One way of looking at a gas turbine centrifugal compressor is that the combustion air compressor must pump sufficient air to support combustion across the turbine blades as needed to spin the gas compressor at its required speed. Any factors which reduce the flow delivered by the combustion air compressor will reduce horsepower available to the gas compressor. The factors which reduce air flow are identical to those parameters which reduce the capacity of any centrifugal compressor:
• High suction temperatures due to elevated ambient temperature.
• Mechanical damage.
• Low suction pressure due to plugging of the air filter suction screen. A pressure drop of 4 inches of water will reduce the air compressor capacity by roughly 2%
• Dirt accumulation in the rotors internals due to inadequate suction filtration and dusty air.
• Slow speed due to the problems listed above with the gas turbine driver.
To remove dirt and dust accumulations from the air compressor rotor, detergent water washing is required. An aqueous detergent solution is squirted into doors provided on the air intake ducting. The machine is running at a reduced speed during this period, and the natural gas process piping is isolated from the compressor. In addition to washing the air compressor rotor, some of the detergent solution may carry-over to the turbine blades and promote some cleaning. During detergent washing, the turbine is powered in the normal fashion—i.e. by firing natural gas.
Frequent cleaning or replacement of the air intake filters will also improve air flow availability for combustion. To simplify this procedure, a so called “Huff & Puff’, self-cleaning air filter may be retrofitted into existing equipment. Such a self-cleaning filter should operate for two years without manual maintenance. Also, it reportedly reduces air filter pressure drop by an average of five inches of water over this two year period for an effective increase in engine horsepower availability of 2-3%.
The combustion air compressor should develop a certain discharge pressure (110 PSIG is typical) as specified by the manufacturer’s data. After correcting for suction pressure, suction temperature and speed, (see manufacturer’s correlations), if the indicated air discharge pressure cannot be achieved, the combustion air compressor should be washed. If washing fails to correct the shortcoming, the rotor should be checked for mechanical damage.
Keep in mind that not only will problems connected with the turbine blades slow down the combustion air compressor, but that deficiencies with the combustion air compressor will indirectly be reflected in lower combustion air compressor speed.