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 coefcient 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 coefcient
Step 2.
To determine ow rate for liquids other than
water or ow rate for differential relief service:
Q = C
v
∆P/G
G = Specic 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 coefcient from table
P
1
= Valve inlet pressure psig
To determine maximum allowable pressure drop for
uids other than water, see Fisher
®
Catalog 10.