TM 5-810-15
(2) Three element control. A three element
DA level control system as shown in figure 9-22(b)
uses a metered condensate flow feedback signal in
a cascaded control loop. This system will maintain
DA level on units that operate under swinging load
conditions.
c. Pump recirculat ion control. Pump recircula-
tion controls are necessary to maintain the mini-
mum flow through a pump when required by the
manufacturer. A breakdown orifice plate sized to
pass the required minimum flow can be installed in
a line from the pump discharge to the pump suction
source. Since this system is a constant recirculation
type, it is a source of lost pump hp. The lost hp can
be eliminated by using automatic pump
recirculation controls. This system requires pump
flow to be metered and an automatic valve to open
when pump flow is at or below the minimum flow
requirement. Automatic recirculation control will
be used only when justified by LCCA evaluation.
b. Deaerator level controls.
9-5. Control panels.
(1) Two element control. A two element DA
a. Control room. A control room isolated from
level control system as shown in figure 9-22(a) uses
the plant environment complete with heating and
feedwater flow as a feedwater signal to make the
air conditioning should be provided for all boiler
system responsive to load changes. A two element
plants. The boiler panels and auxiliaries may be
system for DA level control can be used for most
located at the boiler front for packaged boilers up
multiple unit installations that operate under steady
to 70,000 pph and for stoker fired units. A recorder
load conditions.
panel should be located in the control room. The
9-18
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