Data Sheet

Bulletin 71.2:92C
8
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Typical regulating capacities in pounds of saturated
steam per hour are shown in Table 5 for pilot-operated
regulators. A typical performance curve is shown in
Figure 6. To determine capacities for pressure-loaded
regulators, multiply the appropriate Table 5 value by the
capacity factor listed in Table 7.
1. If the steam is saturated and the pressure drop
across the regulator is critical (absolute outlet
pressure equal to approximately one-half or less
of the absolute inlet pressure), use the equation:
Q = (P
1abs
) (C
s
)
where,
Q = Maximum ow capacity, pounds of
saturated steam per hour
P
1abs
= Absolute inlet pressure (gauge inlet
pressure plus 14.7 psi)
C
s
= Wide-open steam sizing coefcient
(see Specications section)
To convert capacity to kilograms per hours, multiply
the capacity pounds per hour by 0.4536.
2. If the steam is superheated or if the pressure drop
across the regulator is lower than critical (absolute
outlet pressure greater than approximately
one-half the absolute inlet pressure), use the sizing
nomographs in Catalog 10.
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Table 6 gives regulating capacities in U.S. gallons per
minute of water and in cubic meters per hour of water.
To determine capacities for pressure-loaded regulators,
multiply the approximate Table 6 value by the capacity
factor listed in Table 7.
To determine regulating capacities at pressure settings
not given in Table 6, or to determine wide-open
capacities for relief sizing at any inlet pressure, use
the Catalog 10 liquid sizing procedures in conjunction
with the appropriate liquid coefcients (C
v
and K
m
, see
Specications section).
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Where,
Q = Flow in GPM
P = Valve differential in psi
105
1.11
C
v
= Valve sizing coefcient (see Table 6)
G = Specic Gravity
Example,
NPS 1 (DN 25) body
3/4-inch (19 mm) orice size
Glycol (Specic Gravity) = 1.11
P
inlet
= 150 psig (10,3 bar)
P
out
(setpoint) = 50 psig (3,4 bar)
Capacity based on 10% Droop from setpoint
P
out
at full ow = 50 psi (3,4 bar) setpoint - 5 psi
(0,35 bar) Droop = 45 psi (3,1 bar)
P = 150 - 45 = 105 psi (7,2 bar)
C
v
= 6.89 from Table 6
Q = 6.89
= 67.0 GPM (253,60 l/min) Glycol
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Pressure drops in excess of allowable will result in
choked ow and possible cavitation damage.
Choked ow is the formation of vapor bubbles in the
liquid ow stream causing a crowding condition at
the vena contracta which tends to limit ow through
the regulator. The vena contracta is the minimum
cross-sectional area of the ow stream occurring just
downstream of the actual physical restriction.
Cavitation and ashing are physical changes in the
process uid. The change is from the liquid state to
the vapor state and results from the increase in uid
velocity at or just downstream of the greatest ow
restriction, normally the regulator orice.
To determine the maximum allowable pressure drop
for water:
P(
allow
) = K
m
(P
1
)
Where,
P = Valve differential — psi
K
m
= Valve recovery coefcient from Table 7
P
1
= Valve inlet pressure psia
To determine maximum allowable pressure drop for
uids other than water, see Fisher
®
Catalog 10.
P
G
Q = C
v