# Hydrocarbon Compression

## Gas Compressor Problems

Referring back to Figure 10—1, remember that we have compared the actual gas compressor speed to the speed indicated by the curve that passes through point “A”. We calculated point “A” from the natural gas flow, and the observed suction and discharge pressure. We said that if the measured gas compressor speed exceeded the speed indicated by point “A” on Figure 10-1, then the gas compressor was deficient. This is not quite true. The following factors all raise the actual speed as compared to the speed calculated from Figure 10-1:

• Increased suction temperature.
• Increased gas compressibility
• Lower gas specific gravity
• Reduced impeller diameter

It is a relatively simple matter to reduce the diameter of the gas compressor impellers; they can be turned down on a lathe. For instance, on one centrifugal compressor, the impellers were trimmed down from a 12″ to an 11″ diameter. Other factors being equal, the speed of the gas compressor end of machine increased from 11,000 rpm to 12,000 rpm, while the speed of the combustion air compressor held constant at 13,200 rpm.

Figure 10—1 Actual speed vs. the predicted speed based on compression ratio and flow is a measure of centrifugal compressor efficiency.

Written by Jack

January 30th, 2010 at 1:17 pm

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## Gas Turbine Air Compressor Problems

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.

Written by Jack

January 30th, 2010 at 1:02 pm

Posted in Gas Turbine

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## Gas Turbine Exhaust Temperature Unit Troubleshooting

Gas turbines are limited, as are all rotating assemblies, by either speed or power. For an electric motor, the power limit is manifested by maximum amperage, (more precisely, the maximum permissible winding temperature). The situation with gas turbines is similar. The ultimate amount of power (i.e. work, horsepower), that can be developed by the turbine blades, is limited by the turbine exhaust temperature.

A typical maximum turbine exhaust temperature is 1,100°F. This limit is imposed by the metallurgy of the turbine’s blades. Continuous operation above the turbines design exhaust temperature will lead to accelerated deterioration of the blades and a consequent reduction in engine horsepower.

When neither end of the centrifugal compressor is running at its peak speed, and the turbine exhaust temperature is below it’s design limit, there are two other possibilities which may be limiting horsepower output:

• Fuel gas firing is limited by a faulty over-ride on the temperature controller. That is, the exhaust temperature is artifically surpressed by an instrument malfunction.
• The fuel gas flow control valve is wide open; or it is partially plugged by natural gas hydrates.

When the turbine exhaust temperature is at it’s limit, and horsepower output is deficient, other possible causes are:

• Excessive wear to turbine blades.
• Air/fuel ratio problems.
• Carbon deposits on turbine blades. Periodic detergent washing of the combustion air compressor will help reduce this effect.
• Lack of proper flow from the combustion compressor.

Written by Jack

January 30th, 2010 at 12:32 pm

## Troubleshooting Gas Turbine Drivers

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.

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?

Written by Jack

January 30th, 2010 at 12:25 pm

## Gas Turbine Driven Centrifugal Compressors

While the majority of natural gas field and transmission compressors are reciprocating machines, a sizable minority are centrifugal compressors driven by gas turbines. Only on rare occasions can electric, steam or deisel oil drives compete with natural gas as compressor fuel in pipeline service.

A gas turbine works on the same principle as a jet engine. Air is compressed (typically to 110 PSIG), and discharged into a combustion zone. Fuel gas is also injected into the combustion zone. The pressurized, burning gas expands as it passes across the blades of a turbine. The turbine serves two functions:

• One or more wheels of the turbine drives the combustion air compressor.
• One or more wheels of the turbine drives the gas compressor.

The major part of the horsepower developed in a gas turbine is consumed by the combustion air compressor. The gas compressor absorbs about one third of the gas turbines power output. Work done by the combustion air compressors is recycled back to the turbine blades via the pressurized combustion air.

An important feature of the gas turbine driven compressor is that the two ends of the machine are not mechanically coupled. This is called a split shaft design; which means that the combustion air compressor and the gas compressor operate at different speeds. This permits the air compressor end to run at a speed consistant with developing full horsepower, while the gas compressor end may be running at a lower speed due to factors such as high discharge pressure.

Written by Jack

January 27th, 2010 at 8:12 pm