PROLONGING THE SERVICE LIFE
OF A CONTROL VALVE THROUGH
PROPER SELECTION AND
OPERATION
his is due to the fact that
it is the final element that
ultimately manipulates the
value of the variable in the
control process. It alters
the value of the manipulated variable
in response to an output signal from a
controller whether automatic, manual or
by direct human action.
T
The control valve has two main parts
namely the valve bod y and the valve
plug and they differ in geometry and
material of construction. The combined
body and plug geometry determines
the flow properties of the valve. The
valve flow differs in design like the
through-flow types and the blending and
stream-splitting types. Likewise, valve
seats differ in construction. There are the
conventional and the contoured valve
seat types with parabolic and quick-
opening plugs whose internals can only
be inspected during servicing.
A simple control loop as shown in Figure
1 is a close system consisting of selected
instruments that work together as a single
to control fluid flow and minimize flow
control problems. From Figures 1 and 2 a
simple control loop consists of a sensor
which can be an orifice, thermocouple, or
a venture meter, a transmitter which can
be either differential pressure electro-
pneumatic or pneumatic transmitter, an
indicator which can be a pressure gauge,
level gauge or temperature gauge, a
transducer which converts the reported
signal from the manipulated form, to the
form understandable to the final control
instrument or a positioner which gives
70
PECM Issue 31
proportion al positional action to the
valve stem so as to correctly position the
plug in the valve body and finally the
control valve.
S ome of the most common valves
utilized in industries are :
• Ball valves
• Gate valves
• Butterfly valves
B all V alves :
These are stop valves that use a ball to
stop or start the flow of fluid. When the
valve handle is operated to open the
valve, the ball rotates to a point where
the hole through the ball is in line with
the valve body inlet and outlet. When
the valve is shut, which requires only a
90 degree rotation of the hand wheel for
most valves, the ball is rotated so the hole
is perpendicular to the flow openings
of the valve body, and flow is stopped.
Most of ball valves are of the quick
acting type, but many are planetary gear
operated. This type of gearing allows the
use of relatively small hand wheel and
operating force to operate a fairly large
valve. The gearing however increases the
operating time for the valve. Ball valve is
normally found in the following systems:
desalination plants, trim and drain, air,
hydraulic, and oil transfer etc. It is used
for general service, high temperature
conditions and slurries.
G ate
V alves :
Gate valves are usually used when a
straight line flow of fluid and minimum
restriction is desired. They are also
named because the part that either stops
or allows flow through the valve acts
somewhat like the opening and closing
of a gate. When the valve is wide open, it
is fully drawn up into the valve, leaving
an opening for flow through the valve
the same size as the pipe in which the
valve is installed. Therefore, there is little
pressure drop or flow restriction through
the valve. Gate valves are not usually
suitable for throttling purposes because
the control of flow would be difficult due
to valve design and since the flow of fluid