Question: Consider the turbine/compressor process shown in Fig. The motive force for the turbine, T-201, is a high-pressure gas, and the compressor, C-103, compresses a refrigerant gas. The operator sets the valve position of the high-pressure gas valve, which in turn results in a certain compressor speed. A lag unit is used to avoid sudden changes in the high-pressure valve position. Under normal operating conditions, the valve should respond to the operator's set value. However, there are some special conditions that the control system must guard against. l Under normal conditions, the pressure in the refrigerant gas line is about 15 psig. However, during start-up and under other circumstances, the pressure in the line tends to drop below 8 psig, which condition is dangerous to the compressor. In this case the compressor velocity must be reduced to pull in less gas, thus increasing the pressure in the line. The lowest safe pressure in the refrigerant line is 8 psig. l Because of mechanical difficulties, the compressor velocity must not under operation exceed 95% of its maximum rated velocity. Also, it must not drop below 50% of its maximum velocity.
Design, to manipulate the high-pressure gas valve, a control strategy that does not violate the foregoing constraints.