Hydrocarbon Processing - June 2021 - 75

Process Controls,
Instrumentation
and Automation
C. OBERLE and T. BONKAT, Emerson Automation
Solutions, Shakopee, Minnesota
Key instrumentation technologies to tackle
the toughest measurement challenges
Just as some people seek out extreme sports, process manufacturing
has its extreme applications, and engineers who design
for these environments must find ways to safely contain and
monitor all manner of dangerous reactions and products. Consider
a process where two feedstocks come together, generating
a powerful exothermic reaction. The equipment must withstand
what is, in effect, a continuous explosion capable of generating
enormous amounts of heat and pressure, while ensuring safety
for products that may be both highly corrosive and toxic.
Such situations are well beyond the capabilities of gardenvariety
equipment, including instrumentation. The need for
accurate and reliable measurements is crucial to control critical
processes and avoid serious safety incidents. If the continuous
explosion cannot be monitored effectively, it could break out of
its constraints, causing serious damage.
Extreme applications take many forms, some potentially violent,
flammable and toxic, while others have more subtle hazards,
but all these still have the potential to damage or shorten
the service life of conventional process instruments. In this article,
the authors will examine five types of difficult conditions.
These include:
* Excessive vibration
* Difficult flows
* Demanding process fluids
* Extreme temperatures
* Severe pressures.
For each condition, the authors will consider what makes
the process extreme, and then look at pressure and temperature
instruments and associated tools designed to tame them. Most
of the solutions involve specialized materials, physical design
modifications and sensor signal processing, supported by software
able to analyze the situation and offer practical alternatives.
PART 1: VIBRATION
In a refinery or chemical plant, there are few, if any, situations
where vibration is desirable, so maintenance teams go to great
lengths to minimize it. With rotating equipment (e.g., motors
and pumps), excessive vibration indicates misalignment, failing
bearings or other problems. If excessive vibration is transferred
from a pump to adjacent piping, it can cause fittings and flange
FIG. 1. A proprietary wireless pressure gaugea
the mechanical version but with multiple advantages.
does the same job as
bolts to loosen, creating leaks.
Such vibration is tough on mechanical instruments, especially
pressure gauges, which operate using a delicate mechanism with
springs and gears, making them vulnerable to shock and damage.
Most operators have seen typical results of vibration: leaking
bourdon tubes, bent indicator needles or stuck needles from
broken gears. Some models are armored with rubber covers and
use beefed-up mechanisms, but these options add cost and have
limited effectiveness.
Many users like the functionality of a traditional pressure
gauge, but not its finicky mechanism and instability. Electronic
gauges (FIG. 1) use an electronic sensor to provide the capabilities
of a full process transmitter. Some include device diagnostic
functions and WirelessHART connectivity, but in an analog
gauge form factor with a traditional needle display. This approach
checks the boxes for most-wanted features, without the
problems of traditional pressure gauges.
Fluid-induced vibration. Vibrations can be created by flowing
fluids in the piping configuration, causing wake shedding and
peculiar eddy currents. This situation is very common where a
thermowell is inserted into the fluid stream-perpendicular to
the flow-to provide a temperature reading. Vortices form on
both sides of the thermowell and create high- and low-pressure
areas capable of inducing vibration (FIG. 2).
Hydrocarbon Processing | JUNE 2021 75
Hydrocarbon Processing - June 2021

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