Air valve on compressors cracking open often?

The air valve to let pressure off heads for start up will pop off every 10 seconds until the compressor starts back up?

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Let’s look at this issue.

When the compressor motor is off, usually the unloader valve (often part of the pressure switch) is open to atmosphere to allow the pressure over the piston heads to bleed to atmosphere.

When the motor starts, the unloader valve is supposed to close, allowing pressure to build in the cylinder(s) and compressed air to flow into your tank.

Are you saying that the unloader valve is venting air to atmosphere now and again while the compressor is off?

Or, are you saying that the PRV (pressure relief valve) is opening and closing during this period.

I’m guessing that it’s the unloader valve that’s blipping air out, and if that is the case, then I suspect your check valve in the line going into the tank from the compressor head isn’t seating fully, and it’s leaking air from the tank out through the unloader valve. That’s not supposed to happen,and the check valve’s purpose is to prevent exactly that.

Typical compressor pressure relief valve


Emax 3hp air compressor blowing PRV valve

hi, my 50L 3HP compressor builds up pressure to the recommended 8 bar or beyond. it then blows the prv valve until the pressure drops down to about 5 bar. it then continues building pressure until it blows again and so the cycle goes on. i have tried adjusting the prv a few turns but the same thing happens at more or less pressure, depending on how much i adjust it. its been working fine for the last 3 months i have had it.

thanks

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Do I understand correctly that your PRV is opening at 8 bar (120 PSI), this being the normal cut out pressure of your compressor?

That being the case, my first thought is your PRV valve has failed. Don’t monkey with it, replace it.

If, on the other hand, the PRV valve is blowing at, say 140 PSI, when your air compressor is normally shutting off on high pressure cut out at 8 bar, then this points to your pressure switch having failed.


75 PSI pressure valve popping on cross line?

by David
(Cleveland, Ohio)

Speedaire 7.5 hp upright compressor has a 150 psi tank mounted PRV that is ok.

PRV hidden, not on list!

PRV hidden, not on list!

 

By the head, there is another PRV that is 1/8 npt and 75 psi rated. I have to keep the compressor set at a lowly 70 psi to keep that valve from popping.

I had the compressor running at 110 psi for the last 6 months with no problems. All of a sudden in the last two weeks I have had to keep adjusting the pressure down lower and lower to keep it from popping while I figure out the problem.

Do I have a head problem? Or do I just have a bad PRV? Or am I missing something elsewhere?

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David, from the look of your diagram, the “cross-over” line you refer to is the one that runs over to the pressure switch.

The PRV senses the pressure in the system, and is set to “blow off” at a certain pressure, if the pressure in your system approaches what is deemed unsafe.

I’ve never hear of a PRV set to go off at 75 PSI. That doesn’t make sense if it’s purpose is to prevent over-pressurization, and the system pressure is 125 PSI.

If you could identify in the diagram the number of the PRV, maybe I could help identify the problem more closely.

However, if the PRV is letting to at 75 PSI now, and it didn’t when you had the system pressure at 110 PSI, why? I can’t that it would be anything but the PRV, yet I don’t have a lot of experience with one of them failing, unless after it lets go, it isn’t closing properly and then requires less pressure to open again. Here’s more info.


PRV blowing…

by Christian
(Oxford, UK)

I have a 3hp single phase Charnwood 106 compressor for painting a car I’m working on – little used (off eBay).

Charnwood 106 compressor

 

Unfortunately the PRV blows when the pressure enters the ‘red’. The pressure switch has only operated once out of the 10 times I’ve used it so I assume this is where the problem is? I’m a compressor novice so am not sure where the pressure switch is – can it be fixed? I’m pretty good at DIY – I think spares are still available… Thanks

This is the link to page which has the spec for my compressor – http://www.charnwood.net/ProductDesc.jsp?cat=29&stockref=C106

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Hi Christian:

Yes, you want to replace your pressure switch right away, as I suspect that this is your problem. Instead of your compressor shutting of at the cut out pressure level, it barrels on through, continuing to compress air until the PRV blows.

Here is info on pressure switches to help you identify yours.


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