Flow Control Turbo-Machines

Compressor performance curves should strictly be plots of polytropic head H against suction flow measured in actual volumetric units (F ). However, it is common for discharge pressure, or the ratio of discharge to suction pressure, to replace polytropic head. In order to simplify the description of how possible flow controls operate, we will use the curves in this form. These approximations assume suction conditions and molecular weight remain constant. Should either change then the performance curve, drawn on this basis, will move. 

In order to adjust the flow, we have to cause either the process curve or the compressor curve to move. Alternatively we have to remove the condition that discharge and process inlet pressures are equal, or that the flows are equal. 

Because of the nonlinear behaviour of both the compressor and the process the relationship between flow and valve Ap is highly nonlinear. An equal percentage valve or some other form of controller output conditioning (as described in Chapter 5) can be used to avoid problems in tuning the flow controller. 

As might be expected, because we are expending energy to raise the pressure of the gas only to partially reduce it again across the valve, the scheme is not energy efficient. Its range is limited since, as we close the valve, the process curve approaches the end of the compressor curve and surge occurs. 

One concern is that it is possible for the suction pressure to fall below atmospheric pressure and any leaks in the pipework would permit air to enter the compressor. If compressing a flammable gas this could cause an explosion inside the machine. 

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In the event of quick disconnect a torque imbalance occurs and the PCU shaft begins to over speed. If no control action is taken the terminal speed is a function of PCU loop friction losses and the turbo-machine performance curves. To prevent damage from the over speed condition, a bypass valve is opened to redirect flow from the high pressure compressor to the outlet of the turbine. 

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