 The
simplest and least costly method of liquid level indication
is the tubular gage glass. Two slightly different designs are
available. Both are
simply transparent vertical tubes with their lowest visible
point connected to the tank or boiler at the lowest level of
interest. The top of the glass may be open to the atmosphere
if the tank is open or to the unfilled part of a closed vessel
above or at the highest level permitted. Isolating valves are
placed above and below the gage glass connections.
One choice is a gage glass with slow closing valves and the
second choice are valves with quick closing type where a one
quarter turn of the valve spindle will change the valve from
the fully open to the fully closed position. The valve spindles
are fitted with levers to which chains may be attached in order
to operate the valves from ground level if the vessel is located
at a higher position. Drain valves or cocks may also be installed
below the gage glass to remove any solid material that may
collect.
Since
many tanks and pressurized vessels are not under continuous
supervision, a broken gage glass may allow a large amount of
fluids to escape. To prevent this, the lower valve on the gage
glass is often equipped with a safety shutoff device consisting
of a stainless steel ball which closes off the fluid passage
when the glass breaks. One disadvantage of this type of safety
device is that it requires more maintenance.
Under normal conditions, the steel ball remains in the recess
in front of the valve seat. However, when the gage glass breaks,
the sudden rush of fluid through the valve will force the ball
against the valve opening. This shuts off the flow out of the
broken glass.
The gage glass is usually surrounded by a number of metal
rods or transparent shield to protect the glass from breakage
and the operator from flying particles in case the gage glass
shatters.
The use of tubular gage glasses is limited to lower pressures
and temperatures, and restricted to non-toxic and non-hazardous
material. Tubular gage glasses should not exceed 750 mm in
length. If the level range to be observed exceeds this length,
then two or more gage glasses should be installed so that they
overlap.
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