Ultrasonic Flow Meter Proves its Worth on Vacuum Tower Bottoms - Rainer Wetzel

Process conditions at the bottom of vacuum distillation towers make flow measurement challenging. In this case study, a clamp-on ultrasonic flow meter proves more reliable than the traditional orifice plate.

Operating at temperatures of around 350°C, the bottom of a vacuum distillation tower is no place for delicate instrumentation. To measure the flowrate of vacuum residue or bitumen, refiners therefore traditionally rely on
differential-pressure (DP) devices such as orifice plates or venturis. These are simple, robust, and economical in line sizes above 200 mm.

Unfortunately, simplicity and robustness do not always imply accuracy or reliability. The biggest problem with all DP devices is the need for a pair of impulse lines connecting the sensor to the DP transmitter. Impulse lines often become plugged with particles of solid material such as coke, or bubbles of gas. In bitumen service, blockages are most commonly due to frozen product – a particular problem in cold climates, despite trace heating.

Faced with unreliable orifice plates, one refiner sought a better way to measure the flow of bitumen leaving a vacuum tower on its way to a delayed coker. In this case the product starts life as crude bitumen which is diluted with lighter hydrocarbons to create a synthetic crude. The lessons learned, however, apply equally well to refined bitumen produced in the vacuum distillation of conventional crude.

Versatile Technology
The refining company decided to try a clamp-on ultrasonic flow meter as a way to make flow measurement more reliable. The main rationale for the experiment was that because ultrasonic flow meters are non-invasive and do not require impulse lines, they cannot become clogged or blocked. Other important features of this flow measurement principle are that it is independent of system pressure, works with practically any fluid, and remains accurate over a wide turndown range (see boxes).

The company first carried out several tests with a portable ultrasonic flow meter, allowing it to select the best transducer type and positions for the job in hand. Once the engineers were confident that the new technology would perform well, they installed a permanent clamp-on ultrasonic flow meter on the 400mm pipework connected to the bottom of the vacuum tower.

The complete system comprised two sets of FLEXIM Type G low-frequency ultrasonic transducers with hightemperature mountings, and a Fluxus® ADM 7407 transmitter with two input channels . The transmitter
communicates with the refinery’s distributed control system (DCS), making it easy to compare the readings from the ultrasonic flow meter with those of the flow transmitters linked to the three original orifice plates
(Figure 1).