4.3.2 Feed pump duty, margins, and the need for variable speed
The duty of the boiler feed pump as determined by the boiler and turbine pressure flow conditions, is given by Fig 1.71. Shown on the diagram are the boiler feed-pump characteristics, i.e., the relationships between the head produced by the boiler feed pump corresponding to the particular value of feed flow pumped, and the boiler feed pump speed.
Various head/flow characteristics corresponding to the operating range of pump speeds are indicated. The curve denoted as 'system resistance' is the relationship between feedwater flow and the head that has to be produced by the boiler feed pump to pump the feedwater into the boiler. Also shown in the diagram are the design values of feedwater flow for the turbine plant, boiler plant and the feed pump itself.
It is normal practice to design the boiler to produce a higher rate of steam generation than that appropriate to the value used in the turbine-generator design. This margin is normally about 5% and is intended to cover application variations (e.g., site conditions such as available cooling water) at the time that the boiler is designed, and also to allow for a deterioration of plant capability in service. In addition to the effect of the boiler margin, the boiler feed pump is designed to produce a rather higher feedwater flow than that needed by the normal boiler design condition. This is intended to cope with pump wear and transient conditions, and to act as an operational margin.
At the value of feed flow corresponding to the turbine design quantity, the head produced by the pump is in excess of that required to pump the water to the boiler. This excess pressure can be minimised if the pump can be driven at reduced speed: if this is not possible the excess pressure has to be broken down across the feedwater regulating valve. As can be seen from Fig 1.71, at low values of feed flow, the excess of head produced by the pump over the system resistance head is considerable. On typical British sets of 500 MW and over, variable-speed drive is always adopted because the cost of providing it is much less than the operational and financial losses that would be incurred by this breakdown of pressure. In addition to this, all conventional plant is required to be capable of two-shift operation, i.e., to be shutdown overnight and started in the morning. During an overnight shutdown the boiler pressure falls significantly, such that during the start-up period next morning, the boiler feed pump is only required to deliver some 100-200 bar instead of the approximate 230 bar closed-valve full-speed pressure of the pump. For this reason, the pump that is used during the start-up period should have a speed range down to approximately 70% full speed to avoid excessive wear on the feedwater regulating valve.
As the power requirement of a boiler feed pump for a typical 500 MW unit is approximately 10 MW, it can be seen that the choice of an economic and technically acceptable variable-speed boiler feed pump drive is a decision of major importance. The possible types of boiler feed pump variable-speed drives which have been considered by the CEGB for use on large generating units are:
- Fixed-speed electric motor with variable-speed coupling.
- Converter-fed variable-speed (electric) motor (VSM).
- Back pressure steam turbine.
- Condensing steam turbine.
In addition to these choices, the option of slipring induction motor, regulated by resistance in the rotor circuit, has been considered and used in the past. This relatively cheap and simple solution, using large liquid rheostats with mechanically-movable tapping points, has proved unreliable in practice, with a high maintenance burden.
Variable-speed electric motors and condensing steam turbines have only recently become viable options (for future units over 900 MW) due to the trend to slower (6000 r/min or less) 'International Class' boiler feed pumps, to limit erosion and cavitation in the pump, to the NPSH required and de-aerator height, etc. These two types of drive are not feasible for use with the higher speed 'Advanced Class' pumps used on 500 and 660 MW units so far, due to converter size limitations and blade vibration and stressing difficulties.
A technical description of electric motor drives (variable-speed, induction, synchronous, etc.) for feed pumps is given in Volume D. A description of the use of steam turbines for driving feed pumps can be found in Section 9 of this chapter.