High accuracy at slow speeds

Four different types of measurement techniques are commonly used to measure the flow of liquids through pipes. These include the ultrasonic flow meter, the magnetic flow meter, the vortex flow meter and the standard differential pressure meter.

Measurement accuracy is one method that is commonly used to differentiate between the products. And this figure is usually supplied at a given turndown for the technique in question. Standard differential pressure meters usually provide a 2% accuracy over a turndown range of 5:1. Vortex flow meters, on the other hand, offer a 1.0% turndown at a maximum turndown of 15:1. For their part, magnetic flow meters offer a 1.0% accuracy and a turndown range between 10:1 and 40:1.

Traditionally, ultrasonic measurement techniques have used a method whereby sound is guided obliquely through the pipe cross section. Engineers at Siemens wanted to increase the accuracy and the turndown range when they decided to improve upon existing ultrasonic techniques. What is more, they wanted to eliminate another problem associated with ultrasonics: lack of accuracy at low flow rates.

To do so, they invented an unusual method of transmitting the ultrasonic signal down the pipe. Rather than transmit the signal obliquely, they decided to send the signal down the pipe in a helical fashion. In this way, the transmission speed of the signal would measure the complete flow profile.

Because fluid travels at different speed when close to the walls of the pipe than at the centre, the new technique can provide a more accurate reading, particularly for fluid travelling in small bore pipes, at low flow rates, or particularly viscous liquids.

Dubbed the Sitrans F, the new meter has an accuracy of 0.5% at 25:1 turndown rates and even at 100:1, an accuracy of 1% is achieved. The device is equally suited to conductive and nonconductive liquids. Solvent and organic liquids can be measured, even in hazardous locations.

The instrument is Profibus compatible and a HART output is provided as standard. Programming is carried out remotely using a hand held communicator or personal computer. Local programming can also be performed using a four-button touch screen.

Figure 1: The old and the new. Traditionally, sound was guided through the pipe cross section obliquely (right). With the new design (left), a helically shaped ultrasonic signal gives accurate integration using the whole profile

Figure 2: How it looks in practice. The ultrasonic signal helps to measure high viscosity liquids and slow flowing liquids more accurately than before

Figure 3: Conventional ultrasonic flow meters had problems at low flow rates. The new Sitrans F solves the problem by using a new helically shaped ultrasonic signal

Figure 4: The Sitrans F Ultrasonic flow meter

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