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.
Some inline meters, such as multipath ultrasonic meters, make multiple measurements and create a calculated average to determine flowrate.
Insertion vortex meters make a point measurement and then compute the flow through the whole pipe based on flow profile considerations. The formula used to make this calculation is based on extensive testing and can be improved with time and experience.
Early issues
Like many other flowmeters, vortex meters experienced application issues early in their development. One problem for vortex meters was the effects of vibration, which tended to create false readings. Suppliers created software to deal with vibration issues, and this proved to be an effective solution.One important development in recent years has been the introduction of reducer vortex meters. These meters have a smaller line size than the pipe they are placed in, enabling them to measure lower flows. Most incorporate a single line size reduction, al though some incorporate two line-size reductions. Other changes since vortex flowmeters were introduced in 1969 include anti-vibration software and electronics, multivariable flowmeters, reduced bore meters, and plastic flowmeters. Today there is a wide diversity of choices for customers to make when specifying or purchasing vortex flowmeters.
Advent of multivariable meters
Multivariable vortex flowmeters have become increasingly popular since Sierra Instruments introduced them in 1997. A number of suppliers have brought out their own multivariable vortex flowmeters, including ABB (Goettingen, Germany), abzil (Tokyo, Japan), Krohne (Duisburg, Germany), and Endress+Hauser (Reinach, Switzerland).
Multivariable flowmeters house 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.
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.
Mounting types
Vortex flowmeters are available in flanged, wafer, and insertion styles, and the right choice depends on multiple considerations: required accuracy and repeatability, line size, fluid type, and the desired supplier or distributor.
Flanged and wafer (inline) styles offer greater accuracy than insertion meters, but are not practical in large line sizes — vortex meters top out in the 16” range. (The “sweet spot” in terms of size for vortex flowmeters is from 1” to 4”.)Insertion vortex flowmeters offer a viable option to companies that want to measure flow in large pipes, especially pipes with an internal diameter greater than 12”.
Insertion meters are sometimes used to measure flow in pipes that cannot be shut down. Because insertion meters can be hot tapped, the meters can be swapped out or parts can be replaced without shutting down the line. Inline meters do not have this advantage unless a bypass line is installed, and even so the line has to be shut down to install the bypass line. This gives insertion vortex meters additional flexibility over inline meters. Insertion vortex meters cannot achieve the same accuracy as some inline meters because they make a single-point measurement inside the pipe.
While flanged vortex meters are somewhat more expensive than wafer-style meters, they are more secure and provide less opportunity for leakage than wafer-style meters. The longer bolts that are used to secure wafer-style meters have a tendency to expand, creating a possibility of leakage. This could create a safety hazard, potentially resulting in fugitive emissions and lost product. It is easier to install flanged vortex meters correctly than wafer-style vortex meters. Companies that are downsizing may have fewer skilled engineering staff to perform these installations.