Swimming Pool pump/filter sizing

[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?

 

[electricpete]

Come on you guys. Excercize those brain cells. Prove to me that there are a few things mechanical engineers can teach to electrical engineers ;-)

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.

 

[peterpom]

Hi electricpete,

It sounds to me that the system is poorly designed and undersized.

First to your sand filter. The water flow should be in the top and out the bottom. I have never seen a filter that exits at the halfway level. As you say, half the sand is not in the flow path and useless. I don't know what you can do othee than replace the filter.

The pump should be big enough, looking at your available power, and assuming no cavitation (entrained air).

As a rule of thumb, I always design pool filters to turn the water over within 3 to 4 hours, and your pump should be capable of this.

Pool filters always draw from the top of the pool, so as to provide leaf skimming. As you say this aspect is working well. To clean the bottom of the pool requires using a vacuum line. Occassionally add a floculant to the water, (you pool shop will call this a clarifier or water polisher) to preciputate suspended solids and make them available for vacuuming.

In summary, change the filter for a larger unit, that uses the whole depth of sand. Ensure that your chemical balance is kept within spec, and vacuum regularly. My home pool runs 3 hours a day, and is vacuumed once a week, and is always sparkling.

I hope this helps.

 

[235zzzo]

Hello Electricpete!

Following Peterpom, it seems to be correct the values he has mentioned, however I would say, that you don't have to replace your sand filter, but to add another one, (it's cheaper!) in a paralell circuit, both served by the same pump, which is supposed to be oK.
Please give attention, it has to be installed one retention (one sense) valves, and one manual regulation valves in each paralell circuit, in order to balance respective pressure drops and flowrates.

One criterium for calculating the diameter of your sand filter is based on the crossing-section velocity, about 1m/sec or ~3 ft/sec, and the high, being as much as twice that diameter.
I hope your swimming-pool water becomes nice! Good luck!
zzzo

 

[SErgini0]

Christ!
What a poor job! Your pump is ok, maybe a little undersized but it ain't a big problem. Your filter is no good. It sounds like there might be a defect in it. However a thumb rule in swimming pool design, try to slightly oversize things.
I think a parallel filter might solve your problem, but certainly go for a new bigger filter. I would have resized you the filtration system, but I'm quite poor in US units, I usually work in SI.
Best of luck!

 

[recycled]

With the pool pump you have ( estimated 3/4 hp - 45 usgpm) your filter would be too small at 19" dia.

I suggest an additional filter of same size operating in parallel

take care with the suction piping it should be 2" rigid piping to get a reasonable suction velocity ( < 5 ft/sec)with a discharge piping dia of 1-1/2"

If filter backwash is dirty then dump system flush lines and restart

I presume this is a residential pool therfore bather load is desigh at 6 with a max of 15.

Hope this helps

Recycled

 

[TheBlacksmith]

Backflushing at a 1 psi increase over clean 4 times a week! Maybe you are backflushing too much. A lot of the particles that cloud the water are relatively small. Filter sand is relatively coarse. One approach, that costs money, is to add the floculant as mentioned above, it clumps the little stuff into larger clumps that are easier to filter. Another is to add diatomaceous earth, that costs money, it fills in the spaces between the sand and makes it a better filter. If you just let the filter run until the pressure over clean increase 50% to 100%, you accomplish the same thing. The filter may be a little undersized, but letting the sand pack and settle and tighten up between backflushes may help.

Blacksmith