2000 Hook-up Book - Spirax Sarco

Water Valve Sizing
Water valve capacity is directly
related to the square root of the
pressure drop across it, not the
static system pressure. Knowing
the load in GPM water or any
other liquid, the minimum valve
Cv required is calculated from the
allowable pressure drop (∆P):
Cv = GPM S.G.
√ ∆P derived from...
GPM(Water) = Cv √ ∆P
(Other SG Liquids)
GPM = Cv ∆P
√ S.G.
A
Figure 43
Three-Port Mixing Valve in a Closed Circuit
(Constant Volume, Variable Temperature)
A
Boiler
Boiler
Constantly
Open Port
Pump
Figure 43A
Three-Port Diverting Valve in a Closed Circuit
(Constant Temperature, Variable Volume)
B
Temperature Control Valves for Liquid Service
O
X Z
Three-Port Valve
If the allowable differential pressure
is unknown, the following
pressure drops may be applied:
• Heating and Cooling systems
using low temperature hot
water (below 212°F)—
Size valve at 1 psi to 2-1/2 psi
differential.
• Heating Systems using water
above 212°F—
Size valve on a 2-1/2 to 5 psi
differential.
B
Constantly
Open Port
Pump
Balance
Valve
O
Z X
Three-Port Valve
Balance
Valve
C
Heating
System
C
Heating
Plant or
Process
Equipment
• Water for process systems—
Size valve for pressure drop
of 10% up to 20% of the system
pressure.
• Cooling Valves—
Size for allowable differential
up to full system pressure
drop when discharging to
atmosphere. Be sure to
check maximum allowable
pressure drop of the valve
selected. A bellows-balanced
type may be required.
Using Two-Port and
Three-Port Valves
Only two-port valves are used on
steam systems. However, when
dealing with controls for water we
can select either two-port or
three-port valves. But we must
consider the effects of both types
on the overall system dynamics.
A three-port valve, whether
mixing or diverting, is fairly close
to being a constant volume valve
and tends to maintain constant
pressure distribution in the system,
irrespective of the position of
the valve.
If a two-port valve were used,
the flow decreases, the valve
closes and the pressure or head
across it would increase. This
effect is inherent in the use of
two-port valves and can affect the
operation of other subcircuits.
Furthermore, the water
standing in the mains will often
cool off while the valve is closed.
When the valve reopens, the
water entering the heat exchanger
or load is cooler than expected,
and it is some time before normal
heating can commence. To avoid
this, a small bypass line should
be installed across the supply
and return mains. The bypass line
should be sized to handle flow
rate due to mains losses but in
the absence of information, the
bypass should be sized for 10%
of the design flow rate.
29
SYSTEM DESIGN