Chemical Engineering April 2019 - 34

Cover Story
Part 1
Protecting Instruments
from Overpressure
Overpressure in a process can damage the seals in a pressure instrument's sensor or skew the
zero point. Here's how to avoid the problem
Endress+Hauser
Ehren Kiker
Endress+Hauser
P
IN BRIEF
PRESSURE
MEASUREMENT
PROCESS UPSETS
MANIFOLD ISSUES
TOUGH INSTRUMENTS
EXPANDING FILL FLUIDS
CERAMICS
ressure is one of the most
commonly measured variables
in the chemical process
industries (CPI). Accuracy
and, more importantly, stability are
critical to maintaining a safe, reliable
process and maximizing uptime. Modern
pressure instruments are highly
accurate and provide excellent longterm
stability, with a variety of diagnostics
to provide greater insight into
the process, but they are still highly
susceptible to overpressure events. By
better understanding the application
and the sources of overpressure, as
well as knowing options available from
manufacturers, process engineers can better
avoid the possibility of costly process problems
caused by overpressure.
FIGURE 1. Overpressure can damage a pressure instrument's sensor,
such as this differential pressure device
Pressure measurement
Pressure is typically measured by an instrument
with a sensor and a transmitter. The
sensor interfaces to the process to determine
the pressure and sends this value to
the transmitter. The transmitter converts the
signal from the sensor to a type suitable for
transmission to a control and monitoring
system, such as 4-20 mA or a digital fieldbus-communications
protocol.
Modern electronic pressure instruments
(Figure 1) are safe and reliable, with onboard
diagnostics and extremely sensitive sensors
that provide insight into the process and flexibility
in application. They can measure either
differential pressure (that is, the difference
between two pressures in a pipe or vessel) or
static pressure (gage or absolute pressure).
In addition to the sensor, a transmitter
consists of several important components.
The sensor body is the machined component
that houses the sensor. The sensor
diaphragm(s) serves to keep process fluid
from coming into direct contact with the sen34
sor,
while allowing the process pressure to
be measured by the sensor. The system may
also incorporate a manifold that allows the
transmitter to be isolated from the process
for removal or calibration without having to
shut down the process.
In many cases, the complete measuring
system also includes remote process seals
and capillaries with hydraulic fill fluid designed
to provide added protection to the
transmitter in the case of excessive process
temperatures, aggressive process media or
process media that could plug impulse lines.
While these measuring systems are highly
reliable, there are many common issues that
can affect pressure measurement and speed
of response, such as errors caused by ambient
or process temperature swings.
These issues can typically be addressed by
correct selection and installation (for instance,
proper fill fluid selection, insulating and heat
tracing of capillaries, and so on) of the pressure
instrument. But there is one common
issue that is difficult to avoid but can have
severe consequences not only for the sensor,
but for the process as well: overpressure.
Overpressure occurs when excessive
pressure is applied to one, or both in the
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