The Vortex flow meter is the only type of flow meter that can measure all three phases of a fluid – liquid, gas and steam. Vortex flowmeters make use of the only flow measurement technology that is capable of performing steam flow measurement in one integrated package; this ability makes it best and most economical solution for steam flow measurement. The vortex meter can also be offered with integrated pressure and temperature measurement for both compressed gas and steam flow measurement in order to allow the meter to compensate for variations in pressure and temperature that will affect volumetric flows of compressible media. The vortex meter’s no-moving-parts design provides users with maintenance free operation and long service life even in dirty and contaminated media applications.
Vortex flow meters’ principle of operation is based on the work of early 20th-century Hungarian-American physicist Theodore von Karman. Von Karman discovered that when a non-streamlined obstruction (also referred to as a bluff body) is placed in the path of a flowing media, the fluid will alternately separate from the object on its two downstream sides. As this phenomenon occurs, the boundary layer becomes detached and curls back on itself and the fluid forms vortices (also called whirlpools or eddies). Von Karman also noted that the distance between the vortices was constant and depended solely on the size and geometry of the bluff body causing the vortices’ to be formed.
On the side of the bluff body where the vortex is initially formed, fluid velocity is higher and pressure is lower. As the vortex moves downstream, it grows in strength and size, and then eventually detaches or sheds itself. (This is why vortex meters are sometimes referred to as vortex shedding meters.) Alternating vortices are formed on each side of the bluff body 180 degrees apart and are spaced at equal distances. In fluid mechanics, these vortices are referred to as a Karman Vortex Street. The frequency with which the vortices are formed, as well as the magnitude & length of the vortices, are directly proportional to the velocity of the flowing media. The diagram below illustrates this phenomenon. This phenomenon can also observed as wind hits a flagpole and the flag moves from side to side. In this analogy, the flag pole acts as a bluff body causing the vortex formation and the rippling of the flag is the response to the vortices’ formation. In a closed pipe, the vortex effect is dissipated within a few pipe diameters downstream of the bluff body. The vortex meter counts the vortices in much the same way that a turbine flow meter counts rotations of a turbine’s blades, by making use of commonly available electronic components such as a piezoelectric sensor.
The primary materials of construction for the Vortex flowmeter are typically either 304 or 316 stainless steel. Components fabricated from the stainless alloy would include the flow body, the bluff body, and the flanged process connections. Other key components include the sensor which counts the vortices, which is typically a piezoelectric type, and a display/transmitter module which can mounted integrally or remotely. The vortex meter is typically offered as an inline meter with either flanged or wafer-type process connections, but it may also be provides as an insertion style probe. Vortex meters are sensitive to low Reynolds numbers and to velocity profile distortion.
Bluff body geometries (square, rectangular, t-shaped, trapezoidal) and dimensions will vary from manufacturer to manufacturer; these design differences will provide for certain performance trade-offs. The bluff body’s width must occupy a large enough portion of the overall pipe diameter that the entire flow profile will participate in the vortex shedding process. It also must have protruding edges on the upstream face in order to fix the lines of flow separation, regardless of the flow rate. The bluff body’s length along the direction of the flow must be a certain minimum multiple of the bluff body width.
There are a number of different ways that a vortex meter may detect and count the vortices’ formation. The majority of vortex meters use piezoelectric or capacitance-type sensors which detect the pressure oscillation around the bluff body. The sensor detects pressure oscillation with a low voltage output signal which has the same frequency as the oscillation. These replaceable sensors can operate over a wide range of temperatures in order to accommodate all types of flow measurement applications ranging from cryogenic liquids to superheated steam. Sensors can be located inside or outside of the bluff body. Wetted sensors (outside the bluff body) sense the vortex pressure fluctuations and are enclosed in hardened cases to avoid corrosion and erosion effects. External sensors, typically piezo-electric strain gages, sense the vortex shedding indirectly through the force exerted on the shedder bar. External sensors are preferred for highly erosive/corrosive applications in order to reduce maintenance costs, while internal sensors provide better rangeability and low flow sensitivity. The internal sensor is also less sensitive to interference from ambient pipe vibrations. Please refer to the diagram below:
Smart vortex meters employ the vortex flow measurement technique combined with a microprocessor based flow computer which automatically corrects for insufficient straight pipe conditions inside its flow body as well as diagnostic information which can identify problems with both the meter and with the application.
Mass flow vortex meters include integrated pressure and temperature sensors to detect process pressure and temperature in addition to the vortex frequency. With the ability to measure the media pressure, temperature and velocity, the mass flow vortex meter is able to determine the density and the mass flow rate. Typical accuracies for this style of meter are 1.25% of reading for measuring the mass flow of liquids and a 2% of reading for gases and steam. The multi-parameter capability of this meter provides an additional benefit for applications where knowledge of process pressure and temperature is required or is of value for other reasons. In those situations, a mass flow vortex meter provides a convenient, less costly alternative to installing separate transmitters.
Steam measurement is the most popular application for vortex flow meters as they are far more economical and easy to install versus other flow measurement technologies that have the ability to measure steam. However, for compressed gases, the vortex meter has low flow and low-pressure limitations. For liquids, some vortex meter manufacturers are unable to cancel out electrical noise, which can be considerable in liquid applications. Moreover, batch control applications must be avoided as it takes approximately 30 seconds or more to stabilize a flow rate reading. This is because vortex sensing meters count the number of vortices in a moving average technique in order to determine an instantons flow rate.