Hydrocarbon Processing - June 2021 - 80

Process Controls, Instrumentation and Automation
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Process temperature, °C (°F)
FIG. 8. Pressure and temperature figure into selection of a fill fluid in a
high-vacuum application.
where the length of the capillary lines allows the fill fluid to come
to ambient temperature, it can slow response or cut it off entirely.
Some stand-off mounts use two fill fluids, one optimized for
the process temperature and the other for the ambient temperature.
Each fill fluid is tailored to the application and environment,
eliminating the need for heat tracing or other protection methods.
With appropriate fill fluids, dual-fluid mounts can be used for
temperatures up to 410°C (770°F) or down to -105°C (-157°F).
PART 5: SEVERE PRESSURES
Like temperatures, extreme pressures can be high or low,
and both ends of the spectrum present their unique challenges.
A pressure gauge or transmitter must be able to operate continuously-potentially
for 1 yr-within its expected operating
range, and it should also withstand pressure spikes. Countless
mechanical gauges have suffered damage in this way due to their
delicate internal mechanisms. A bourdon tube pushed past its
limits will not return to the correct shape or may simply burst.
Fortunately, electronic transmitters provide more robust construction,
allowing them to absorb severe increases in process
pressure and return to normal operation as if nothing happened.
Most general process instrumentation providers max out at
20,000 psi (1,379 bar), since applications beyond this are rare
and highly specialized. A proprietary electronic pressure gaugea
does not have a conventional mechanical mechanism. Instead, it
uses the same sensor technology as electronic transmitters, thus
avoiding common traditional gauge failures. This pressure gauge
can measure up to 10,000 psi (690 bar), with overpressure protection
up to 1.5 times its range.
When a more conventional pressure transmitter is needed,
in-line pressure transmitters cover a variety of ranges-up to
20,000 psi (1,379 bar) gage pressure or absolute pressure. Many
are SIL 2/3 certified for use in the most safety critical applications.
In addition, many include basic and advanced device diagnostic
capabilities.
Panel-mounted transmitters can also handle up to 20,000 psi
(1,379 bar) gage pressure or absolute pressure. These devices
are compact, lightweight and can come with all-welded, stainless-steel
construction, providing a stable and robust solution
for harsh environments.
Tough dP flow measurement. Consider the challenge of measuring
flow of a high-pressure line-e.g., 5,000 psi (345 bar)-
using a dP transmitter and conventional primary element. There
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will be a pressure drop across the primary element, but if the application
is engineered well, it should only be around 30 psi (2 bar)
at normal operating range. This presents a serious challenge for
the transmitter to produce a precise reading of a very small pressure
differential in an environment where the static pressure can
easily be hundreds of times more than the differential.
Some dP transmittersf
are designed exactly for this scenario.
They have a modest measuring range up to 150 psi (10.3 bar)
but can withstand static line pressures up to 15,000 psi (1,034
bar). Their construction can resist overpressure and line pressure
effects to maintain high accuracy and stability, even in
harsh environments.
Working in a vacuum. Low pressures are anything below atmospheric:
14.7 psi (1 bar) absolute. Measurements made in
these conditions still use the same instruments and associated
equipment, including diaphragm seals, but special consideration
must be given to the fill fluid. When enough vacuum is applied to
a liquid, it can reach its boiling point and vaporize. If this happens
to the fill fluid, it can cause the whole system to fail. These issues
are exacerbated when ambient or process temperatures are high.
When the process is under vacuum conditions, the fill fluid
will vaporize under a lower temperature than when it is operating
under normal atmospheric or greater pressure. Fortunately,
there are a variety of fill fluids for remote seal systems, each with
a specific vapor-pressure curve (FIG. 8) indicating the pressure
and temperature relationship where a fluid remains in a liquid
state. When a diaphragm seal is used in a vacuum application, allwelded
construction avoids drawing air into the capillary system.
The right tools. Any reasonable person participating in extreme
activities, whether for pleasure or work, will want to have
the right protective equipment because it is critical to success
and even survival. In a similar manner, products for extreme applications
are designed to ensure reliable performance and safety
in the most challenging applications. The technologies and practices
suggested here are designed to improve instrumentation
performance, increase reliability and reduce maintenance. None
require major capital projects, nor do they have to be executed
on a grand scale. Each improvement can be implemented oneby-one
to deliver incremental gains and a quick return on investment,
eliminating obstacles to effectiveness and profitability.
NOTES
a Rosemount™ wireless pressure gauge
b Emerson's Thermowell Design Accelerator
c Rosemount Twisted Square™ thermowells
d Emerson's dP Flow Sizing and Selection Tool
e Rosemount X-well™ technology
f Rosemount 3051S High-Static DP Transmitter
CONNOR OBERLE is a Global Pressure Product Manager
for Emerson's Automation Solutions business in Shakopee,
Minnesota, with responsibility for Rosemount pressure
transmitters. He earned a BS degree in mechanical engineering
from the University of North Dakota.
TIMCHAN BONKAT is a Global Temperature Product Manager
for Emerson's Automation Solutions business in Shakopee,
Minnesota, with responsibility for Rosemount sensors and
thermowells. He earned a BS degree in electrical engineering
from St. Cloud State University.
Vapor pressure, psia
Vapor pressure, mbara (torr)
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Hydrocarbon Processing - June 2021

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