WattMaster WM-WCC3-TGD-01B - Manual (Page 261)

WCC III Technical Guide 3-147
WCC3.EXE SCREEN DESCRIPTIONS
Analog Actuator/Valve Control
Useful Analog Actuator/Valve Control
Information
Manually or solenoid operated ball valves can be closed quickly,
and thus there is a danger of water hammer. Some ball valves
are equipped with an analog controlled actuator that may be then
motor operated. These valves can be used either for on/off or fl ow
control. A fl ow control valve may also be equipped with a feed
back potentiometer for additional feed back control.
It is important to realize that most analog controlled valves are
either controlled by a 0 to 10 volt or 2 to 10 volt control source
which then must correspond to an actual 0 to 90 degree opening or
closing of a typical quarter turn valve.
All analog valves have what is called a throttling range. This
analog valve throttling range is typically non linear and should be
thought of as the useful control range for the analog valve. This
range is typically rated at 20% to 80 % of the valves open position.
There is no useful fl ow until the valve is at 20% open, and for all
intensive purposes when the valve position is over 80% there is no
more useful fl ow increase either.
A ball valve is a valve with a spherical disc with a hole in it and
is the part of the valve which controls the fl ow through it. There
generally are three types of ball valves: Straight through, two way,
and three way ball valves. Three-way ball valves have an L- or
T-shaped hole through the middle of the valve.
There are fi ve general body styles of duplex ball valves: single body,
three piece body, split body, top entry, and welded. The difference
is based on how the pieces of the valve—especially the casing that
contains the ball itself—are manufactured and assembled. The
valve operation is the same in each case.
In addition, there are different styles related to the bore of the ball
mechanism itself:
 A full port or more commonly known full bore ball
valve has an over-sized ball so that the hole in the
ball is the same size as the pipeline resulting in lower
friction loss. Flow is unrestricted but the valve is
larger and more expensive, so this is only used where
free fl ow is required, for example, in pipelines which
require pigging.
 In reduced port or more commonly known reduced
bore ball valves, fl ow through the valve is one pipe
size smaller than the valve’s pipe size, resulting in
fl ow area being smaller than the pipe. As the fl ow
discharge remains constant and is equal to area of
fl ow (A) times velocity (V), A
1
V
1
= A
2
V
2
, the velocity
increases with reduced area of fl ow.
 A V port ball valve has either a ‘v’ shaped ball or a
‘v’ shaped seat. This allows the orifi ce to be opened
and closed in a more controlled manner with a closer
to linear fl ow characteristic. When the valve is in
the closed position and opening is commenced, the
small end of the ‘v’ is opened fi rst allowing stable
fl ow control during this stage. This type of design
requires a generally more robust construction due to
higher velocities of the fl uids which might damage a
standard valve.
 A trunnion ball valve has additional mechanical
anchoring of the ball at the top and the bottom,
suitable for larger and higher pressure valves (say,
above 10 cm and 40 bars).
 Cavity fi ller Ball Valve—Many industries encounter
problem with residues in the ball valve. Where the
fl uid is meant for human consumption, residues may
also be a health hazard, and when the fl uid changes
from time to time, contamination of one fl uid with
another may occur. Residues arise because in the
half open position of the ball valve a gap is created
between the ball bore and the body in which fl uid can
be trapped. To avoid the fl uid getting into this cavity,
the cavity has to be plugged, which can be done by
extending the valve seats in such a manner that they
are always in contact with the ball. This type of ball
valve is known as Cavity Filler Ball Valve.