Drain Testing using Dye For Effect

[JamesW]

Drain Testing using Dye For Effect on Pool Circulation - YouTube

Maybe the test in this video is real, but in my opinion it does not look right. I don't think that the dye would float by if the drain was pulling 800 gpm.

The video was posted by a channel called "Suction Entrapment ". Maybe they have a motive to fake the video.

Maybe it's real. However, I'm not convinced. Maybe if the people who did the test had some sort of expertise in fluid dynamics, they would be more believable.

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Note: Split from this thread.

Pentair performance curve for dual speed, low head limit - Page 2

 

[atttech-2]

I think it may actually be real, I have seen other dye tests in an actual deep end do the same thing. Past a certain amount of water over the drain I don't think the depth of the water has much to do whit it. Suction entrapment doesn't happen because someone swims by and is suddenly pulled in from several inches away by the massive flow. It happens when people come in contact with the drain and it gets very nearly 100% covered that is why the dome shaped drains work well, it is difficult to for someones skin to totally cover the dome shape and when the drain is elongated or connected to a second drain like modern builds the probability drops even further.

The video strikes me as a demonstration that might be used in court to show how deceptively dangerous the drain can be. It is fairly clear that drain does not meet modern standards and while it is tragic it is not hard to believe someone died as a result.

 

[pooldv]

My pool has modern dual drains with 2" pipe teed together under the pool. I sometimes brush the pool toward the drain to clear "dust" from the bottom of the pool. I set the skimmer/drain valve to 100% drain and set the VS pump to 2500 rpm. As I am brushing the dust it builds up as you get to the drain just like sweeping the floor, the more you sweep the bigger the pile gets. When I get to the drain there is an easily visible pile of fine debris that can be sitting on the bottom less than an inch from the drain and not be sucked into the drain. If you brush too fast it will float up several inches and float right over the drain with out getting sucked into the drain. The "dust" has to be very slowly brushed across the bottom directly into the drain for it to sucked into the drain.

I haven't calculated by head loss, but my plumbing is simple and this was before I installed the heater with solar bypassed. So, somewhere around 90-100 gpm is my guess.

 

[slickraft]

You would think that if the drain in the video were really flowing 800GPM that you would see some whirlpools trying to form since the water was shown as only 11" deep.

My pool was built in 94. It has a single main drain that does not meet current standards but the drain is fully disabled with a threaded drain plug inside the skimmer basket. It's been that way for years. No worries at all when kids are in the pool. The only time I enable the main drain is when I empty the pool to do water exchanges due to calcium levels.

 

[mas985]

That video is referenced in a paper written by the APSP so I have a hard time believing they would publish a paper with a fake video. You can explain what you are seeing simply by look at the velocity of the water which is quite low because the volumetric area around the grate is so big (~ 6 sq-ft) so I think it is very plausible.

http://apsp.org/portals/0/PDFs/Case%20Study%20-%20The%20Option%20of%20Pools%20without%20Main%20Drains.pdf

 

[duraleigh]

that you would see some whirlpools trying to form

There is a small whirlpool in the video.

Similar to a skimmer being ineffective when the water is too high (well, sort of similar), I think 11" of water is enough to buffer any surface affect.

 

[JoyfulNoise]

Drains typically have very high flow rates around the edges and it falls off as you get to the middle of the opening. There is NOT uniform water velocity across an orifice. I would expect a test particle in the water crossing the drain to experience very low forces once it is a few inches away.

To test this, I filled up the utility sink in my wash room (15"x21" and 13" deep or ~17 gallons with a 1-1/2" drain opening). I calculate the flow rate of water through that drain to be ~ 44.6 GPM assuming no plumbing loss from the p-trap. Let's just call it 40GPM for arguments sake. I placed my arms into the sink and held a very thin and flexible plastic reed near the drain. I pulled the drain plug and observed the reed's deflection.

At the edges of the drain, the reed deflected a lot (nearly sucked into the drain). A few inches (maybe two or three) directly above the drain and the reed hardly bent at all (until the top surface of the water came near and vortices formed).

So based on those simplistic observations, it does not surprise me that the dye floats by the drain in that movie.


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[JamesW]

Maybe it's a legitimate video.

I get that the velocity drops off exponentially as distance increases.

I would suspect that it's probably a cube factor. For example, if the water velocity is V at 1 unit of distance, then it would be V÷8 (2^3 = 8) at 2 units of distance and V÷ 27 (3^3 = 27) at 3 units of distance.

I suppose that at 800 gpm, I expected more action.

While I think that it's possible, I still have some doubt.

 

[JamesW]

Regarding the paper written by the APSP, we don't know that they did anything to authenticate the video.

Maybe they just found the video, believed it and ran with it. They even say that they do not claim that the information is accurate.
Here is what they say near the end of the article:

The Association of Pool & Spa Professionals makes no claim, promise or guarantee about the
accuracy, completeness or adequacy of the information contained in this article

The APSP position is biased in the paper against main drains.

I would find the video much more believable if it was done by someone with good credentials.

There are lots of fake videos online and people need to be cautious not to believe everything they see.