Chemical Engineering April 2019 - 35

Endress+Hauser
case of differential pressure, of the sensor
diaphragms of the instrument. This can be
either directly to the process diaphragm of
the instrument, or to a diaphragm seal either
directly connected to the instrument, or remotely
connected via a capillary with a noncompressible
fill fluid.
Typically, there are three main causes of
overpressure in a process: process upset
conditions, excessive cycling of valves and
incorrect use of manifolds.
Process upsets
Process upset conditions occur when the
pressure, temperature or both exceed the
acceptable limits for the pressure instrument's
sensor. This could occur during normal
operation but is most likely to occur
during startup or shutdown. The most obvious
effect occurs when there is an actual
overpressure applied to the sensor. The less
obvious, but more likely cause, is when the
transmitter experiences elevated ambient or
process temperatures. This can cause the fill
fluid in the seals and capillaries to expand,
while also increasing the internal pressure of
the fluid such that an overpressure is applied
to the sensor.
Even when process conditions are within
acceptable limits, there is the possibility of
overpressure being applied due to cyclic
pressures on the sensor. The repeated " on/
off " cycling of pressure from a high pressure
to a low pressure can have the same effect
as overpressure, causing damage to both
the diaphragm and the sensor.
Manifold issues
More common than process upset conditions
is when manifold valves (Figure 2) are
improperly closed and opened. Manifolds
are used to isolate the instrument's sensors
from the process for re-zeroing/recalibrating
or removing the instrument without the need
to shut down the process. Three-valve and
five-valve manifolds are regularly used with
differential pressure (DP) instruments on DP
flow or DP level applications.
When closing the process manifold valves,
it is important to make sure the valves are
fully
closed and seated
before opening
the equalizing valve. A common mistake is
opening the equalizing valve before completely
closing one or both process valves,
allowing line pressure to leak past the valve
and impact the sensor diaphragms. Even
the most seasoned instrument technician
has probably made this mistake on more
than one occasion.
In the best-case scenario, an overpressure
event has no discernible effect on the
sensor's measurement. Most overpressure
events, however, result in a shift of the zeropoint
of the sensor, resulting in the need to
re-zero the instrument's transmitter. In some
cases, the effect can be severe enough that
the instrument must be recalibrated - most
likely by removing it from the process and
placing it on a bench calibrator.
While neither of these types of incidents
should require replacement of the instrument,
if left unchecked they can have unforeseen
impacts on process operations
that could result in unplanned shutdowns. In
addition, repeatedly having to re-zero or recalibrate
a pressure instrument due to overpressure
events uses manpower that could
be employed elsewhere in the facility.
In situations where the overpressure event
is severe enough or prolonged enough, the
diaphragm or the sensor can be irreparably
damaged. When this occurs, the only option
is to replace the damaged instrument. It can
be especially costly and time consuming if
the instrument has remote diaphragm seals
and capillaries.
Tough instruments
While the effects of overpressurizeation sound
dire, it's incorrect to assume that all instruments
are built the same way and have the
same limitations when faced with an overpressure
event. It's important to understand if the
specific instrument has
internal systems
to minimize
the effects of
overpressure,
or if the sensor
itself can
handle high
overpressure
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APRIL 2019
FIGURE 3. Remote seals, as
on this pressure instrument,
protect the sensors
FIGURE 2. Five-valve manifold
(to left of transmitter) is
used to isolate a pressure
instrument's sensor so it can
be removed from the line, rezeroed
or calibrated
Endress+Hauser
35
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Chemical Engineering April 2019

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