Vortex flowmeter sensors

Vortex flowmeters consist of various components, including measuring tube, bluff body, sensor, preamplifier and meter electronics (see Fig. 1). In the majority of meters on the market, the sensors have no moving parts and, therefore, are not subject to wear and require no maintenance.

The two most common vortex meter body designs are those with flanges and those without. The latter is known as the wafer type and is designed for installation between two pipe flanges. Some wafer designs have a standardized overall length of 65 mm (2.5") which allows them to directly replace orifice assemblies.

The standard range available on the market covers nominal diameters from DN 15 to 300 (1/2 to 12"), with some versions up to DN 400 (16"). Pressure ratings can be up to PN 250 (ANSI Class 1500). The vortex frequencies of nominal diameters larger than DN 300 (12") are very low, and powerful signal processing is required in order to obtain a stable signal. For large diameter applications, vortex meters are relatively expensive in comparison to orifice plates. Many manufacturers also offer designs for very low or very high temperatures (–200 to +450 °C / –330 to +842 °F).

Devices with two independent sensors and electronics constitute a special case (Fig. 1). This design is primarily used in industries where redundancy measurement is considered important.

The vortices produced by the bluff body create local pressure fluctuations in the flow which can be measured by a variety of sensors and converted into electric signals. The types of sensors favored by individual manufacturers vary widely and include capacitive, piezo-resistive, ultrasonic, thermistor, mechanical, pressure and strain-gauge sensors. In most instances, the sensor is either integrated into the bluff body or situated immediately behind it. Most sensors today measure the vortex shedding capacitively or piezo-electrically.

The DSC sensors used by Endress+Hauser have a paddle-shaped sensor protruding behind the bluff body (Fig. 2). This paddle (a) transmits the vortex pressure fluctuations to a sleeve-like center electrode (c) that forms capacitors C1 and C2 together with the outside electrode (d), which consists of two half-shells. The change in the width of the gap produces a periodically varying change in capacitance that is proportional to the vortex pressure differential, which is processed by the meter electronics. The mechanical balancing of the sensor makes such measuring systems largely unaffected by pipeline vibrations.

The primary advantages of the DSC sensor are:

• Resistant to temperature shocks, for example in cryogenic applications (see Fig. 3) or in steam systems. The stainless steel DSC sensor has no moving parts or very sensitive components and is extremely robust.
• Resistant to water hammer, e.g. in steam systems.
• Unaffected by pipe vibrations. The distance between the center electrode and the outside electrodes is not influenced by the acceleration effects caused by vibrations. Sensor paddle and center electrode are exactly balanced, so the acceleration forces produced by vibrations always act at the sensor system’s center of gravity and, consequently, do not generate additional, vibration-related signals.
• Largely unaffected by foreign matter, because the DSC sensor is mounted freely in the measuring tube. In worst-case conditions, deposits on the sensor paddle itself can produce a marginal reduction in measuring range (turndown), but do not influence the accuracy of (the) measurement.