some control algos use (Pressure ratio)/Delta P.
where delta P is dp across flow measuring element and pressure ratio is Pd/Ps.
This is OK when the design conditions remain constant but the delta P (flow measuring device) needs to be corrected by changes in suction pressure, therefore we then get (P ratio)/(DP/Ps)
Surely this is the same as ((P ratio)*Ps)/DP and since pressure ratio is pd/ps, dont we get ((pd/Ps)*Ps)/dp which means the Ps cancels out on the top line and we end up with Pd/DP. Doesn't this infer we dont have to use suction pressure since the change in suction pressure is self cancelling in the error from the flow measutring device (which is a mss flow device) and pressure the pressure ratio

 

Steve,

There are a number of ways of doing surge control on compressors. I'm not familiar with the algorithm your using, but many of the systems use Pratio (plotted on the y axis of the compressor map) versus flow (which is orifice or annubar differential pressure plotted on the x axis of the compressor map). In order to make the equation invariant to suction pressure, the flow differential pressure (dP) is divided by the Psuction.

In order to do a reasonable control, you need inputs for dP, Psuction, and either Pdischarge or Pdifferential across the machine so Pratio can be calculated. In some air machines, you can get by without measuring suction pressure since it is ambient condition and doesn't change substantially. Temperature inputs are also used in some algorithms.

Depending on the process, the machine, and a number of other variables, you may want to look at different control algorithms

Byron Broussard
[email protected]