Vortex flow meters are the only flow meter that can measure all three phases of a fluid such as liquid, gas and steam. Vortex flowmeters are the only flow technology that can measure steam in one package making it best solution for steam flow measurement. It can also measure mass flow compensating for changes in pressure and pressure in a flow application. Having no moving parts and wide turn down makes vortex flowmeters the most versatile flow meter in the industry.
Vortex flow meters are based on the early 20th-century Hungarian physics Theodor von Karman who discovered that, when a non-streamlined object (also called a bluff body) is placed in the path of a fast-flowing stream, the fluid will alternately separate from the object on its two downstream sides, and, as the boundary layer becomes detached and curls back on itself, the fluid forms vortices (also called whirlpools or eddies). He also noted that the distance between the vortices was constant and depended solely on the size of the bluff body that formed it.
On the side of the bluff body where the vortex is being formed, the fluid velocity is higher and the pressure is lower. As the vortex moves downstream, it grows in strength and size, and eventually detaches or sheds itself. A vortex is formed on each side of the bluff body 180 degrees and the alternating vortices are spaced at equal distances. This phenomenon is observed as wind hits a flagpole and the flag moves side to side 180 degrees apart. The flag pole acts as a bluff body causing the regular rippling of a flag. In a closed pipe, the vortex effect is dissipated within a few pipe diameters downstream of the bluff body. A pressure sensing devise generally a piezoelectric sensor is strategically located to count the number of vortices. These vortices are called Karman Street see diagram below:
Vortex Flow Meter Construction:
Vortex flowmeter is typically made of mostly of either 304 & 316 stainless steel or materials which includes the bluff body, a sensor typically piezo electric and a transmitter which can mounted integral or remotely. They are typically available in flange, wafer or even insertion style. Bluff body shapes (square, rectangular, t-shaped, trapezoidal) and dimensions have been experimented by various manufactures to make them unique for patent reasons. Vortex meters are sensitive to low Reynolds numbers and to velocity profile distortion.
The bluff body must have a width that is a large enough fraction of the pipe diameter that the entire flow participates in the shedding. It also must have protruding edges on the upstream face to fix the lines of flow separation, regardless of the flow rate. The bluff body length in the direction of the flow must be a certain multiple of the bluff body width.
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 temperature ranges om cryogenic liquids to superheated steam. Sensors can be located inside or outside 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 on highly erosive/corrosive applications to reduce maintenance costs, while internal sensors provide better rangeability (better low flow sensitivity). They are also less sensitive to pipe vibrations. Please see diagram below:
Smart Vortex Technologies:
Smart vortex meters are microprocessor based which automatically correct for insufficient straight pipe conditions inside its flow body as well as diagnostic which can identify problems with both the meter and with the application.
Mass Flow Vortex Meters:
Mass flow vortex metes are designed with multiple sensors to detect not only the vortex frequency, but also the temperature and pressure of the process fluid. Based on that data, it determines both the density and the mass flow rate. This meter offers a 1.25% of rate accuracy when measuring the mass flow of liquids and a 2% of rate accuracy for gases and steam. If knowledge of process pressure and temperature is of value for other rea sons, this meter provides a convenient, less costly alternative to installing separate transmitters.
Vortex meters are best used in steam application as there are no flow meters measure multiphase fluids (between liquid and gases). However, for gases it has low flow and low-pressure limitations and for liquids as some manufacturers are unable to cancel out electrical noise which in liquid application can be considerable. Moreover, batch control application must be avoided because of it takes about 30 seconds or more to stabilize a flow rate. This is because vortex sensing device mostly piezo electric sensors count the number of vortices in a moving average technique to come up with instantons flow rate.