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b. The pressure drop across an open valve should be
about half of the pressure difference between
system supply and return, enough so that the
valve, not the friction through the coil or radiator,
controls the volume of water flow or the valve
pressure drop should be equal to or greater than
the pressure drop through the coil or radiator,
plus the pipe and fittings connecting them to the
supply and return mains.
c. Verify allowable full open and full closed
pressure drops for all proportional and two-
position water valves with appropriate
manufacturer literature.
d. Make an analysis of the system at maximum and
minimum rates of flow to determine whether or
not the pressure difference between the supply
and return mains stays within the limits that are
acceptable from the stand point of control
stability and close-off rating.
2. For two- and three-way valves consider the following
guidelines for valve pressure drop:
a. In load bypass applications (Fig. 13) such as
radiators, coils, and air conditioning units, the
pressure drop should be 50 to 70 percent of the
minimum difference between the supply and
return main pressure at design operating
conditions.
b. A manual balancing valve may be installed in the
bypass to equalize the load drop and the bypass
drop.
3. When selecting pressure drops for three-way mixing
valves in boiler bypass applications (Fig. 13), consider
the following:
a. Determine the design pressure drop through the
boiler including all of the piping, valves, and
fittings from the bypass connection through the
boiler and up to the three-way valve input.
b. The valve pressure drop should be equal to or
greater than the drop through the boiler and the
fittings. If the valve drop is much smaller than
the boiler pressure drop at design, effective
control is obtained only when the disc is near one
of the two seats. The mid-portion of the valve lift
will be relatively ineffective.
c. A manual balancing valve may be installed in the
boiler bypass to equalize the boiler drop and the
bypass drop.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
VALVE SELECTION AND SIZING
WATER VALVE SIZING EXAMPLES
EXAMPLE 1:
A two-way linear valve is needed to control flow of 7 C
chilled water to a cooling coil. The coil manufacturer has
specified an eight-row coil having a water flow pressure
drop of 22 kPa. Further, specifications say that the coil will
produce 13 C leaving air with a water flow of 3.32 m
Supply main is maintained at 275 kPa, return is at 200 kPa.
Select required capacity index (K v ) of the valve.
Use the water valve K v formula to determine capacity
index for Valve V1 as follows:
K v = Q
P • 10
Where:
Q = Flow of fluid in Cubic meters per hour
required is 3.32 m
= Density of water is 1000.
P = Pressure drop across the valve. The
difference between the supply and return is
75 kPa. 50% to 70% x 75 kPa = 37.5 to
52.7 kPa. Use
40 kPa for the correct valve pressure drop.
Note that 40 kPa is also greater than the coil
pressure drop of 22 kPa.
Substituting:
K v = 3.32
Select a linear valve providing close control with a
capacity index of 5.4 and meeting the required pressure
and temperature ratings.
EXAMPLE 2:
A bypass valve is required to prevent flow through the
chiller from dropping below 90 percent of design flow.
When sizing valves for pump or chiller bypass
applications (Fig. 16), system conditions that cause the
valve to open or close completely must be considered
before a pressure drop can be selected.
439
3
/h.
3
/h.
= 5.2
• 10
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