source a air to open pneumatic valve at a certain temperature. The pressure at the source is a large hiss, by the time I get to the valve, it's just a draft, maybe 1/3-1/2 of source pressure.

The line is a 1/4in plastic tube which extends 10 meters to the thermostat from the source, then back to the source (10 meters) which connects to a 5/32 copper tube going to the valve (15 meters). So in total the valve is 35 meters in tube away from the pressure source. Will this length cause significant pressure drop? See illustration.

I am afraid there will not be enough pressure to close the valve. I was not able to detect any air leaks in the tube.

If I cut the copper tube and connect it closer to the stat, will I be able to solve my problem? See Red. We originally did not do this in order to leave the copper tubing intact. If however, we have to, we will cut it so we can eliminate the extra distance.

 

[mendonsy]

The extra length certainly will not help anything. The pressure should build up enough eventually to close the valve but the extra length will reduce the flow dramatically and make it slow to respond.

 

[kamenges]

First of all, do you know you have a problem? It depends to a large degree on the volume at the valve and how well sealed the system is. I wouldn't get too concerned unless you know there is an issue. If there is, decreasing the piping between the thermostat and the valve as you have indicated will help.

Keith

 

[Pete.S.]

Pressure and flow are two different things.

With no flow, the pressure at the valve will (eventually) be the same as the pressure at the source.
With an open end you get maximum flow but very little pressure. That is normal.

There are tables and calculations you can use to determine the pressure at a certain flow or how long it takes to build up a certain pressure. Easier than that is to use software like Festo Fluidsim to simulate the circuit.


From a design point of view it would be better use an electrical thermostat and a solenoid controlled valve.

 

[Tom Jenkins]

I think you need to do some additional investigating.

35 m is not a long length for a pneumatic control signal. As Pete S. pointed out, with no flow there is no pressure drop anyway.

Do you really have a problem? In other words, is the valve/thermostat system no working? Or are you just not seeing the pressure you think you should?

These systems usually work in three steps. The pressure source should provide 3 to 15 psig to the thermostat. The thermostat modulates that signal, sending a signal to the valve that corresponds to % open. Then a separate actuator uses a higher pressure to move the valve.

There are, of course, many variations. More info will give you better answers.

 

[danw]

I am afraid there will not be enough pressure to close the valve.

Change the setpoint of the thermostat so that it 'calls for heat' and see how long it takes or the valve to open. If it never opens, then the system lacks sufficient air pressure.

The cost of a pressure gauge, a Tee and some fittings is peanuts. It might be worth it to have a gauge at the supply end and one at the valve end.

5/32" OD copper tube is pretty small diameter tubing, but eventually the flow should fill whatever actuation mechanism is on the valve and the pressure should rise to near supply.