Vortex Flow Meter Installation Guide
Why are flow meter straight lengths important?
Vortex flow meter requires a well developed and symmetrical flow velocity profile, free from any distortions or swirls if it is to give good accuracy and repeatability. To achieve good accuracy and repeatability it is standard practice to place the meter some distance from sources of turbulence. Common sources of turbulence include pumps, valves, changes in line direction (i.e. bends), changes in line size etc.
How are flow meter straight lengths defined?
Most manufacturers provide the user with minimum distances for their particular products. These distances, referred to as straight lengths, are indicated in Pipe Diameters (D). For example, 10 D means place the flow meter ten times the pipe's internal diameter away from the source of turbulence. Because turbulence both upstream and downstream can reduce accuracy, manufacturers provide straight length requirements for both upstream and downstream of the meter.
Installing a Vortex Meter
Different manufacturers claim differing requirements, with fewer straight lengths being marketed as an advantage for the end user.Ideally the flow transmitter should be sited with as many upstream and downstream straight pipe lengths as possible, preferably more than recommended by the manufacturer, but definitely not less. Obtaining the necessary straight lengths can be difficult, especially in compact plants. Therefore it is worth remembering that Vortex meters can be installed vertically, horizontally, or at any angle, as long as they are kept flooded. So if installing vertically try to have upward flow.If you are compensating for pressure and temperature then allow 3 to 4 pipe diameters between the meter and downstream pressure taps, and thermowells should be small and located 5 to 6 D downstream of the meter.
As a general rule of thumb straight lengths should be about the same as that required for an orifice installation with a beta ratio of 0.7 - see the table below
Vortex Meter Straight Length Requirements
Vortex Flow Meter Operating Principle
How does a Vortex flow meter work?
A vortex flow meter's working principle exploits the phenomenon of "Karman vortex streets". A Karman vortex street is a repeating pattern of swirling vortices, and in a vortex flow meter the rate of pressure oscillations caused by these vortices is measured and correlates to the fluid velocity. A simple calculation using fluid velocity and cross sectional area of the pipe allow the volumetric flow rate to be determined. Pressure and/or temperature compensation can be used to allows mass flow measurements to be made.
The Vortex Meter
In a vortex flow meter, the bluff body is known as a "shredder bar". The shedder bar is shaped to allow process fluid to separate and generate vortices around the back side of the shedder bar. Sensors, generally piezoelectric or capacitance-type, located in or just behind the shredder bar are used to detect the pressure oscillation caused by the vortices leaving the shredder bar. The sensors respond to the pressure oscillation with a signal which has the same frequency as the oscillation pressure, and the frequency of the pressure oscillations is directly proportional to the fluid velocity.
What does a Vortex Meter Look Like?
There are many vortex flow meter manufacturers and each manufacturer's flow meter will look different. However in general a vortex flowmeter will look like one of the three shown adjacent:
Wafer type - the meter is clamped between two flanges in the pipe,
Flanged type - the meter is bolted inline using its own flanges and those in the pipe,
Insertion type - the meter is inserted into the pipe, similar to the way a thermowell is inserted.
Vortex Flow Meter Turndown Ratio
The turndown ratio (i.e. the ratio of maximum flow rate to minimum flow rate) of most vortex meters will be somewhere between 10 and 15, depending on the viscosity of the fluid being metered. Vortex flow meters measuring the flow of a high viscosity fluid will have a lower turndown ratio than when measuring flow of a low viscosity fluid.
Vortex Flow Transmitter Applications
Vortex flow transmitters are well suited for a variety of applications and can be used on liquids or gasses. They can also be used over a wide range of temperatures, from cryogenic liquids to superheated steam. They are best suited to continuous flow measurement and not usually recommended for batching or other intermittent flow applications.
Vortex Meter Advantages
Vortex flow meters have no moving parts, therefore unlike some other types of flow meter, e.g. turbine meters they do not require bearings to be lubricated or replaced.
Their ability to be mounted at any angle makes them a popular choice in congested plant locations. See our guide on Vortex Meter Installation for further discussion on best practice for installation.
The permanent pressure loss through a vortex meter is about half that of an orifice plate, i.e. only a few psi. However, the size of meter selected is often "one size down" from that of the pipe e.g. a 4" meter in 6" line, so that a sufficiently high Reynolds number is achieved. This practice of changing flow meter line size can lead to permanent pressure drops of around 10psi.
Vortex meters are readily available from many manufacturers and are available as SIL rated instruments. See our page on Safety Integrity Levels for further information on SIL ratings, and their importance.
Vortex Flow Measurement Limitations
Vortex meters do not like low flow, or slow moving fluids. In these situations the Reynolds number may be too low to allow vortices to form. It is for this reason that they are not usually recommended for batching or other intermittent flow applications.
They like relatively clean fluids. Sludge or slurry may coat the shredder bar disrupting the formation of vortices therefore they are not recommended in these applications.
They require straight lengths of pipe both upstream and downstream of the flowmeter to properly characterize the flow. Although the straight length requirements for a vortex meter are lower than required for many flow instruments this still could be prohibitive in some applications. See our guide on Vortex Meter Installation for further discussion on best practice for installation.
