Specifications

Relief Valves
Vessel Relief Valves
Relief valve connection sizes are 1 inch FPT and
are in the quantity shown in
Table 9. Relief Valve Piping Sizes
Table 7 and Table 8.
Relief valves must be piped to the outside of the
building in accordance with ANSI/ASHRAE 15-
2001. The new 2001 standard has revised the
calculation method compared to previous issues.
Pipe Size
inch (NPT)
1 1/4 1 1/2 2 2 1/2 3
Moody Factor 0.0209 0.0202 0.0190 0.0182 0.0173
Equivalent
length (ft)
2.2 18.5 105.8 296.7 973.6
NOTE: A 1-inch pipe is too small to handle these valves. A
pipe increaser must be installed at the valve outlet.
Twin relief valves, mounted on a transfer valve,
are used on the condenser so that one relief valve
can be shut off and removed for testing or
replacement, leaving the other in operation. Only
one of the two valves is in operation at any time.
Figure 21, Typical Vent Piping
Vent piping is sized for only one valve of the set
since only one can be in operation at a time.
Relief Pipe Sizing (ASHRAE Method)
Relief valve pipe sizing is based on the discharge
capacity for the given evaporator or condenser and
the length of piping to be run.
Since the pressures and valve size are fixed for
McQuay chillers, the sizing equation can be
reduced to the simple table shown below.
The discharge from more than one relief valve can
be run into a common header, the area of which
shall not be less than the sum of the areas of the
connected pipes. For further details, refer to
ASHRAE Standard 15. The common header can
be calculated by the formula:
D
Common
DDD
n
=+
⎛
⎝
⎜
⎞
⎠
⎟
1
2
2
22
05
....
.
32 Catalog WMC-4