BTH Electric Boilers
USE & CARE MANUAL
(Revision August /06)
, Page
7.
the
ASME Boiler and Pressure Vessel Code
and
limit the maximum operating boiler pressure. It is
a safety device, not an operating control.
The BTU per hour rating of the relief valve must
equal or exceed the BTU per hour input of the
boiler(s) or heat source(s) as marked on the
boiler(s) rating plate.
Connect the outlet of the relief valve to a
discharge line with its lower tip at most 6” above a
floor drain, well clear of any live electrical parts.
The discharge line must pitch downward from the
valve to allow complete draining by gravity of the
relief valve and discharge line, and be of a
diameter no smaller than that of the valve outlet.
The tip of the discharge line should not be
threaded or concealed and should be protected
from freezing. No valve of any type, restriction or
reducer coupling should be installed on the
discharge line. Local codes shall govern the
installation of relief valves.
SYSTEM PRESSURE CONTROL
Expansion tank
Pressure control devices within the system ensure
that each component operates within minimum
and maximum allowable pressures and maintain
minimum pressure for all normal operating
temperatures. They also allow air bleeding,
prevent cavitation at the pump inlet and prevent
water from boiling within the system; all this is
accomplished with minimal addition of new water.
The increase in boiler water volume resulting from
higher temperature is stored in the expansion tank
during periods of high operating temperature and
is returned to the system when the temperature
decreases.
The expansion tank must be able to store the
required volume of boiler water during maximum
design operating temperature without exceeding
the maximum allowable operating pressure, and
to maintain the required minimum pressure when
the system is cold. Contact your installing
contractor, plumbing supply house, or local
plumbing inspector for assistance.
The point where the expansion tank is connected
should be carefully selected to avoid the
possibility that normal operation of automatic
check or manual valves will isolate the tank from a
hot boiler or any part of the system. Pre-charged
diaphragm expansion tanks are preferable to air
control.
These tanks incorporate a balloon-like bladder or
diaphragm. It is inflated, prior to filling the system,
to a pressure equal to the setting of the water
pressure makeup regulator.
The expansion tank should be located on the
suction or intake side of the pump. The pump can
be located either just upstream or just
downstream from the boiler.
