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| David W. Spitzer, P.E.
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Selecting
flowmeters for pipelines where liquid flow is downward requires careful
consideration, as such applications can pose measurement problems. For
example, there is a pressure drop across flowmeter elements, such as an
orifice plate, vortex shedder, or Coriolis mass flowmeter. In
horizontal pipes, the pressure at no flow is the same throughout the
pipe at the same elevation. At no-flow conditions in a vertical pipe,
the pressure is lower at higher elevations. With upward flow, the total
pressure drop is the sum of the pressure loss due to elevation and the
pressure loss due to the flowmeter element.
Conversely, at no-flow conditions in a vertical pipe, the
pressure is higher at lower elevations. With downward flow, the total
pressure drop is the sum of the pressure gain due to elevation plus the
pressure loss due to the flowmeter element. When the pressure loss
across the flowmeter element is less than the loss due to elevation,
the differential pressure across the flowmeter will be positive and
result in a pressure gain. This can affect different flowmeters in
different ways. In one downward flow installation, a vortex shedder was
reputed to be in error by as much as 10 percent. In another
installation, a Coriolis mass flowmeter measured accurately. When in
doubt, the results of downward flow testing in a flow laboratory would
likely provide useful information as to the suitability of the
flowmeter for downward flow installations.
Accurate liquid flow measurement is generally predicated on the
flowmeter being full of liquid. This poses a problem because many pipes
that flow downward are not necessarily full of liquid. For example, a
vessel overflow installation may keep the vessel full, but the pipe
leading downward is likely partially full for at least part of its
downward run. Flowmeters requiring a full pipe that are located where
the pipe is not full will be in error. This is not necessarily due to
the downward flow, but rather the partially full pipe.
Configurations where the pipe is partially full of liquid can be
insidious. In one such application, the pipe rose, ran horizontally,
and then fell vertically into a vessel. The flowmeter was located
upstream of the control valve in the vertical pipe with flow downward.
The flow measurements were in error by 30 percent to 50 percent. The
problem was not in the technology because the same type of flowmeter
performed accurately in a downward flow application where the pipe was
full of liquid.
The lesson here is that you should be very careful when you apply flowmeters in downward flow applications.
David W. Spitzer,
P.E., is a regular contributor to Flow Control. He has more than 30
years of experience in specifying, building, installing, startup, and
troubleshooting process control instrumentation. He has developed and
taught seminars for over 20 years and is a member of ISA and belongs to
the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number
of books concerning the application and use of fluid handling
technology, including the popular "Consumer Guide" 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. He can be reached 845 623-1830.
www.spitzerandboyes.com
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