Surge flow 1

[all2win]

Hi Everyone,
We have a centrifugal compressor (4 stage)for chlorine service.it has a electronic Anti-surge
controller which takes inputs from discharge pressure and flow to calculate the surge flow.Can anyone tell me
how to calculate this flow.Any help will be appreciated.
Thanx

 

[Guidoo]

I know that there are much more sophisticated algorithms for anti-surge control, following will do provided that composition (MW), suction temperature and pressure do not change greatly.

First, the anti-surge parameter (ASP) is calculated as follows:

ASP = 1 - ((dP-c)/h)/K

Where:
dP = compressor differential pressure: discharge minus suction pressure [bar]
K = slope of surge control line [bar/mbar]
h = pressure drop across orifice plate (as a measure of volume flow)[mbar]
c = intercept at h=0 [bar]

The anti surge parameter (ASP) clearly indicates where the operating point is relative to the surge control line:

ASP=0 The compressor is operating on its surge control line
ASP>0 The compressor is operating in the safe region of the operating envelope. [maximum ASP=1]
ASP<0 The compressor approaches the surge limit or is already surging.

Intercept ‘c’ and slope ‘K’ are unique parameters for each compressor. These parameters should be adjustable in the field to allow a better fit to installed compressor characteristics and to correct for changing operating conditions.

The surge control line should be chosen such that there is sufficient margin before compressor really starts surging. How large this margin should be depends on things like controller sampling time, valve stroke speed etc. The faster your anti-surge control system, the smaller the margin can be. So by selecting a more sophisticated (faster) anti-surge control system, you can have a smaller margin, which may result in economical benefits (less recycling required).

The calculated ASP is the PV (process variable) for the anti-surge controller. This anti-surge controller is a normal PID (Proportional Integral Differential) type controller.

The set point of each anti-surge controller (with the ASP defined as specified above) is normally zero (0).

In case your system does not meet the above mentioned requirements (inlet temperature/pressure/composition more or less constant), or you if you want to check whether a more sophisticated system would be attractive, it may be wise to seek advise from vendors of such anti-surge control systems.

It should also be noted that the effectiveness of your anti-control system largely depends on the overall system (so not only the control algorithm). For example, volume between compressor, recycle valve and non-return valve should be as small as possible. The recycle valve should be a fast acting, linear control valve (air failure open).