July 2017, Vol. 244, No. 7


With Coriolis, Ultrasonic Flowmeters, Design Really Matters

In many ways, Coriolis and ultrasonic flowmeters have similar applications and serve similar needs. Yet, despite their similarity, they are quite different in many ways. The Coriolis and ultrasonic markets are the two fastest growing of any flowmeter markets. So it is natural to ask if they are complementary or competing. While there are cases where the two flowmeter types go head to head, they are more complementary than competing for many applications.

Coriolis flowmeters were first introduced to the commercial market by Micro Motion in 1977. Though Micro Motion is still a dominant supplier, Endress+Hauser and KROHNE have also become important suppliers to this market. Both flowmeter types are widely used in the oil and gas industry, but they also play a major role in the chemical, food and beverage, and pharmaceutical industries.

Fluid Types 

Some of the differences between Coriolis and ultrasonic flowmeters are rooted in their different operating principles. Coriolis flowmeters have one or two vibrating tubes, and the flowing fluid deflects the tubes depending on the fluid momentum. The flowmeter detects the amount of deflection caused by the fluid and this amount is proportional to mass flowrate.

Since Coriolis meters depend on fluid momentum, they are more effective with liquids than with gases. Liquids are denser than gas and can more easily deflect the tubes of the Coriolis meters. While Coriolis meters have had success with gas, gas flow measurement accounts for less than 15% of total Coriolis applications. The rest are made up of petroleum and non-petroleum liquids.

Ultrasonic flowmeters have a different operating principle. They use an ultrasonic transducer to send a beam diagonally across the flowing liquid. Though different meters have different designs, a receiver is required so that the time it takes for the signal to travel from one side of the pipe to the other can be recorded. A signal is sent in the opposite direction and its transit time is also recorded. The signal travels faster with the flow than against the flow. The difference between these two values is proportional to flowrate, and the flowmeter uses this value to compute flowrate.

The transit time difference works with both liquids and gases, so ultrasonic flowmeters are widely used to measure the flow of natural gas, petroleum liquids, and non-petroleum liquids. Unlike Coriolis meters, ultrasonic meters do not rely on the density of the fluid – instead, the transit time of the signal is the relevant factor. As much as 40% of ultrasonic flowmeters are used for gas flow applications.

Inline, Clamp-on, and Insertion

Ultrasonic flowmeters are often divided into three categories: inline, clamp-on, and insertion. Inline ultrasonic meters have a meter body and their ultrasonic transducers are embedded in the meter body. To install the flowmeter, the pipe section is cut and replaced with the flowmeter, which typically has the same diameter as the pipe it is being inserted into. Inline meters work especially well with gas applications, and the majority of inline ultrasonic meters are used for gas applications.

Clamp-on ultrasonic flowmeters might seem at first glance like the world’s most perfect flowmeter. The ultrasonic transducers and receivers are mounted outside the pipe, and yet the meter can determine flowrate in the same way as other ultrasonic flowmeters – by comparing the difference in upstream and downstream flowrates. There is no need to disturb the flow, no meter body is required, and the transducers can be installed almost anywhere on the pipe.

Clamp-on meters have their place, and they are widely used as check meters to verify the performance of other meters. But the presence of the pipe wall can attenuate the signal, leading to some uncertainty in the reading. Also, clamp-on meters can have difficulty in taking into account any buildup within the pipe, which affects the volume of flow passing through the pipe. Also, clamp-on meters have difficulty making reliable gas flow measurements, although some companies claim to have done this successfully. Suffice it to say that the large majority of ultrasonic clamp-on meters are measuring liquid flows.

Insertion meters are widely used for flare and duct stack gas measurement. The transducers are typically welded into the stack at the appropriate angles and the flowrate of the exhaust gas is determined. Insertion flowmeters are ideal for this application because it is impractical to cut the pipe or duct to fit in an inline meter. The ultrasonic transmitter that analyzes the output from the transducers is generally positioned fairly near the stack or duct. Other technologies used for stack gas measurement are thermal flowmeters and averaging Pitot tubes with differential pressure transmitters.

Insertion flowmeters are also used in process applications in which high accuracy is not required and there is no need to place a meter body in the pipe. While they may be less accurate than inline meters, insertion meters are typically less expensive than inline meters. They are used for both gas and liquid applications.

