|By David W. Spitzer, P.E.
Some years ago, I
was asked to look into the operation of a new orifice plate flowmeter
installation in a chemical plant where two gases were being mixed prior
to being reacted. The problem cited was that the gases were not
reacting properly and the plant surmised that the gases were not being
added in the proper amounts to achieve a good and complete reaction.
One of the gases
was fed from high-pressure gas cylinders, while the other gas was
generated in a process unit located approximately 100 meters from the
flowmeters. Both flowmeters had sufficient straight run and were
installed properly. There were pressure and temperature measurements
downstream and upstream of the orifice plate respectively. Flow
controllers were used to control the gas flows, and I seem to recall
that a multiplier was used to ratio the flows.
Observation of the
operation and its recordings revealed that the controlled flow rates of
both gases were both controlling at set point, but the process gas flow
was less steady than the cylinder gas flow. Both gas temperatures were
steady. The pressure of the cylinder gas was steady and at its design
pressure. However, the pressure of the gas from the other process unit
would vary somewhat near its design pressure and occasionally drop off
and recover a few minutes later.
As in many
applications, the problem lies in more than one location. First and
foremost, these flow measurement systems were not compensated for
pressure and temperature variations. The pressure of the gas from the
cylinders should be compensated because even though it could easily be
controlled, the pressure gauge used to control its pressure could be in
error (or be read incorrectly) and cause significant flow error. In
this application, the temperature of the gas varied with ambient
conditions, so flow errors of a few percent were likely.
That said, the most
ominous problem in this scenario was that of the process gas where the
pressure at the flowmeter could vary between 60 percent and 120 percent
of its design pressure and cause significant flow measurement errors.
Even when steady, the operating pressure introduced errors because the
pressure was only close to its design condition. Temperature variations
were less dramatic than the cylinder gas, but could still produce
measurement error. Pressure and temperature compensating this gas flow
would mitigate the problem and likely eliminate the symptoms.
temperature instruments for most flowmeters locate these measurements
upstream and downstream of the flowmeter respectively. Note that the
designer of this installation located the pressure transmitter
downstream on the downstream tap of the orifice plate and the
temperature transmitter upstream of the orifice plate. This did not
cause problems with this particular installation (but it could with
other installations) because orifice calculations were available for
the downstream pressure tap location.
transmitter location did not cause a problem because, it was installed
far enough upstream so as not to distort the velocity profile in the
flowmeter, and the temperature difference between the upstream and
downstream locations was small.
It is apparent that
the person(s) who designed this installation likely did not consider
the need for pressure and temperature compensation; and if they did,
they did not understand where the transmitters are commonly located.
Recognizing that a temperature variation of 3 C will alter the volume
of a gas by approximately 1 percent at near atmospheric conditions,
temperature compensation should be considered in virtually all gas
applications. Similarly, relatively small pressure variations can cause
significant volume changes, so pressure compensation should be
considered in virtually all gas applications.
Even if it is
decided not to pressure and/or temperature compensate the flowmeter, it
is important to properly locate and install taps for pressure and
temperature transmitters. They are relatively inexpensive … and you
never know when you will need them.
About the Author
David W. Spitzer,
P.E., is a regular contributor to Flow Control. He has more than 25
years of experience in specifying, building, installing, start-up, and
troubleshooting process control instrumentation. He has developed and
taught seminars for almost 20 years and is a member of ISA and belongs
to ASME MFC and ISO TC30 committees. Mr. Spitzer has published a number
of books concerning the application and use of fluid handling
technology, including the popular The Consumer Guide to… series, which
compares flowmeters by supplier. Mr. Spitzer is currently a principal
in Spitzer and Boyes LLC, offering engineering, product development,
marketing, and distribution consulting for manufacturing and automation
companies. He can be reached at
For More Information: www.spitzerandboyes.com