Need some air compressor help

[jamm]

I have a 5 hp Sanborn air compressor that has not been used for 7+ years (actualy had it for ~12 yrs). I cleaned it up and checked the oil level. Everything seems to work fine except when it reaches its preset 125psi it shuts off and the relief pressure valve opens. Once it hits 90 psi it will start up again like it should and repeats itself once it hits 125 psi again.

Anybody ever have this problem? Could the pressure switch be faulty? It works fine starting and stopping the compressor, it's just that when it cuts out it opens the pressure relief valve. Very annoying to hear the valve bleeding off just after the compressor shuts off.

Anyone have any thoughts?

Thanks

 

[jimvannoy]

Sounds like the pressure relief valve is bad or your pressure gauge is not reading correctly and it's building pressure over the relief valve rating.

 

[Charles (in GA)]

This is NORMAL. The pressure switch is bleeding off the pressure trapped on the head of the compressor. If it did not do that, the motor would stall the next time it started up. Does the air bleed off and quit, or does it keep bleeding off till it restarts? It should bleed off and quit in a few seconds. If if just keeps hissing till the pump restarts, then the inlet check valve at the tank is bad.

The overpressure relief valve is in no way connected to the pressure switch, it is the one with the little key ring thru it, that will pop off if the pressure in the tank gets too high, or if you pull the ring it will blow off. The pressure switch has a relief valve designed to dump the head pressure when the switch opens.

Charles

 

[jimvannoy]

What it sounds like he is saying to me is that it stays open and lets the pressure drop to 90 and then it starts up again.

 

[jamm]

Does the air bleed off and quit, or does it keep bleeding off till it restarts? It should bleed off and quit in a few seconds. If if just keeps hissing till the pump restarts, then the inlet check valve at the tank is bad.Charles

The air continues to bleed off until it restarts so I'll check the inlet check valve on the tank. Thanks for the quick responses.

Jerry

 

[nissan_crawler]

Ok, I'm not following. Are you saying:

A. The pressure relief valve that's on it's own is leaking it down to 90 p.s.i.?

or

B. The unloader on the pressure switch itself keeps letting air leak through?

If A is happening, you need to verify that the gauge is reading correctly. If it's not, replace it. If it is, replace the pressure relief valve.

If B is happening, then the check valve between the pump and tank is bad, as has been said.

 

[dxdexter]

I'm somewhat in the same boat as Nissan. I am not intirely sure waht valve is leaking. Is it the actual pressure relief valve on the tank or is it the unloader valve on the pressure switch . the later will have a smaller hose from the switch to the main line from the pump to the tank to relieve pressure on the head.

If the unloader is still leaking once the compressor reaches its predetermined pressure then the likely cause is the one way check valve between the tank and the compressor head. If this leaks by then it will continue to drain through the unloader until the compressor restarts.

 

[jamm]

Ok, I'm not following. Are you saying:

A. The pressure relief valve that's on it's own is leaking it down to 90 p.s.i.?

or

B. The unloader on the pressure switch itself keeps letting air leak through?

If A is happening, you need to verify that the gauge is reading correctly. If it's not, replace it. If it is, replace the pressure relief valve.

If B is happening, then the check valve between the pump and tank is bad, as has been said.

I'm somewhat in the same boat as Nissan. I am not intirely sure waht valve is leaking. Is it the actual pressure relief valve on the tank or is it the unloader valve on the pressure switch . the later will have a smaller hose from the switch to the main line from the pump to the tank to relieve pressure on the head.

If the unloader is still leaking once the compressor reaches its predetermined pressure then the likely cause is the one way check valve between the tank and the compressor head. If this leaks by then it will continue to drain through the unloader until the compressor restarts.

"B" is what is happening. My original post used terminology that was incorrect. The pressure relief valve with the ring attached that can manually be pulled open it not the one leaking. What you are calling the unloader valve opens once the compressor hits its' 125 psi limit and remains open until it closes at the 90 psi limit when the pressure switch restarts the motor. THerefore I am constantly losing pressure. If I shut off the compressor at any psi, the tank looses all its' pressure through this unloader valve.

I didn't know there was a check valve on the tank previously, but I'm thinking that is the problem.

 

[nissan_crawler]

that valve is working properly, there's a bad check valve.

Here's how it works:

Between the pump and tank, there's a check valve in the line.

Between the pump and that check valve, that valve that's on the pressure switch is plumbed in.

When the compressor hits 125 p.s.i. and stops pumping, that check valve closes, and the valve on the pressure switch opens, bleeding off all air between the pump and check valve.

That makes it easy for the pump to restart since it has no pressure on it.

Your check valve isn't fully closing, and it's letting the valve on the pressure switch constantly leak tank pressure.