Calculating head loss for AG pools?

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Rangeball

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May 25, 2007
766
Was at my dad's last night, as I'm taking care of their 13,500 gallon 15'x30'x4' AG pool while they're on vacation. It has a Hayward 1 hp powerflo LX and a Purex (now pentair) triton star ST50 de filter.

Looked up the specs on the pump and filter this morning, and found a potential problem. The max flow rate on the filter is 50 gpm. The flow rates at various head loss for the pump are-

10 dhl- 86 gpm.
20 dhl- 86 gpm.
30 dhl- 75 gpm.
40 dhl- 58 gpm.
50 dhl- 27 gpm.

Unless they have a very high dhl, there pump is too powerful for their filter.

The pump and filter are immediately below the deck of the pool. All pipe is 1.5". For the suction line, a 90* elbow comes off the pool, 4' of pipe straight down the another 90* then 2' of pipe into the pump. About 3' of pipe/hose (can't remember) between the pump and filter, then 2' of pipe from the filter to the pool, a 90*, then 4' of pipe up the pool into another 90* that feeds the return in the wall.

So, suction is 6' of pipe with 2 90* elbows.
Return is 8' of pipe and 2 90* elbows.

Any guesstimate to what the DHL is?

Filter pressure reads 23 psi, however I'm not sure the gauge is working. My dad said it doesn't move much. I haven't turned off the pump and relieved the pressure to see if it's still working or not. Another possibility is he said when he broke it down to clean it (after he fired the pool store maintenance), there was no DE in it that he could see, and none in the yard by the drain hose either. Not good signs. Filter elements could be clogged or damaged from being run without DE, but there's now DE back in the pool, so I have my fingers crossed.

Anyone know what the expected PSI should be in this set up?

Thanks :)

Oh, one more thing. There is a valve on the suction side ahead of the pump. If it turns out the flow rate is too high, can I partially close this valve to reduce flow?
 
If the filter pressure is correct, then I would say the head loss is between 69-80 feet but that sounds too high for the configuration so the guage is probably broken. Without at least a good working filter guage, it is pretty hard to get to a head loss.
 
Mark, I've been reading different places about how to measure how many feet of what size pipe, add so many feet per # of elbows, etc, then "refer to manufacturers charts for head loss at desired flow rate" to calculate dhl for the return side. Once this is done, add to suction side, add in filter, skimmer, main drain, any other equipment to get total DHL then look at pump curve charts.

The first chart is throwing me. I assume it's something put out that says what head loss would be at a given length of pipe of a certain size, but I haven't seen these charts. Any idea where I can find them?

I'll take a closer look at the gauge. If I can, I may replace it and put it on their tab :)

There is very good flow in the pool, the water circulates pretty fast.
 
I Wouldn't Worry Too Much

The 1.5" piping will limit the flow, probably to less than 50 gpm. I read somewhere that the max flow through 1.5" PVC is somewhere around 60 gpm, and that's without any bends. Using the usual guess of 65 ft of head would also put the flow below 50.
 
Rangeball said:
Mark, I've been reading different places about how to measure how many feet of what size pipe, add so many feet per # of elbows, etc, then "refer to manufacturers charts for head loss at desired flow rate" to calculate dhl for the return side. Once this is done, add to suction side, add in filter, skimmer, main drain, any other equipment to get total DHL then look at pump curve charts.

The first chart is throwing me. I assume it's something put out that says what head loss would be at a given length of pipe of a certain size, but I haven't seen these charts. Any idea where I can find them?

I'll take a closer look at the gauge. If I can, I may replace it and put it on their tab :)

There is very good flow in the pool, the water circulates pretty fast.
Click to expand...

I've gone down this route before without much success. I tried this with my pool and the head loss came out way off. The problem is that the fitting estimates tend to be way off as well as there are no head loss estimates for pool specific fittings such as skimmers and return eyeballs which, as it turns out, contributes quite a bit.

You could always put your guage temporarily on his filter to get a reading and then swap them back. This will get you an accurate reading on the return side at least. Then you can estimate the suction side.
 
Re: I Wouldn't Worry Too Much

ssabin said:
The 1.5" piping will limit the flow, probably to less than 50 gpm. I read somewhere that the max flow through 1.5" PVC is somewhere around 60 gpm, and that's without any bends. Using the usual guess of 65 ft of head would also put the flow below 50.
Click to expand...

As Rangeball pointed out, those limits are recommendations only. It does not mean that the pipe won't allow flow rates more than that because they will. It has more to do with water hammer damange than anything else. I explain it here.
 
Had an interesting experience with this pool last night. Went to vaccuum, vac head looks brand new, but the brushes on the bottom are all but worn out. Got the hose filled with water, attached to the vac head, installed the skimmer plate, and started vacuuming. Vac head kept getting stuck to the pool floor, too much suction, which makes me think maybe this pump is pulling very high gpms, as my pool never has this issue at about 48 gpm flow.

Needless to say, it was a PITA... well, until the head stuck firmly to the floor, wouldn't budge, pump started to cavitate and I pulled up on the pole and the top of the vac head around the pole attachement broke and came off in a complete circle. That's how strong the suction was... :shock:

I'm reluctant to take my vac head out, just in case they have mustard algae (I'm pretty sure they don't), but now it looks like I might have too. Hopefully, since my brushes are in much better shape, I won't have similar issues, but if I do, I was wondering if I could simply partially close the valve ahead of the pump to lower gpms and reduce suction?
 
I would restrict the flow on the return side vs the suction side to reduce flow. Restricting flow on the suction side is likely to cause more problems and perhaps cavitation. Pumps prefer more head loss on the return side so it is usually better to restrict it there.
 

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Mark, relating to the DHL = 4 x PSI for 1.5" pipe, if a pump is way oversized for a filter, would you expect PSI to read higher than the DHL actually is?

I'm still not certain my dad's gauge is reading correctly, but he says it has always read 20 psi with a clean filter. That would put his DHL at 80, which for his pumps flow chart would put him at very low flow, which isn't the case.

I still haven't had a chance to put a different gauge on it, and know I need to, but was wondering about this.
 
The 4x fudge factor is based upon a standard well designed pool so that may not apply here if the filter is way undersized. The filter will make the pressure read extra high with even a low flow rate.

If the gauge is working properly, then we know that the return head loss is at least 46 feet of head (2.31*20). If the undersized filter is adding significant head loss, it would be included in the 20 PSI.

If you use a minimum of 5 feet for the suction, the total head loss has to be at least 50 feet of head and probably a bit more. So I would use 50-55 feet as a range. So even at 50 feet of head, it looks like the flow may be below 50 GPM.
 
But that is only return head. Suction head accounts for something. I usually use at least 5 feet but even with 3 feet total is still only 40 feet and 58 GPM.
 
Hadn't considered that.

However, since this is an AG pool and the water level is 3' above the pump, perhaps suction head is minimal since gravity and water pressure are helping out?

Either way, 58 gpm is close enough. Still more than the filter's max rate, but it seems to be ok. For now... :)

Thanks again :)
 
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