Chemical Engineering March 2019 - 51

VORTEX METERS
Vortex flowmeters work on the vortex shedding principle as it was observed and noted by
Theodore von Kármán, a Hungarian-American physicist. He first described the effect he observed
when a non-streamlined piece of material - often called a bluff body - is placed in a
fast-flowing stream. The liquid, or steam, will separate from the object on its two downstream
sides; when detached, it will curl back on itself, forming vortices, also called eddies or whirlpools.
You can see this principle in action if you put a rock in a fast-moving stream, for example.
On the side of the bluff body where the vortex is formed, the fluid velocity will be higher and the
pressure lower. As the vortex moves downstream, it gets stronger and bigger, eventually detaching
or shedding itself. A vortex is then formed on the other side of the bluff body. The alternating
vortices are spaced at equal distances. The frequency of vortices shedding is what is measured.
A vortex meter has no moving parts, which makes it highly suitable for corrosive materials.
The meter has external sensors that sense the vortex shedding indirectly through the force
exerted on the shedder bar - the bluff body - that is inside the meter.
Vortex meter sensors are modular, inexpensive and easy to replace. They can operate
under a wide range of temperatures - from cryogenic liquids to superheated steam.
New developments
New configuration designs are making implementation of SIL-rated vortex sensors easier and
safer for use in extreme or aggressive applications governed by stringent safety requirements.
Today, there is the dual vortex option and the newer quad vortex option (Figure 1). Both
have strong capabilities and fit a variety of systems and applications.
The dual vortex is a safe and simple drop-in solution for SIL 3 applications. The dual has
two independent transmitters and flow sensors, providing a 1:1 or 1:2 voting configurations,
which determines the number of error readings before the system triggers a shutdown.
The quad vortex has four independent transmitters and sensors with two separate shedder
bars acting as bluff bodies to create the vortices needed to accurately measure the
flow. This configuration is recommended for SIL 3 applications where 2:3 voting is needed.
The benefits of a quad vortex over a dual vortex are that a quad gives the ability to put
many sensors and transmitters in a single spool piece that makes installations easier. Less
space is required for flow conditioning within the pipe, giving the chemical producer more
flexibility in how and where to apply it within the safety system.
The 2:3 voting offers a significant benefit in that it prevents a single reading that is off from
tripping the system into a shutdown. With the 2:3 voting, it will take at least two skewed readings
to trigger a system shutdown, thereby preventing false trips, which can become costly.
In each instance though, it's important for engineers to determine which option will best
meet the requirements they must work within when designing the plant safety system. ❑
vortex flowmeters have overcome
these issues, thereby boosting the
capabilities of the meters and making
SIS-rated vortex sensors a good
fit in most chemical applications (see
sidebar on this page). However, it is
important to make sure the device
selected has a third-party certification
of its SIL capability. The exida
company (Sellersville, Pa.; www.
exida.com), for example, is an industry
leader in providing SIL certification
for devices.
Manufacturers of vortex meters
strive to ensure their meters and
sensors meet the SIL certification
and are tagged as such, including
shipping devices with a certificate
stating the usability in SIL applications.
These are important considerations
to a chemical processor when
designing a robust SIS system.
An application example
A chemical producer in the northern
part of the U.S. had an application in
a unit that utilized a feed stripper to
remove dissolved oxygen from storage
feed tanks, which is an important
step in the process that ensures
fewer process upsets. The producer
had installed a safety instrumented
system for the protection of their plant
personnel, aiming to eliminate the risk
of two-phase flow from high-pressure
hydrotreating reactors through the
feed drum back to the feed stripper.
When first engineering this safety
instrumented system, two out of
three voting (2:3) was selected to reduce
the chance of a spurious trip
and to simplify online critical alarm
testing. The differential pressure
flowmeter solution they initially opted
for consisted of multiple pairs of impulse
lines off a single orifice plate.
Those impulse lines had a number of
challenges; chief among them was
that they were very difficult to zero at
no flow. In addition, they were also
prone to plugging and, especially
in the winter months, freezing. The
lines had to be heat traced due to
the cold winters, and added to that,
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
MARCH 2019
there were a large number of potential
leak points. Challenges like
these bring added headaches and
increased complexity to installations,
along with unexpected failures and
the potential for false trips.
The SIS quad vortex solution provided
this user with a more efficient
solution. Some of the benefits they
realized after implementing the quad
vortex included the following:
* The elimination of impulse lines
and complex heat tracing, thereby
reducing capital expenditures and
install time of associated additions
* No potential for plugging or leaking
due to the all-cast, all-welded
design that cuts down on operating
expenditures
* Online removable sensors to keep
the process up and running, thereby
maximizing availability
* The simple bolt-in installation reduces
installation time and reduces
potential leak points to only two
Putting it all together
With vortex meters now a proven
technology for chemical producers
capable of measuring the flow of
extreme materials over a very wide
range of temperatures, they can now
be seriously considered when creating
a safety instrumented system.
Regulations governing the environment
and worker safety have grown
more stringent over the past few
decades. Having the right tools and
instruments to meet - and even exceed
- those regulations has benefits
beyond the obvious of meeting
those regulations. It means more efficient
and reliable operations, greater
worker safety, fewer shutdowns and
less unexpected expenses connected
with unplanned shutdowns.
Having a strong and well-designed
safety system in place just makes
good sense.
n
Edited by Gerald Ondrey
Author
Mike Klein is vice president of
Global Chemical Industry Marketing
for Emerson's Flow Solutions
business ( 6005 Rogerdale Road,
Houston, TX 77072; Email: mike.
klein@emerson.com).
He has
worked at Emerson for 18 years
and in the industry for 25 years.
Klein received a B.S. degree in
quantitative analysis and methods
from Penn State University.
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Chemical Engineering March 2019

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