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Relief Valves
As a safety precaution and to meet code requirements, each chiller is
equipped with pressure relief valves located on the condenser,
evaporator, and oil sump vessel 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 (large condensers will have two such sets).
One valve remains active at all times and the second valve acts as a
standby.
Figure 12, Condenser 3-Way Valve
Refrigerant Vent Piping
Relief valve connection sizes are one-inch FPT and are in the quantity shown in Table 2 and Table 3 on page 22. 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. Where four valves are shown in the
table, they consist of two valves, each mounted on two transfer valves. Only two relief valves of the four are active at
any time.
Vent piping is sized for only one valve of the set 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
derived from it, is contained in ASHRAE Standard 15-2001.
