If inlet conditions in a low-pressure centrifugal compressor change, there is a reduction in operating efficiency but no surge problem. However, in a high-pressure compressor, variations in inlet condition can introduce an error of more than 30 percent (greater than the usual safety margin of 10 percent).
As a result, protection systems may become inefficient and this could potentially damage the compressor. Using the correct software algorithm, however, it is possible to predict the performances of a centrifugal compressor under varying thermodynamic and inlet gas conditions. These predictions have been found to be accurate at high pressures.
Centrifugal compressor anti-surge protection systems are based on the Surge Limit Line (SSL). SSL is the description of the surge point locus and, in current practice, it is assumed to be invariant regardless of centrifugal compressor inlet conditions. A numerical study was done to investigate the effect of inlet condition variations on the SLL.
Protection methods have been designed with the intention of running the compressor safely by preventing it from nearing surge points. One such method for anti-surge systems would consist of opening, partially or totally, a control valve (anti-surge valve) located on a line that recycles the gas from the discharge to the compressor suction. In this way, the control system could reduce the overall line resistance and increase the elaborated flow, moving the compressor operative point to the right of the characteristic curve corresponding to the actual operative speed.
The anti-surge valve is commanded by a dedicated proportional–integral–derivative controller (PID controller), usually embedded in the protection system Programmable Logic Computer (PLC). The surge protection logic embeds the SLL and the correlated Surge Control Line (SCL).
Using field readings, the protection logic calculates the actual operative compression ratio and uses the stored SCL data to determine the corresponding actual flow limit parameter. This value is then used as set point for the anti-surge valve PID controller.
More in March/April 2013 issue of Turbomachinery International