DAVID W SPITZER'S E-ZINE (May 2023)

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 disappointing court ruling prompted me to recommend that the first sewage district install a superior flowmeter in each of the forced mains located immediately upstream of each of two billing flowmeters. Therefore, each new flowmeter was installed to measure the same sewage as its respective billing flowmeter so their measurements could then be compared and used to quantify measurement error. Money was tight but the first sewage district installed these two flowmeters.

Measurements from one of the billing flowmeters showed that the billed flow was approximately 50% higher than the measurements obtained by the new superior flowmeter. I remember that the other billing flowmeter measured higher (perhaps 20% to 30%) than its respective superior flowmeter. Further, these errors had been occurring for years, so the first sewage district had clearly been overbilled for years.

Despite these superior measurements of the same sewage streams, attempts made to have the sewage flowmeters fixed fell on deaf ears at the second sewage district, which would not correct the flowmeter installations nor change calibration techniques. The judge had ruled that they did not have to change anything, so they did nothing and therefore knowingly continued to overbill.

More next month about fixing one of these flowmeters.

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

A. 5% high

B. 2.5% high

C. No error

D. 2.5% low

E. 5% low

The steam header is located approximately 25 feet above grade. Transmitters are typically mounted approximately 4.5 feet above grade. Therefore, the pressure transmitter calibration will introduce an error in the amount of its condensate leg of approximately 20 feet, or approximately 10 psi where 2 feet of water column roughly corresponds to 1 psi.

The pressure transmitter will measure high by approximately 5% (10/200) but slightly less so when considered in absolute pressure terms (10/215). The output of differential pressure flowmeters changes by approximately -0.5% per percent density change where increasing pressure increases density. In this application, the measured pressure is higher than the actual header pressure so the differential pressure flowmeter will tend to measure lower than the actual flow by approximately 2.5% (Answer D).

Additional Complicating Factors

The density of the condensate is dependent on its temperature, which is usually assumed to be uniform along the length of the condensate leg. However, the temperature of the condensate (and thereby its density) can vary with the seasons in outdoor locations in some applications. In other applications, the condensate temperature can be significantly different in different parts of the condensate leg such as when the differential pressure transmitter is located outdoors, and the pressure transmitter is located indoors.

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