siemens - Public Service Commission

4.3.5 Condensate Drainage
As mentioned above, the condensate drainage is accomplished in two steps. First,
the water inventory condensing in the 1 st and 2nd passes is withdrawn from the
lowermost area of the head via a drain nozzle to the existing drain tank. No excess
steam is withdrawn at this point. Second, the condensate generated in the 3rd and
4th passes is discharged into the sump of an internally welded box from which it is
discharged, together with the excess steam through a discharge pipe and a valve
directly into the receiving feedwater heater.
The drain and discharge nozzles are sized properly to allow a fully vented gravity
flow from the 2nd pass and well established flows from the 4th pass. The sizing,
which constitutes a very important design requirement, is based on the proven
Froude Number criterion for the drain nozzle and on a two-phase critical flow design
procedure for the 4th pass. We do not anticipate any change in the existing 2nd pass
drain since after the uprate to 3030 MWth power; it will still carry less condensate
than its original design capacity.
This method of discharge is a simple and proven method. It is simply to discharge
the two-phase flow to a feedwater heater through a critical two-phase flow valve.
This method only requires one valve in the existing pipe line per reheater. It is a
pressure reducing valve which can be adjusted.
The desired flow rate can then be determined by monitoring the amount of
subcooling at the exit of the 4th pass with the thermocouples installed in the header.
The valve is trimmed to restrict flow until the onset of subcooling occurs and then it
is trimmed open slightly to provide for more vapor flow. This technique requires
thermocouples to be installed in discharge legs of the tubes comprising the 4th pass.
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