Pentair DE Filter Issue

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DHinNC

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Apr 26, 2007
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I have a Pentair FNS 60 square foot DE filter on an inground pool. Anyone have any problems with internal corrosion on the SS nuts that are used on the rods that holds the filter grid assembly together? The pool is a salt water pool that is just now 5 years old. Today when I disassembled the filter I noticed that the nuts that hold the SS rod to the bottom grid rack were corroding. One just crumbled apart when I tried to remove it. The rod hasn't corroded at all. Both nuts on the each of the 2 rods had corroded. Neither rod had any corrosion on it. I haven't had any issues with pool chemistry. Chlorine and PH numbers are normal and salt is ~3000 ppm. Chlorine generator is working fine and no other problems with the pool.

Anyone have any ideas?
thanks-
DH
 
DHinNC said:
I have a Pentair FNS 60 square foot DE filter on an inground pool. Anyone have any problems with internal corrosion on the SS nuts that are used on the rods that holds the filter grid assembly together? The pool is a salt water pool that is just now 5 years old. Today when I disassembled the filter I noticed that the nuts that hold the SS rod to the bottom grid rack were corroding. One just crumbled apart when I tried to remove it. The rod hasn't corroded at all. Both nuts on the each of the 2 rods had corroded. Neither rod had any corrosion on it. I haven't had any issues with pool chemistry. Chlorine and PH numbers are normal and salt is ~3000 ppm. Chlorine generator is working fine and no other problems with the pool.

Anyone have any ideas?
thanks-
DH
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I've seen that even on non SWG pools. I think it is just something that happens over long periods of time. Better quality steel nuts will help; the higher the quality the better. A thought though..... the metal rods and bolts do need to be the same metal to help prevent this and the two "welding" themselves together so I guess it would be best to let our experts comment on that. My old filter did that but they didn't disintegrate; just welded together on the bottom side of rods. That was probably because I always took the top nut off and left the bottom nuts in place when taking assembly apart.

My friend's pool guy used non- pool standard nut/bolts, mixed metals for her tank band. They were so "welded" together I could not get them off after an hour of struggling. (That's how I knew he hadn't inspected and/or cleaned the grids.) Her pool is not SWG but the filter is exposed to the elements. He kept insisting that he had inspected and cleaned the grids and they were fine. So why did it continue to spew all the DE back into the pool I asked? After taking to him once on the phone I resorted to leaving her notes to show to him, worded sot that I didn't seem so confrontational. Next time he was over there he spent the whole afternoon getting them off and locating the right kind. He also left the grid assembly out, as I requested, but still assembled, and they were a mess. He kept resisting dissembling the grids from the manifold; I don't know why other than he had to be "right". After I took them apart. Most had holes near the top tube which he did repair with glue and they are holding. The replacement grids cost over $500. I had her soak them in dishwasher detergent for two days but he told her they didn't need an acid soak. They did and it didn't get done. At least the DE is staying in the filter now and the starting psi is 15 instead of 25. It would probably be 10 psi if they had gotten a short acid soak.

:rant: BTW... this "pool guy" is a long time friend who stops by several times a week so she and her DH, who are both disabled, really need to stay on good terms with him. One great thing has come out of our interactions..... he's now using bleach in several of the pools he attends to in her neighborhood :whoot: and has stopped throwing loads of cal hypo into those pools. Her fill water is low in CH but the water was CH 800 and no CYA when I tested. He never considered the constant green and yellow algae to be a problem as, "That's just expected and normal around here in summer." :hammer: :grrrr:

gg=alice
 
Thanks- that's one of the thoughts that I had that the 2 nuts that corroded were not the same quality of SS as the rods or the captivated nuts on the bottom of the plastic grid support nor the thumb nuts at the top of the grid. None of those had any corrosion on them. I'll be keeping an eye on the replacement nuts each time I disassemble the filter from this point on. I was a little concerned that there may be some stray leakage voltage/ current issues going on, but when I checked all the grounds were good. Fortunately I'm able to do most all the maintenance on the pool, pump, filter, etc. Just replaced the leaking pump seal last weekend. Wasn't leaking much but still enough to be an annoyance.
 
Any sort of steel will corrode over time. 5 years is a pretty long time for stainless to hold up to salt.

The nuts corrode more quickly than the rod b/c of how the threads are formed in them. They are the same type of stainless, but depending on how "worked" the stainless is, it will corrode more quickly.

The goal of the design is to have the nuts fall apart as opposed to "welding" on as the other poster described. This way you can just replace the nut instead of the whole grid pack.
 
Brice thanks for the reply. I found out today that the material of the nuts was plated brass not SS as originally thought. This is from the Pentair FNS -Plus Manual. I feel fairly confident now that there was some galvanic interaction between the different materials and that the plated brass ended up the sacrificial piece. Not sure why we shouldn't expect SS parts to last more than 4-5 years in a salt water pool environment as the salinity is relatively low as compared to that of sea water (3,000 ppm vs. 35,000 ppm). Also, there are other metallic parts exposed to the pool water that haven't experienced any signs of corrosion.
 
It all depends on how salty, and what other contributing factors there are. For example, add ozone to the mix, and you'll see even more corrosion.

Are you talking about the wing nuts? The nuts at the bottom should be stainless.
 
No- the wing nuts at the top are SS. The captivated nuts underneath the lower grid support are SS. The plain nuts on top of the lower grid support are plated brass per the FNS Plus owners manual "Item 12 Part Number 98211400 Nut, 1/4-20, plated brass". These are the ones that were corroded. Not sure why these were the only fasteners inside the filter that weren't designed as SS. Seems like a potential design flaw.
 
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