Coriolis Meters

Optimal line sizes are quite different for Coriolis and ultrasonic flowmeters. About two-thirds of Coriolis flowmeters sold have a diameter of 2 inches or less. Flowmeters of this size are widely used in the chemical, food and beverage, and pharmaceutical industries. Different types of tubes, including stainless steel and Hastelloy steel, work well with hygienic applications. In this respect, Coriolis meters are like magnetic flowmeters, which have sanitary liners for food and beverage, and pharmaceutical applications.

As the diameters of Coriolis flowmeters get larger, they become heavier and more difficult to manipulate and install. Coriolis meters become unwieldy in line sizes at 4 inches and up. For many years, Rheonik was the only company making Coriolis flowmeters in line sizes above 6 inches. In the past 10 years, Emerson, Endress+Hauser, KROHNE, and Shanghai Yinuo have begun manufacturing Coriolis flowmeters in the 8- to 14-inch size range. Endress+Hauser has even introduced a Coriolis flowmeter that will accommodate a 16-inch line size.

These large line size flowmeters are primarily aimed at the market for custody transfer of natural gas and petroleum liquids. They tend to be quite expensive, and sell in the $50,000-75,000 range. Some of these meters may work well in an upstream environment where line sizes of 4 to 8 inches are not uncommon. However, they are less competitive in the midstream environment for custody transfer of natural gas, where line sizes of 20 inches and up are quite common. Ultrasonic, turbine and differential pressure flowmeters are mainly used to measure flow in these large pipes.

Ultrasonic Meters

As is the case with Coriolis meters, the line sizes most suitable for ultrasonic meters are rooted in their principle of operation. Because ultrasonic flowmeters rely on the amount of time it takes an ultrasonic signal to travel from one side of a pipe to the other, line size can actually be an advantage for ultrasonic meters. Just as Coriolis meters are most practical in line sizes below 4 inches, ultrasonic meters perform best in line sizes above 4 inches. This makes them ideal for applications involving custody transfer of natural gas where line sizes of 12 to 42 inches are not uncommon.

Ultrasonic meters have another feature that gives them an edge over other flowmeters. Those designed for custody transfer have multiple paths, meaning three or more pairs of transducers sending signals back and forth across the meter body. This avoids a problem shared by some insertion flowmeters and also some other types of flowmeters – measuring flow at only one point in the flow stream. Multiple paths generally yield higher accuracy, which is why they are required for custody transfer applications that use ultrasonic flowmeters. Suppliers have introduced ultrasonic flowmeters with three, four, five, six, eight, 12, and even 18 paths.

Another design modification from Coriolis manufacturers helps compensate for their large sizes, especially for bent tube meters. Straight tube meters operate on a similar operating principle to bent tube meters, but the meter body is straight rather than bent. This makes for a less bulky flowmeter. It also addresses an issue that arises for some hygienic applications. Fluid can build up around the bends in a bent-tube Coriolis meter while a straight tube meter does not have this problem. KROHNE brought out the first commercially successful straight tube Coriolis meter in 1994 and since that time other suppliers have followed suit.

Table 1: Advantages and Disadvantages of Coriolis Flowmeters

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Table 2: Advantages and Disadvantages of Ultrasonic Flowmeters

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Author: Jesse Yoder, Ph.D., is president of Flow Research, Inc. in Wakefield, MA (www.flowresearch.com), a company he founded in 1998. He has 30 years of experience as an analyst and writer in process control. He has authored over 250 market research studies in industrial automation and process control, and has written over 280 published journal articles on instrumentation topics. He is a regular contributor to Pipeline & Gas Journal and has also published in Flow Control, Processing, InTech, Control, Fluid Handling, and other instrumentation publications. Study topics include Coriolis, magnetic, ultrasonic, vortex, thermal, differential pressure, positive displacement, and turbine flowmeters.  He has authored two separate six-volume series of studies on gas flow and oil flow. Dr. Yoder is a regular speaker at flowmeter conferences, both in the United States and abroad.  He has most recently written a book with Dick Morley called The Tao of Measurement, which was published by ISA in March 2015.  Flow Research has recently published a study on ultrasonic flowmeters called The World Market for Ultrasonic Flowmeters, 5th Edition. This study is described at www.flowultrasonic.com.


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