plastic tank is leaking

It won't be easy to repair a leaking tank. With no clue where you are it's hard to help.

You might try welding it. Can you apply a partial vacuum long enough to clean/dry the area, then weld?

I agree with your assessment.

It may be possible to arrest the leak with EVA glue application. EVA glue sticks with applicators are commonly available in most hard ware out lets.

Do you maintain any pressure in the tank?

Hi, If you can get inside the tank place a large thin plastic patch over the hole. The internal water pressure should press it into the leaking hole. This will allow you to dry the outside ant then apply a patch there. You could also use a hollow container on the inside, for instance a glass jar, fit foam around the rim to act as a seal then proceed as above. If an external patch does not work you could screw a thick plastic patch (made from a piece of plastic packing or a CD case) to the tank using self tapping screws. Put epoxy resin on patch to seal it.

A, "a piece of plastic packing or a CD case". Really, I don't think this is good advice.

Since there is no BA (bad answer) response, I gave you an OT. I doubt that you know what this means either.

When you say "foam plastic tank", I am assuming that "foam plastic" describes the purpose of the tank. What is the tank made of (carbon steel, polypropylene, ...)? Is it flat bottom, or horizontal, or spherical, is it rated for pressure or is it open to the atmosphere?...

It's a plastic tank contain foam for fire fighting atmospherique pressure 3m hight

Keep going tell us more, where is the leak, can you tranfer the contents to IBC's [intermediate bulk containers] or another tank?

There are so many questions

Polyethelyene tank?

what liquid does the tank contain?

What temprature?

Where is the leak?

On large poly tanks the connections are sometimes made using a square metal [SS] flange with a foam gasket that bolts through the wall of the tank. You could make or buy a flange & use it for a blind. The foam gaskets are madefor chemical resistance & come in various material formulations.

Get a new tank!

I'm going to do

But seriously, is it crack, puncture or pin hole? Are you trying to repair it with fluid in it? I have used this stuff called "Seal All" and it has worked on small scale tanks such as a car gas tank, but for your case to fix it properly it should probably be done from the inside with plastic weld, assuming it is a PP tank. (Be aware of your confined space and take the appropriate safety precautions!) Pin hole? How about running a screw in it?

As far as temporary solutions are concerned what I have observed from a plumber, the possible solutions can be,

Em seal 2 component adhesive[versalitile sealant], and if it is the metal part EPOXY 2 COMPONENT[ARALDITE & HARDENER] adhesive, and also one metal sealant green paste is also avilable[follow mixing and application advice of manufacturer].In relevent cases follow sufficient curing time to get good results.

One word of caution...Don't try welding a cross-linked polyethylene tank, the molecular structure will not allow welding...it will just melt away with the heat.

Could plug leak with cake soap or sealing wax depending on chemical composition of tank contents.

Some plastic tanks can be welded. Some must be patched. Some must be replaced. It all depends upon the type of plastic, the service and the available resources.

For a tank used in fire suppression, I would think that it needs to be repaired with a durable repair or replaced. You don't need the thing splitting and dumping your suppression agent on the road prior to arriving at the destination. If it is in a fixed location, patching it up may be an option if the cost of losing a charge of suppression agent is not a significant expense compared to replacing the tank.

Sometimes the best repair is replacement!