10.1 HP feed system
The increase in unit size and possible use of supercritical steam conditions will increase the HP heater hemi-head diameter and thickness. With current HP header design using a hemi-head with U-tubes, the tubeplate thickness will increase because of the heater diameter being 1.25 times that of present units to maintain the velocity of feed in heater tubes at existing values.
A further increase in tubeplate thickness will be needed for heaters used in a supercritical cycle as the feedwater pressure will be approximately 1.5 times that used when steam conditions are sub-critical. The increased tubeplate thickness will also need further development of present techniques for deep-hole (gun) drilling. One disadvantage of thicker tubeplates and hemi-head walls is the increased susceptibility to damage caused by thermal transients inducing high stress levels.
The perforated area of the tubeplate responds almost immediately to the feedwater temperature change caused when preceding heaters are by-passed. The un-perforated volume and the hemi-head respond slowly, thereby creating a temperature gradient between the perforated and unperforated sections of the tubeplate with the potential of creating high local stress levels.
Although thermal transients can be reduced to a minimum by individual by-passing of preceding heaters and rigorous reinstatement procedures, with specified rates of temperature increase for a cold heater, the use of such measures imposes cost penalties due to the extra equipment involved as well as imposing operational restrictions.
The limitations imposed by the thermal inertia of the large mass of metal in a conventional hemi-head type of heater can be overcome by using a modern version of the header type of heater similar in principle to that illustrated in Fig 3.55. The use of the latest manufacturing techniques for a similar design has resulted in heaters which are currently giving satisfactory service in Europe and America as shown in Fig 3.70. To increase throughput of feedwater in such a heater, the number of tubes is increased and the header length is adjusted to allow the attachment of these extra tubes. The heater shell diameter has to be increased to accommodate the extra tubes but, as the shell is only subjected to bleed pressure, it will require only a modest increase in thickness. As the header diameter is small, an increase in pressure of 1.5 times does not result in a header wall thickness which imposes operational restrictions due to thermal transients inducing unacceptable stress levels within the header. The use of header type heaters is considered to be an option for future stations as an alternative to conventional hemi-head HP heaters.