Is there any way to externally dampen a pressure switch?

[EJG73]

Hello,
I had some issues with a pressure switch on the discharge side of a fan the other day. It was cycling often so I removed the switch and replaced it with a freshly calibrated switch. After I removed the switch I tried it on the bench, it worked great! I then hooked a 754 with a pressure module up to the inservice pressure switch by putting a T fitting in the sensing line. I datta logged the switch for 2 days acumulating well ofer 14K points of data. Not once has the switch cycled. The only change made was hooking in the meter. Has anyone come by a situation where increaing volume on the sensing line of a switch had a damping affect on it? Do they make a product like a tubing fitting to dampin a switch?

 

[MikeHalloran]

See "Heisenberg".

Let's clarify your description of the experiment.

You removed a pressure switch from service because it was rapid cycling. It tested okay on the bench. Problem Not Found, not unusual.

You replaced the PNF switch with a second switch, not bench tested, i.e., it could be stuck.
You measured the pressure near the second switch's inlet, and never detected a pressure change that should have changed the switch's state. So the switch's inlet could be plugged. Or your tee could be plugged. Or it could be spliced into the wrong hose. Or the second switch could have been removed because it didn't work at all, and returned to a shelf for future repair that never happened. If you monitored the switch's state while logging pressure, you didn't say so.

Okay, I'm guessing about a lot of that, because you left a lot of obvious questions unanswered.


Writing a more detailed account of what you did, and verifying things you need to check in order to make factual assertions, may lead you to solve your problem even without our intervention. But do please complete the story for us.

To answer your question, there exist dampers for high pressure hydraulic system gauges, but I don't think they'll be effective on what I conjecture is a very low pressure switch (specifications also not stated).



Mike Halloran
Pembroke Pines, FL, USA

 

[LittleInch]

You may have just happened upon a section of sensing pipe which was being subject to pressure waves created within it from the nozzle causing pressure buffeting. Changing the internal make up and volume may just have stopped it. Try putting the original tubing length and size back and see if your new switch then starts tripping.

Sensing lines from air systems can be sensitive to length of tube.

This is all a bit of a guess as we have no info on sensing hole location, air speed, distance from fan etc

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[DRWeig]

As for the OP's question at the end of his post, adding extra tubing will provide a damping effect -- unless it's just the right length to set up a standing pressure wave. We often use a coil of tubing to damp noisy air.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[danw2]

Damping coils seem to be available premade in the UK, but in the US I find that people roll their own from 1/4" tubing.

Remember that a pressure switch actuation requires some volume to change, so using tubing as small as capillary tubing might delay the switch response.

 

[racookpe1978]

Or, going the other way around, adding the tubing (but a larger diameter, longer length or more flexible (thinner wall) tubing) in the test rig may let each surge of air to "bellows out" the thinner-walled tubing, and so damping the impulse passed by a stiff-walled, short tube.

 

[Artisi]

Have you considered a commercial pulsation dampner?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[MikeHalloran]

No number has been revealed yet, but I'm guessing the OP is working with pressures on the order of a few inches of water column, maybe less.

In that pressure range, I'd recommend sense line tubing of a minimum 1/4" OD, or around .17" ID. Smaller tubes can retain slugs of water that adhere to the walls well enough to produce substantial stiction and hysteresis in manometric devices.



Mike Halloran
Pembroke Pines, FL, USA

 

[racookpe1978]

Good point - At that low a pressure, I would expect no swelling of the tubes.