Data sheet
VFS 2
6 | VD.HC.L3.02
© Danfoss | 2016.11
Control valve sizing diagram
for steam
(continued)
1 For saturated steam
Design data:
Flow rate: 1000 kg/h
Absolute inlet pressure: 5 bar (500 kPa)
- follow dashed line -
The absolute inlet pressure is 500 kPa. 40% of
this is 200 kPa.
Locate the diagonal line corresponding to the
pressure drop of 200 kPa (line A-A).
Read the absolute inlet pressure on the lower left
hand scale (point B), and draw a horizontal line
across until it meets the pressure drop diagonal
(A-A) at point C.
From this point extend a vertical line upwards
until it meets the horizontal line representing
the steam flow of 1000 kg/h from point D. The
intersection of this is point E.
The nearest diagonal kvs line above this is line
F-F with a k
vs
of 25 (point E’). If the ideal valve size
is not available the next largest size should be
selected to ensure design flow.
The pressure drop through valve at the flow rate
is found by the intersection of the 1000 kg/h
line with F-F (point E’) and dropping a vertical;
this actually hits the horizontal line for 500 kPa
(point E’’) inlet pressure at a pressure drop
diagonal of 70 kPa. This is only 14 % of the inlet
pressure and the control quality will not be
good until the valve has partially closed. As with
all steam valves this compromise is necessary
since the next smaller valve would not pass the
required flow (maximum flow would have been
about 600 kg/h).
The maximum flow for same inlet pressure
is found by extending the vertical line (C-E)
through point E until it crosses the k
VS
25 line F-F
(point E’’’) and reading off the flow (1700 kg/h).
2 For superheated steam
Design data:
Flow rate: 500 kg/h
Absolute inlet pressure: 5 bar (500 kPa)
Steam temperature: 190 °C
The procedure for superheated steam is much
the same as for saturated steam, but uses a
different flow scale which slightly elevates the
readings according to the degree of superheat.
- follow dotted line -
As before, the diagonal pressure drop line
A-A is located as before for 40 % of 500
(200 kg/h). The horizontal inlet pressure line
through point B is now extended to the left to
read off the corresponding saturated steam
temperature at point G (150 °C). The difference
between the saturated steam temperature and
the superheated steam temperature is
190 °C - 150 °C = 40 °C.
The superheated steam flow is found on the
upper right hand scale, point H, and the diagonal
line is followed down from here until it meets
a vertical line from the steam temperature
elevation (40 °C) at point J.
As before, the horizontal line through point B is
drawn to cut line A-A at point C and the point
where the vertical line from this point meets the
horizontal line from point J is the operating point
(point K). This horizontal line, J-K, is the corrected
flow line. The nearest diagonal line above this is
for k
vs
10 (point K’). A vertical line dropped from
the intersection of J-K with the 10 k
vs
line intersects
the 500 kPa inlet pressure line (point K’’) at a
pressure drop diagonal of about 150 kPa. This
is about 30% of the inlet pressure which will
give reasonable control quality (compared to
recommended ratio of 40%).
