Honeywell AUTOMATIC CONTROL SI Edition User Manual

ENGINEERING MANUAL OF AUTOMATIC CONTROL
VALVE SELECTION AND SIZING
443
The critical pressure drop is found using the following
formula:
∆P
critical
= 50% x kPa
∆P
critical
= 0.50 x 650 kPa
= 325 kPa
The critical pressure drop (∆P
critical
) of 325 kPa is used in
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
o
=P
1
– ∆P = 650 kPa – 325 kPa =325 kPa
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.
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:
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
1
– P
2
)
Where:
P
1
= Upstream pressure in supply main is
650 kPa.
P
2
= Pressure in return is atmospheric pressure or
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
K
v
=
Q
0.224
∆P • P
o
K
v
=
329.7
0.224 325 • 325
=
329.7
0.224 x 325
= 4.53
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.

Honeywell AUTOMATIC CONTROL SI Edition Specifications

General

Brand	Honeywell
Model	AUTOMATIC CONTROL SI Edition
Category	Control Systems
Language	English