Compressor Discharge Pressure

If gas properties and suction pressure remain the same then the only thing that affects the discharge pressure is the system downstream of the compressor. For instance a control valve opening or closing.

GA Sir Slack .... nice style change, but I have taken your words to heart, "if all else fails.... read the instructions / call the manufacturer".

Just to add:

Centrifugal engines (compressor here) normally can work with fixed speed amd their performance curves are plotted for a fixed speed.

The change of discharge pressure is due to the downstream conditions: More or less flow is taken by opening or closing some valves. More flow request will drop the pressure at the discharge point (and vis-versa).

Look at the performance curve, Flow / Head pressure. The Motor will take more or less current to keep the speed the same (or near the same since maybe Ascynchronous motor).

Centrifugal comressors can operate over certain range of discharge pressure and flow rate which are very much inter related. Centrifugal comressors can operate with variable speed drives also for effective capacity control. But when you have a fixed speed motor as a drive, other means of capacity control is adopted, because we are not expected to run the compressor always at rated capacity. The other means of capacity control in the order of efficient control are: 1. Inlet guide vanes (IGV) 2. Suction Throttle 3. Discharge Throttle Capacity should be varied according to the requirement of the system in the down stream. Suppose your capacity control is by IGV (pneumatic or elecric operated), and it opened for 90% capacity at fixed position. If consumption of your system in downstream decrases abnormally (due to sudden throttling or closing of some valve), then discharge pressure may suddenly rise. In contrary,If consumption of your system in downstream incrases abnormally (due to sudden opening of some valve), then discharge pressure may suddenly prop. When system consumtion suddenly decreases, surge may occure which is detrimental to the health of compressor. So, to take care of this, every centrifugal compressor are equiped with an anti-surge device, in which a by-pass valve to the suction opens automatically in such situations. At a perticular position of IGV opening, a minimum flow through the compressor can be obtained from its performance curve. Anti-surge bypass valve setting should be little higher from this value. Process Compressors are also fitted with automatic capacity control where IGV is operated automatically to get constant flow rate or constant discarge pressure.