Todays ‘part of the day’ is the minimum pressure valve. It’s been some days since my last ‘part of the day’ post, mostly because I have been busy with the new air compressor buying guides and my new eBook: Your Rotary Screw Compressor: Troubleshooting and Maintenance
The minimum pressure valve can be found on rotary screw compressors. It is that valve that sits on top of the separator vessel (the small compressed air tank inside your compressor).
This minimum pressure valve has two functions. Actually, it is two valves in one:
- It’s a minimum pressure valve – it opens at a certain minimum pressure
- It’s a check valve – air can only pass through in one way (to outside the compressor).
Why do we need a minimum pressure?
We need a minimum pressure to circulate the oil. Circulation of oil is critical for cooling of the rotary screws and for the lubrication of the bearings.
A rotary screw compressor has no oil pump. The oil is pumped through the system by the differences in pressure that exists inside the compressor.
When the compressor just starts up, we need to built up a pressure as soon as possible, for the above reasons.
If the compressor is connected to an empty air receiver or piping system, than it would take a long time before pressure is built up (a very long time). Therefore, the minimum pressure valve stays closed until a minimum pressure has been reached inside the air compressor. That minimum pressure is usually round 4 bar.
Check valve function
As I said earlier, the minimum pressure valve also acts like a check valve. This is to make sure that no air can flow back into the air compressor.
This could happen for example when the compressor is stopped or running unloaded.
Some people think it’s necessary to install a check valve after a rotary screw compressor. This is not the case. The check valve is already there, the minimum pressure valve.
Oil carry over
If you experience too much oil carry over (oil in your compressed air), it could be due to a bad minimum pressure valve.
If you checked everything, oil separator, oil level, scavenge line, temperatures, but you still experience oil carry over… check the minimum pressure valve.
If the minimum pressure valve is malfunctioning, there will be a ‘rush’ of compressed air blown through it at start up. The separator element will not function properly at these high flow rates and low pressure.
The result is that every time the compressor starts up, oil is blown into the compressed air system.
The minimum pressure valve doesn’t need any real maintenance. It is advised to change the seals every few thousand running hours.
If the valve is malfunctioning, changing the seals usually solves the problem.