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 Pressures from 450 to 2000 psi - Steam generating boilers are equipped with at
least one gage glass. It indicates the level of water  in
the boiler so the operator can be assured that the level is
within safe limits. The level must be high enough to completely
cover all parts of the heating surface to prevent overheating
and low enough so that water is not carried over with the steam.
All such boiler installations must comply with ASME and CSA
regulations. See
Picture of FG909 (shown without end stems)
Note the drain valve which permits all the connections
to be blown through daily to be sure that they are not plugged
with sludge or sediment. The valves can be slow or quick opening.
This method of attachment is commonly used only on boilers
that provide a straight vertical surface, unobstructed by reversing
chamber or smoke box, such as the cast-iron heating boiler
or some types of vertical boilers. See
Picture of FG909 (shown with end stems)
A more usual construction is to have the gage glass connected
to a water column which
in turn is connected to the boiler. The water
column acts as a reservoir to dampen agitation in the water.
In addition, the column traps any sludge or sediment and prevents
it from collecting in the glass connections. The column also
provides a place for installation of high and low level alarms
and controls. Trycocks are installed
on the column to provide a means of point level detection when
the gage glass is being replaced.
Note that the bottom of the gage glass must be 76 mm
above the top row of firetubes, flue or crownsheet, to prevent
overheating. This will ensure that the tubes are covered even
when the gage glass shows zero level. Heating boilers have
a lowest permissible water level marked by the manufacturer.
The lowest visible level in the gage glass should be 50 mm
or more above this marker.
When a water column houses
an alarm, a water level control, or a low-water
fuel cutoff device, isolating valves are not permitted
in the piping between the column and the boiler.
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