Data Sheet
Bulletin 71.4:98
7
Answer from Equation 2, Q = 14 500 SCFH
(389 Nm
3
/h) rated capacity. Since this capacity is
greater than the 13 200 SCFH (354 Nm
3
/h) required,
the relief valve may be used or the calculation could be
repeated to try for a smaller body size.
Example 2.
Service Conditions:
Setpoint, 165 psi (11,4 bar)
Build-up allowed, 10 psi (0,69 bar)
Outlet discharges into pressured system of
50 psi (3,4 bar)
Require 3000 SCFH (80,4 Nm
3
/h) air ow rate
From Table 11, select NPS 3/4 or 1 (DN 20 or 25)
body, blue spring. For 150 psi (10,3 bar) setpoint and
10 psi (0,69 bar) build-up, the Type 98H will ow
9000 SCFH (241 Nm
3
/h) air.
Calculate C
g
for these table conditions using Equation 1:
P
1
(inlet) = 150 psig (10,3 bar) + 10 psig
(0,69 bar) build-up = 160 psig or
174.7 psia (11,0 bar g or 12,0 bar a)
∆P = P
1
- P
2
= 160 psig (11,0 bar) - 0 =
160 psid (11,0 bar d)
Q = 9000 SCFH (241 Nm
3
/h)
C
1
= 35
Answer from Equation 1, C
g
= 51.5*
*Since build-up relates to travel, this coefcient is
valid for all pressure settings on the spring used in the
calculation whenever the build-up is 10 psi (0,69 bar).
Now, solve for actual ow, Q, from the original service
conditions using Equation 2:
P
1
= 165 psig (11,4 bar) + 10 psig (0,69 bar)
build-up = 175 psig or 189.7 psia
(12,1 bar g or 13,1 bar a)
∆P = P
1
- P
2
= 170 psig (11,7 bar) - 50 psig
(3,4 bar) = 120 psid (8,3 bar d)
C
g
= 51.5 (from above)
C
1
= 35
Answer from Equation 2, Q = 9500 SCFH (255 Nm
3
/h)
rated capacity. Since this capacity is greater than the
3000 SCFH (80,4 Nm
3
/h) required, the relief valve may
be used or the calculation could be repeated to try for a
smaller body size.
Liquid Sizing for Liquids Other than Water
Step 1.
Determine C
v
at build-up conditions of application.
C
v
=
Q
∆P
Q = Flow in GPM from capacity tables
∆P = Pressure drop in psi (setpoint and build-up)
C
v
= Valve sizing coefcient
Step 2.
To determine ow rate for liquids other than
water or ow rate for differential relief service:
Q = C
v
∆P/G
G = Specic gravity of uid
Maximum Allowable Pressure Drop for
Liquid Service
Pressure drops in excess of allowable will result in
choked ow and possible cavitation damage.
To determine maximum allowable pressure drop
for water:
∆P(allow) = K
m
(P
1
)
∆P = Valve differential - PSI
K
m
= Valve recovery coefcient from table
P
1
= Valve inlet pressure psig
To determine maximum allowable pressure drop for
uids other than water, see Fisher
®
Catalog 10.