Control Valves
Michael Bell
4 August 2019
Honor Statement
I hereby certify that I have neither given nor received answers to any of the questions in this assignment. I
have not copied, in whole or in part, any available pony. All information is my own work and was obtained
directly from referenced materials, interviews, or investigations.
Signed:
1. Explain the events that take place in a Leslie self-actuated reducing valve after a drop in
pressure occurs
A pressure-reducing valve takes a high-pressure fluid and steps it down to a relatively
lower pressure on the outlet side. In order to achieve this, Leslie self-actuated reducing
valves proceed through a series of events due to mechanical linkages that regulate
discharge pressure. Fluid enters through the inlet of the valve – a majority of the flow
proceeds to the chamber. A small portion splits off through a small channel leading to a
chamber where a pilot is present, hence the self-actuating nomenclature. Simultaneously,
a portion of fluid from the outlet side of the valve proceeds through a similar channel
leading to a chamber with a diaphragm. When a drop in pressure on the outlet side
occurs, the force of the fluid acting on the pilot’s operating spring exceeds the force of
flow from the outlet acting on the diaphragm. The diaphragm will then act downwards
upon the control valve/pilot, forcing it off the seat. This allows fluid from the supply side
in the chamber with the pilot to act upon the piston attached to the main valve spring. At
this point, with the valve off the seat, fluid will flow from the supply side to the outlet of
the valve. This causes pressure on the outlet side to increase. Outlet pressure will increase
until it can sufficiently overcome the force of the pilot on the diaphragm, at which point,
outlet flow is throttled to provide the proper pressure for service downstream of the
control valve.
2. Compare the operation of a turbine driven fire pump Constant Pressure pump governor
with a MFP Excess Pressure pump governor
A constant pressure governor maintains a constant discharge pressure from a turbinedriven pump by throttling the steam provided to the turbine. An excess pressure governor
maintains a constant pressure differential rather than a constant pressure. A constant
pressure governor works in a similar manner to the self-actuating reducing valve
described above; however, it contains another diaphragm and sensing line attached to the
pump discharge.
Both CP and XP governors are utilized to control steam flow administered to a turbine to
maintain discharge pressure on the outlet of a pump. A CP governor utilizes a diaphragm
assembly with a port connected to the discharge line of the pump. This sensing line will
determine the deflection of the diaphragm, which then controls how much steam input is
allowed to the turbine. The steam flow to the turbine consequently increases or reduces
RPM to maintain the desired pump discharge pressure.
An XP governor, on the other hand, has an additional diaphragm assembly when
compared to the CP governor. There are two sensing lines attached to an XP governor –
one at the discharge of the feed pump and one coming from the steam drum. The feed
pump discharge sensing line acts similarly to the CP governor sensing line. The sensing
line from the boiler, however, acts on the top of the diaphragm assembly. This allows the
governor to maintain a pressure differential between the feed pump discharge and the
steam drum. If this was not the case, no feed water would reach the feed pump due to the
lack of pressure driving the water.
3. a. Reverse-Acting Leslie Valve
The diaphragm control valve in this system is a normally closed, reverse acting valve.
The valve’s set pressure changes by varying the air pressure acting upon the diaphragm’s
upper surface. An internal port within the valve diverts flow from the outlet side of the
valve to act upon the lower surface of the diaphragm. Fluid flows through the outside of
the valve, where the reverse-acting pilot controller sends a control air signal to the valve,
lifting the valve off the seat to allow flow through until the pressure reaches the set point.
b. Hotwell Level Control
The condenser hotwell level control valve utilizes a differential pressure pilot that
regulates flow through the valve. The outlet side of the valve has a channel that diverts
flow to the underside of the diaphragm inside the valve. The sensing line attached to the
upper portion of the hotwell applies a pressure to the pilot diaphragm that varies with the
tank level. The sensing line attached to the other side of said diaphragm is a static
pressure generated by the liquid in the tank at the low level.
c. Feed Pump XP Governor
The feed pump XP governor maintains a pressure differential between the steam drum
and the discharge from the feed pump, guaranteeing water will circulate into the steam
drum. The governor maintains steam flow to a turbine-driven feed pump or drive-end
cylinders of a direct-acting, reciprocating feed pump. The XP governor senses pressure
via a sensing line from the steam drum and another from the pump discharge line. These
sensing lines act on two diaphragms – the steam drum line on an upper diaphragm and
the feed pump discharge on a lower diaphragm. This provides a greater degree of control
of discharge pressure, which creates the constant pressure differential between the steam
drum and feed pump discharge.
d. Heater Temperature Control Valve
Temperature-regulating valves control the flow of fluid entering a heat exchanger to
maintain the exit temperature of either fluid exchanging heat. The valve utilizes a
temperature sensing bulb that is either directly submerged or encased and submerged
within the fluid. When the bulb senses a temperature greater than the set point
temperature, a force is exerted on a bellows due to the expansion of fluid due to heat.
This expansion or contraction positions the valve stem of the control valve appropriately
to regulate the temperature of the fluid in question.
References:
Hunt Modern Marine Engineers Manual Volume 1 Chapter 11