Advances in ultrasonic flow measurement

Oil, gas and water are vital natural resources, and they are distributed very unevenly around the globe. Therefore, our world is crisscrossed by a network of pipelines in which these raw materials are transported over long distances. For the operation of such supply arteries, seamless monitoring of operating parameters such as pressure, temperature and flow rate is fundamental. But reliable leak detection is also essential in the transport of these vital goods. Thanks to modern technology, measuring systems can now be used for this purpose that can replace or supplement established measuring devices in terms of efficiency and convenience. Do these new systems stand a chance against the established ones?

Due to their relevance for supply security and their geopolitical explosiveness, pipeline routes are rarely in the spotlight. Operators keep a low profile, safety first. But the fact is that there are thousands of kilometres of these pipelines made of steel or plastic around the world. Mostly buried or laid under water, they cross countries and nature reserves, conquer mountains and national borders. Depending on the medium being transported, natural gradients, compressors or pumping stations provide movement in the pipes.

Especially in oil pipelines, there is a metrologically significant difference between the individual batches, because not all crude oil is the same. Depending on the composition, density and viscosity change and thus the flow behaviour also differs. Many parameters are crucial for safe pipeline operation, and, in addition, operators must always record the quantity of their precious cargo to the exact liter.

"Siemens has been developing flowmeters for decades," explains Wolfgang Ottinger, product manager for ultrasonic flowmeters. "Most pipeline operators among our customers use established flowmeters, which we now complement with a modern system that is superior to many existing solutions."

The system Ottinger is talking about is based on the Sitrans FS230, an ultrasonic measuring device in clamp-on design. Unlike classic measuring devices such as turbine meters, its operation does not require cutting open the pipeline: the sensors are attached to the outside of the pipes without interrupting operation. This also eliminates direct contact with the medium, and regular cleaning and maintenance are not necessary. Structural measures such as narrowing the pipe diameter – and hence reducing the throughput – are also unnecessary.

For accurate measurement of volumetric flow, ultrasonic flowmeters operate according to the transit-time difference principle. Sensor pairs send or receive acoustic signals directly through the pipe wall. The volumetric flow rate of the crude oil is determined by the detected flow velocity. The flow profile, which can be described by the Reynolds number, is used to correctly calculate the volume.

Since different crude oil mixtures differ greatly in their viscosity and flow behaviour, a further correction function is necessary. In the transmitters, therefore, the so-called liquident variables are stored in an instrument-integrated table. Together with the current temperature and the measured sound velocity, the measuring instruments are thus able to automatically recognise up to 30 different batch types and thus precisely determine the individual standard volume.

The calculation methods mentioned compensate for the volumetric differences due to changing operating conditions. But is ultrasonic measurement by clamp-on sensors at all suitable for measurements of media with different flow profiles? Definitely yes, if multi-path measurements are taken! Sitrans FS230 allows up to four measurements from different angles at one measuring point. This improves the flow profile averaging and, due to the multiple passing through the pipe, deviating flow velocities outside the center of the pipe are also considered. Values averaged in this way give accuracies better than one percent. This was also confirmed by TÜV, the German Technical Inspection Association, in a customer project for a crude oil pipeline.

One advantage of this sensor arrangement is redundant measurements, each measurement path has the same valence. The measurement paths monitor each other via the measured sound velocity as well as identical measured values at zero flow. The use of so-called WideBeam sensors ensures an improved signal-to-noise ratio as well as a larger dynamic range, which makes these precision sensors less sensitive to changes in the medium.

Precise measurement results with a high variance of flow profiles, easy retrofitting during operation and virtually maintenance-free – the Sitrans FS230 systems combine standard operating measurements with several advantages. "For our customers, however, the solution is also extremely attractive for another reason," explains Ottinger: "The devices are ideal for fast and reliable leak detection. After all, pipelines can be found all over the world – from the Wadden Sea to the Alps. Should leaks occur, they must be detected and localised immediately." Here, too, the ultrasonic flowmeter proves its technical superiority: segment sections of up to 70km are equipped with two measuring devices for this purpose. If there is a difference between the input and output measurement, the higher-level system raises an alarm and provides information about the amount of leakage.

With an update rate of the measuring cycles of 100Hz, the Sitrans FS230 is one of the fastest ultrasonic measuring instruments. Only this high measuring frequency enables the reliable detection of pigs, even if they only interrupt the measuring paths for a truly short time due to their design. These cleaning or inspection devices are clearly detected, and possible false alarms are avoided. Thanks to the high-resolution of the ultrasonic flowmeters, other pig detection systems can be avoided.

"Our system will certainly not be able to replace all previous flow measurements in pipelines, but a lot of advantages await every operator. Thanks to the clamp-on technology, Sitrans FS230 systems can be retrofitted at any time without risk and with manageable effort. I am sure that this new milestone in technology will increasingly replace or at least supplement existing types of flow measurement," Ottinger sums up.