VALVES AND ACTUATORS
37
Valve Sizing Diagram for Steam
Saturated steam flow rate kg/h
Overheated steam flow rate kg/h
Q = flow rate m
3
/h
D
pv = valve leakage
Pu = absolute pressure downstream the valve (bar)
Kvs = Q
22.8 •
D
pv • Pu
Pressure drop
Critical pressure drop
Overheating degree
NOTE: The recommended valve presssure drop is about
30% of the absolute supply pressure.
Example for saturated steam:
FLOW RATE:
4700 Kg/h of saturated steam
ABSOLUTE PRESSURE
UPSTREAM:
850 kPa
PRESSURE DROP:
160 kPa
Use the diagram as follows:
- Identify the crossing point between the line starting from absolute
pressure upstream the valve (850 kPa) and the inclined line corre-
sponding to the pressure drop value (160 kPa).
- Identify the crossing point between the line starting from the cross-
ing point above and the line from the flow rate value (4700 Kg/h).
This point corresponds to the required flow rate coefficient: Kvs 63.
Example for overheated steam:
FLOW RATE:
140 Kg/h of overheated steam
ABSOLUTE PRESSURE UPSTREAM: 350 kPa
TEMPERATURE:
209 °C
PRESSURE DROP:
100 kPa
Calculate the overheating degree of steam as follows:
- On the left side of the diagram, read the temperature value corre-
sponding to 350 kPa (139 °C).The overheating degree is:
209 – 139 = 70 °C
Use the diagram as follows:
- Identify the crossing point“A”(right side of the diagram) between the
line starting from the overheating value (70 °C) and the inclined line
corresponding to the flow rate value (140 Kg/h).
- Identify the crossing point“B”between the line starting from the val-
ue of pressure upstream (350 kPa) and the inclined line correspond-
ing to the value of pressure drop (100 kPa).
- Identify the crossing point between the line starting from the points
“A”and“B”.
