Honeywell AUTOMATIC CONTROL User Manual

ENGINEERING MANUAL OF AUTOMATIC CONTROL
VALVE SELECTION AND SIZING
448
The critical pressure drop is found using the following
formula:
h
critical
= 50% x (psig + 14.7 psi)
h
critical
=0.50 x (80 psig upstream + 14.7 psi)
=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
Fig. 19. Linear Valve Steam Application.
Use the steam valve C
v
formula to determine capacity
index for Valve V1 as follows:
per pound at average pressure in valve (P
avg
):
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.
P
avg
= Pm –
h
2
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.
= 80 –
47.4
2
= 80 – 23.6 = 56.4 psig
C
v
=
(1 + 0.00075 x 0) x 808.5 x
2
63.5 47.4
=
1745.6
63.5 x 6.88
= 4.6
VALVE VI
C2336
5 PSI
SUPPLY
1.96 PSI
(VACUUM)
RETURN
STEAM
COIL
30% PRESSURE DROP, Cv = 41
80% PRESSURE DROP, Cv = 25
C
v
=
(1 + 0.00075s)Q
V
63.5
h
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:
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.
Hg.
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
critical
= 50% x (psig + 14.7 psi)
h
critical
=0.50 x (5 psig upstream + 14.7 psi)
=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
avg)
:
P
avg
= Pm –
h
2
= 5 –
5.6
2
= 5 – 2.8 = 2.2 psig
The specific volume of steam at 2.2 psig is
23.54 and the square root is 4.85.

Honeywell AUTOMATIC CONTROL Specifications

General

Brand	Honeywell
Model	AUTOMATIC CONTROL
Category	Controller
Language	English