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# Calculating water velocity in hot water pipe2

## Calculating water velocity in hot water pipe

(OP)
Hi All:

I'm currently running some water velocity tests on a hot water pipe loop and came across somethin strange that would like to get some help from the forum:

The team used 2 methods to measure pipe velocity and we are getting very different results. Both of them seem adequate, so I'm struggling to understand why results are so different.

The first method is simply converting the flow into velocity. We teed of the return hot water pipe and measured the time that it took to fill a known volume
The second is measuring the flow on the balancing valve using a Comdronic Electronic manometer.

The problem is that we are consistently getting lower values with the Comdronic method, sometimes 3 or 5 times less. which is way too big of a difference.

I think that the known container volume method is quite low tech but is fairly accurate, so I'm more inclined to suspect on the Comdronic, but still couldn't nail what they are doing wrong.
Any help would be appreciated.

Thanks

### RE: Calculating water velocity in hot water pipe

Flow rate measurement by measuring filling time of a tank is more accurate.
For the Comdronic method please check:
1. Whether right kind of online valve (across which pressure difference is measured) has been used.
2. Calibration of digital meter is OK.
3. Whether proper procedure of measurement has been used as per manual.

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### RE: Calculating water velocity in hot water pipe

Never use more "tech" than you need.

### RE: Calculating water velocity in hot water pipe

It looks like the manometer is set to calculate flow based on DP and I would guess a valve CV.

Hence you need to be sure that the valve is the same one as in its large database, but if this is a variable valve then it probably won't work because the manometer has no way of knowing what the valve opening is unless you tell it. For variable valve it can tell you how much to close it to achieve a set flow based on the DP, but not flow unless it knows the opening percent.

Also note that the manometer needs purging for every connection before use and seems to be quite sensitive to this.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Calculating water velocity in hot water pipe

It seems to me that the volume measurement method, while valid for an open pipe, might not necessarily be representative of the closed loop system.

I would want to know for sure that there isn't some sort of flow restriction or back pressure that affects your inline instrument in the closed loop configuration.

TTFN (ta ta for now)
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### RE: Calculating water velocity in hot water pipe

Can you use a temporary orifice plate of known ID and edge sharpness in place of the valve as a check of both?

Yes, it would require a separate test - which is not cheap, but hot water is not an expensive media and it appears the validity of the main test runs require an independent verification of flow. Or the whole series of test results is wasted effort.

### RE: Calculating water velocity in hot water pipe

Low tech method already knows the flowrate.

### RE: Calculating water velocity in hot water pipe

To follow up on IRStuff's suggestion...Does your volumetric measurement involve free discharge at atmospheric pressure? If so, and assuming that your manometer measurement is measuring flow in a closed and pressurized system, you should see different results because the hydraulic conditions are different.

============
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### RE: Calculating water velocity in hot water pipe

(OP)

I was expecting some differences between the methods for the reason pointed above by IRstuff and others.

What I wasn't expecting were factor 4 to 10 differences in the measurements, so I'm scratching my head over this...

### RE: Calculating water velocity in hot water pipe

It's unclear, after all this time, what your system actually looks like.

My understanding of "balancing valve" is that it's intended to throttle flow in a system to control the temperature downstream; by definition, then, its flow, under normal operating conditions, would be nowhere close to what an open pipe can sustain.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

### RE: Calculating water velocity in hot water pipe

(OP)
Thank you all for your input.

After some head scratching and hints from this forum. mystery was solved.

I spoke with the guys that actually did the work and after all, these tests are not comparable.

They teed off before the balancing valve for the volume container test, so on top of pressurized vs atmosphere issue, naturally the water volume is completely unrelatable.

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