Safety/pressure relief valves on air compressor:How much margin above the pressure switch?

[Henry W]

For context - I bought the two compressors referenced in this thread:
http://www.ncwoodworker.net/forums/showthread.php?t=48015

The larger unit, a tank belt driven unit from Sears with no apparent model number shown, seemed to require a new pressure switch. The motor did not cut out before the safety valve released the pressure.

I have since installed a new pressure switch; it cuts out as expected at the stated 125 PSI. However in order to get the pressure to that 125 PSI, I had to hold in the safety valve. I did not try this with the initial pressure switch. It now seems to me that the initial problem may have been a mismatch of this pressure switch and the safety relief valve (or a safety relief valve that let go too early).

So the question is - there seems to be a lot of choices at a place like Grainger. If the pressure switch is set for 125 PSI max, what pressure should the chosen for the safety valve? The Grainger tech team just suggested a 150 PSI valve. Does that sound right to you?

Henry

 

[nn4jw]

Depends a lot on what the tank is rated for maximum pressure. Exceeding that would risk making a bomb out of your compressor. Then there's the condition of the tank to consider as well. If there is rust inside or out then I'd think you'd want to be very conservative on maximum working pressures.

 

[Henry W]

Thanks Jim - there is no rust evident, at least on exterior. Not sure how to inspect the inside though.

Henry

 

[nn4jw]

Googling "air compressor tank pressure ratings" will bring up quite a few links about testing tanks and "normal" ratings for single stage compressors. Assuming your compressor is a single stage unit then max working pressure should be no more than 100 psi and the tank itself rated closer to 300 psi for safety. Then the over pressure valve should probably be no more than 125 psi and you really shouldn't run it over 100 psi under normal conditions. None of my air tools require more than 100 psi anyway. There are various ways of checking the tank's pressure holding capability, usually involving filling it with water and manually pumping it up with a calibrated pressure gauge to leak test it. If anything blows that way it's not as dangerous as just air alone. One way I'd suggest to check the internal condition of the tank is to remove the safety valve, fill it with water, let it sit for a few minutes and then drain it into a light colored bucket or tub looking for any rust to run out. Rust could have formed simply by condensation as the tank is used normally, especially if the original owner never periodically drained the tank to get rid of condensation. Rust would make me nervous, especially if large flakes showed when tested. Better yet would be to look into it with a bore scope through the safety valve hole. You have a bore scope sitting around, right? (Me neither.) That pretty much exhausts (no pun intended) my limited knowledge on the subject. Good luck and here's hoping you got a good buy.

 

[Henry W]

Thanks Jim - that's helpful.
As you suspected no bore scope - only because my neighbor borrowed it last year and I have not seen it since

Henry

 

[Acelectric]

The maximum working pressure of the tank should be stamped on it somewhere. That is the number to go by for the safety valve setting.

 

[buildintechie]

Thanks Jim - that's helpful.
As you suspected no bore scope - only because my neighbor borrowed it last year and I have not seen it since

Henry

Harbor Freight has them for less than $100...I know because we just got one at work to scope a gas tank.

http://www.harborfreight.com/digital-inspection-camera-67979.html

 

[Henry W]

OK the tank is stamped for 150 psi - what Grainger suggested seems right then.

Thanks

Henry