21
2.7
M
ECHANICAL
A
CTUATOR
In order to control the outlet pressure of a PRV the Sentinel 2 system is required to
modify the PRV behaviour so that it produces downstream water pressure which
follows the programmed pressure profile. This is achieved with the
help of the HWM Mechanical Actuator, shown opposite.
The actuator consists of a diaphragm sandwiched between two
concave plates. The one side of the actuator has threaded rod which
has been drilled through its length to form a tube. Inside the tube is a
plunger which is in contact with the diaphragm. On the other side of
the diaphragm, the actuator forms a sealed chamber which can be
inflated (or deflated) using hydraulic pressure; hydraulic pressure is
supplied through a water-pipe connection, as shown below (via the
green hose). The actuator therefore converts an applied hydraulic pressure into the
mechanical position of the plunger. The end of the plunger exits the
thread.
A PRV normally has its pressure regulated via some control
mechanism (which may include a pilot valve), with its outlet pressure
being set mechanically; Typically, this is by the manual adjustment of
a setting bolt which acts on a spring.
The actuator used within a Sentinel 2 system replaces the setting
bolt. It allows the Sentinel 2 to adjust the PRV pressure.
2.8
S
UMMARY OF
PRV
F
UNCTION
(P
ILOT OPERATED
)
(Readers familiar with PRVs controlled by pilot valve operation may skip to section 2.9).
2.8.1
Main valve
The main components of a Pressure Reducing Valve (PRV) are shown opposite.
A plunger within the valve body is able to close
onto the valve seat (closing off water flow) or open
(allowing water flow). Depending on the position
of the plunger, the flow has a variable restriction.
The restriction reduces the outlet pressure when
water is flowing. The plunger position is influenced
by the water pressure on both the inlet and outlet
sides of the valve seat, internal springs acting on
the plunger, and also the position of a diaphragm
which is attached to stem of the plunger.
The diaphragm changes position in a manner
dependent on the difference in pressure either side it. The pressure on the diaphragm
is usually under control of components external to the main valve body, including a
pilot valve.
Inlet
Pressure
Outlet
Pressure
Main valve
chamber
(Installed)
