Oil and Gas Flows

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Vortex flowmeters were first introduced to the market in 1969. Since that time, the number of worldwide suppliers has grown to at least 35. Between the time they were first introduced and today, many changes have taken place in the vortex flowmeter market. These include anti-vibration software and electronics, multivariable flowmeters, reduced bore meters, plastic vortex flowmeters, and much more. Today there is a wide diversity of choices for customers to make when specifying or purchasing vortex flowmeters.

Vortex flowmeters operate on a principle called the von Karman effect. This principle concerns the behavior of fluids when an obstacle is placed in the path of flow. Under the right conditions, the presence of the obstacle generates a series of alternative vortices called the von Karman street. This phenomenon occurs in liquid, gas, and steam, and has been observed in many diverse contexts including cloud layers passing an island and whitewater rapids.

In vortex flowmeters, the obstacle takes the form of an object with a broad, flat front called a bluff body. The bluff body is mounted at right angles to the flowstream. Flow velocity is proportional to the frequency of the vortices. Flowrate is calculated by multiplying the area of the pipe times the velocity of the flow.

In order to compute the flowrate, vortex flowmeters count the number of vortices generated by the bluff body. They use a variety of techniques for sensing the presence of a vortex. The majority of vortex flowmeters use a piezoelectric sensor; however, some use a capacitive sensor and others use an ultrasonic sensor to detect vortices.

Despite what has been slow growth in the vortex flowmeter market, there are signs that this flowmeter is breaking out of its slump. One sign is the major product enhancements that have occurred in the past five years. One perennial problem with vortex flowmeters has been susceptibility to vibration error. Vibrations in the line can cause a vortex flowmeter to falsely generate a vortex signal, or to incorrectly read an existing vortex. Suppliers have responded to issues surrounding vibration by implementing software and electronics, including digital signal processing, that have reduced the susceptibility of vortex meters to interference from vibration.

Another positive sign is the growing availability of multivariable vortex flowmeters. Sierra Instruments introduced the first multivariable vortex flowmeter in 1997. This flowmeter includes an RTD temperature sensor and a pressure transducer. By using information from these sensors, together with detection of vortices generated, the flowmeter can output volumetric flow, temperature, pressure, fluid density, and mass flow. Multivariable flowmeters measure more than one process variable, and typically use this information to compute mass flow. This makes the flowmeter measurement more accurate in changing temperature and pressure conditions.

In the past five years, a number of new suppliers have brought out their own multivariable vortex flowmeters. These include ABB (Goettingen, Germany), Yokogawa (Tokyo, Japan), Krohne (Duisburg, Germany), and Endress+Hauser (Reinach, Switzerland). Even though multivariable flowmeters are somewhat more expensive than their single-variable counterparts, they enable users to obtain significantly more information about the process than single-variable volumetric meters. This additional information can result in increased efficiencies that more than make up for the additional cost of the multivariable flowmeter.