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Pump drivers include electric motors, steam turbines, expansion turbines, gas turbines, and internal combustion engines.
Three-phase alternating-current induction motors are the most commonly used driver for pumps because of the desirable characteristics of electricity as a power source and because the standard rotative speeds (1,750 and 3,500 rev/min) are well suited for driver centrifugal pumps. See Electrical Systems
Large gas plants containing boilers use steam turbines to drive large pumps such as lean-oil pumps, boiler feed-water pumps, and solvent-circulation pumps. It is a common practice to select a turbine rated at pump speed and power requirements and to rely on the inherent flexibility of the turbine to provide for a margin of error.
Expansion and hydraulic turbines
High-pressure process streams in gas plants commonly have pressure reduced for further processing. This energy can be recovered by expansion turbines, which in turn drive pumps or generators. If the stream to be expanded is liquid (i.e., rich oil or solvents), the type of turbine is generally a centrifugal pump with the inlet and outlet reversed and is called a hydraulic turbine. If the stream to be expanded is gas, the type of turbine is called an expander.
High-pressure liquid enters the hydraulic turbine through the centrifugal pump’s discharge flange and the low-pressure fluid exits through the pump’s suction flange. Hydraulic turbines have proved economical when the developed power exceeds 160 BHP, the flow exceeds 200 gal/min, and the inlet pressure exceeds 200 psig.
The most important design consideration is handling flow and pressure-differential fluctuations so that adequate shaft power can be provided to the driven equipment at all times. This is done in one of two ways: a supplemental electric motor can be provided in tandem with the hydraulic turbine, or a continuous bypass can be installed around the hydraulic turbine that absorbs all process flow variation, always leaving the minimum required flow through the turbine.
A critical factor in the design of the hydraulic turbine is the rate of gas evolution as it flows through the turbine. Design specifications should include a complete analysis of the liquid stream so that the designer can optimize the flow passages. A pump failure by overspeed exists when the flow through the driver pump is suddenly reduced or stopped before flow through the turbine is shut off. To avoid this type of failure, an overspeed trip device should be specified.
Hydraulic turbines normally use mechanical shaft seals identical to those applied to pumps. External flushing is generally required to either prevent gas evolution within the mechanical seal area or to minimize dirt and other solids from entering this seal area.
The trend toward plants with minimal operator attendance indicates that steam may become obsolete. Gas-fired turbines are the logical choice to replace the steam turbine for large pump drives. (See Prime Movers).
Internal combustion engines
Internal-combustion engines are used to drive pumps when other power is not available or when a standby energy source is desirable. The most common application in upstream production operations is for fire water pumps and for crude-oil shipping pumps in offshore applications. See Prime movers
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