4.4.2 By-pass capacity
Sizing of the by-pass system is based on the most arduous duty, in this case the ability to run-through a trip to house load. The HP by-pass size is usually expressed as a percentage of the HP stop valve flow at rated full-load conditions. To cater for a load rejection to house load, the size of the by-pass will be given by:
The LP by-pass size is generally expressed as a percentage of the interceptor valve flow at rated full-load conditions. For a trip to house load, rapid closure of the interceptor valves causes a pressure rise and the LP by-pass opens to contain this. The LP by-pass is usually sized at well below the 100% capability since, under the worst case of rejection from full-load, any excessive pressure rise is accommodated by lifting of reheater safety valves for a few minutes.
A by-pass system sized on the basis of load rejection capability can readily accommodate the duties imposed for start-up and independent boiler operation.
To reduce the enthalpy of the steam, it is necessary to inject spraywater, derived from the condensate system, after both HP and LP by-pass valves. The spraywater flow adds to the HP by-pass flow to give the mass flow rate through the LP by-pass system; this is therefore used in sizing calculations.
The supply of a by-pass system reflects on the design of other plant in a major way. To cater for the supply of spraywater on top of the maximum HP by-pass flow, it is necessary to increase the rating of the condensate extraction pumps.
The increased flow and high enthalpy of the by-pass steam increases the size and cost of the condenser significantly. Figure 1.77 shows how the additional heat load relates to the HP by-pass size.
The additional throughput of condensate flow must also be taken into account in the design of the LP heaters.