4.2 Low pressure system configuration part 2
The principle of forward pumping of drains is illustrated in Fig 3.39 and shows how the enthalpy into the upstream heater is increased by the injection of the drains water. The consequent decrease in steam used in the upstream heater improves cycle efficiency.
An alternative drain route to the condenser is also provided, so that the loss of a heater drain pump or the malfunction of associated equipment does not result in the forced isolation of the heaters which are drained by the drain pump because of lack of an alternative destination for the drains. The short term loss of efficiency is accepted until the drain pump is returned to service.
Distribution of bleed point pressures and hence enthalpy rises over the heaters, determines the gain in efficiency by forward pumping of the drains of each individual heater or group of heaters (see Chapter 1).
The grouping of two or more LP heaters into a bank to provide a common by-pass on the feedwater side makes it convenient to forward pump the cumulative drains from the lowest pressure LP heater. The choice as to which LP heaters are provided with a drains pump is made by the feed system manufacturer at the inception of the feed cycle design, taking into account the factors described above in conjunction with the value of efficiency given in the relevant enquiry document.
To meet the functional needs for the protection against water ingress as given in Section 1 of this chapter, each heater is provided with duplicate level sensing devices, which can be on-load tested to verify their health.
In the event of an HWL in LP3 heater, it is isolated and by-passed on the condensate side and also isolated on the steam side. An HWL in LP1 or LP2 heaters will result in isolation on both steam and condensate sides, with the condensate by-pass valve opening to supply water to LP3.
Also shown in Fig 3.38 is the portion of the LP feed system which connects the condenser to the LP feedheaters. Part of this system is the insurge group, by which means feedwater is admitted to the condenser. Water is fed into the condenser from the reserve feedwater tanks (RFT) under the control of the insurge group.
There are three routes by which feed can be admitted via the insurge group:
A fixed orifice which allows 1% CMR flow on a continuous basis. Any small leakages and water losses are made up by this quantity.
Via the control valve which receives its signal from de-aerator level. The capacity of the valve is usually about 2-3% and varies according to the static head available from the RFT (especially relevant for a low level tank).
A manually-operated by-pass valve which can pass anything from 5-7% and is only used for start-up or in an emergency.
The outsurge valve and associated equipment is connected to the condensate system downstream of the sectionalising valve. The rate of discharge through the control valve is determined by the signal from the level controller on the de-aerator. The maximum capacity of the outsurge valve is normally about 10%. A non-return valve is provided in the outsurge line to prevent the contents of the RFT leaking into the condensate system. A motorised by-pass valve with an orifice in series is connected around the outsurge valve to allow the quantity outsurging to be trimmed by the operator as needed.
To supply the de-aerator with feed from the RFT under emergency conditions a 30% make-up pump may also be provided. The pump can also be used to fill the de-aerator, avoiding the use of the extraction pumps for this duty when initially filling the tank. Upstream of the make-up pump and outsurge connections is a sectionalising valve, which is used to isolate the condensate system during system flushing and clean up.