TABLE OF CONTENTS Let s assume that we are driving a centrifugal compressor with a constant-speed electric motor. We are compressing natural gas, coming right off a thousand wellheads, in Laredo, Texas. This is not a good idea. There is entrained brine (salty water) in the gas. The brine will dry out inside the compressor case, due to the heat of compression. The resulting salts will deposit on and inside the wheels or stages of the compressor s rotor.
The compressor efficiency will be adversely affected. As a consequence:
The flow of gas compressed will be reduced.
The discharge temperature of the compressor will increase.
The amp load on the electric motor driver will go down.
Why, though, does it take less work to drive the compressor when its rotor wheels are encrusted with salt? It is true that it takes somewhat more work to compress a mole of gas with a fouled rotor. But the fouled rotor also compresses a lot fewer moles of gas. Therefore, the net effect of rotor fouling is a reduced workload for the motor driver.
Let s now assume that I am driving the same compressor with a gas-fired turbine. The fuel-gas regulator to the turbine is 100 percent open. The turbine is spinning at 10,000 rpm. As the compressor s rotor fouls with salt, what happens to the speed of the turbine?
Answer it runs faster! It is easier to spin the compressor rotor when its efficiency is impaired. The salt-encrusted wheels do not bite as hard into the gas...
