Specifications

IMM WMC-2 Centrifugal Chillers 17
Relief Valves
As a safety precaution and to meet code requirements, each
chiller is equipped with pressure relief valves located on the
condenser and evaporator for the purpose of relieving excessive
refrigerant pressure (caused by equipment malfunction, fire,
etc.) to the atmosphere. Most codes require that relief valves be
vented to the outside of a building and this is a desirable
practice for all installations. Relief piping connections to the
relief valves must have flexible connectors.
Note: Remove plastic shipping plugs (if installed) from
the inside of the valves prior to making pipe connections.
Whenever vent piping is installed, the lines must be run in
accordance with local code requirements; where local
codes do not apply, the latest issue of ANSI/ASHRAE
Standard 15 code recommendations must be followed.
Condensers have two relief valves as a set with a three-way
valve separating the two valves. One valve remains active at all
times and the second valve acts as a standby.
Figure 13, Condenser 3-Way Relief Valve
Refrigerant Vent Piping
Relief valve connection sizes are one-inch FPT and are in the quantity shown in Table 1 and Table 2 on page
8. Twin relief valves mounted on a transfer valve are used on the condenser so that one relief valve can be
shut off and removed, leaving the other in operation. Only one of the two is in operation at any time.
Vent piping is sized for only one valve of the set (but connected to both) since only one can be in operation at
a time. In no case would a combination of evaporator and condenser sizes require more refrigerant than the
pumpdown capacity of the condenser. Condenser pumpdown capacities are based on the current
ANSI/ASHRAE Standard 15 that recommend 90% full at 90°F (32°C). To convert values to the older ARI
standard, multiply pumpdown capacity by 0.888.
Sizing Vent Piping (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. Discharge capacity for R-134a vessels is calculated using a complicated equation
that accounts for equivalent length of pipe, valve capacity, Moody friction factor, pipe ID, outlet pressure and
back pressure. The formula and tables are contained in ASHRAE Standard 15-2001.
The McQuay WMC centrifugal units have a relief valve setting of 200 psi.