Vent Piping
38 CVHH-SVX001A-EN
• L = equivalent length of discharge piping, feet
• C
r
= rated capacity as stamped on the relief device in
SCFM (conversion: lb/min = SCFM * 0.0764)
C
r
= C value in lb/min from
Tab le 9, p . 37
• f = Moody friction factor in fully turbulent flow
• d = inside diameter of pipe or tube, in.
• ln = natural logarithm
• P
2
= absolute pressure at outlet of discharge piping, psi
(atmospheric pressure)
• P
0
= allowed back pressure (absolute) at the outlet of
pressure relief device, psi
P
0
= (0.15 P) + atmospheric pressure
Figure 24. Rupture disk vent pipe sizing (IP units); for use with
Table 9, p. 37
Note: This figure, provided as a refer ence, is based on ASHRAE St andard 15. Vent line size is typically dict ated by stat e or local code which m ay be different
from ASHRAE Standard 15 requirements.
Pipe Size ( I .D.)
friction factor
6 NPS (150 DN)
6.06 in. (154 mm)
f= 0.0149
4 NPS (100 DN)
4.03 in. (102 mm)
f= 0.0163
3 NPS (80 DN)
3.07 in. (78 mm)
f= 0.0173
5 NPS (125 DN)
5.05 in. (128 mm)
f= 0.0155
Pipe size as a Function of “C” Value and Length of Run
1000
100
10
100
100010
L = Pipe Length ( Equivalent Feet)
( Feet x 0.30 5 = Meters)
“C” Value (lb/min)
ASHRAE Standard 15
L =
0.214d
5
(P
2
0
- P
2
2
)
-
d * ln(P
0
/ P
2
)
fC
2
R
6f
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