Ultrasonic technology used in interface detection

Siemens Process Instrumentation has explained the key factors and considerations that should be taken into account when selecting clamp-on ultrasonic measurement technology.

Ultrasonic flow measurement has been used in the oil and gas industries for many years, however, the different technologies offer varying advantages that mean specific types are more suitable for certain applications.

Since its invention in the early 1970s, ultrasonic technology has advanced with regards to available configurations, electronics performance and sensor design.

Today's ultrasonic meters are versatile and capable of meeting the high accuracy and reliability demands of the oil and gas markets.

It has also become clear that various ultrasonic meter technologies have different advantages that make certain types more appropriate for certain applications.

This also applies to the field of interface detection where different ultrasonic technologies can be utilised in numerous variations based on the type of configuration.

Each offers their technology-specific pros and cons and offer functionalities to enhance the use of the technology even further.

For this reason, many manufacturers choose to offer multiple types for where specific application challenges require certain technology or configuration types.

One of these technologies is based on clamp-on ultrasonic measurement.

Before choosing this technology, however, certain key factors and considerations have to be taken into account.

Clamp-on ultrasonic flowmeters are mounted on the outside of the pipe.

They inject a signal into the pipe wall, using it as a carrier of the ultrasound signal.

These are flexible - they can be mounted virtually anywhere with little effort and with minor configuration changes, and can be fitted onto very small or very large pipe diameters.

They also have the advantage that the sensors can be physically located exactly on the part of pipe where measurement is needed.

This benefit is realised with the ability to 'adjust' the meter for a heavy or light liquid, making each meter meet the specific application needs.

This feature greatly improves the ability to measure interfaces or multiple grades of liquids within the same pipe as the sensors can be precisely located on the pipe, achieving the best performance under a full range of liquid types.

Lastly, clamp-on meters are less sensitive than some meters to suspended solids, which may scatter the ultrasonic signal, limiting their ability to perform in applications where the liquid is not pure.

When using interface detectors in hydrocarbon applications, several factors influence whether or not the measurement result can be deemed reliable.

Liquid density, temperature and type of velocity all have to be taken into account to enable an interface detection meter to distinguish one liquid from another.

Clamp-on technology, however, allows the inclusion of liquid identification, otherwise known as sonic signature.

When combining a liquid's sonic velocity and temperature, it is possible to determine the temperature-compensated sonic signature; which is a more sensitive, cost-effective and precise method of calculating density (API) compared to using an insert densitometer.

A liquid's sonic signature can be compared to a human fingerprint in that it provides an extremely accurate way of distinguishing between products that have very close densities.

To determine the sonic signature, the interface detector analyses the liquid's temperature and sonic velocity.

When this has been calculated, it is used to identify the density, which is available to the user, and also for a user-settable rate of change in the measured liquid over a specific time.

If this rate of change is reached, the meter identifies it as an interface change and therefore triggers an alarm, alerting an operator or an automated system to perform a certain action or set of actions, such as opening or closing a valve.

The sonic signature is not the only factor that has an impact on the precision of a clamp-on interface detector.

The choice between narrow- and wide-beam ultrasonic signal transmission also makes a difference.

Narrow beam clamp-on meters generate a high but narrow energy acoustic signal, typically around 1MHz into the pipe wall, which is received by the opposing sensor.

However, the high transmit voltage combined with the anti-pipe frequency usually produces a high level of noise injected into the pipe wall.

This noise is ultimately present on the receive signal and, therefore, extensive software filtering is required to reduce the excessive signal-to-noise ratio, which can possibly attack performance.

Wide-beam technology utilises the resonant frequency of the pipe wall.

It assists and transmits the sound signal into the flowing media with the wall of the pipe acting as a waveguide.

This method allows for a low transmit voltage of approximately 30V and produces a focused, coherent signal that covers a large axial area of the inside diameter.

Because this technology makes use of a wide beam it is much more resilient to suspended solids and/or bubbles.

It would simply require a much higher concentration of interference particles to disturb the entire beam to a point where failure would occur.

Another factor to consider when evaluating clamp-on interface detectors is the two sensor-mounting methods: direct or reflect.

Direct-mounted sensors can be physically located on opposite sides of a pipe, offset axially at a predetermined spacing based on the pipe size and liquid being measured.

This principle allows a direct transmission between sensors across the liquid stream.

Reflect-mount sensors, on the other hand, allow sensors to be placed on the same side of a pipe, spaced axially apart.

By using the reflect mount method, two or four paths are created for a given pair of sensors across the liquid, allowing the benefit of additional averaging.

It also facilitates installation of numerous paths, resulting in the higher accuracy that is so sought after in interface-detection installations.

As mentioned above, it is important to select the correct meter for the application, as one Malaysian oil company discovered.

When adding an additional pipeline to an oil terminal carrying various oil products from vessels to land-based storage tanks, the company investigated the most economic and efficient method of building the pipeline.

Solutions were considered, including more traditional ones such as insertion-type meters that detect the interface through density (API) measurement, but due to the narrow density difference and overlapping of the gasoline grades carried in the pipeline, these solutions were not considered precise enough.

To meet the requirements, the company opted for a multi-function mass flow measurement system that included a built-in interface detector.

The system was based on the direct displacement method without the use of a buffer, such as water, MTBE (methyl tertiary butyl ether) or a scraper device ('pig').

The only way to make this possible was to include a sonic signature calculation as opposed to relying solely on the liquid density and temperature.

This approach enabled the company to save the installation of a slop oil treatment system and optimised the overall efficiency of its pipeline operation.

Another application where clamp-on interface detectors proved valuable was on a pipeline operated by an American petroleum and natural gas pipeline company.

It was experiencing various issues with insert meters; particularly with regards to the intrusive nature of the installation, the amount of required maintenance and problems with scraping the pipe due to transducers in direct contact with the media.

The company tested several options and opted for a clamp-on ultrasonic multi-function interface detection system.

The reflect-mounted meters are primarily used for terminal delivery scheduling and advanced mainline batch tracking of interfaces upstream of a loading terminal or pipeline junction.

These offered a number of benefits, including low maintenance, scraper detection accommodation, no need for periodic recalibration, no need for straight-run piping and improved accuracy.

The results are substantial savings attributed to the reduced maintenance, cleaning, recalibration, quality control improvement and manpower efficiency.

Understanding the considerations when choosing between the various ultrasonic technology offerings available for interface detection is key in achieving a high-performance and successful application.

If this is combined with a thorough analysis of the installation requirements, providing a 'best fit' will greatly increase the satisfaction and metering performance.

The clamp-on ultrasonic interface detection technology offers many of those requirements.

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