Steam Valve Sizing Examples - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

STEAM VALVE SIZING EXAMPLES

EXAMPLE 1:
A two-way linear valve (V1) is needed to control high-
pressure steam flow to a steam-to-water heat
exchanger. An industrial-type valve is specified. Steam
pressure in the supply main is 650 kPa with no
superheat, pressure in return is equal to atmospheric
pressure, water flow is 5.2 L/s, and the water
temperature difference is 10 kelvins.
Use the steam valve K v formula to determine capacity
index for Valve V1 as follows:
Q
K v =
0.224 P • P o
Where:
Q = The quantity of steam required to pass
through the valve is found using the
converter valve formula:
Q = L/s • Tw • 6.462
Where:
L/s = 5.2 L/s water flow through exchanger
Tw = 10 kelvins temperature difference
6.462 = A scaling constant
Substituting this data in the formula:
Q = 329.7 kilograms per hour
P = The pressure drop across a valve in a
modulating application is:
P = 80% x (P – P
1
Where:
P = Upstream pressure in supply main is
1
650 kPa.
P = Pressure in return is atmospheric pressure or
2
101.325 kPA.
Substituting this data in the pressure drop formula:
P = 0.80 x (650 – 101.3)
= 0.80 x 548.7
= 439 kPa
ENGINEERING MANUAL OF AUTOMATIC CONTROL
)
2
VALVE SELECTION AND SIZING
The critical pressure drop is found using the following
formula:
P = 50% x kPa
critical
P = 0.50 x 650 kPa
critical
= 325 kPa
The critical pressure drop ( P
calculating K v , since it is less than the pressure drop ( P)
of 439 kPa. Always, use the smaller of the two calculated
values.
Q = 329.7 kg/h
P = P critical = 325 kPa
P = P – P = 650 kPa – 325 kPa =325 kPa
o 1
0.224 = A scaling constant.
Substituting the quantity of steam and pressure drop in the
K v formula shows that the valve should have a K v of 4.5.
329.7
K v =
0.224 325 • 325
329.7
=
0.224 x 325
Select a linear valve providing close control with a
capacity index of 4.5 and meeting the required pressure
and temperature ratings.
NOTE: For steam valves downstream from pressure
reducing stations, the steam will be superheated in
most cases and must be considered.
EXAMPLE 2:
In Figure 19, a linear valve (V1) is needed for accurate
flow control of a steam coil that requires 325 kilograms
per hour of steam. Upstream pressure in the supply main
is 150 kPa and pressure in the return is 90 kPa minimum.
443
) of 325 kPa is used in
critical
= 4.53