6.7 Horizontal high pressure heaters
An HP heater, as manufactured by GEC for the Littlebrook D 660 MW units, is shown in Fig 3.52.
The flow paths for feedwater, bled-steam and drain water are indicated. The heater illustrated has a steel hemispherical water head with mild steel U-tubes welded to the tubeplate by the Foster-Wheeler method. The shell, baffles, etc., are all made from mild steel. The heater is of all-welded construction, except for items such as inspection doors, etc., which need regular removal. The heater has desuperheating, condensing and drain cooling sections provided in the form of suitably-baffled compartments. To provide a steam flow path in the condensing section above the top of the tubenest and also a space below the tubenest for drain flow, a segment at the top and the corresponding segment at the bottom of the tubeplate which could be used for tubes is left blank. This shows as a 'flat' on the top and bottom of the generally circular tubenest. A dividing baffle is placed in the water head to provide the two-pass flow path for the feedwater.
The horizontal dividing plate which runs the full length of the tubenest is used to support the baffle plates and also to carry the air extraction ducts.
The desuperheating section is in the form of a shroud over the return half of the water flow adjacent to the tubeplate encompassing nine baffles. A plate is provided to seal the bottom to form a steam-tight compartment. The steam inlet is at the top centre of the desuperheating section. A diffuser is provided to prevent direct impingement of steam on the desuperheating section tubes. The steam flows in both directions from the central entry point, the baffles having cut-outs (illustrated in Fig 3.52 (b)) to provide as near cross-flow of the heating steam as attainable. The end baffles have no cut-outs and form the end of the section. The end baffles are also thicker than normal baffles and have combined sealing and anti-vibration clips fitted in the clearance between the tubes and the holes through which the tubes pass. A removable bellows piece connects the steam inlet of the heater and the stub branch on the desuperheating section, the bellows piece being welded in place after the shell is in position over the tubenest. The steam leaves the desuperheating section by four windows cut in the shroud as shown in Fig 3.52 (b). The condensing section baffles have no cut-outs but take advantage of the space between the tube bundle and the shell to distribute the steam over the length of the condensing section where it is condensed, the condensate draining to the bottom of the shell. A drain water level is maintained in the shell by the external drain control system.
The drain cooling section occupies the space directly below the desuperheating section. It comprises 13 baffles with cut-outs suitable to promote cross-flow of the drains, all enclosed by a semicircular shroud, with the full baffles at each end forming the end of the drain cooling section. A plate is provided to seal the top of this section. The end baffles have combined sealing and antivibration clips fitted between the tubes and the holes through which the tubes pass.
Figure 3.52 (c) and (d) shows how the 'dip tubes', which form the seal between the condensing and drain cooling sections, are welded to the drain cooler shroud. The dip tubes and the bellows piece which seals the drain cooler outlet are welded in position after the shell has been placed over the tubenest.
The space below the tube bundle in the condensing section is used to maintain a fixed water level above the drain cooling section dip-tubes by means of the drain water level control system.
The drains from the bottom of the condensing section flow via the two dip-tubes into the drain cooling section, from where they flow to the centre and are discharged by the central drain branch.
The venting arrangements are illustrated in Fig 3.53 which shows the vent paths within the heater. The air and non-condensables in the condensing section are vented by the hollow tubes incorporated in the horizontal centre baffle. The drain cooling section also has air vents just above the drain cooler outlet.
The heater has a single relief valve sized in accordance with BS5500 Appendix J .
Support feet are provided at each end of the heater; the fixed foot is at the water header end, with the other foot being free to slide to allow for thermal expansion.
A water header manhole door is provided to allow access to the feed side of the tubeplate for inspection and tube plugging. The heater internals are not intended to be maintained in situ, but to have the whole heater cut out of the pipework and moved to some convenient maintenance area for the shell to be cut adjacent to tubeplate and removed. This is only necessary if a heater has to be retubed.