A safe repair would likely exceed the cost of a new filter, and very, very few people, even professionals, would have any idea how to go about such a repair. Because of that, the risk of shrapnel and the fact that the filter has already cracked once, I'm on the no repair side.
I could see patching a small crack for a season and trying to make it work if a new filter just wasn't in the budget. Use a water proof, strong epoxy. I wouldnt want to patch it with anythign that could get clogged in the plumbing if it failed.
Here's a lil fun math next time somebody wants to repair a crack in a pressure vessel.
Lets say average filter pressure is 15psi. So for every square inch of surface area of filter housing there is 15 pounds of force pushing out on it. There are 144 square inches in every square foot.
15 x 144 = 2160 pounds of force per square foot.
In a 20" sand filter I'm going to estimate (no real dimensional measurements taken) there is roughly 15 square feet of surface area. That's based on the assumption of a 20" diameter cylinder 24" tall.
So that un assuming little thing out by everyones pool is containing about 30,000 pounds of force. But the likely hood of the whole filter shattering all at once is pretty slim so that 30k number is pretty meaningless.
The meaningful number is that 2160 number. Its not so unheard of for a large piece say around 1 square foot in size to come of in a failure propagated by a crack. What ever happens to be within about 24" of that piece when it lets go is going to get hit with about a ton of force. That's enough to knock a grown man down and break all kinds of expensive pool equipment.
Since you are dealing with a vessel designed to withstand pressurization up to a defined point, any cracks that occur without having reached that pressure limit indicate a structural failure of the vessel. It is very rare for a filter under normal operation to be pressurized to the extent of failure, and you would more likely see a catastrophic failure (explosion) rather than a crack. When you see a small crack in a filter vessel it is likely a result of prolonged UV exposure, abuse (such as striking it with a hammer to loosen stuck pieces) or a manufacturing defect that has weakened over time. However it was caused, the structural integrity of the vessel has been compromised and cannot be restored.
If the vessel has cracked prior to reaching the limits of its pressurized capacity, why would a fiberglass or epoxy patch which has not been pressure tested at all be seen as a safe repair?
I also vote NO to fixing it with a patch. Just too dangerous. Yes, a new filter is expensive. But so is a hospital stay from a fracture skull and concussion, damaged equipment and/or an injured bystander suing you for personal liability.
There is a reason why pool filters have these types of warning labels on them and in the manuals. I don't know how many people have died from malfunctioning pool filters but I'm sure it isn't zero.
THIS FILTER OPERATES UNDER HIGH PRESSURE
When any part of the circulating system, (e.g., closure, pump, filter, valve(s), etc.), is serviced, air can enter
the system and become pressurized. Pressurized air can cause the top closure to separate which can
result in severe injury, death, or property damage. To avoid this potential hazard, follow these instructions:
This filter must be installed by a qualified pool serviceman in accordance with all applicable local codes
and ordinances. Improper installation could result in death or serious injury to pool users, installers, or
others and may also cause damage to property.
Do not operate the filter until you have read and understand clearly all the operating instructions
and warning messages for all equipment that is a part of the pool circulating system. The following
instructions are intended as a guide for initially operating the filter in a general pool installation. Failure
to follow all operating instructions and warning messages can result in property damage or severe
personal injury or death.
Never exceed the maximum operating pressure of the system components. Exceeding these limits
could result in a component failing under pressure. This instantaneous release of energy can cause the
filter to separate and could cause severe personal injury or death if they were to strike a person.
In a former life and in a galaxy far, far away I was a DOT certified to conduct hydrostatic testing on SCUBA cylinders. A crack in a,pressure vessel is cause for immediate condemnation. It should be removed from service immediately.
While SCUBA cylinders operate in the 2250 - 3500psi range, I have personally seen the results of a catastrophic failure. In one case the entire front wall of a store was destroyed. In another case the person filling the tank (a friend) had just submerged the tank in a water bath when it let go. The water did absorb most of the force, but he lost about a third of his hand.
Awesome posts about the math/science/logic in play as well as the pros (none yet) and cons (repair cost greater than replacement, unsafe/illegal, risking further damage to home/equipment/life, if it failed in one place more are coming) but especially grateful for this post that I believe helps those less familiar with the relevant science reach a common sense answer for themselves:
I agree with the consensus not to repair a cracked filter case if the damage has affected the structural integrity of the case. But it you had something that didn't weaken the case you might be able to plastic weld it. For instance if you took a sand filter and drilled a 1/4" hole in it I think that I could probably patch it up with a hot soldering iron. Especially if you have a piece of plastic scrap as donor material and can get to the damage both inside and out. But a crack that is spreading might be impossible.
Last summer our filter housing developed a crack starting from the lip edge/top and ~3" long near the seam. It was 15 years old. Looked all over the internet for methods of patching the leak. We tried the fiberglass patch, hot soldering iron, plastic weld, marine epoxy, JBWeld. It stopped the leak for only a few hours or slowed it down considerably. The fiberglass patch on both the outside and inside failed. We limped thru each different attempt at patching for several days, only running the pump for several hours a day until we gave up. Needless to say nothing worked so we shut it down for the summer in Mid August since we were headed on vacation out west. We bought a replacement filter on Craig's list before we left but could not use it since the guys laid the filter on its side and rolled it to the car. Sand in the valve regulator (if that's what it's called). This year, cleaned it all up and the replacement works great. A friend took the old cracked filter housing and plans on making it a planter. Probably the only thing its good for. LOL
So, long story short, no use trying to fix the crack. It won't work.