Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

Page: 454 / 512

ENGINEERING MANUAL OF AUTOMATIC CONTROL
VALVE SELECTION AND SIZING
444
Fig. 19. Linear Valve Steam Application.
Use the steam valve Kv formula to determine capacity
index for Valve V1 as follows:
Where:
Q = Quantity of steam required to pass through
the valve is 325 kilograms per hour.
∆
P = The pressure drop across a valve in a
modulating application is found using:
∆
P = 80% x (P
1
– P
2
)
and:
P
1
= Upstream pressure in supply main is 150 kPa.
P
2
= Pressure in return is 90 kPa.
∆
P = 0.80 x (150 – 90)
= 0.80 x 60
= 48 kPa
The critical pressure drop is found using the following
formula:
hcritical = 50% x kPa (upstream)
hcritical = 0.50 x 150 kPa
= 75 kPa
The pressure drop (
∆
P) of 48 kPa is used in calculating
the Kv, since it is less than the critical pressure drop
(
∆
P
critical
) of 75 kPa.
P
o
= Absolute outlet pressure of the valve in kPa
= P
1
-
∆
P
= 150 kPa – 48 kPa = 102 kPa
0.224 = A scaling constant.
Substituting the quantity of steam and pressure drop in the
Kv formula shows that Valve V1 should have a Kv of 22.3
or the next higher available value.
Kv =
Q
0.224
∆
P • P o
Kv =
Q
0.224
∆
P • P o
Kv =
325
0.224
48
• 102
=
325
0.224 x 69.97
= 20.7
Kv =
325
0.224
18
• 132
= 29.8
Select a linear valve providing close control with a
capacity index of 22.3 and meeting the required pressure
and temperature ratings.
EXAMPLE 3:
Figure 20 shows the importance of selecting an 80 percent
pressure drop for sizing the steam valve in Example 2.
This pressure drop (48 kPa) approximates the linear valve
characteristic. If only 30 percent of the available pressure
drop is used (0.30 x 60 kPa = 18 kPa), the valve Kv
becomes:
This larger valve (18 kPa drop) has a steeper curve that is
further away from the desired linear valve characteristic.
See LINEAR VALVE under VALVE SELECTION for
more information.
Fig. 20. Effect of Pressure Drop in Steam Valve Sizing.
100%
0% 100%
STEM TRAVEL
VALVE OPENING/
STEAM FLOW
M15323
Kv = 34
Kv = 22
LINEAR VALVE
CHARACTERISTIC
VALVE VI
M15322
150 kPa
SUPPLY
90 kPa
(VACUUM)
RETURN
STEAM
COIL
30% PRESSURE DROP, Kv = 34
80% PRESSURE DROP, Kv = 22