Flowmeter Separation Distance Include Fittings?

2/7/2020

Split case pump - flow meter installation question:

As indicated in NFPA 20, the distance from the flowmeter to either isolation valve should be as recommended by the meter manufacturer.

Based on your own experience, do fittings (elbows or bends) need to be included in this distance calculation?

For example - should the distance recommendation length of 10-diameters be calculated from the last fitting or is the fitting ignored and only calculated from the isolation valve? Thank you.

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3 Comments

Dan Wilder

2/7/2020 10:13:54 am

That minimum distance is in reference to the length minimum of STRAIGHT pipe, as based on 2 or 5 diameters of pipe before and after the flow meter. If you meet that minimum, the configuration outside of those minimums isn't an issue with that type of flow meter. In this case, the butterfly valve disc is the point measured (center of the valve) but if you were to have an elbow or tee (changing water direction), that point would be from the start of STRAIGHT pipe, not the center of fitting.

Good practice would be that you minimize the supply side turbulence as much as possible but after the valve should not be near as much an issue.

Mike L

2/7/2020 11:41:04 am

Agreed with Dan's response. The minimum pipe length before and after the flow meter is the straight length provided to allow the water to return to laminar flow and get a more accurate reading on the meter.

Mike

2/9/2020 09:52:44 pm

It's best to have laminar flow so the meter reads accurately. It's even better to use two valve - inlet that is required and downstream that is suggested.

Then fully open the upstream valve and use the downstream valve to throttle flow for the most accurate reading.

This suggestion was given to me by a pump manufacturer and makes a lot of sense.

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