Hydrocarbon Processing - June 2021 - 77

Process Controls, Instrumentation and Automation
stacks and deep-sea risers to solve similar problems. This type
of thermowell does not depend on a specific orientation when
inserted and reduces the need for excessively-thick thermowells
and large-diameter process penetrations.
PART 2: CHALLENGING FLOWS
Applications can become extreme based on fluid flow characteristics,
with these three areas posing challenges for instrumentation:
high-velocity flows, wide flow-turndown range and
large line sizes.
High velocity. Historically, piping guidelines called for liquid
velocities below 7 ft/sec to avoid excessive pressure loss, pipe
wear and high pumping costs. Much of this practice has been
abandoned as too expensive, with engineers increasing flowrates
and/or using smaller pipes to save initial costs, resulting in
higher velocities. The following are some ways velocity-related
problems can be avoided or mitigated.
High-fluid velocities are particularly problematic for flow
measurement. Many plants that routinely use differential pressure
(dP) methods experience this directly. For those situations,
there are straightforward cures. First, it is important to minimize
the pressure drop across the primary element. Second, velocity
gradients within the pipe increase with velocity, so an averaging
reading across the full cross-section must be taken to ensure an
accurate measurement, especially with large line sizes.
One way to solve both problems is to use a specific type of primary
element design instead of a conventional single orifice. An
averaging pitot tube sensor is minimally intrusive in the pipe, can
work symmetrically for bi-directional flow and self-averages velocity
gradients for high accuracy. It is suitable for liquid, gas and
steam with velocities up to 300 ft/sec, and determines volumetric
flow-which can be combined with a temperature measurement
and known fluid density characteristics-to calculate mass flow.
Widely variable flow range. Most process units are designed
to operate within a relatively narrow production range. With
that knowledge, engineers can size instruments to fall in the
measuring sweet spot during normal operation, but applications
with a wide flow range can still create challenges. Coriolis
flowmeters have an especially wide flow range but are not suitable
for every application.
When conventional dP flow measurement techniques are
used, some users resort to an outdated practice of double-stacking
two dP transmitters with different measuring ranges on the
same primary element. This works but is cumbersome for installation,
maintenance and signal processing.
Today's dP transmitters are available with electronics that
extend the measuring range to keep the percentage error range
far more uniform. This avoids the problem of reduced accuracy
at the low end of the range due to percentage-of-span accuracy
characteristics.
Large line sizes. While many flowmeter types are highly scalable,
creating versions for large pipe sizes can get expensive. An
averaging pitot tube sensor is very well suited to large line sizes
since it can be built for cross-sections up to 96 in.
Temperature measurements have different complications.
Large line sizes call for long thermowells able to reach to the
pipe center, but these are especially vulnerable to VIV. Again, using
square helical thermowells verified by suitability calculations
makes these difficult applications far easier to implement.
Help with dP flowmeter selection. DP meters have long
been the most widely used technology for flow measurements
thanks to their range of configurations and adaptability, but they
can present challenges for a novice instrumentation engineer trying
to choose from the variety of primary elements. Which best
suits the application? Fortunately, any instrumentation engineer
trying to make the best selection can use new online software
toolsd
to simplify the choice. These tools streamline product sizing
and configuration by generating flow calculations faster and
with high accuracy.
Once the initial operating scope parameters are settled and the
specific application is identified (instrument location, tag number,
etc.), a flowmeter or primary element selection must begin with
a detailed understanding of the application conditions, including
piping size, process fluid and normal operating parameters.
Once these variables have been characterized, more subjective
elements come into play. Questions to ask include:
* What degree of accuracy and turn-down range is expected?
* How much pressure loss can be tolerated?
* How easy is it to install a given type of primary element?
* How much straight pipe run is available and practical
to deliver accurate readings?
These tools augment the limited experience of younger engineers,
while expediting the selection process for their more experienced
counterparts. The final presentation includes a table
of data illustrating the operating characteristics in the application
context. When the process is complete, the designer will receive a
full configuration description and part number based on vendor
catalog data.
PART 3: DEMANDING PROCESS FLUIDS
When considering instrumentation, it is important to look at
what is flowing through the pipes-the process fluid itself-liquid,
gas or steam. Characteristics that make a fluid extreme include
some mix of abrasiveness, usually due to fine particulates
or corrosiveness, due to highly acidic or caustic process media,
or one containing aggressive chemicals, such as chlorine.
Some products can be highly toxic, flammable or environmentally
dangerous, but these characteristics do not necessarily
attack instruments and equipment. This section will concentrate
on elements able to affect instruments and equipment directly.
The damage caused by these fluids is loss of metal, either
eaten away chemically, worn away by abrasiveness or both. Solutions
call for materials that are chemically inert relative to the
fluid and/or hard enough to withstand the constant scraping action.
When materials reach their limits, the solution may call for
ways to live with a shorter service life.
For corrosiveness, the solution often requires wetted parts in
a specialized alloy designed to withstand the service, including
a large family of stainless-steels with high nickel (Ni), molybdenum
and chromium content. However, the answer is not always
so simple. Pressure instruments are particularly vulnerable to
attack, so this calls for a closer look.
With severe fluids, containment is very critical, so engineers
often use remote seals to eliminate the possibility of process
Hydrocarbon Processing | JUNE 2021 77
Hydrocarbon Processing - June 2021

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