Troubleshooting gas turbine drivers

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A centrifugal compressor driven by a gas turbine at a pipeline booster station is moving 80 MMSCFD of natural gas. It used to move 95 MMSCFD. What’s wrong? As the troubleshooter, consider whether the problem is with the driver or the compressor. Actually, there are three primary components involved:

• The combustion air compressor.
• The turbine blades.

Actual speed vs. the predicted speed based on compression ratio and flow is a measure of centrifugal compressor efficiency.

First, plot the current operating condition for the gas compressor on the curves supplied by the manufacturer. A typical family of compressor curves is shown in Figure 10—1. Point “A” shown in this figure falls on the curve for 12,500 rpm. If you had measured a gas compressor speed of about 12,600 rpm, you would conclude that the gas compressor was all right. On the other hand, if you had observed a speed of 13,400 rpm, you could be reasonably positive that something was amiss with the gas compressor. The preceeding statements assume that the actual gas specific gravity, suction temperature, compressibility, as well as the diameter of the impellers (wheels), match the parameters stated in Figure 10—1. The effects of deviations from these assumptions will be quantified later.

Having proved that the gas compressor end of the machine is performing properly, next decide if the driver is delivering as much horsepower to the gas compressor as can be expected at current ambient conditions. Assume the rated horsepower of the gas turbine is based on an ambient temperature of 90*F. As a rule of thumb, for each increase of 10°F in ambient conditions, the horsepower of a gas turbine drops by 5% (only assuming that neither the gas or combustion air compressors are operating at maximum speed). Thus, a 110°F air temperature cuts the engine horsepower 10% below design.

After accounting for the effects of ambient temperature (barometric pressure, while also important, does not change very much) compare the gas compressor horsepower indicated on the manufacturer’s curves against the rated gas compressor horsepower, after derating for ambient temperature.

Let’s say that the turbine is rated for 3,000 horsepower. After derating by 10% for 110°F air the turbine should be providing 2,700 horsepower to the gas compressor. Unfortunately, based on the current suction pressure, discharge pressure and flow you only calculate 2,500 horsepower. We have already decided that the gas compressor section of the machine is okay. What factors account, then, for the reduction in driver horsepower from 2,700 to 2,500?

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