DAVID W SPITZER'S E-ZINE (May 2022)

Ultrasonic flowmeters use sensors to generate ultrasonic waves and direct them into the flowing stream. Information from the remnants of these sound waves is used to determine the flow of fluid passing through the flowmeter. Ultrasonic flowmeters have no moving parts.

Focusing ultrasonic energy into the flowing fluid and detection of its remnants is predicated upon a complete ultrasonic circuit. The ultrasonic circuit can consist of the transmitting device, entry pipe wall, entry pipe liner, fluid, reflections off pipe walls, exit pipe liner, exit pipe wall, and receiving device. For the flowmeter to function properly, all parts of the ultrasonic circuit and their interfaces must allow the passage of sufficient ultrasonic energy. If a part of the circuit attenuates ultrasonic energy excessively, the ultrasonic signal at the receiving device may weaken. A weak received signal may cause the flowmeter to be erratic or cease to function.

For example, an ultrasonic circuit could be compromised by paint on the outside of the pipe, drying of the ultrasonic coupling compound, pipe material, coating or corrosion on the inside of the pipe, and a poorly bonded pipe liner. Tuberculation, or the growth of barnacles on the inside pipe wall, can also compromise the ultrasonic circuit.

Most designs utilize Doppler, transit time (time-of-flight), pulse repetition (sing-around), or phase shift sensing techniques. While dependent upon signal processing techniques, many designs are capable of measuring flow in both the forward and reverse directions.

It should be noted that some designs are dependent on the speed of sound in the fluid. Therefore, changing composition, temperature, and/or (gas) pressure can change the speed of sound in the fluid, and hence affect flowmeter performance. Some ultrasonic flowmeters measure the speed of sound of the fluid and correct for this effect. Other designs avoid this issue by using flow equations in which the speed of sound is not needed.

Excerpted from The Consumer Guide to Ultrasonic and Correlation Flowmeters.

The billing rate in a given year was apportioned between the sewage districts based on the flow measurements — four outfalls from the first district and one at the inlet to the sewage treatment plant. The previous year’s flow from the first sewage district could be calculated by summing the flows measured through each of the four flowmeters during the previous year.

The total plant flow for the previous year was similarly calculated using the total plant flow measurements during the same time period. The sewage generated by the second district was calculated by subtracting the first sewage district flow from the total plant flow. The resultant proportion of sewage generated by each district could then be used to allocate the cost of operating the sewage treatment plant and calculate the next year’s billing rate for each district.

For example, if the first and second sewage districts deliver 40% and 60% of the liquid to be treated, the customers in the first and second sewage districts would be expected to pay 40% and 60% respectively of the expenses associated with operating the sewage treatment plant. This allocation would then be used to calculate the next year’s billing rate. In addition to being invoiced for the normal daily treatment charges — billing rate multiplied by the daily flow — a surcharge was added to the second district’s bill for flow that exceeded a set amount of sewage during that day.

Read more next month about how wide billing variations can occur on different days.

This article originally appeared in P. I. Process Instrumentation magazine.

A. U-tube down

B. Inverted U-tube

C. Horizontal (parallel to grade)

D. Flag position

Coriolis mass flowmeters in liquid service must be completely full of liquid to measure accurately. The inverted U-tube orientation (Answer B) could accumulate gas and should not be used for liquid applications.

The U-tube down orientation (Answer A) and horizontal orientation (Answer C) could be acceptable but would entail modification of the upstream and downstream piping. Therefore, the flag position (Answer D) would appear to be the most practical answer;  however, the flowmeter must be kept full of liquid for accurate measurement.

Additional Complicating Factors

Liquids are generally pumped upward or horizontally from one location to another. Pumped downward flow can sometimes occur, for instance, in order to get around an obstruction. However, most flow downward occurs using gravity, so keeping the flowmeter full of liquid can be a challenge in many installations.

For example, a U-tube Coriolis mass flowmeter oriented in a flag position (Answer D) located in piping that is hydraulically submerged would be an acceptable installation. However, if the flowmeter is not hydraulically submerged in the downward pipe (as is often the case), a hydraulically submerged condition for the flowmeter should be created, such as by modifying the piping to flow down, then up through the flowmeter and then down again.

In addition, not all Coriolis mass flowmeters have U-tube geometry, so the piping orientation should be designed to ensure that gas is not allowed to accumulate in the flowmeter.

This article originally appeared in P. I. Process Instrumentation magazine.