This article originally appeared in iWWD May 2020 issue as "Ultrasonic Clamp-On Flowmeters"
Process water and wastewater operations play an important role in an increasingly demanding industrial environment. Selecting proper flow instrumentation is essential to ensure tight control of treatment processes.
Many industrial end users of flow measurement technology think ultrasonic flowmeters are a relatively recent invention. In fact, ultrasonic flowmeter history spans more than six decades.
Ultrasonic Flowmeter Background & History
In 1959, a Japanese physicist, Shiego Satomura, designed an ultrasonic flowmeter for use in blood flow analysis. It was not until four years later that the earliest ultrasonic flowmeters for industrial applications were introduced. Initially, ultrasonic flowmeters were misapplied and not well understood. This hurt their reputation for some end users. With technological improvements, and the correct usage, ultrasonic meters now are better understood.
Ultrasonic flowmeters are ideal for applications involving dirty process fluids that are water-based or sonically conductive. Unlike electromagnetic meters, they can measure flows of non-conductive fluids. They are also ideal for applications where chemical compatibility is essential.
Numerous industry approvals have opened the door for wider use of ultrasonic flow measurement, and the meters have gained greater acceptance due to advancements in microprocessor technology which enables improved measurement accuracy and greater success rates out of the box.
How Ultrasonic Meters Work
An ultrasonic flowmeter measures the velocity of a fluid with ultrasound to calculate volumetric flow. Using ultrasonic transducers, the it measures the average velocity along the path of an emitted beam of ultrasound. It averages the difference in measured transit time between the pulses of ultrasound propagating into and against the direction of the flow or by measuring the frequency shift from the Doppler effect.
In the 1990s, transit time ultrasonic technology, also called the time-of-flight method, was introduced as a way to measure liquids in waste and clean water industries. These meters measure the difference in time from when an ultrasonic signal is transmitted from one transducer until it crosses the pipe and is received by another. A comparison between upstream and downstream measurements is then made. If there is no flow, the travel time will be the same in both directions. When flow is present, sound moves faster if traveling in the same direction as the flow and slower if moving against it. The difference in the upstream and downstream measurements taken over the same path is used to calculate the flow through the pipe.
A clamp-on ultrasonic flow meter consists of at least two ultrasonic sensors mounted externally to a pipe. The sensors measure flow by transmitting and receiving ultrasonic signals directly through the pipe wall and the medium. How the flow rate is determined varies depending on the nature of the application. Transit-time is the preferred mode of ultrasonic operation for relatively homogeneous liquids and is accurate up to 0.5% of flow.
Transit-Time Ultrasonic Flowmeter Advancements
Developments of transit time ultrasonic flow meters have continued for several decades now. The current breed of clamp-on ultrasonic meter is highly advanced, with critical improvements such as increased data acquisition speed, speed of sound measurement and digital communications.
The latest generation of transit-time ultrasonic flow meters offer an increased resolution capability, making it possible to precisely track fluid movements down to a true zero flow condition. Transit time meters do not require reflective material in the flow stream and thus have wider applicability.
They can be employed for a host of data logging tasks, allowing users to select multiple parameters to log, including flow rate and total flow, signal strength and alarms. Meter diagnostics take the guesswork out of isolating process or application problems. Users are alerted to out-of-specification flow conditions and can access a history with the most recent alarm, error and event codes. Configuration software also enables in-field flow meter configuration, calibration and troubleshooting via a laptop PC.
Additionally, some modern transit-time ultrasonic flowmeters support an encoder output for use with cellular endpoints for connecting to cloud-based advanced metering infrastructure (AMI) software. This solution enables the user to collect measurements at various intervals and upload them to the cloud on a regular schedule to help with tracking and analyzing water and wastewater information.
Industrial End User Solutions
Industrial firms of all sizes are faced with new, stringent standards for water quality and pollution controls. They also have an urgent need to save energy and upgrade aging infrastructure. Plant operators want to reduce raw water consumption, decrease energy and chemical costs, minimize toxic waste treatment and discharge, and improve regulatory compliance.
Transit time ultrasonic flow meters have found greater use in industrial flow measurement as they’ve become less expensive, more accurate, smaller, and more widely available. Clamp-on transit-time meters are a good alternative to traditional mechanical or inline devices when shutting down the process is costly; when ± 1% of rate accuracy is sufficient for controlling the treatment process; or when water contains chemicals or other substances that may foul the electrodes of an electromagnetic meter.
With advanced integrated analog circuits, transit-time ultrasonic flowmeters can perform real-time capture and processing of ultrasonic transducer waveforms to get accurate time-of-flight information. Furthermore, non-intrusive ultrasonic flowmeters do not have moving parts, making them a preferred choice to replace maintenance-prone devices.
Clamp-on flow technology is sometimes avoided in process water and wastewater treatment applications due to the misconception that transit-time ultrasonic meters can only measure liquids without particles or bubbles, leaving Doppler-style ultrasonic meters as the only clamp-on option for these applications. In truth, a modern transit-time meter can tolerate a high level of solids without an adverse effect on performance. With advances in signal processing and more powerful transducers, transit-time ultrasonic technology has surpassed earlier predecessors in measuring commonly dirty flows.
Typical Measurement Applications
Oil and gas refinery, chemical industry and power generation facility development is increasing the demand for ultrasonic flow measurement technology. The growing acceptance of transit-time ultrasonic flowmeters can be attributed to the need to accurately and cost-effectively determine the characteristics of process water and wastewater flowing through pipelines.
Typical applications in industrial water and wastewater include process water measurement; influent and discharge monitoring; water consumption monitoring; reservoir flow monitoring; chemical dosing; leak detection; and network load monitoring.
Ultrasonic transit-time clamp-on meters have two key attributes for deployment in industrial water and wastewater processes. First, by avoiding contact with the process, the meters cannot become fouled or damaged by caustic fluids unlike wetted sensors. Second, by not requiring pipe intrusion, the ultrasonic device eliminates the need to manage any waste products left in the line that may spill or mandate additional personal protective equipment.
For example, in saltwater disposal applications in the oil and gas industry, well-produced saltwater is considered hazardous waste because of its high salt content, hydrocarbons and other compounds. Operating companies can recycle the water, injecting it back into working reservoirs for reuse in gathering any remaining oil or gas, or discard it at a saltwater well disposal site. Clamp- on transit-time ultrasonic flowmeters can provide bi-directional flow measurement and allow for installation without interrupting production or cutting into process piping. This non-contact solution also eliminates pressure loss and potential meter fouling.
Advanced ultrasonic metering technology offers exact and continuous flow measurements in industrial process water and wastewater applications. Although they are not a “universal” flowmeter, they are versatile and accurate for many industrial applications.
Transit time ultrasonic flow meters provide a non-contact means of measuring flow velocity. They are clamp-on devices that attach to the exterior of the pipe and enable measurement of corrosive liquids without damage to sensors. This approach offers an overall lower cost of ownership because meters can operate under a wide range of operating conditions due to their externally oriented sensors and lack of moving parts.