Empirically measuring water flow

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kiro

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Aug 29, 2014
5
Whittier, CA
Doing my best but new to this forum... so all the usual noob considerations requested.

My (old) filter has a leak at the bottom so I'm looking into replacing it (because I've been told and read repairs on older equipment isn't really worth it). All equipment issues/discussions begin with flow rates. I'm trying to determine my actual flow rather than what every contractor has been telling me: "look it up the manufacturer website and take their word for it."

The guy quoting me on a new filter said I'm getting 80gpm per pump spec.

My test has been: Submerge a 5 gallon bucked in the pool well below the water line. Insert the squid hose (minus the squid) into bucket. Raise the bucket levelly. As the lip of the bucket crosses above the waterline, start a timer. When the hose sucks the last of the water out, stop the timer. (Quickly submerge hose to suck as little air as possible.) Repeat until you get an average number. (Yes, I'm bleeding the air out of my filter.) (Yes, I'm trying to induce as little resistance as possible when I'm holding the hose in the bucket.)

My average time is around 13 seconds.

That's around 23gpm and 1,400gph and 33,000 gpd. Which is a lot less that 80gpm and since my pool's roughly 41,000gal pool: I'm screwed.

Is my experiment ideal? Absolutely not. Is it subject to a 71% error? Doesn't seem like it should be.

What, if anything, am I doing wrong? What, if anything, can you suggest?

Thanks,
 
I would suggest a flowmeter such as the blue white F-300 pitot tube acrylic.

Alternatively, you could put a vacuum gauge on the suction pipe in front of the pump and use that with the filter pressure gauge to determine the total dynamic head and then use the pump curve to calculate the flow rate.
 
kiro said:
What, if anything, am I doing wrong? What, if anything, can you suggest?
Click to expand...
Despite what you think, you are probably changing the characteristics of the plumbing measuring that way.

But as a rough order of magnitude a WFE-4 will produce about 64 GPM on 1 1/2" plumbing setup and 82 GPM on a 2" plumbing setup. I can get closer if you can tell me the following:

Current clean filter pressure
Distance from pool to pump
Pump elevation relative to the pool
Suction size plumbing and if you have more than one run pool to pump
 
53 GPM @ 74' of head give or take. You have fairly high head loss plumbing. Suction side is 15' and return side is 59'. I assumed a 1 1/2" multi-port valve on the filter too.

That pump is really too big for 1 1/2" plumbing. If you decide on a new filter, I would downsize the impeller to 1/2 HP. That would drop your flow rate to 46 GPM at 56' of head.
 
JamesW: Thanks and an actual flow meter seems like a good thing to have in your system one way or the other. This a good, long term meter? I don't mind paying a few extra bucks for accuracy and longevity.

mas985: Thanks for the calculations. The pool has been perfect for ages so I assumed I was doing something wrong.
 
mas985: The filter has a rotary valve for backwashing and the system has no separate multi-port valve.

JamesW: Yes, the pressure gauge zeroes when the pump is off.


Part of my confusion lies with the piping (all PVC unless specified):

SECTION 1:
There is a 2" pipe coming out of the ground and into the pump.
A 2" pipe from pump to filter.
A 2" pipe from filter which necks down to a 1.5" metal pipe and then necks back up to a 2" fixture on the heater.
A 2" fixture on the heater which necks down to a 1.5" metal pipe which necks up to a 2" pipe in the ground.
In the pool, there are two, what-seem-to-be 1" pipes returning the water to the far side of the pool.
At the bottom of the skimmer (near side of pool), there are two, what-seem-to-be 1.5" pipes. To one, I attach a valve with has a spring-load port (for some half-@$$ skimming action) and connects to the squid. The narrowest point of these connectors is 1.25".

SECTION 2:
There is another what-seems-to-be 1" hole at the bottom of the skimmer that, I'm told by all contractors, connects to the drain at the bottom of the deep end. This line is unused.

SECTION 3:
There what-seems-to-be 1" port at the middle of the pool, that all contractors tell me, goes to the sealed-off 1" pipe at the pump (ie. not used). They say this was for the old-fashioned pool sweep that swam around the surface and had tentacles that writhed along the bottom.

SECTION 4:
There is a 2" pipe from the filter which necks down to 1.5" pipe in the ground (which goes to a P-trap used for backwashing).



1) So is my pumping system limited by the port at the bottom of the skimmer which seems to be 1.25" (thanks to the connectors)? Given this bottleneck, is there any benefit from replacing the two section of 1.5" metal pipe at the heater with 2" PVC?

2) Is there anyway to put SYSTEM 2 into use to my advantage?

3) The drain at the bottom of the deep end has no purpose? (Used be a leaf catcher down there in the Dark Ages).
 
1) Reductions are not an issue but pipe size can be if you have a lot of it. But since it is such a small length, it would not be worth your effort. Although it is metal pipe and that can be an issue if the PH gets too low so there is that risk.

2) Not sure what you are asking here. Can you elaborate?

3) Not much of a purpose. It adds a little to circulation but if you cap off your MD, you are unlikely to see any difference in water quality.

But given most of your plumbing is 2" that changes the head loss calculations some. Now I am getting a more reasonable 66 GPM at 67' of head. But still a little high for 2" plumbing.
 
I have an existing one inch line from the pool to the pump that isn't being used. It wouldn't be difficult to splice it into line before the pump or after the heater. It would become an addition supply or return line, whichever could use it more. Wouldn't decrease my resistance/increase my flow/decrease my power consumption?
 

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Actually, when you decrease head loss, it increases flow rate but it also increases power usage. However, gallons/watt-hr does go up but that is usually not much of a benefit unless there is a way to reduce run time. But run time seems to be dictated more by how long it takes for the skimmers to clean the surface and the pool cleaner to sweep the floor which are not impacted much by small changes in flow rates.

So improving your plumbing to improve flow rates is somewhat self defeating because you could end up paying more in energy costs. That is why I usually don't recommend plumbing changes unless something is not working well (e.g. spa jets).

But one thing you could do that would have a pretty big impact on energy costs is a change in impeller size and/or changing your motor to a two speed.

Your pump motor probably draws close to 2000 watts. With a two speed, on low that would only be about 430 watts. If you change the impeller to a 3/4 HP, the power becomes 1600 watts on high and 370 watts on low.
 
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