sand/silt in pool

[uaguyua]

Hi I have a 2hp pump with an older 400lb sand filter (4.9sqft). I am seeing about a pint of sand appear in the pool per week and the filter seems to return a high fraction of silt/dead algea and even very small leaves back into the pool. I've taken the filter apart and replaced the sand and checked for broken laterals. My question: if I have a small crack which I missed last time I disassembled, could that explain both the sand and debris? Or would a small crack in a lateral/hub only explain the sand? I could understand how silt/dead algeae might back it back into the pool, but how could something as large as a small mesquite leaf make it back through the filter?

thanks so much,
dan

[PoolDoc]

Hi "Dan";

Question #1: Did the sand leakage start before, or after, you replaced the sand?
Question #2: Does the sand leak into the pool only after you backwash, or anytime the pump is on?
Question #3: What make / model filter do you have . . . and do you remember the air vent assembly?
Question #4: Why do you have a 2HP pump with a 30" filter? What make / model pump is it?
Question #5: Did you really put only 400# of sand in a 30" filter?
Question #6: Was the sand you added labeled as screened, quartz, filter sand . . . or was it something else?

[uaguyua]

Question #1: The sand leaked less before I replaced the sand. it is definately worse now, but it was always leaking since i bought the place.
Question #2: The sand leaks when the pump is on. I replaced the gaskets on the purge value to ensure i wasn't seeing sand leak in from backwashes.
Question #3: It is a Sta-rite HRPB30 ; the air vent is a small metal tube at the top with a metal valve on it.
Question #4: this is the pump/filter combination i had when i purchased the house. The pump is a sta-rite. I expect I am getting about 80 gpm with the setup, which is inside the specifications for the filter (according to the owners manual, at least).
Question #5: The manual called for 400 lbs of sand....and that filled up half of the bell (to where the bolts are)
Question #6: The sand was labeled as 'pool filter silica sand #20'. I tired to find the sand the owners manual said to use (45mm-55mm) but could not find sand graded that finely.

[PoolDoc]

Ok.

+ If it leaks sand ALL the time, it's in the filter, not the valve.

+ That pretty much means it's EITHER a cracked lateral or manifold OR a problem with the internal air vent. That's an old filter, and I've never worked on one, so I can't go much further. I did take one apart once, and replace it, but that's all.

+ A 2HP pump @ 80 GPM is too much for any 4.9 sft sand filter. At 15 GPM per sft, (73.5gpm), you already get suboptimal performance from sand filters. But, when you go above that, you blow more and more dirt through the sand. I personally size filters at 12 GPM (so I still get 15 gpm/sft backwash) and maximize the sand bed depth. The higher NSF approved ratings only prove who owns the NSF, not that filters work well at those flow rates.

+ Split case sand filters, including both old ones like yours and new ones like the Sta-Rite System3 units have extra shallow effective sand bed depth, and so require even LOWER flow rates to perform well. Your filter is going to blow silt through the sand EVEN if everything is right. Not leaves, though. The fact that it took only 400# to get up to the line is an indication of the problem. A good 30" sand filter, like the TR100 takes 600# of sand.

[uaguyua]

Thanks Ben. I didn't realize that my pump/filter were mis-sized (the manual says the acceptable flow rate is between 73gpm and 98gpm). I had assumed this was okay since the HR in HRPB30 stands for 'high rate'. I will look into buying a smaller pump.

I was kinda guessing that either I missed a problem with a lateral or the hub or I somehow damaged the laterals when I put the new sand in (I followed the instructions and filled it 1/4 way with water). But my main question for the forum is 'Do the tiny mesquite leaves blowing back into the pool indicate that something else really serious is wrong with the filter or could they also be caused by a broken hub/lateral? I don't want to spend a lot of $ fixing the lateral/sand issue only to find something else is also wrong. At this point should I just bite the bullet and buy a new filter or is there anything more I could do to diagnois this?"

thanks again,
Dan.

[PoolDoc]

The "high rate" thing references some old changes in filter ideas, that were mostly bogus at the time (30+ years ago).

The leaf problem points to something like the filter air vent. If there is an air vent, it should have a screen or something like it, on the upper exposed end. A small leaf could transit that tube, under certain conditions. Look at the part #12a in the image -- that's what WOULD keep out the leaf, if it were there.

Regarding the pump, depending on the particular pump model, it may be possible to replace the impeller / diffuser so that it will run as if it were a 1.5" HP pump -- the electric motor won't care, and your electrical costs will go down, while your filter efficiency goes up.

BUT . . . before you do that, you'll need to send photos to poolforum@gmail.com of the pump, filter AND piping. I'll post them, and we'll see what's possible.