14.2 Feed pump developments
Boiler feed pumps installed on early 500 MW units were typically as shown in Fig 4.55; they included a long flexible shaft with about six stages and the residual thrust from the impellers taken by a conventional balance disc arrangement. The bearings were mounted on separate pedestals which had to be removed before the main pump internals could be withdrawn. Glands were of the fixed labyrinth or floating ring type, and the main bolted casing joint was subjected to full discharge pressure.
With these designs, loss of water or reduction in NPSH (even momentarily) is likely to lead to metallic contact resulting in pump seizure. The bolted casing joint arrangement then results in a substantial outage for replacement of the pump internals.
To overcome the shortcomings of the multi-stage flexible shaft design of feed pump, and with an awareness of overall economic considerations, a new concept was developed with the emphasis on achieving maximum availability coupled with a design life of at least 45 000 h for all components. The essential features were a rugged high speed unit capable of surviving dry running and thermal shock operation without damage, and having a cartridge construction that permitted rapid replacement of the complete rotor/stator assembly.
A dry running capability, which forms part of the design specification, means that the pump must be capable of accepting either of the following conditions without damage:
- A transient reduction in suction pressure for as long as a pump continues to deliver and generate a substantial head. After this the pump is expected to accept, without distress, the re-establishment of normal suction conditions with no necessity for pump shutdown.
- A complete loss of water due to incidents such as the inadvertent complete closure of the suction valve. In this extreme case, it is recognised that the pump should be shut down and only restarted after it has been fully reprimed.
These requirements have been met by the development of an advanced class pump which incorporates the following basic features:
A substantially stiffened shaft with the number of stages reduced, preferably to two but not more than three, giving improved rotor rigidity and lower shaft deflections.
- Internal clearances enlarged to ensure that the pump is capable of dry running without damage.
- Replacement of the heavy externally-bolted discharge cover by an internal self-sealing high pressure joint system.
- Use of a balance drum to oppose the axial hydraulic thrust, with residual unbalance being carried by an external oil-lubricated thrust bearing.
- 'Cartridge' design, permitting rapid replacement of the pump internals with a spare element (an alternative design was also developed using the concept of removal and replacement of the complete pump unit, having a special bolted arrangement on the adjacent suction and discharge pipework).
The result of the stiff shaft is that, even with the maximum permitted worn internal clearances, the rotor transverse critical speeds in water are well above the operating speed range. This permits relatively high rotational speeds (typically in the range 6500-8700 r/min) to achieve the necessary high head per stage. Slow speed booster pumps are therefore required to provide sufficient NPSH to limit cavitation problems on the main pump.
With regard to NPSH, a conservative design approach was adopted. De-aerator storage tank height and suction pipework layout were arranged to provide a substantial margin of NPSH over the measured pump 3% head drop NPSH, even under the worst transient situation.