Hydrocarbon Compression

Archive for the ‘Troubleshooting’ tag

Gas Turbine Exhaust Temperature Unit Troubleshooting

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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

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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.

 Troubleshooting Gas Turbine Drivers

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

Dehydration and Compression Station Troubleshooting

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Natural gas transported through common carrier pipelines must meet a moisture specification of 7 pounds of water per MMscf. Gas is usually dried to meet this requirement by scrubbing with a concentrated glycol solution. Figure 6-1 shows a standard glycol contactor tower, regenerator, and pump.

Gas flows into the bottom of this tower where entrained water and naphtha drop out and are withdrawn under level control. The upflowing gas is contacted with the circulating glycol and dried. The glycol is pressured from the contractor to the regenerator, where it is heated to its boiling point to drive off water. Typically, 100 pounds of circulating glycol absorbs 3—4 pounds of water. After cooling, the reboiled glycol is pumped back to the contractor tower.

On the surface it would not seem possible that much could go awry with such a simple system. But, of course, the experienced process operator knows that it is only a matter of time for anything that can go wrong to go wrong. As a case in point, consider the operation of the glycol circulating pump.

This ingenious positive displacement pump is driven by expanding gas withdrawn along with the wet glycol, from the contactor tower (see Figure 6-1). The speed of this pump is set by a small valve that controls the amount of expanding gas emitted into the pump. An operator judges the amount of glycol circulation based on the audible strokes made by the pumps internals. The quicker the strokes, the greater the glycol circulation.

But suppose the pump has developed mechanical problems that reduce the volume of glycol normally pumped per stroke? Or perhaps the pump internals have deteriorated to the point that glycol circulation has stopped. Since glycol drying units are not normally equipped with flow meters on the circulating glycol, how can the process operator of the troubleshooting engineer recognize the problem.

 Dehydration and Compression Station Troubleshooting

Written by Jack

October 25th, 2009 at 8:28 pm