TECHNICAL AND MARKETING SERVICES
FOR INSTRUMENTATION SUPPLIERS AND END-USERS
Ultrasonic Flowmeter Applications (Part 1 of 2)by David W Spitzer and Walt Boyes
Ultrasonic flowmeters can be applied to fluids that are ultrasonically conductive. Doppler ultrasonic flowmeters may be applied to fluids that have a sufficient number and size of discontinuities that reflect ultrasonic energy, such as particles, bubbles, and eddies in the flow stream. Transit time and pulse repetition ultrasonic flowmeters can be applied to measure fluids that do not have these discontinuities. Ultrasonic flowmeters generally add little, or no pressure drop to the piping system.
Ultrasonic flowmeter sensors are available with wetted and non-wetted designs. Clamp-on sensors have no wetted parts, and are desirable when the fluid is corrosive, toxic, or when contamination is an issue. Clamp-on sensor installation is often more convenient and less expensive because few piping modifications are typically needed to attach the sensors to the outside of the pipe.
Clamp-on technology cannot be used to directly measure the transit time across the flowing fluid because the ultrasonic energy also takes time to travel through the pipe walls and interfaces. In addition, the angle of refraction of the ultrasonic energy at the inside pipe wall can vary with the speed of sound of the fluid. Some ultrasonic flowmeters measure the speed of sound in the fluid and perform calculations to correct the flow measurement.
Wetted sensors directly interface the sensor to the fluid to provide superior ultrasonic connections and eliminate the effects of the pipe walls and their interfaces on the measurement. Hot-tapped wetted sensor designs allow sensor installation and removal without shutting down flow. Retractable wetted sensor designs allow sensor removal without shutting down flow.
Excerpted from The Consumer Guide to Ultrasonic and Correlation Flowmeters.
Outside of the Process Line: The Effect of Ambient Conditions on Flow Measurementby David W Spitzer
The entire flow measurement system should be
operated within its constraints in order to provide accurate flow measurements.
This means that each component of the flow measurement system should be
considered to ensure that it is operated properly.
turbine flowmeters should be operated above a certain Reynolds number. Failure
to do so will cause K-factor shifts that can result in measurement error. Some
turbine flowmeters can use flow computer functionality to correct this error by
calculating the Reynolds number in conjunction with a theoretical or
experimental relationship between Reynolds number and the flowmeter K-factor.
However, the correction is not necessarily performed perfectly because there is
an uncertainty associated with the parameters used to calculate Reynolds number
as well as an uncertainty associated with the K-factor calculation.
additional parameters that are typically ignored but should be considered.
Process and ambient temperature can have a profound effect on the quality of
measurement. For example, a flow transmitter designed to operate in ambient
temperatures up to 60ºC may operate properly in a boiler house during the
winter when the temperature is 30ºC. However, it can operate sporadically in
the summer when the ambient temperature approaches 70ºC in an enclosed space
near the steam pipes in the middle of the afternoon on a sunny day.
can also profoundly affect flowmeter operation. In one such instance, a
flowmeter operated properly during the winter but would suddenly measure zero
flow in the middle of the afternoon on warm summer days. The cause of the
problem was traced to the sun warming the pipe to the extent that the liquid
would vaporize and cause the flowmeter to stop operating. This was a major
issue because zero flow would cause the reactor to trip. By the time the
reactor was restarted a few hours later, the ambient temperature was lower and
the flowmeter operated properly (until the next warm day).
electrical power supply can cause problems albeit not often. What if the power
supply voltage is outside of its specifications or varies? Flowmeter operation
and performance can suffer. Did you ever calibrate a low range differential
pressure transmitter outdoors in the sun and then notice a calibration shift a
few minutes later when a cloud shaded the transmitter?
In general, we tend to concentrate on how the process
affects flowmeter operation in the context of a given flowmeter technology.
However, we should not forget that other seemingly trivial factors can play
havoc with the measurement.
This article originally appeared in Flow Control magazine.
Where is Maximum Flow in a Pipe?by David W Spitzer
Insertion flowmeters typically measure the flow
rate at one or more strategic locations in the pipe in order to determine the
total flow in the entire pipe. They are often used in large pipes to obtain
economical flow measurements. However, locating the sensor incorrectly can
cause the flow measurement to be in error by as much as 10 to 30 percent or
more. At which location (expressed as a percentage of pipe radius from the pipe
wall) will the sensor register the maximum flow rate?
Any of the above
Fully developed flow in the pipe
will exhibit a velocity profile that is not flat, that is, the fluid velocity
at different locations in the pipe will be different. Therefore, Answer E is
The velocity profile has the boundary condition that
the fluid velocity at the pipe wall is zero. The fluid velocity will increase
as the sensor is moved away from the pipe wall. The maximum velocity will occur
at the center of the pipe where the effects of the pipe wall are low.
Therefore, the location where the fluid velocity is at its maximum is the
center of the pipe that is 100 percent of the pipe radius from the pipe wall.
Answer D is correct.
Additional Complicating Factors
The effects of a distorted velocity profile can
be profound --- even when the sensor is properly located. Locating the sensor
without sufficient upstream and downstream straight run can distort the
velocity profile and cause the flow measurement to exhibit exorbitant errors.
Locating the sensor in a velocity profile that is jetting due to a partially
open valve or similar restriction poses particular problems for insertion
flowmeters because only one or (at best) a few measurements are used to infer
relatively large flow rates.
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.