max distance the pump can handle with garden hose

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danielsonway

Active member
Aug 15, 2022
26
Arizona
Pool Size
13000
Surface
Plaster
Chlorine
Liquid Chlorine
Hello,

I am getting ready to SLAM my pool. Before that though, following the advice of fellow forum experts, I'd like to replace about half the water in my pool.

I ordered a Superior Pump 91330 1/3 HP and the user guide shows flow rates up to 25 ft. I need to dump the water 50 ft away. I am wondering if anyone has any experience whether that is possible and what the flow rate would be?

Many thanks.
 
For a 3/4" garden hose at 50 feet long, the flow will be about 14 GPM.

With a 1" hose, the flow will be about 22 GPM.

This assumes that the dump point will be at about the same level as the pool surface.

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PVC Pipes Schedule 40 - Friction Loss vs. Water Flow

Water flow in thermoplastic PVC and CPVC pipes Schedule 40 - friction loss <i>(ft/100 ft, psi/100 ft)</i> and flow velocities at dimensions ranging 1/2 to 16 inches.
www.engineeringtoolbox.com www.engineeringtoolbox.com
 
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Mechanical Engineer here. When they talk about 25 ft, they're talking about vertical lift (height), not horizontal distance. The head pressure to flow rate relationship for pumps is always identified by their pump curve. Yours can be found in your manual. I also located it here. Looking at your pump curve, at the maximum head of 25 ft, you will get 2 gpm. After that, the pump will stall. At zero head (no pressure), your pump will deliver 40 gpm. The head pressure will be determined by two things, your "static head" which is essentially the height above the surface of the water in your pool you are trying to pump plus the "dynamic head" which is the pressure loss caused by the friction of the water moving through your garden hose. So the shorter you make your hose, the better. But let's go with the 50 foot of garden hose you mentioned.

Let's make a few assumptions: Let's assume you aren't pumping "uphill" and are merely lifting the water from the surface of your pool to the ground. We can conservatively estimate the "static head" to be 2' (probably less). Based on the pump curve, and interpolating between the 5 psi and 0 psi points, we can estimate your pump can deliver approximately 37.6 gpm of flow at a static head height of 2' with NO HOSE attached.

Now, determining the dynamic head is a little more tricky because the length of the hose determines the pressure drop, but the pressure drop influences the flow. So this solution has to be done iteratively. I found a nice little irrigation calculator here. We will plug the starting pressure which will give us a flow rate, we will then look up the pressure at that flow rate on the pump curve, and plug it back into the calculator. We will repeat this until the flow rate determined by the calculator no longer changes.

After plugging several values into the calculator 12.5 gpm will give just under 20 foot of head which, on the pump curve yields about 12.5 gpm. This is the point of zero error. Add your 2' of static head and you will be at roughly 20' of head pressure on the pump which will give you 12 gpm exactly. We could break out formulas and get a lot more accurate, but this will suffice for the kind of estimate we're doing.

So expect about 12 gpm out of your little pump. To drain half of your 12,500 gallon pool, you can expect it to take about 8 hours 45 minutes.

Hope that was helpful.
 
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Wow, I am amazed by this forum. Many thanks JamesW and lucasaltic.

My bad in misreading the chart for static head as if it was for dynamic head.

JamesW - I am still confused about your hand-drawn magenta and black lines for dynamic head vs flow rate though. Should they not also have negative slopes?
 
That's called a system curve.

The more flow you try to put through the line, the more head loss or resistance there is.

The intersection of the system curve and the pump curve is called the operating point.
 
You can use a chart or a head loss calculator to determine the dynamic head loss at each flow rate and plot that on the pump curve so that you can find the intersection.

Since the head loss increases as the square of the flow according to the affinity law, you can calculate a system curve equation from 1 or more known points.

If you double the flow, the head loss is 2^2 = 4 times and the power is cubed or 2^3 = 8 times.

Hazen-Williams Friction Loss Equation - calculating Head Loss in Water Pipes

Friction head loss (<i>ft<sub>H2O</sub> per 100 ft pipe</i>) in water pipes can be estimated with the empirical Hazen-Williams equation.
www.engineeringtoolbox.com www.engineeringtoolbox.com

If you are draining and the pool surface is at the same level as the end of the hose, the dynamic head is the total head.

If the end of the hose is higher or lower, you have to add or subtract the static head loss.

As the pool drains, the static head loss changes by the surface elevation decrease.

For above ground pools, the surface is usually higher than the end of the hose at the outlet.

For inground pools the surface is usually lower than the dump point but sometimes it is higher.
 
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danielsonway said:
Wow, I am amazed by this forum. Many thanks JamesW and lucasaltic.

My bad in misreading the chart for static head as if it was for dynamic head.

JamesW - I am still confused about your hand-drawn magenta and black lines for dynamic head vs flow rate though. Should they not also have negative slopes?
Click to expand...
You’re in good hands at TFP! You’re best to go with @JamesW ’s solution as it was much more rigorous. He took the time to plot the curves. That said, it was kinda cool we got kind of close with two completely different methods. Let us know what your actual pumping times turn out to be!
 
BTW - what method are you planning to use for replacing water.... Just making sure you know that it is not a good idea to lower the water level in a FG pool.
 
I ran the pump for 11 hrs last night. I did water exchange. Though my supply was a little lower than my drain flow. So at some point I had to stop the pool pump.
With this exchange I was able to drop my CYA from 90 to 30! Now I am ready to start SLAM.
 
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JamesW said:
Were you able to estimate the flow rate?

What hose diameter did you use?
Click to expand...
If he dropped it from 90 ppm to 30 ppm, we can assume he replaced 2/3 of his water, or about 8,000 gallons in 11 hours (this assumes negligible short circuiting). That comes out to 12.1 gpm.
 
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