14 Boiler feed pumps
The design and arrangement of boiler feed pumps has a significant impact on overall unit availability, In determining the optimum arrangement of feed-pumping plant, the economic assessment needs to take account of capital costs, capitalised running costs, repair and maintenance costs, and the likely effects of loss of availability.
Other constraints which should be considered are:
- The need to ensure that failure of a single pumpset does not impair the start-up of the main unit or affect output capability. Standby capacity equivalent to the largest duty pumpset is indicated with a rapid start-up capability, sufficient to prevent the loss of boiler drum level and consequent unit trip.
- The need to ensure that the plant is able to operate satisfactorily during and after a large load rejection by the turbine-generator unit. This requires that the drives for the duty pumps and their power supplies must be suitable for this operating condition. Alternatively, a suitable rapid start/standby pumpset is necessary.
- The need to provide adequate NPSH margins, taking into account that the pumps are supplied from a direct contact heater (de-aerator), which can be subject to pressure decay following a reduction in turbine load.
- There should be at least two pumpsets capable of starting the unit. If a turbine drive is to fulfil this function, then a steam supply independent of the main boiler (i.e., an auxiliary boiler) is required.
- If two or more pumps are required to operate in parallel, then the pumpsets should be able to accommodate run-out duties following loss of an operating pump.
The provision of sufficient pumping capacity to meet flow requirements under all operational circumstances. It is normal practice to include a flow margin to accommodate additional demand by the turbine above its design rating during transient flow disturbances. A margin on pump generated head is also appropriate to cover for deterioration resulting from internal wear during periods between overhaul. In the interests of keeping pumpset sizes and powers to a reasonable minimum, consistent with maintaining the pump best efficiency close to the duty point operation, these margins have been optimised as 5% on flow and 3% on generated head.