A.O. Smith HW 300 Instruction Manual page 10

PRESSURE RELIEF VALVE
An ASME rated pressure relief valve is furnished with the boiler.
A fitting for the pressure relief valve is provided in the top of the
boiler. Never operate the heating elements without being certain
the boiler is filled with water and a properly sized pressure relief
valve is installed in the pressure relief valve opening provided.
The pressure rating of the pressure relief valve should be equal
to or less than the rated pressure capacity of any component
in the system including the boiler. Should the valve need to be
replaced, call the toll free phone number listed on the back of this
manual for further technical assistance.
A discharge pipe from the pressure relief valve should terminate
at an adequate floor drain. Do not thread, plug, or cap the end of
the drain line.
CAUTION
Water Damage Hazard
•
Pressure Relief Valve discharge pipe must
terminate at adequate drain.
The Discharge Pipe:
• Shall not be smaller in size than the outlet pipe size of the
valve, or have any reducing couplings or other restrictions.
• Shall not be plugged or blocked.
• Shall not be exposed to freezing temperatures.
• Shall be of material listed for hot water distribution.
• Shall be installed so as to allow complete drainage of both
the pressure relief valve and the discharge pipe.
• Must terminate a maximum of six inches above a floor
drain or external to the building. In cold climates, it is
recommended that the discharge pipe be terminated at an
adequate drain inside the building.
• Shall not have any valve or other obstruction between the
pressure relief valve and the drain.
Once the boiler is installed and filled with water and the system
is pressurized, manually test the operation of the pressure relief
valve.
Explosion Hazard
Pressure Relief Valve must
comply with ASME code.
Properly sized Pressure Relief
Valve must be installed in
opening provided.
Can result in overheating and
excessive tank pressure.
Can cause serious injury or death.
If any pressure relief valve is replaced, the replacement valve
must comply with the current editions of the ASME Boiler and
Pressure Vessel Code, Section IV or CSA B51, as applicable.
Select a pressure relief valve with a discharge NOT less than the
boiler input, and a pressure rating NOT exceeding the working
pressure of any component in the system.
A. O. Smith supplies a 125 psi pressure relief valve for hot water
supply systems and 50 psi pressure relief valve for space heating
application.
An ASME rated temperature and pressure relief valve must be
installed on each and every water storage tank in a hot water
supply system.
The storage tank temperature and pressure (T & P) relief valve
must comply with the applicable construction provisions of the
Standard for Relief valves and Automatic Gas Shutoff Devices
for Hot Water Supply Systems, Z21.22 - CSA 4.4. The T & P
valve must be of the automatic reset type and not embody a
single-use type of fusible plug, cartridge or linkage.
The T & P relief valve should have a maximum temperature
rating of 100°C (210°F), a pressure rating NOT exceeding the
lowest rated working pressure of any system component, and a
discharge capacity exceeding the total input of the water boilers
supplying water to the storage tank.
Locate the T & P relief valve (a) in the top of the storage tank or
(b) in the side of the tank on centerline within upper 6 inches from
the top of the tank. See Figure 29 to Figure 42 on Pages 47 to 60.
Tapping shall be threaded in accordance with the latest version
of the Standard for Pipe Threads, General Purpose (inch), ANSI/
ASME B.120.1.
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