Hi All,
In the power plant that i work have two instrument air compressors ,the problem is that they start and stop very frequently the starting pressure is 5 bar and the unloading pressure is 7 bar so that diff of 2 bar.while investigating the problem i found that when the compressor start unloading at 7 bar it drop to 6 bar rapid and slowly drop to 5 bar to start the cycle again.
This is what i suggest there is not pressure regulator after the air receiver (thank) so the rapid drop from 7 - 6 bar is the surge in the system ,so if u have pressure regulator it would eliminated the surge .Your help would be greatly appreciated thank you.

Whether a regulator will help or not depends on wh the pressure drops. If it is because of use of the air downstream, it won't help. If it is because the compressor is unloading, it is unloading at a final pressure of 6 barg and the regulator would not hold it at 7, or raise it to 7 again. The regulator may help to hold the pressure at 6, provided you can still meet your downstream air supply needs with the regulator set at 6.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

It sounds to me like your problem is a large pressure drop between your compressor outlet (where the pressure sensor is) and your receiver vessel. Are your driers located here?
Another possibility is that the compressed air is not being cooled adequately before getting to the receiver, which then fills with hot air. The air cools and looses pressure when the compressor stops.

@ Biginch that's what i am try to figure out if the compressor are too small, @ Compositepro the sensor is on the compressor and the receiver vessel is about two feet away after the air dryer.

Could be the receiver is too small. If that was larger, pressure would decrease to 5 barg more slowly. Is that what you need?

If it ain't broke, don't fix it. If it's not safe ... make it that way.

The compressor sees a pressure equal to the receiver pressure plus the pressure drop in the dryer. As soon as the compressor stops the pressure drop goes away. So your receiver is actually only getting to 6 bar rather than 7 bar. Thus you are losing half your storage capacity.

Frequent load/unload may not be a problem. Check with the manufacturer.

Good explanation. "minus" instead of plus would have simplified the algebra

If it ain't broke, don't fix it. If it's not safe ... make it that way.