4 Low pressure feed system
The low pressure (LP) feed system is defined as the heaters and the equipment between the outlet from the condenser hotwell and the condensate inlet to the de-aerator system.
Two types of LP feed system are in current use. The older systems use direct contact heaters where, as the name implies, the bled-steam and the condensate mix in the heater and are pumped forward together, as opposed to the traditional design of system with tubed surface LP heaters, where the bled-steam and the condensate are separated by the tube walls.
DC LP heaters were originally adopted in preference to the traditional vertical tubed surface LP heaters for the following reasons:
Because the steam and condensate are mixed, zero steam and drain temperature differences are achieved with a consequent reduction in the heat rate of about 0.5 to 0.25%.
The cost of a DC heater, which is basically a tank with an internal water distribution system, is considerably less than a tubed LP heater.
The boiler feedwater requirements for the AGRs necessitated the use of an in-line 100% water treatment (polishing) plant. Heaters downstream of the polishing plant must not contain copper-bearing materials, so a DC heater of all-steel construction seemed the logical choice.
As briefly mentioned in the Introduction, a study of the cost effectiveness of the DC low pressure heater system was undertaken in the early 1970s, from which it was concluded that the factors which resulted in their adoption in the late 1950s no longer applied and that the gain in cycle efficiency was outweighed by the following factors:
Operational experience has shown that the original designs needed considerable modifications to make them perform acceptably. These included increasing the depth of the loop seals and the upgrading of the turbine protection against the possibility of water ingress from the heaters.
A high water level in any of the heaters in a DC heater train results in loss of feed to the de-aerator because of closure of the condensate isolating valve placed just prior to the first (lowest pressure) DC heater. As a consequence, unless a by-pass is provided as at Hinkley Point B, Hartlepool, Heysham 1 and Grain, the unit usually has to be taken off load with resultant loss of revenue. Even if a by-pass is provided, the flow of cold condensate to the de-aerator causes pressure decay in the de-aerator tank, with consequent restrictions in unit operation.
Because of the vertical separation required between DC heaters, already illustrated in Section 1 of this chapter, long condensate and bled-steam lines were needed. Extensive steelwork was also needed to support the heaters at the required levels. These extra costs were found to outweigh the cost difference between DC and surface type LP heaters.
As it was evident that a revision of traditional practice was cost effective, a complete review of previous practice was undertaken by the manufacturers. A new concept evolved which followed international practice of using surface LP heaters in the horizontal attitude. Figure 3.36 shows the position finally adopted, which allowed easy bled-steam pipework drainage and makes use of the entire heater shell to contain condensate in a flooding situation. Another important factor is the compact layout, which allows the heaters to be factory built, complete with pipework fittings, etc., and shipped to site as a unit. The only site work necessary is to connect to the system on the steam and water sides and to provide air and electricity to the valves and instruments as needed, with consequent savings in erection costs.
The cost of modification and upgrading of plant after or during construction has vindicated the decision to change to surface LP heaters for future units: however, there are forty-nine 500 and 660 MW units in operation with DC heaters. To complete the picture of current plant, Fig 3.37 shows a typical DC three-heater cascade as employed at Hartlepool and Heysham 1, both AGR stations.
Water is pumped from the condenser hotwell via the condensate system up to DC 1 heater. From this heater, it cascades into DC 2 and then cascades into DC 3 heater, from where it is pumped by the DC heater extraction pump to the de-aerator heater. A 60% capacity by-pass is provided to prevent the unit tripping on low de-aerator water level in the event of a DC heater train trip.