4.3.1 Feed pump size and number
The choice of feed pump size and number is based upon the principle of ensuring that full turbine-generator unit output can be maintained with the loss of one main duty feed pump. This ensures maximum plant availability.
For the majority of 500 and 660 MW units, the most economic solution has been a 100% duty steam turbine-driven feed pump with two x 50% electric motor-driven start/standby sets (higher capital costs but lower running costs than all-electric pumps).
The situation is now changing for units over 660 MW in that the saving in capital costs of having 150% (three x 50%) feed pump capacity compared with 200%, begins to outweigh the increased complication of having two pumps running in parallel for full-load operation. Other advantages of having three identical 50% duty pumps include reduced spares holdings, quicker maintenance, modular construction with a more compact layout occupying less station floorspace, and less complex and expensive pipework.
Feed pump size has more recently been influenced by two other factors:
- International market appeal — 50% duty pumps for large (over 660 MW) units have the greatest worldwide market potential, whereas 100% duty pumps would require new pumps to be designed that have very little export potential.
- The choice of feed pump driver influences, to a certain extent, the pump size — for example, the current international trend to electric motors and 50% duty pumps means that no variable-speed motors or couplings large enough for 100% duty have yet been developed.