the top of the lower crosshead. Steam is led from the
Two opposing forces are involved in the operation
governor outlet to the bottom of the lower diaphragm
of a constant-pressure pump governor. Fluid from the
and also through a needle valve to the top of the
pump discharge, at discharge pressure, is led through an
intermediate diaphragm. A steam chamber provides a
actuating line to the space below the upper diaphragm.
continuous supply of steam at the required pressure to
The pump discharge pressure exerts an UPWARD force
the top of the intermediate diaphragm.
on the upper diaphragm. Opposing this, an adjusting
Any up or down movement of the crosshead
spring exerts a DOWNWARD force on the upper
assembly is therefore opposed by the force of the steam
diaphragm.
pressure acting on either the intermediate diaphragm or
When the downward force of the adjusting spring
the lower diaphragm. The whole arrangement prevents
is greater than the upward force of the pump discharge
extreme reactions of the controlling valve in response
prcssurc, the spring forces both the upper diaphragm
to variations in pump discharge pressure.
and the upper crosshead downward. A pair of
Limiting the movement of the controlling valve in
connecting rods connects the upper crosshead rigidly to
the manner just described reduces the amount of
the lower crosshead, so the entire assembly of upper and
hunting the governor must do to find each new position.
lower crossheads moves together. When the crosshead
Under constant-load conditions, the controlling valve
assembly moves downward, it pushes the lower
takes a position that causes the main valve to remain
mushroom and the lower diaphragm downward. The
open by the required amount. A change in load
lower diaphragm is in contact with the controlling
conditions causes momentary hunting by the governor
valve. When the lower diaphragm is moved downward,
the controlling valve is forced down and open.
until it finds the new position required to maintain pump
discharge pressure at the new load.
The controlling valve is supplied with a small
A pull-open device, consisting of a valve stem and
amount of steam through a port from the inlet side of
a handwhcel, is fitted to the bottom of the governor.
the governor. When the controlling valve is open, steam
Turning the handwheel to the open position draws the
passes to the top of the operating piston. The steam
main valve open and allows full steam flow to the
pressure acts on the top of the operating piston, forcing
turbine. When the main valve is opened with the
the piston down and opening the main valve. The extent
handwhcel, the turbine must be controlled manually.
to which the main valve is opened controls the amount
Under all normal operating conditions, the bypass
of steam admitted to the driving turbine. Increasing the
remains closed and the pump discharge pressure is
opening of the main valve therefore increases the supply
raised or lowcrcd, as necessary, by increasing or
of steam to the turbine and so increases the speed of the
decreasing the tension on the adjusting spring.
turbine.
The increased speed of the turbine is reflected in an
CONTROL AND MAIN VALVE.--If there is
increased discharge pressure from the pump. This
pressure is exerted against the underside of the upper
leakage in the generator through the control valve or its
bushing, steam will flow to the top of the operating
diaphragm. When the pump discharge pressure has
piston, opening the main valve, and holding it open,
increased to the point that the upward force acting on
the underside of the upper diaphragm is greater than the
even though there is no tension on the adjusting spring.
downward force exerted by the adjusting spring, the
The main valve must be able to close off completely or
upper diaphragm is moved upward. This action allows
else the governor cannot operate properly. The only
a spring to start closing the controlling valve, which, in
remedy is to disassemble the governor and stop the
turn, allows the main valve spring to start closing the
steam leakage. In most instances, you must renew the
main valve against the now-reduced pressure on the
control valve. If the leakage is through the bottom of the
operating piston. When the main valve starts to close,
bushing and its seat, you must lap the scat. A cast-iron
the steam supply to the turbine is reduced, the speed of
lap is best for this type of work.
the turbine is reduced, and the pump discharge pressure
Rotate the lap through a small angle of rotation, lift
it from the work occasionally, and move it to a new
is reduced.
At first glance, it might seem that the controlling
position as the work progresses. This will ensure that
valve and the main valve would open and close
the lap will slowly and gradually rotate around the entire
constantly and the pump discharge pressure would vary
scat circle. Do not bear down heavily on the handle of
continually over a wide range. But this does not happen
the lap. Replace the compound often, using only clean
because the governor prevents such excessive opening
compound. If the lap should develop a groove or cut,
or closing. An intermediate diaphragm bears against an
redress the lap. Continue lapping only long enough to
intermediate mushroom, which, in turn, bears against
remove all damaged areas.
13-11