Sump pressure – not blown down during onload

Questions & AnswersCategory: GeneralSump pressure – not blown down during onload
Guest Staff asked 5 days ago

Good day Sir,

Our IR SSR EP 200 compressor separator sump does not de-pressurize when the compressor unloads.
The pressure remains at slightly above the package air discharge pressure.

The compressor has an intellisys controller.

The compressor runs constantly and loads up at package air pressure ( plant air pressure ) of 6.9 bar, ( blowdown solenoid closes, air inlet suction valve opens) and loads up. The package air discharge pressure increases to the unload pressure of 8.0 bar ( at this point the sump pressure is around 8.8 bar).

At the 8.0 bar setpoint, the blowdown solenoid opens, ( and supposedly the suction valve closes)…. but the separator sump pressure does not decrease down to the 1.7 – 2.4 bar regulated pressure for the unload cycle.
The coolant temperature is above 49 degrees C so the unloaded sump pressure should be in the region of 1.7 to 2.4 bar. This does not happen and the sump pressure remains slightly above the package discharge pressure during the unload cycle ( around 7.5 – 7.8 bar )

The separator MPCV has been proven correct ( the valve was changed out for a new one ).

The Unload solenoid is correct ( can be hear unloading )

The suction valve stepper motor coupling has been checked for slippage ( all tight )

The pressure transducer 3APT has been replaced.

The sump oil level is correct.

The oil separator has been replaced as our initial warning alarm was ( change oil separator). We subsequently also has a”sump high/ line diff”alarm. which has since cleared.

The question is why the separator sump pressure remains at around 7.8 bar during the unload cycle.

Is it perhaps the stepper motor not closing the suction valve sufficiently to reduce the air pressure being supplied to the separator during the unload cycle?

There are 3 compressors in our plant and with both of the other compressors, the sump pressure falls to around 1.9 bar during the unload cycle.

Your assistance is appreciated.

Kind regards
Simon Dobson.

1 Answers
Cas Staff answered 5 days ago

Hi Simon,

Thanks for your question.

When it unloads, you hear the ‘ssshhhhhhh’ unloading sound. How long does it last? It normally lasts anywhere from 10 seconds to 30 seconds (guessing here).

If the ssssshshhhhh sound continues non-stop than it means that the inlet valve stays open too much. But I think that on this compressor this is regulated by the controller, as it has different unload pressure settings depending on the temperature (which is quite unique).

If the sshhhh sounds stops abruptly after x seconds, but the sump pressure is still high, than the controller has closed down the blow-down solenoid too soon, for some reason.

Normally, the blow-down valve should always remain open during unloading. The amount that is sucked in through the slightly-open inlet valve (or, on most compressors, through a small suction hole), is blown-out again through the blow-down valve. This is to make sure there’s oil circulation for lubrication and cooling.

(And just some extra info: in your case, the internal pressure stays at 8.8 bar even at unload, which means the compressor will use the same amount of energy during unloading as it is running loaded! )

Another possibility (which is often the problem in cases like this) is a leaking Minimum Pressure Valve (mpv valve), but you already exchanged it.

Please double-check by closing the outlet valve of the compressor (if it’s there), that way you are 100% sure that no air is flowing back.

And also, when you close the outlet valve.. you can double check the inlet valve, because if it stays open too much (and there’s no blow-down ssssshhhh sound), the compressor is essentially still producing air, and since it has no way to go, outlet pressure will go sky high (pop the safety valve on your separator vessel).

If easy to do, check power consumption (current) and compare loaded and unloaded current.

Hope this was helpful.. please let me know when you found the problem or did some more tests… thanks