Honeywell AUTOMATIC CONTROL Engineering Manual page 458

VALVE SELECTION AND SIZING
The critical pressure drop is found using the following
formula:
h = 50% x (psig + 14.7 psi)
critical
h = 0.50 x (80 psig upstream + 14.7 psi)
critical
= 0.50 x 94.7 psi
= 47.4 psi
The critical pressure drop (h critical ) of 47.4 psi is used in
calculating C v , since it is less than the pressure drop (h) of
64 psi. Always, use the smaller of the two calculated values.
V = Specific volume (V) of steam, in cubic feet
h
P avg = Pm –
2
47.4
= 80 – = 80 – 23.6 = 56.4 psig
2
per pound at average pressure in valve (P
The specific volume of steam at 56.4 psig is
6.14 and the square root is 2.48.
63.5 = A scaling constant.
Substituting the quantity of steam, specific volume of
steam, and pressure drop in the C v formula shows that the
valve should have a C v of 4.6.
(1 + 0.00075 x 0) x 808.5 x 2
C v =
1745.6
=
63.5 x 6.88
NOTE: If P avg is rounded off to the nearest value in Table
5 (60 psi), the calculated C v is 4.5 a negligible
difference.
Select a linear valve providing close control with a capacity
index of 4 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 750 pounds per hour of
steam. Upstream pressure in the supply main is 5 psig and
pressure in the return is 4 in. Hg vacuum minimum.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
63.5 47.4
= 4.6
5 PSI
SUPPLY
30% PRESSURE DROP, Cv = 41
80% PRESSURE DROP, Cv = 25
Fig. 19. Linear Valve Steam Application.
Use the steam valve C v formula to determine capacity
index for Valve V1 as follows:
(1 + 0.00075s)Q V
C v =
Where:
Q = Quantity of steam required to pass through
the valve is 750 pounds per hour.
h = The pressure drop across a valve in a
modulating application is found using:
):
avg
h = 80% x (Pm – Pr)
and:
Pm = Upstream pressure in supply main is 5 psig.
Pr = Pressure in return is 4 in. Hg vacuum.
NOTE: 1 in. Hg = 0.49 psi and 1 psi = 2.04 in.
Therefore,
4 in. Hg vacuum = –1.96 psig.
h = 0.80 x [5 – (–1.96)]
= 0.80 x 6.96
= 5.6 psi
The critical pressure drop is found using the following
formula:
h = 50% x (psig + 14.7 psi)
critical
h = 0.50 x (5 psig upstream + 14.7 psi)
critical
= 0.50 x 19.7 psia
= 9.9 psi
The pressure drop (h) of 5.6 psi is used in calculating
the C v , since it is less than the critical pressure drop
(h critical ) of 9.9 psi.
V = Specific volume (V) of steam, in cubic feet
per pound at average pressure in valve (P
P avg = Pm –
= 5 –
The specific volume of steam at 2.2 psig is
23.54 and the square root is 4.85.
448
1.96 PSI
VALVE VI STEAM
(VACUUM)
COIL
RETURN
C2336
63.5 h
Hg.
h
2
5.6
= 5 – 2.8 = 2.2 psig
2
) :
avg