|FLOW METERS: PIPE LAYOUT RECOMMENDATIONS
Nearly all flow meters must be installed so that there is a significant run of straight pipe before and after the
location of the flow meter. This is intended to allow the straight pipe run to "smooth out" any turbulence
produced by the presence of valves, thermowells, chemical injectors and diffusers, and changes in pipe direction. This
type of turbulence produces error in the reading of most flow meters. For more information on Global Water's
flow meters click, HERE
How Much Can The Flow Meter's Error Be?
The flow meter's error can be quite large. The error produced by a thermowell installed immediately upstream
of a flow meter can be in the range of 5-10%. That of a gate valve or a butterfly valve upstream of a flow
meter can be as much as 50-60%! The error produced from a partially closed ball valve can be as much a 50%
for flow meters. Chemical injectors can produce significant error in the flow meter reading also. For
example, a chlorine injector-diffuser may produce enough entrained undissolved chlorine bubbles to produce an error
in the 10- 20% range of the flow meter output.
Good Flow Meter Installations
We suggest at least 10 diameters of straight pipe run upstream and 5 diameters of straight pipe run downstream of any
flow meter installation in order to achieve proper accuracy. These are minimum suggested values. As the
pipe layout diagram shows, you may need much more straight run prior to the flow meter, under specific circumstances.
Preventing Flow Meter Cavitation
Cavitation can be caused by entrained air, and it can seriously damage the rotor on a turbine flow meter. An
amount higher than about 100 mg/l of entrained air or gas can produce error. In addition, cavitation can be caused
by too little backpressure on the flow meter. For turbines and other rotary impeller flow meters, you should
provide a backpressure (downstream pressure) of at least 1.25 times the vapor pressure, plus 2 times the pressure drop
through the flow meter. This can usually be accomplished by providing the minimum downstream straight run
after the flow meter and prior to discharge.
When In Doubt, Straighten The Flow Into The Flow Meter
There are a number of ways to straighten the flow. If you need the highest possible accuracy from your flow
meter installation, you should install a separate flow-straightening device upstream of the flow meter. Any flow
straightener requires additional diameters of straight run. Usually the mimimum requirement is 7 diameters
before the flow straightener and 5 diameters between the straightener and the flow meter itself. Using a flow
straightener is not a substitute for correct amount of straight pipe run!
What If You Can't Provide Adequate Straight Run For Your Flow Meters?
If you can't provide enough run to smooth out the turbulence caused by valves, fittings, and changes in direction, you
will have to live with the inaccurate effects this turbulence will create for your flow meters. This does not mean
that the flow meter's reading is meaningless, however. In the majority of applications, the flow meter may be
enough to provide a repeatable reading, if not an accurate one. In applications where the flow meter is a control
device, operating a valve or controlling chemical addition, repeatability of reading is more critical than absolute
accuracy. You may find that you can get excellent results without excellent accuracy.
Some Rules of Thumb For Flow Meters
To the right are several diagrams with some recommendations to assist you in properly locating your flow
meters. Remember these are only Rules-of-Thumb for flow meters. If you do experience error in spite of
observing these recommendations, you may be able to perform an "on site" flow meter calibration to attempt to
minimize the error.
Please visit our flow meter page for more information on these products.