Chemical Engineering February 2012 - 34

Cover Story
Level bridle
The measuring range of a transmitter
is typically specified to be the same
as or slightly less than (usually 4-6 in.)
the measuring range of the local level
gage. This allows the output of the
transmitter to be directly compared to
the visual indicator to check its calibration
and ensure it is functioning
properly. The transmitter range should
rarely exceed the gage visible range,
since this makes verifying the high and
low limits of the transmitter in situ difficult,
if not impossible. Conversely, any
vessel volume that is viewable by the
local gage but not measurable by the
transmitter is generally not available
for use in the operating range.
The working operating range of the
liquid level should not extend past the
alarm points, and by definition cannot
extend past the shutdown points.
Typically the vessel percent full as
read by the level instrument correlates
to percent of level instrument span,
and does not equate to percent of actual
vessel volume. A vessel strapping
table should be developed and used to
convert percent level readings to percent
of vessel volume. Most modern
control systems can be programmed
with an equation to convert automatically,
if desired.
The alarm and shutdown setpoints
for high level generally should be no
greater than 90% of the transmitter
span. Similarly, the alarm and shutdown
setpoints for low level should be
no less than 10% of the transmitter
span. The reason for this is to ensure
that these critical points can be read
by the transmitter under all operating
conditions.
For example, when using a differential-pressure-style
level transmitter, a
10% decrease in the density of the process
fluid - such as might occur in vessels
containing fluid mixtures of varying
composition or temperature - can
result in the vessel being completely
full (filled above the top transmitter
connection), but the transmitter may
indicate only 90% output. This is because
the weight of the liquid column
between the transmitter connections is
less than that for which the transmitter
was designed, and the liquid above
the top connection exerts its weight
on both the top and the bottom connections
equally, so it does not create
Gage
glasses
Connections from the
level bridle to the vessel
and to the instruments
often have to be at
different elevations so
that the welds do not
interfere with each other
Level
transmitter
FIGURE 4. This level bridle with gage glasses and level transmitter on a vertical vessel
depicts a common (but generally not recommended) installation practice
a differential pressure to be measured.
If the alarm or shutdown setpoint is
greater than 90%, the transmitter may
be incapable of reading it during process
upset conditions when temperature
or composition changes affect the
liquid density.
The selection of the level technology
to be used should include consideration
of how the specific physical
property of the liquid being measured
may change during normal operating
or process upset conditions. If the liquid
density can change significantly,
then differential-pressure-style measurements
may not be the best choice.
If the electrical conductivity of the liquid
can vary, or is extremely low, then
technologies that rely on this property,
such as radar or capacitance, may not
be suitable.
Any shutdown setpoint should generally
be preceded by an alarm to alert
the operator to take action to bring
the process back into control and prevent
safe operating limits from being
exceeded. The difference between the
alarm and shutdown setpoints needs
to be adequate for the operator to have
response time to identify that an issue
exists, then diagnose and correct the
problem. The alarm and shutdown setpoints
also need to be sufficiently far
away from the normal operating level
that minor process upsets do not create
nuisance alarms.
To ensure that the level-instrument
design and the vessel process design
are consistent with each other, the
drum levels corresponding to low and
high alarms and shutdown setpoints
should be indicated on the process and
instrumentation diagram (P&ID).
32 CHEMICAL ENGINEERING WWW.CHE.COM FEBRUARY 2012
Level bridles
Level bridles are often used as a means
to mount multiple level instruments to
a single pair of vessel process connections,
rather than providing each level
instrument with independent connections
to the vessel. This is sometimes
unavoidable and necessary, especially
with existing pressure vessels or highpressure,
thick-walled vessels where
the addition of nozzles cannot be cost
justified.
Mounting instruments to a level
bridle generally requires that the elevations
of the process connections (for
each instrument to the bridle) be staggered
to avoid interference with each
other, and this results in a loss of usable
vessel volume as the instrument
span is narrowed. This consequence
can be avoided if each instrument has
its own connections to the vessel.
In addition, avoiding the use of level
bridles will reduce the total number of
welds required, generally reduce the
number of valves required to install
all the instruments, and will virtually
eliminate an interface between
the process control and piping design
disciplines. All of these factors reduce
the complexity and cost of the levelinstrument
installation.
Level instrument best practices
In order to maximize the volume of a
vessel that is usable and within the
measurable and controllable limits of
the instrumentation, take the steps
outlined here. Each of these three
steps will add approximately 6 in. to
the usable operating range of the liquid,
or about 18 in. total.
1. Use magnetic level gages instead of
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Chemical Engineering February 2012

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