Compressor "easy start" valve - what's it properly called? Where can I get one?

[ivan]

Most small compressors unload the actual pump (loud PSSS) when motor stops, so that the motor, when starting again, is not driving the pistons directly against the tank's psi. When the motor starts again, the actual compressor output is wasted for a few seconds, through a spring loaded leak valve (mine has two). This valve is normally open, releasing some air, but is slowly closed as the pressure builds up to the point that the tank non return valve opens, and air is then pumped into the tank.
Something similar is found on the underside of filters. This little valve is open when unpressurised so water can drain out. When the air is turned on, there's a short leak, which blows out any remaining water, and eventually the valve seals tight.

I'd like to fit something similar in my drop pipes, whose drain cocks I frequently forget to open... I can't find anything googling, probably I need the correct name?

 

[Doug B]

Do you mean an automatic drain Clarke CAT169 ½" BSP In-line Automatic Drain Air Filter - Machine Mart - Machine Mart

 

[Inspector]

It would be called an unloader valve here.

Pete

 

[Argus]

It would be called an unloader valve here.

Pete

That's exactly what a similar device is called on a Refrigeration compressor.... does the same thing on machines that are very much larger.

 

[novocaine]

It would be called an unloader valve here.

Pete

Called that here too.

 

[julianf]

You can buy electric unloaders, so, when you turn off the compressor main power, the valves all open up to depressurise.

Or you can fit little auto ones that are, basically, a sprung loaded seal, so, when the pressure goes to nothing, the spring pushes open the valve, but as soon as there is air pressure again, it closes up itself.

They are commonly fitted to the bottoms of commercial water traps. But will, of course, never open if your system maintains pressure. You would need one of the electronic ones to overcome that.

 

[Spectric]

It is an unloader valve, once the tank is at pressure and the pressure switch turns off the motor then you want to dump the pressure between the compressor and tank, there is a one valve at the tank end. On my last big compressor I used an electrical solenoid valve that opened when the tank was at pressure controlled via a timer relay that just opened it for a 10 sec window. Even if you have a large motor it is better to initially start with no load, it can really reduce the initial current, I was using a two cylinder B & W with an 18Kw motor and that valve made a big difference on startup.

 

[baldkev]

Pressure relief valve, or sometimes pressure release valve

 

[user22161]

I'd like to fit something similar in my drop pipes, whose drain puffins I frequently forget to open... I can't find anything googling, probably I need the correct name?

Automatic drain? G1/2 DN15 Automatic Electronic Timed Drain Valve Air Tank Water Moisture for Air Compressor Condensate Management, Separated Valve (AC 230V): Amazon.co.uk: DIY & Tools

 

[XH558]

Decompression valve. Big single cylinder motor cycles often had them. No idea how they'd work on a compressor though.

 

[ivan]

"Or you can fit little auto ones that are, basically, a sprung loaded seal, so, when the pressure goes to nothing, the spring pushes open the valve, but as soon as there is air pressure again, it closes up itself. "
That's the boy I'm looking for; the unloader dumps when the motor shuts off, and the "little auto one" leaks when the motor starts up, so initial motor load is free piston movement.