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When one or
both impulse lines to a differential pressure device are filled with a
stable, process compatible fluid, the installation is called a "wet-leg"
installation. The net effect of the legs' height above the instrument
and specific gravity of the fluid must be considered in the calibration.
Wet leg design must also allow for the filling and draining of the leg(s).
Seal pots are used with wet legs when the instrument displaces a large
volume of liquid as the measurement changes. A seal pot is a small
pressure vessel about one quart in volume that is mounted at the top of
the wet leg line. If two wet legs are used in a differential
application, the pots must be mounted at the same elevation. Each pot
acts as a reservoir in the impulse line where large volume changes will
result in minimal elevation change so that seal liquid is not dumped
into the process line and elevation shifts of the wet leg liquid do not
cause measurement errors.
Seal Pot used
when measuring steam pressure or in processes in which a vapor will
condense into liquid at ambient temperature. Acts as a condensate
chamber and provides a large area of liquid contact between process and
measuring instrument. Lead lines to the measuring instrument should be
full of liquid to protect the instrument from high temperatures. In some
cases, the seal pot, line and instrument are filled with a sealing fluid
to prevent freezing. When a sealing fluid is used, the measuring
instrument should be zeroed with the lines full of the seal fluid; ‘wet
leg’ installation
Dry & Wet
Leg Designs
When measuring the level in pressurized tanks, the same d/p cell designs
(motion balance, force balance, or electronic) are used as on open
tanks. It is assumed that the weight of the vapor column above the
liquid is negligible. On the other hand, the pressure in the vapor space
cannot be neglected, but must be relayed to the low pressure side of the
d/p cell. Such a connection to the vapor space is called a dry leg, used
when process vapors are non-corrosive, non-plugging, and when their
condensation rates, at normal operating temperatures, are very low . A
dry leg enables the d/p cell to compensate for the pressure pushing down
on the liquid's surface, in the same way as the effect of barometric
pressure is canceled out in open tanks.
It is important to keep this reference leg dry because accumulation of
condensate or other liquids would cause error in the level measurement.
When the process vapors condense at normal ambient temperatures or are
corrosive, this reference leg can be filled to form a wet leg. If the
process condensate is corrosive, unstable, or undesirable to use to fill
the wet leg, this reference leg can be filled with an inert liquid.
In this case, two factors must be considered. First, the specific
gravity of the inert fluid (SGwl) and the height (hwl) of the reference
column must be accurately determined, and the d/p cell must be depressed
by the equivalent of the hydrostatic head of that column [(SGwl)(hwl)].
Second, it is desirable to provide a sight flow indicator at the top of
the wet leg so that the height of that reference leg can be visually
checked.
 Any changes in leg fill level (due to leakage or
vaporization) introduce error into the level measurement. If the
specific gravity of the filling fluid for the wet leg is greater than
that of the process fluid, the high pressure side should be connected to
the reference leg and the low to the tank.
If the condensate can be used to fill the reference leg, a condensate
pot can be mounted and piped both to the high level connection of the
tank and to the top of the vapor space. The condensate pot must be
mounted slightly higher than the high level connection (tap) so that it
will maintain a constant condensate level. Excess liquid will drain back
into the tank. It is also desirable either to install a level gage on
the condensate pot or to use a sight flow indicator in place of the pot,
so that the level in the pot can conveniently be inspected.
Either method (wet or dry) assures a constant reference leg for the d/p
cell, guaranteeing that the only variable will be the level in the tank.
The required piping and valving must always be provided on both the tank
and the reference leg side of the d/p cell, so that draining and
flushing operations can easily be performed. When a wet reference leg is
used, a low thermal expansion filling fluid should be selected.
Otherwise, the designer must correct for the density variations in the
reference leg caused by ambient temperature variations.
If smart transmitters are used and if the filling fluid data is known,
wet-leg temperature compensation can be provided locally. Alternatively,
the host or supervisory control system can perform the compensation
calculations.
If it is desired to keep the process vapors in the tank, a pressure
repeater can be used. These devices repeat the vapor pressure (or
vacuum) and send out an air signal identical to that of the vapor space.
The measurement side of the repeater is connected to the vapor space and
its output signal to the low pressure side of the d/p cell. If the tank
connection is subject to material build-up or plugging, extended
diaphragm Type 1:1 repeaters can be considered for the service (Figure
7-2).
While repeaters eliminate the errors caused by wet legs, they do
introduce their own errors as a function of the pressure repeated. For
example, at 40 psig, repeater error is about 2 in. At 400 psig, it is 20
in. In many applications, the former is acceptable but the latter is
not.
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