10.3 Review of air extraction equipment
The main development of air extraction equipment for large condensers has evolved through the use of air extraction pumps. Steam-jet air ejectors were in common use, but, for the higher pressure and temperature cycles adopted today, these have proved to be less economic than air pumps, both in capital and running costs.
Many different air extraction equipment designs have been developed, all of which combine the functions of vacuum maintaining and vacuum raising, either in full or in part, depending upon the volume to be evacuated and the time specified for synchronising the machine.
It is interesting to note that the last-row blades of a 660 MW turbine will overheat if they run at speed and at low load in a poor vacuum. Therefore, a vacuum of 677 mbar(a) must be obtained before steam is admitted to the turbine, and a vacuum of 880 mbar(a) for full speed. It is important that the time taken to bring boiler and turbine plant onload shall not be increased by insufficient extraction capacity.
The main systems in use today are:
- Rotary pumps (e.g., Le Blanc), which use water as the motive fluid for vacuum maintaining duties, with separate booster pumps for vacuum raising.
- Hydraulic pumps, which work on the water-jet principle, for vacuum maintaining. Pumps of this type have reasonable vacuum raising characteristics, but usually require to be supplemented by liquid-ring pumps for vacuum raising duties.
- A system which consists of an ejector and a pump. The ejector provides the first stage of compression, and a liquid-ring pump provides the final stage of compression to atmospheric pressure. This system uses both the ejector and the pump for vacuum maintaining duties, but the pump is used in isolation for vacuum raising.
A summary of the development of air extraction equipment is given in Table 4.7, which indicates the number of installations of each type of equipment associated with 500-660 MW turbines in the UK.
It is quite clear from Table 4.7 that the use of rotary and hydraulic air pumps has virtually been phased out in the 660 MW designs. The reasons for this change in policy are discussed below.
In the initial stages of the 500 MW programme, orders were placed for two air pump systems on which there was previous British experience; these being Le Blanc pumps and hydraulic air pumps. Both systems, however, could only offer a small contribution during vacuum raising, and relied on supplementary equipment to enable units to be loaded within 20 minutes.
The 'ejector plus pump' extraction system was introduced into Britain during the early 1960s. Initially this system only used atmospheric air as the motive fluid in the ejector stage, and incorporated a liquid-ring pump for second stage compression. This gave better vacuum raising characteristics than the hydraulic air pump, which offset the slightly higher operating costs of the system for vacuum maintaining duties.
Developments in ejectors have more recently produced an alternative design which uses low pressure waste steam from the de-aerator vent. This system has proved to be very competitive and has captured a large percentage of the 660 MW market, largely due to its low power requirement (about 0.727 kW/kg/h of air), which is approximately half that of other systems.
A more detailed account of these systems follows, with the exception of the rotary Le Blanc pumps which are now of little relevance to modern UK practice.