Question for those knowledgable about compressors

I've had my compressor for about 10 or 12 years. It's been used sparingly over that time, mostly for my sandblaster and has always worked well. When I turned it on the other day and it started to fill I suddenly heard air escaping out of it. It seems to start at about 60 psi. If you look at the pics, the air is coming out of the part with the white arrow pointing at it. Not sure what it is as my owner's papers I got with the compressor don't say anything about it. I thought at first it might be the pressure relief valve but the pressure relief valve is behind the on/off switch as shown in the second pic and it isn't bleeding off any air. Your thoughts at what is going on are appreciated.

6PAX - it kind of looks like a Fusible Plug. They are filled with a low melting point solder/lead material and are designed to melt and relieve the pressure during a fire or other high temp event. I found a pic online of some, see below. Not sure where to buy them. You might see if there is any writing on the fitting itself for an idea.
Ron

What brand/model of compressor do you have? My 5 hp Saylor-Beall knockoff does not have that.

I just went through the same or similar situation with my 60 gallon Dewalt Compressor. All these compressors have a check valve threaded into the tank, it allows air to flow from the compressor pump into the tank, but will not allow it to flow back out of the tank toward the pump. If this check valve gets stuck open the compressed air will move backwards toward the pump and forced through a tube and escape out a port meant to release pressure in the case of a check valve failure. This port IS NOT the HIGH PRESSURE safety release valve that opens in the case the tank pressure becomes unsafe.

Your problem sounds exactly like a check valve failure.

Check valves can fail on their own, but in my case, my year old compressor had a reed valve break which pushed debris into the check valve and caused it to sick open.

I replaced both my Check Valve and then my Valve Plate to resolve my issue.

Hope this helps.

Not to be a PITA but I think what you were hearing was air escaping from the unloader valve. When the compressor shuts down, the main line check closes and the unloader valve opens to de-pressurize the compressor. That way when it starts, it's starting against no pressure. Depending how the unloader valve is configured, it will then shut and the compressor begins operating. If the main line check fails but the unloader valve operates, there will be air discharged from the unloader when the compressor stops until the tank is empty.

The unloader is usually operated via a centrifugal valve in the crankcase but I think what you're seeing is the compressor head over-pressure relief valve which opens at a much higher pressure than the tank relief valve. Is the escaping air a steady stream or does it pulse?

Is that part with the arrow similar to the same one that looks like it on the other side with the green end? I haven't seen those on any compressor I've dealt with before and hard to see from the pics what they are.




A lot of those check valves have a hex part you can unscrew to take it apart to clear them. There's big rubber seal and spring inside so it's pretty easy to take apart and put back together. One of my compressors had a problem when I first got it where there was a bunch of goop inside it on the seal that was stopping it from closing all the way.

It is a steady stream. There is a small hole on the face of it where you see the black sticker but that is not where the air is coming out. It's coming out on the side or from the threads where it screws into the pipe.

[quote=5thAve]Is that part with the arrow similar to the same one that looks like it on the other side with the green end? I haven't seen those on any compressor I've dealt with before and hard to see from the pics what they are.[/b]

The green one on the left looks the same but it has different specs on it. It says PT15 bar PN25 D .6mm Q3338 I.min The black one says PT 4 bar PN 25 D .6 mm Q1038 I.min They air both Air Tek brand.

It is a U.S. General model US7580V.

https://aircompressorpartsonline.co...essor-parts-us7580v-90836-p-1548012.html

Thanks John. It looks like on the schematic that the part may be what they call the intercooler safety valve. Doesn't look quite the same as what is on my compressor though plus there are two different ones as I mentioned previously.

I talked to a guy at a compressor company today that knows about U.S. General compressors. Before I had a chance to tell him the relief valve starts bleeding off pressure at 60 psi, he said that the pressure relief valve is supposed to activate at 60 psi so it is not the valve. He said that either a bad valve in the head or a bad head gasket will trigger the relief valve to bleed off pressure.