Hydrocarbon Processing - June 2021 - 78

Process Controls, Instrumentation and Automation
fluid getting into impulse lines or the pressure instrument itself.
These thin diaphragms have unique flexing characteristics to
provide the necessary measuring sensitivity and accuracy. Fortunately,
diaphragms can be manufactured from a variety of materials,
including stainless-steels, tantalum and Ni-based alloys
such as alloy C-276 or alloy 400.
All diaphragm materials have their limits for protection
against abrasive fluids. The durability of these diaphragms can
be increased with coatings of hard-carbon nanostructured material.
The micro-thin coating withstands abrasion, extending
service life up to 10 times compared to a standard 316 stainlesssteel
diaphragm in abrasive service but does not impact remote
seal accuracy or sensitivity. The range of coating options also
includes a variety of metallic and polymer materials.
Another problem is a seal's permeability to atomic hydrogen
if present in the process fluid. Hydrogen atoms can migrate
through the diaphragm and once in the fill fluid, form molecular
hydrogen. Because molecular hydrogen is too large to permeate
back through the diaphragm, it gets trapped and forms hydrogen
bubbles in the fill fluid. These bubbles can severely affect
transmitter performance. Depositing a 5-micron thick layer of
gold to a stainless-steel diaphragm provides protection against
hydrogen permeation.
Avoiding wetted parts. Where conditions make adding a conventional
thermowell impractical, some users clamp a temperature
sensor to the pipe. This provides a reading, but heat dissipation
keeps the external temperature from fully reaching the
internal value. Adding insulation can help, but the reading will
likely never reach the degree of accuracy needed and changing
external conditions will cause unpredictable fluctuations.
Specially designed temperature transmitterse
make measurements
outside of the pipe while solving the normal problems by
using a different methodology to correct for heat loss (FIG. 5).
These transmitters use an algorithm to compensate for heat
transmission and external conditions. The user enters factors
for the pipe material and thickness, and the instrument provides
a process temperature measurement, often as accurate as a traditional
thermowell installation.
When clamped on a pipe, the bracket holds a resistance
temperature device (RTD) in contact with the pipe surface to
ensure consistent heat transfer. Once insulated and operating,
an algorithm compares the reading from the pipe's RTD to a
second RTD in the transmitter. These two readings are continuously
compared to precisely calculate the temperature inside
the pipe, even when ambient and other conditions change significantly.
The result is a highly-accurate temperature reading
without the need for a process penetration.
Designed for durability. Flammable or dangerous fluids require
equipment designed with features that ensure reliable and
safe operation in these environments. This often calls for construction
using industrial strength proportions able to take the
punishment, while allowing for changing out parts when necessary,
with minimal disruption. Since dP flowmeters are frequently
used in tough refinery service, this technology has been
beefed-up to withstand the abuse.
This flowmeter is built upon a fully welded spool section
with built-in isolation valves and temperature input. It comes
fully assembled to the dP transmitter and is leak tested at the factory,
and it uses a multi-orifice primary element to minimize the
need for upstream and downstream straight pipe run. The impulse
lines are short, and the dP transmitter is close-coupled to
minimize the potential for plugging. The impulse lines also have
gate-type isolation valves and clean-out ports so they can be rodded
out while the unit is in operation. The entire dP transmitter
can be removed and replaced without a process shutdown.
The assembly can include an integral temperature sensor
that sends its information to the main dP transmitter, which can
report the value to the automation host system. When the fluid
temperature is combined with the dP volumetric flowrate and
a known fluid density, the transmitter can provide a mass flow
measurement.
The dP transmitter can also provide static line pressure values
to enhance the mass flow measurement without the need for
an additional pipe penetration. A setup such as this can typically
handle fluids up to 315°C (600°F) or higher, depending on the
application and installation.
PART 4: EXTREME TEMPERATURES
When considering difficult temperatures, hot usually comes
first, but cold also presents challenges, particularly in cryogenic
ranges. With either extreme, the first step is protecting electronics
since circuit boards and components have limited temperature
operating ranges.
As for instrument configurations, many of the same techniques
used to protect from aggressive fluids protect from cold
and hot temperatures, although there are some specialized variations.
The following will detail how these solutions work together.
FIG. 5. A specially designed temperature transmittere
provides
a highly accurate process temperature reading through a pipe wall
without a penetration.
78 JUNE 2021 | HydrocarbonProcessing.com
Low environmental temperatures. The cold end of the
temperature range is doubly problematic because it can reflect
not only the process fluid, but also the environment. Some oil
production areas in Canada and Russia routinely experience
winter temperatures of -40°C (-40°F), which affect equipment
as well as people.
Many electrical devices work better in mildly colder weather.
However, problems begin to develop when sophisticated
http://www.HydrocarbonProcessing.com
Hydrocarbon Processing - June 2021

Table of Contents for the Digital Edition of Hydrocarbon Processing - June 2021
