Water Valve Pressure Drop; Water Valve Sizing Examples - Honeywell AUTOMATIC CONTROL Engineering Manual

WATER VALVE PRESSURE DROP

To determine valve pressure drop:
1. For two-way valves consider the following guidelines for
valve pressure drop:
a. Include the pressure drop in the design of the
water circulating system.
— In systems with two-way valves only, it is often
necessary to provide a pump relief bypass or
some other means of differential pressure
control to limit valve pressure drops to the
valve capabilities. For control stability at light
loads, pressure drop across the fully closed
valve should not exceed triple the pressure drop
used for sizing the valve.
— To avoid high pressure drops near the pump,
reverse returns are recommended in large
systems.
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
minim um diff erence between the suppl
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.

WATER VALVE SIZING EXAMPLES

EXAMPLE 1:
A two-way linear valve is needed to control flow of 45F
chilled water to a cooling coil. The coil manufacturer has
specified an eight-row coil having a water flow pressure
drop of 3.16 psi. Further, specifications say that the coil
will produce 55F leaving air with a water flow of 14.6 gpm.
Supply main is maintained at 40 psig, return is at 30 psig.
Select required capacity index (C v ) of the valve.
Use the water valve C v formula to determine capacity index
for Valve V1 as follows:
Cv =
Where:
Q = Flow of fluid in gallons per minute required
G = Specific gravity of water is 1.
h = Pressure drop across the valve. The
Substituting:
C v =
y and
Select a linear valve providing close control with a capacity
index of 6 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.
443
VALVE SELECTION AND SIZING
Q G
h
is 14.6 gpm.
difference between the supply and return is
10 psi. 50% to 70% x 10 psi = 5 to 7 psi.
Use 6 psi for the correct valve pressure drop.
Note that 6 psi is also greater than the coil
pressure drop of 3.16 psi.
14.6 1
= 6
6
ENGINEERING MANUAL OF AUTOMATIC CONTROL