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Straight Length Requirements for Vortex Meters

Why are straight lenghts important?
Like most flow devices, a 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 straight lenghts defined?
Most manufacturers provide the user with minimum distances for their particular products. These distances, refered to as straight lenghts, are indicated in Pipe Diameters (D). For example, 10 D means place the flow meter ten times the pipe's inside diameter away from the source of turbulence. Because turbulence both upstream and downstream can reduce accuracy, manufacturesrs provide straight length requirements for up 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 definately 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.
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
 
Source of TurbulenceUpstream DiametersDownstream Diameters
Line reduces in size15 D5 D
Line increases in size35 D5 D
Single 90? bend30 D5 D
Two 90° bends
in same plane
30 D5 D
Two 90° bends
in different planes
40 D5 D
Fully open valve35 D5 D

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Further Reading

Flow Measurement
 
An Introductory Guide to Flow Measurement
A typical vortex meter

C&I Resources

 
ATEX
Cable Gland Selector
CE Marking
Ingress Protection
Pipe Schedules
Relief Valve Orifice Size
Temperature Class
Temperature Conversion
Thermocouple Types
Valve Leakage
Vortex Meters
Wires and Cables
 

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