Pool pump/fileter sizing 3

[electricpete]

Sorry if this is a little off-topic. I tried searching the normal search engines and only came up with a lot of commercial bs. I'm looking for a more technical reference.:

Does anyone know of a place to find information on sizing of a swimming pool pump and filter?

In particular thumbrules for pump and filter requirements based on pool volume.

Here's the situation. Relatively new pool provided by the cheapest bidder (not in town). It started up pretty well but have been having lots of problems keeping the water clean. We tried lots of chemistry treatments etc which I'd prefer not go go in to. The local pool company has worked through it with us and their conclusion is that the pool and filter is undersized. Of course they'd be happy to fix it at a cost of $700. I'm looking to confirm or disprove.

Here's the data.
24' diameter above ground pool 52" tall (water level 48&quot. I believe this gives approx 13,000 gallons.

Motor Nameplate:
Waterway Hi-Flo 48 Frame Pump
Century Pool/Jetted Pump Motor.... 1081/1795 Pump Duty
AO Smith
Cat BN24
Part 7-177893-24
Type SP Frame V489
rpm 3450
60HZ, 115V 9.8a

Pump Marking (no nameplate)
Waterway
p/n 315-1160
p/n 315-2500 w/ Female THD (???)

Filter Nameplate (spherical sand filter)
Clearwater High Rate Sand Filter
Tank Diameter 19"
Max Flo - 45 gpm
Front Clearance 12"
Top Clearance - 18"
Max Working Pressure - 35 psi
Max Filter Area - 2 square feet

I once opened the filter and saw that the normal flow path water comes in the top, and leaves at screened connections half-way up the sphere. I filled with sand to 3/4 since it seems like only the sand above the outlet connection has any purpose (sand below is not in the flowpath). Of course this reduces my sand surface area exposed to water, but it worked very poorly with less sand. No documentation came with the thing and the folks who sold it aren't helping either.

Both inlet/outlet connections are near the top of the pool.
Pool outlet/pump inlet has a skimmer which collects lots of stuff. Pool inlet/filter outlet has a jet which creates fairly high velocity which keeps the water circulating pretty well (a leaf on the surface goes all the way around in about a minute). Not sure how well it's circulating down below.

Pump discharge filter after going thru downstream filter is 10psig. It increases about 1 psig in a few days... we backflush about 4 times a week. Everytime we backwash we get tons of discolored water out (the filter is removing
stuff... but pool company says not enough). Pump is running 24 hours per day.

Also we have about 10' of hose on suction and discharge of the pump/filter. I'm thinking about moving the pump/filter closer to the pool cutout connections and reducing that length.

Any ideas or links? Actually I'm just looking for any thoughts, good, bad, or ugly. What's posted above is not enough to do any calcs on. I need a starting point that relates pool volume to required pumping or filtering capacity.

 

[cvg]

I'm not an expert on pools, but some knowledge on chemistry and hydraulics. where is the "dirt" coming from? common sense says that your pool should not be getting that dirty unless - leaves, dirt or other source of material is getting into the water. you could cover the pool to reduce this, trim nearby trees, etc. another possibility is that algae growth is out of control, creating large quantities of material in your water. In this case, treat the water to kill the algae. another possibility is a chemical reaction in the water causing calcium or other chemical to precipitate out and cloud the water. This also requires a chemical treatment of the water.

It sounds like your pump is doing a good job, so your filter could be undersized. You shouldn't have to pump 24 hours a day. Moving the filter closer to the pool will probably not make any difference, since your pump appears to be doing a good job already. You might be able to install a second filter (in parallel) to double your capacity. This would reduce the flow velocity and volume of water through the filter media, possibly allowing better results. You may also want to experiment with the sand media by trying a finer sand.

Chuck
cgopperton@stantec.com

http://www.stantec.com/

 

[b]

I have had a look at the information provided.There are a few pieces of information that will be required in order to provide more details comments.

The following factors must be considered in the design of the recirculation sytem:

1. What is the required circulation rate. This is generally determined from relevant swimmming pool codes. This will establish the required minimum flow rate through the filters.

2. The second factor to be determined is the total head in the recirculation system. This will be used to size the pump motor required to do the job.

3. The minimum filter surface area required is calculated using the manufacturer's recommended maximum filtration rate for the type of filter you are using and the design flow rate through the filter.


The reduction of the pipe length will not solve the problem since as you stated it is believed that the filter is under sized. You will actully be reducing the head in the system and hence allowing the pump to push more water throught the filter. The opposite of what you want.

I can send you a copy of a spread sheet that I used to evaluate swimming pool designs if you so desire. You can email me at a_b@candw.ky or abvaughan@hotmail.com so that we can discuss the relevant calculations in more details.

 

[electricpete]

Thanks for two outstanding info.

We've talked with local pool store quite a bit about the chemistry... we've done all that has been recommended.

The pool is located near a huge spawling tree which drops lots of stuff including large clumps of spanish moss that we fish out as we can. We have a well which produces fairly "hard" water.. at least judging from the reddish stains that occur regularly on our tubs and toilets. We have a whole-house pleated-paper cartridge filter and use only the filtered water to fill the pool (but that's still the same water that turns the sinks and toilets colors).

Cutting down the tree would be a good option for another reason... it will also decrease likelyhood of destroying the pool if a limb blows down. I have been considering that but still will run $400 unless I do it myself (also somewhat of a risk).

Also considering adding a more sohpisticated filter system to the house. But it's not in the near-term budget.

I believe the local pool store told us we needed to replace BOTH the pump and filter.

Why is it that you both don't think it would help to trim the tubing lenghth?

I contact you for the spreadsheet and more info.
Thanks again

 

[cvg]

reducing the piping length will increase the pressure and flow only slightly - if the pump is undersized, this won't be enough to compensate for it. As "b" stated, until you do the calculations - circulation rate, required pressure at the filter, and filter size - you can't verify what the problem is. You need to do one of the following: either remove the source of contamination, or upgrade the pump/filter to handle this high volume of particulates. This will require pressure, flow rate and filter area calcs. Also, consider filling the sand filter to the top, as the capacity of the filter is somewhat related to the amount of sand media.

Chuck
cgopperton@stantec.com

http://www.stantec.com/

 

[icw]

Undersized ..... Here are some basic calculations:
1. Pool capacity = 13,500 gallons
2. Flow rate thru Sand Filter should not exceed 5 to 8 gpm per square foot surface area.
3. Should circulate all water approx 6 times per day.
4. At 45 gpm this will take about 30 hours per day (a bit of catching up to do each day)
5. Maximum flow rate thru your filter should not exceed 16 gpm max.
6. Pump size too small for circulation - too high for filter.
7. Local pool guy seems to be right!!