- Question Title: ALUP Screw Compressor SCK25 residual pressure
- Asked by: Guest
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- Answers: 2
Having problems with Motor Temperature High.
After the compressor reaches cut out pressure of 7.5bar the compressor changes over to Idling mode for 20 minutes. By this; the Suction valve closes - controlled by the solenoid valve.
During Idling mode the compressor appears to be creating pressure - not fully unloading. This can be seen by opening the Safety Valve Manually. (On load there is high pressure - on idling there is medium pressure but consistent) Once the motor stops after 20 minutes - and attempts to start again, there is residual pressure acting against the motor. In the Star configuration the motor is creating 70-80amps. This is causing the compressor to trip on function 8 - Motor Temperature.
I have replaced all seals on the Suction Valve with the service kit.
Fitted a new Solenoid valve.
I've replaced all components in the discharge valve (block valve).
The Minimum Pressure Valve and Non-Return Valve have also been checked and proven functional.
With no manual this has become a real chore.
Any advice would be greatly appreciated.
Please Help!
Answers & Replies
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Hello Bain,
Thanks for your question.. it sounds like a difficult problem to diagnose from here, but I will do my best..
I see you already replaced and checked a lot of things.. that's good.
During unload running, there should always be a pressure inside compressor. I don't know the exact details of your machine, but I'm guessing it should be around 2.5 bar.
This internal pressure is needed to keep the oil flowing around. Oil pressure/flow is always needed to lubricate the rotors and bearings and for cooling.
Now there are a lot of different designs of inlet valves and regulating systems, even with machines from the same manufacturer. So I can tell you how it works in general and you should check your service manual how it is exactly implemented on your machine.
Normally, at unload running, there is always a small amount of air drawn in through the inlet valve. Usually this is done through a small by-pass valve.
The same amount of air is blown off. This keeps the air pressure in the compressor constant, while ensuring a constant flow of oil. So, a balance is created during unload running. Air blown in = air blown off at the balance pressure (the unloading pressure)
During load, this blow-off valve/opening is closed, otherwise it would lower the efficiency of the compressor.
When the compressor stops, the blow-off stays open and continues to blow off pressure until the internal pressure in the compressor is 0. (since the compressor is stopped, there is no new air sucked in, only air blown off).
Now, how exactly this blow-off mechanism is implemented differs between manufacturers and compressor models. Sometimes it's a separate valve, sometimes it's integrated in the inlet valve. On new machines it's often a dedicated solenoid valve.
I have attached two screen-shots of how the mechanism works. This is from a different machine (Atlas Copco), but the idea behind is the same for all screw compressors (except variable speed compressors maybe)
Now, what can disturb this mechanism / balance?
In your case this would both explain the high unload pressure and the residual pressure. But you should hear a continuous ssssshhhhhh when the compressor is stopped (compressed air leaking back into the compressor is constantly blown off through the blow-off valve).
If there is a valve installed after the compressor, you can check this by closing it. Now no air can flow back. See if this solved the problem.
There is too much air drawn in during unload. The inlet valve is leaking. In your case, this would explain the high unloading pressure, but not the residual pressure when the compressor is stopped.
The blow-off valve/opening is stuck/blocked.
The air can not be blown off during unload running. Pressure inside the compressor will rise.
When the compressor stops, the pressure can not be blown off and stays inside the compressor.
This last one should explain your problems. I think this is what is wrong with your compressors.
When you stop the compressor, do you hear some air blowing of 'sssshhhhh' ? This is the blow-off valve. If you do not hear the air blowing of when the compressor is stopped, the blow-off is blocked. During unloaded running, the compressor makes to much noise to be able to hear the blow-off.
Please check in your manual how this blow-off is implemented on your machine and check the blow-off valve or opening.
Again, see screenshots how it works on another machine.
Hope this was helpful :) Please let me know when you fixed the problem :) And if you need more info, just leave a comment below here.
Good luck,
Cas.
Hi, I think Cas has explained the compressor operation in every detail and easy to understand language. I do agree with him that Blowing off valve not functioning when compressor stops, is the problem. While restarting the compressor minimum idle period is required to cool the motor windings as per motor manufacturers recommendations. Thanks Prakash