Repairing a Cracked PVC Tank

This might work. It's pretty common. I don't know the specifics for your application.

http://en.wikipedia.org/wiki/Plastic_welding

My experience tells me to abandon any hope of "...repairing a wide crack at the bottom of a PVC tank."

Is there some overpowering reason why repair is considered over replace?

I would give this a try http://en.wikipedia.org/wiki/Fiberglass_sheet_laminating

kramarat has the right idea, I know. Weld a PVC plate over the crack. DO NOT use any adhesives here. Compatible welding rod and plate stock are available.

I worked at a company that made large chemical process equipment. It was all welded sheet. Some one inch thick.

Google plastic welding.

Agreed, and you shouldn't have put that off topic - accept an 'un-vote'

There will be a reason for the crack. Might be an idea to weld on the patch and then band the whole thing. Replacement is often the cheaper option. If so, depending on your concentration; make it clear that the density of the liquid may be several time water.

Thanks.

Fill with one of these.

http://www.graysonline.com/lot/0033-570246/manufacturing/5-x-plastic-bladder-tanks-assorted-sizes-unused

Then take your time to look out for a new tank.

Good luck

Search Plastic Fabrication. There may be a proffesional plastic fabricator in your area that will be able to evaluate your tank and give you an estimate for repair. The cause of the crack is important to establish as it could be from chemically induced stress cracking, mechanical reasons, or poor welding. If it is from chemical stress cracking it will likely have spider cracks close together at general right angles to the main crack. If it is from poor welding the weld bead will be cleanly separated from the material for at least part of the crack. The mating faces of the crack will have a polished look. If from mechanical stress there will be no above mentioned spider cracking and while the crack may be near or through a weld the weld/structure interface will not be separated at any point. If the crack has evidence of chemical stress cracking the tank is garbage. If it is from poor weld you should assume that the rest of the welding is similar and not reliable. If the tank is otherwise in good shape, i.e. the crack is from some type unusual mechanical stress the tank may be well worth saving by hot gas welding of a patch, pulling the crack together and rewelding, or filling the crack with weld depending on the circumstances.

The tank is in good shape and reliable robust. The crack might be due to mechanical stress of some kind. Thinking of patching with reinforced fiberglass.

Thanks for all the comments.

I have removed many fiberglass patches that failed in relatively short time in order to do hot gas welding. It may work but if it doesn't, grind off the patch before you get it welded to save some money. The trouble is the difficulty getting a bond that will work long term with the different expansion rates of the materials. PVC expansion is about 6" per 100 feet per 100 degrees farenheit or .006"/1.2"/100 degrees F.

LiquidHandling is correct about mismatched expansion rates. Glass reinforced polyester or epoxy hardly expands at all compared to PVC.BIG MISTAKE!

My main concern would be; is the plastic brittle? If it is, that would cause it to crack and signal time to replace it. You don't want a tank holding acids and alkalies to leak and become a safety hazard.

Sorry for the question.... but, HOW BIG is the tank we are talking about? Unless you want to repair it for fun, seriously consider dumping your broken tank....

How about a couple of closeup pictures and an overall picture with dimensions? They would help with a more specific recomendation. Here are pictures of a few jobs I've worked on in the last 38 years. (Industrial Plastic Works of Knoxville, Tennessee, USA.) Many shops will do field work within reasonable distance.

I have little experience whit PVC and PP.Repairing is possible.First you most clean good place were is damage.Then use PVC wire and extruder to weld tank.more information you cant see on http://omi.elmedia.net/en/herz-bulgaria_00948.html. I hope i help you little.

You can weld PVC with a hot air gun and PVC rods. This takes practice, you need to heat the rod and pieces you are welding simultaniously. If you don't heat it enough it won't weld properly and if you heat it too much it burns and becomes brittle.

With practice you can make a strong weld and seal most splits but you can never guarentee it is as good as new.

Depending on what pressure and what fluid is present you could always try a quick fix with silicon. It is suprisingly tough and I found often was better than PVC welding in low pressure situations.

Hi,

Go to an auto parts store and ask for a wet fiberglas patching kit. The patching material is sealed in a clear plastic pouch with the hardener already added. Just open the pouch slap it on and let it cure. These are normally used for auto body repair, but should work for you.

I am concerned that you are considering repairing the tank without first understanding why it failed. PVC is used widely in chemical plants, but great care must be taken to ensure compatability with every chemical that it might come into contact with, including the concentration and temperatures. PVC is considered safe with dilute acids, but you said you have medium strenght acids. Also, there are certain chemicals that are not compatible with PVC. If you have a chemical crack (as mentioned earlier) you definately need to replace the tank and you will need to use a different material/different formulation to prevent having the same type of failure again. If you are not sufficiently familiar with chemical cracking of plastics, you might need to have the tank inspected by an expert.

Next, you need to be very careful in repairing the crack. There are MANY different formulations of PVC. They use many different systems of additives - stabilizers (thermal, UV, etc), pigments, extenders, reinforcements, impact modifers, etc. Even if you are able to get a perfect weld, the material formulation needs to be appropriate for the application. Also, the welding process may degrade both the fill material and the surrounding material thermally. The cost of repairing this properly needs to be carfully weighed against the potential risks of another failure.

If and when you decide to weld the crack, follow this old standard method before welding:

Drill a small hole at each end of the crack before welding.This will prevent the crack from reappearing beyond the boundaries of the weld due to stress riser.