TECHNICAL AND MARKETING SERVICES
FOR INSTRUMENTATION SUPPLIERS AND END-USERS
Types of Ultrasonic Flowmeters (Part 2 of 2)by David W Spitzer and Walt Boyes
Variation in certain characteristics of ultrasonic flowmeters has allowed this technology to be applied to many processes. The following sections define flowmeter design categories and how they fit the needs of their intended applications.
Clamp-on Ultrasonic Flowmeters
Many ultrasonic flowmeter designs use sensors that are clamped onto the pipe. The concept of clamping a device onto a pipe to determine the flow inside the pipe has a certain appeal. This not only reduces piping changes, but also allows flow measurement using an instrument that has no wetted parts.
Spool Ultrasonic Flowmeters
Some ultrasonic flowmeters designs utilize sensors mounted on a spool piece that is inserted in the pipe. This design requires that the pipe be taken out of service for installation. The spool piece design allows the spool piece, sensors, and transmitter to be calibrated under flowing conditions in a flow laboratory prior to installation.
Probe Ultrasonic Flowmeters
Probe designs use wetted sensors that are in contact with the fluid. Some designs allow the probes to be installed (hot-tapped) onto an existing pipe without taking the existing pipe out of service. Most designs require that the pipe be taken out of service for installation.
Gas, Stack Gas, and Flare Gas Ultrasonic Flowmeters
Ultrasonic flowmeters are typically designed for use in liquid service. Applying this technology to gas service introduces additional sources of uncertainty that include (but not limited to) the effects of pressure, temperature, and composition. To overcome these issues, many gas ultrasonic flowmeters are designed for multi-path operation for gases with well-known properties.
Excerpted from The Consumer Guide to Ultrasonic and Correlation Flowmeters.
Inspector Cleau'fleau': Investigating a DP Meter in Steam Serviceby David W Spitzer
A lawyer hired me to investigate the "health" of differential pressure flow measurement system in high-pressure
steam service. The flowmeter elements
were in operation for about 50 years whereas parts of the secondary equipment
were modified over the last few decades.
There were "issues" associated with the system, but the impulse tubing
was surprisingly correct.
In hot condensing vapor
service such as steam, the transmitter should be located below a liquid
seal. The liquid seal forms a barrier
between the hot vapor and the transmitter that protects the transmitter by not
allowing hot vapor to contact the transmitter.
Typically, the taps are
located on the side of the pipe and the impulse tubing is constantly sloped
downward to the transmitter. This
configuration allows a liquid seal to form in the impulse tubing without
pockets of non-condensable gas that can affect the measurement. Alternatively, the tap can be located on the
top of a horizontal pipe with the impulse tubing rising and then constantly
sloping downward to the transmitter to form the seal.
In addition, the
temperature of both impulse tubes should be the same in non-horizontal segments
of the impulse tubing runs. Different
impulse tubing temperatures can result in different liquid densities in each
seal and can cause measurement error.
The error associated with relatively short impulse tubes will generally
be small. However, the error associated
with a flowmeter element located 15-30 meters above its transmitter (typical of
older installations) can cause significant measurement error.
The two impulse tubes are
typically routed to the transmitter in parallel with each other. If they are installed separately, somewhat
different routing, supports, and degradation of their insulation can result in
measurement error caused by temperature differences between the liquids in the
tubing. As a practical matter, impulse
tubing of significant length is typically installed in a tubing bundle with
common insulation and heat tracing (if needed).
This tends to reduce the temperature difference between the liquids in
the impulse tubing and hence the measurement errors that the difference can
cause. This installation also tends to
reduce the installed cost of the impulse tubing.
The impulse tubes that I inspected were installed
together in tubing bundles. I did note
that there were some slight dips in relatively short horizontal runs where
someone appeared to have stepped on the impulse tubing. This is not bad considering the age of the
This article originally appeared in Flow Control magazine.
Why the Vortex Meter Measured Zeroby David W Spitzer
Liquid flow of a process fluid into a vessel was
visually confirmed. However, a vortex
shedding flowmeter in the feed pipe measured zero flow. The flowmeter was removed from service and
was found to function properly on the flow bench in the instrument shop. Which of the following problems could cause
this to occur?
Steam tracer off
High pressure and high
temperature should not affect the flow measurement --- presuming that the
pressure and temperature limits of the flowmeter have not been violated. Therefore, Answer A and Answer B should not cause
this problem. In addition, a number of
wiring problems (Answer C) and configuration problems (Answer D) that could
cause the flow (signal) to be zero can occur.
Liquid composition changes
(Answer E) can also cause this problem to occur. In general, process conditions whereby the
liquid becomes more viscous can cause Reynolds number to fall sufficiently to
cause the vortex shedding flowmeter to turn off and "measure" zero flow.
Surprisingly, turning a steam tracer off (Answer F)
can also cause this problem to occur. In
particular, turning off a steam tracer will reduce the liquid temperature and
increase its viscosity. Increasing
viscosity reduces Reynolds number that can (as in Answer C) cause the vortex
shedding flowmeter to turn off and "measure" zero flow.
Additional Complicating Factors
Liquids are typically assumed to have a constant
viscosity at a given temperature. However, some liquids are non-Newtonian whereby their viscosity changes
at a given temperature depending upon how the liquid is handled. For example, the apparent viscosity of some
liquids (such as ketchup) is initially high but tends to fall drastically once
flow occurs. Non-Newtonian fluids are
typically more difficult to measure.
This article originally appeared in Flow Control magazine.
ABOUT SPITZER AND BOYES, LLC
In addition to over 40 years of experience as an instrument user, consultant and expert witness, David W Spitzer has written over 10 books and 500 articles about flow measurement, level measurement, instrumentation and process control. David teaches his flow measurement seminars in both English and Portuguese.
Spitzer and Boyes, LLC provides engineering, technical writing, training seminars, strategic marketing consulting and expert witness services worldwide.
Copyright 2024 Spitzer and Boyes, LLC
The content of this message is protected by copyright and trademark laws under U.S. and international law. All rights reserved.