Listed Stream-Straighteners for a Flow Meter?

8/13/2021

We are having problems getting a flowmeter to accurately read the flow rate that matches what we are measuring downstream out through a test header. A reading using calibrated pitot gauges outside at 1,500 gpm, for instance, is showing 1,900 gpm on the flowmeter inside.

Military specifications for the project require that the flowmeter read accurately, and that the system be piped such that flow must go through the flow meter out through the pump test header.

The flowmeter manufacturer has recommended using a stream straightener upstream of the flowmeter to better streamline the flow through the meter and get more accurate readings.

Does anyone know of a stream straightener that could be used in a fire pump test loop arrangement?

I would think it would have to be listed, but that's just a gut feeling. I don't currently know of any listed stream straighteners. We generally need to meet the black and white code and specifications mandates spot-on without any code alternatives, if possible, being that it's a military project. Thanks in advance.

Sent in anonymously for discussion. Click Title to View | Submit Your Question | Subscribe

10 Comments

Pete

8/13/2021 06:45:17 am

Crestar's line of stream straighteners seem to be made for firefighting equipment and are "NFPA 1964 compliant", but I don't see a UL listing on them.

Can you get the AHJ or whoever is in charge of overseeing the military specifications to approve the use of one?

8/13/2021 07:41:33 am

We probably wouldn't be able to get approval on anything outside the spec unless it was our only option. It's been very black and white to the spec to date.

Alex

8/13/2021 07:34:23 am

Hi Anonymous,

I’m in the same boat as Pete. There are a few stream straighteners that I’m aware of but all are for firefighting use. I have never seen or specified one to be used within a wet system / your scenario.

If the flowmeter manufacture believes you have turbulent flow, could you possibly relocate the existing flowmeter elsewhere on your system? Somewhere you have a longer straightaway, rather than right off of an elbow?

Tough one this morning!
Alex

8/13/2021 07:44:28 am

Right! It's been a tough project for sure.

The flowmeter is placed with exactly 5x diameter upstream, and 2x diameter downstream, which is what the install instructions require.

The manufacturer's engineering team is saying that we're likely getting an inconsistent stream because of the water bending through a tee on the upstream side. They said a stream straightener can help straighten the flow and clean up the readings.

Unfortunately we've maxed out the room and are very limited on space. Was hoping to find something that would help clean up the flow before the flowmeter to solve our issue here.

8/13/2021 08:34:39 am

I will keep an eye out for a stream straightener to fit in your case but sadly haven't heard of one. Maybe the manufacture can recommend one since they are guiding you to go in that direction?

Keep us posted.

Dan Wilder

8/13/2021 07:58:23 am

I would recommend pulling the flow meter and send it in for calibration.

Secondly, we have placed a second flow meter (same MFG and type) at the end of a system and tested through both to verify GPM.

Without knowing what the system setup looks like at the flowing end
-are smooth bore nozzles being utilized and have they been verified for size (1⅛ vs 1¼ is sometimes mixed)
-is the setup between the pitot tube and the gauge utilizing flexible tube (remote read) and is the gauge at the same elevation as the pitot tube, was the tubing manifold flushed and verified clean?
-is the flow setup using an in stream pitot or are these pitotless setups? I've has the in stream pitot tube bent due to debris and not perpendicular to the stream causing a reduction in the GPM readings

8/13/2021 08:10:45 am

All good questions - thanks Dan.

This is our third calibrated meter we've used. They all read the same, so we know the issue has more to do withour system than the meter.

We've flowed outside using 50-ft hoses with diffusers, and with 18-inch 2-1/2" open pipe to reduce backpressure, measuring flow with pitot gauges. In both cases the flow readings did not match the flowmeter inside.

On the flowmeter itself, there is flexible tube between the pitot tube and the gauge. The gauge is higher than the pitot tube (gauge is vertically above the top of the pipe). Tubing has been flushed and cleaned.

This meter has an instream pitot. We can definitely check to see if it's been bent or clogged. We'd be thrilled if that was the fix.

Manufacturer's team has also suggested altering the angle of the flow meter to a 3 o'clock or 9 o'clock position and verify accuracy, since the water flow around the bend on the upstream side is affecting the velocity coming through. We're going to try this first before we go with any kind of stream straightener situation.

8/13/2021 08:26:50 am

I have also had luck increasing the size of the supply side of the flow meter (before the 5 diameters of straight pipe) and using a concentric reducer (don't forget a means to vent the trapped air) making kind of a reservoir for the turbulent water to calm a bit. Most of my riser rooms weren't that big but got lucky on one to be able to try it.

Brad K

8/13/2021 02:46:28 pm

I agree with Dan. We have seen these flow meters perform poorly and be corrected with calibration by the manufacturer.
The laminar flow is key and sounds like you have met the required distance if verified in the device manual.

John G

8/13/2021 08:37:29 am

All good answers. I put an air relief on my flow meter systems, nfpa 20 has a diagram, that seems to even the stream out. At the highest point.

Listed Stream-Straighteners for a Flow Meter?

Leave a Reply.

ALL-ACCESS

SUBSCRIBE

COMMUNITY

YOUR POST

PE EXAM

FILTERS

ARCHIVES

PE PREP SERIES