14.6 Pump layout and drive - part 2
Back pressure turbines have been standard on CEGB stations for feed pumping duties and these have proved very reliable (see Chapter 1 for technical description). Back pressure turbines permit the feed pumps to be driven at speeds in excess of 7000 r/min. The alternative condensing steam turbine drives, while offering a potential improvement in heat rate, are limited to less than 6000 r/min to prevent last-stage blade erosion. Their use with two-stage advanced class feed pumps would therefore necessitate the additional complication of a step-up gearbox.
The electric motor drive is simpler than the steam turbine drive from both physical and operational requirements. The pump can be used during any phase of operation and the control arrangements are more straightforward. Initially, slipring induction motors with liquid resistance controllers were used to drive 50% feed pumps on starting/standby duties. In addition to providing variable speed, the resistance controller on these units also limits the starting current, which has a direct effect on starting torque and run-up time. Slipring induction motor drives have now been replaced by the fixed speed motor/ fluid coupling arrangement which has the following advantages:
- Better speed control resolution.
- Greater motor reliability by the elimination of the sliprings, brushgear and the liquid resistance controller.
- Elimination of frequent outages for brushgear maintenance.
When using the fluid coupling, ihe power developed by the driving motor is transmitted by the kinetic energy of the oil flowing between the input impeller and the output turbine of the coupling. The output speed is controlled by the operation of an adjustable scoop tube, which varies the quantity of oil transmitting the torque between the input and output elements.
The overall unit (Fig 4.64) comprises, within the casing, a fluid coupling and step-up gears. For the high output speeds required on advanced class pumps, step-up gearing is included on both the input and output sides of the coupling.
The fluid coupling incorporates two separate oil systems drawing from a common sump. One system provides lubricating oil to the fluid coupling and to the bearings of the booster and pressure stage pumps and the driving motor. The oil is drawn from the sump by a gear-driven pump with a motor-driven pump provided for start-up and standby duties. In the second system, oil used as the working fluid is circulated by the dynamic head generated at the scoop tip in a closed loop. Make-up for this closed loop is provided by a gear-driven centrifugal pump during normal operation and a motor-driven gear type pump for starting and standby duties. Both circuits include oil coolers.