Similar to multistage centrifugal pumps, centrifugal compressors, as shown in Figure 10-9, use a series of rotating impellers to impart velocity head to the gas. This is then converted to pressure head as the gas is slowed in the compressor case. They are either turbine or electric motor driven and range in size from 1,000 hp to over 20,000 hp. Most larger compressors (greater than 4,000 hp) tend to be turbine-driven centrifugal compressors because there is such a first cost advantage in that size range over integrals. Centrifugal compressors have high ratios of horsepower per unit of space and weight, which makes them very popular for offshore applications.
As shown in Figure 10-10 they can be either horizontally split case or vertically split case (barrel). To develop the required gas velocities and head they must rotate at very high speeds (20,000 to 30,000 rprn), making the design of driver, gear, and compressor extremely important. Turbine drives are also high speed and a natural match for centrifugal compressors.
There is a disadvantage in centrifugal machines in that they are low efficiency. This means it requires more brake horsepower (bhp) to compress the same flow rate than would be required for a reciprocating compressor. If the compressor is driven with a turbine, there is even a greater disadvantage because the turbines are low in fuel efficiency. The net result is that turbine-driven centrifugal machines do not use fuel very efficiently. This fuel penalty can be overcome if process heat is needed. Waste heat can be recovered from the turbine exhaust, decreasing or eliminating the need to burn gas to create process heat.
As with electric motor and engine-driven high-speeds, turbine and electric motor-driven centrifugals can be easily packaged for use in oil and gas fields. They are very common in booster compressor service (high volume, low ratio) and for very high flow rate gas-lift service. Centrifugal compressors cannot be used for high ratio, low-volume applications.
The major characteristics of centrifugal compressors are:
* Starts about 500 hp.
* 1,000 hp increments to 20,000 hp.
* High horsepower per unit of space and weight.
* Turbine drive easily adapted to waste-heat recovery for high fuel efficiency.
* Easily automated for remote operations.
* Can be skid mounted, self-contained.
* Low initial cost.
* Low maintenance and operating cost.
* High availability factor.
* Large capacity available per unit.
* Lower compressor efficiency.
* Limited flexibility for capacity.
* Turbine drives have higher fuel rate than reciprocating units.
* Large horsepower units mean that outage has large effect on process or pipeline capabilities.