Bubbler tubes
provide a simple and inexpensive but less accurate (±1-2%) level measurement
system for corrosive or slurry-type applications. Bubblers use compressed air
or an inert gas (usually nitrogen) introduced through a dip pipe. Gas flow is
regulated at a constant rate (usually at about 500 cc/min). A differential
pressure regulator across a rotameter maintains constant flow, while the tank
level determines the back-pressure. As the level drops, the back-pressure is
proportionally reduced and is read on a pressure gage calibrated in percent
level or on a manometer or transmitter.
The dip pipe should have a relatively
large diameter (about 2 in.) so that the pressure drop is negligible. The
bottom end of the dip pipe should be located far enough above the tank bottom
so that sediment or sludge will not plug it. Also, its tip should be notched
with a slot or ÒVÓ to ensure the formation of a uniform and continuous flow of
small bubbles. An alternative to locating the dip pipe in the tank is to place
it in an external chamber connected to the tank.
In pressurized
tanks, two sets of dip pipes are needed to measure the level. The two
back-pressures on the two dip pipes can be connected to the two sides of a
u-tube manometer, a differential pressure gage or a d/p cell/transmitter. The
pneumatic piping or tubing in a bubbler system should be sloped toward the tank
so that condensed process vapors will drain back into the tank if purge
pressure is lost. The purge gas supply should be clean, dry, and available at a
pressure at least 10 psi greater than the expected maximum total pressure
required (when the tank is full and the vapor pressure is at its maximum). An
alternative to a continuous bubbler is to use a hand pump (similar to a bicycle
tire pump) providing purge air only when the level is being read.
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