SOUND WAVES CUSTODY TRANSFER

Ultrasonic meters do not rely on kinetic energy from the fluid.

Ultrasonic flowmeters are one of the fastest growing precision flow measurement technologies in the world. They work in a wide range of applications. They've gained popularity as new technological developments have substantially increased their accuracy and the range of process conditions they are able to handle.

Improvements in highly accurate transit time measurements formed a breakthrough for ultrasonic flowmeters in the early 1980s. Improved signal processing in the early 1990s resulted in the ability to measure fluids not totally clean.

With the introduction of a multiple channel design, liquid ultrasonic flowmeters are accurate enough for custody transfer applications of oil and oil products.

The use of highly sophisticated techniques enables us to detect very small time differences. This is the difference in transit time principle. These small time differences make it possible to reach a measuring resolution as low as 1 millimeter per second (mm/s).

Propagates at a faster rate

The ultrasonic flowmeter takes advantage of the principle that an ultrasonic pulse travels faster downstream while slower upstream. The basic idea of the transit time differential method is not difficult, and the following example makes it clear.

Imagine two canoes crossing the river over the same diagonal line in two opposite directions, one going downstream with the flow and one going upstream against the flow. The canoe going with the flow will take less time to reach the other side of the river than the one moving against the flow.

Ultrasonic waves behave in exactly the same way. A sound wave traveling in the direction of the flow of the fluid propagates at a faster rate (velocity [V] than one traveling against the flow (V^sub AB^ > V^sub BA^).

Transit times T^sub AB^ and T^sub AB^ collect and register continuously. The difference (T^sub BA^ - T^sub AB^) in time traveled by the two ultrasonic waves is directly proportional to the mean flow velocity (V^sub m^) of the fluid. The larger the difference in time between the two pulses, the more fluid passes by.

Ultrasonic flowmeters use acoustic waves or pulses sent through the medium to establish the volumetric flow rate. One transducer transmits a signal downstream with the flow. A second transducer transmits a signal upstream against the flow along the same path. A sound wave going with the flow travels faster than one propagated against the flow. Meters can very accurately measure the times the acoustic pulses take to travel across, with, and against the flow. The difference in transit times is directly proportional to the medium's mean flow velocity. The volumetric flow rate is the product of the mean velocity multiplied with the cross section of the pipe.

Ultrasonic technologies

Ultrasonic flowmeters can either operate according to the transit time principle (also called the time of flight principle) or to the Doppler shift principle. The transit-time variety gives better utility and accuracy. Much of the growth in the ultrasonic flowmeter market is the result of increased use of ultrasonic flowmeters for custody transfer of natural gas. Ultrasonic meters are an innovative and fast-growing enabling technology in oil and gas production. The space and weight savings associated with the technology play an important role in the economics of marginal field developments.

In recent years ultrasonic flowmeters have become increasingly attractive in oil & gas production applications owing to the economic benefits that come with the technology and its increasing maturity. The technology is well suited for replacement of turbine meters and volume provers thus reducing maintenance requirements. Ultrasonic meters can also cover a wide range of viscosity and flow rates, making them useful in maturing fields and applications where fluids come from various fields. Ultrasonic flowmeters further delineate according to:

* Medium type, such as gas, liquid, and steam

* Number of measurement channels, single versus multipath ultrasonic flowmeters

* Configuration of measurement channels

* Clamp-on (nonwetted) and inline (wetted) design ultrasonic flowmeters

Beam me up ultrasonics

With single-beam ultrasonic flowmeters, the sensors A and B are located in a symmetrical arrangement on the outside of the measuring tube at an angle of 180°C. The average flow velocity measures over one line between two sensors. The measurement of the total flow is a product of the average velocity and the diameter of the pipe. For accurate readings, it is important to have a symmetrical and fully developed flow profile. Ideally one would like to measure the average velocity over the total cross section of the pipe, instead of the average velocity over one line. The figure shows a schematic of a single beam ultrasonic flowmeter.

In practice, the requirement of a symmetrical and similar flow profile cannot always happen. To decrease the effects of the flow profile and changes in viscosity, ultrasonic flowmeters with two or more beams came into being. The increased number of measurement has led to more information on the flow profile, better performance with respect to non-ax symmetrical flow profiles, and subsequently to more accurate readings. The advantages of two beams are demonstrable. Sufficient straight inlet and outlet runs before and after the meter body can improve a symmetric flow profile.