2.3 System configuration part 1
The number of HP feedheater stages is determined by cycle economics and can vary between two and four. The standard configuration of HP heaters at all 660 MW stations (with the exception of those with three units) is two banks of half-duty heaters in parallel.
Conventional hemispherical water headers are difficult to manufacture in a size greater than that capable of accepting 50% of the feed flow for a 660 MW unit. However, there is one 660 MW station which has a single line of HP heaters where a special toroidal water head design was used. It has not proved cost effective and has not been repeated. The limiting factor is tubeplate thickness which is dependent on tubeplate diameter and feed line pressure. To reduce the diameter, the feed flow is divided between two 50% banks of heaters.
To minimise the greatest pressure to which the HP water headers are subjected, they are placed downstream of the feed regulating valves. As there are no isolating valves between the HP heaters and the boiler with the heaters in this position, the maximum pressure they can be subjected to is boiler relief valve pressure and cannot be subjected to feed pump shut-off head. Nevertheless, the design pressure is in the region of 207 bar, which results in a typical 50% heater water header shell thickness of about 120 mm and a typical tubeplate thickness of 350 mm.
The difficulty in the manufacture of a thick tubeplate is in drilling the holes through the tubeplate. Conventional methods of drilling holes do not provide the straightness nor roundness needed. The gun drilling technique originally developed for ordnance work, which allows deep drilling with a high degree of hole straightness and without the need for subsequent reaming or honing, is the usual method used tо deep-drill thick tubeplates. Details of the water head and of the gun drilling method can be obtained by reference to Spence, Ryall and McDonnell . To date, even using the gun drilling nethod, tubeplate thickness has been limited to about 50 mm.
The configurations of a single and double line of HP heaters are shown in Fig 3.18. The main advantage of a double line of heaters is their flexibility in operation. Assuming a typical feed system, where each line or 'bank' normally accepts 50% flow, but on isolation of the active bank will accept 60%, then with one bank by-passed the FFT will be approximately 215°C. on the loss of the top heater of a single bank configuration, the FFT falls to the temperature at the outlet of the next HP heater of 209°C. While the boiler can accept both of these conditions, there is consequent loss in cycle efficiency for the lower nal feed temperature.
One bank of a double-bank configuration of HP waters (as used at Drax Completion) is shown diagrammatically in Fig 3.19, with all major valves and control devices indicated. In the event of a high water level being detected in a heater, the bank associated with that heater is automatically isolated on the steam / water sides. The isolation procedure is in accordance with provisions (a) to (d) of Section 1.8 of this Chapter (Protection against ingress of water/steam turbines), and follows from receipt of a signal from bу one of the duplicated water level sensing devices any heater in the bank:
- The power-assisted bled-steam non-return valves are given a closing signal.
- The bled-steam isolating valves close on all heaters in the bank.
- The feedwater isolating valves for the bank are closed.