Pressure Regulation

Hello everyone,

I am trying to size an air starting system for a gas engine driven compressor. I have a question regarding air receiver and pressure regulator downstream of the receiver.

Let’s say for instance upper limit of pressure for receiver is 120 psig and lower limit is 90 psig. I have set my self contained pressure regulator at 120 psig for example i.e. it will maintain 120 psig at its downstream. The air compressor cut in pressure is 90 psig and cut off pressure is 125 psig. When there is a demand of starting air, as per my understanding the pressure in the air receiver will go down…lets say the pressure at one instant is 110 psig…will the regulator work ..which is set to maintain 120 psig downstream? Since it will have 110 psig at its inlet, I assume it would not allow any flow through it

Pressure regulator valves such as you are using work by reducing pressure. The pressure downstream of the valve is lower than upstream. If regulator is set at 120 psig, the valve will open more if the downstream pressure starts to drop below 120 psig. If the downstream pressure starts to get higher than the set point, then the valve will open less. In your design, there are times when the air supply is only 90 psig. So no matter how much the regulator is open, the downstream pressure will not be greater than 90 psig when the source air is only 90 psig.

What's going on? You posed the question as a student with a different name. That's not nice.

Bobby

The gas flow direction is from high pressure to low pressure except where mechanical work (compressor) is added to the system. 

  The other confusing thing about your hypothetical scenario is the receiver has an upper pressure limit of 120 psig, but the compressor does not cut off until the pressure reaches 125 psig. That is not a workable system. Understanding cause and effect for pressure systems is essential for figuring out what would happen if conditions change. 

hi sorry bobby i lost my password for other account. 

hi pilesar...the compressor cuts off at 125 psig and theres a pr. drop of 5 psi b/w compr and receiver.

actually i am just trying to understand that when receiver is supplying air and say pr. regulator is set to maintain 120 psig downstream and somehow pr. upstream of regulator is less than 120 psig..what happens in that case. 

That depends on the regulator type, we can't answer that for you without the part number.

Some regulators are fail-open (ie the spring normally keeps the valve open and downstream pressure acts against a diaphragm to counteract the spring tension) or they are fail-closed (which is the opposite).

I would assume for an air starter system, the regulator would normally be fail-closed (to stop unplanned engine starts). However you have said the supply pressure to the regulator is 120 psig and the downstream pressure SP is also 120 psig (which you would never achieve due to valve pressure drop from the spring tension) so I can only assume it is a fail-open valve, in which case your scenario of 110 psig supply, the regulator be mostly open (trying to "increase" the pressure downstream)!

Edited by NathanMac, 29 July 2018 - 09:35 PM.

A pressure regulators is a valve that controls the pressure of a fluid or gas to a desired value. Regulators are used for gases and liquids, and can be an integral device with a pressure setting, a restrictor and a sensor all in the one body, or consist of a separate pressure sensor, controller and flow valve.
They are of two types:

1. Pressure reduction regulator 

2. Back-pressure regulator