tank repair options?

[vipereyes]

I need help in selecting the correct repair method of our waste alkaline degreaser holding tank in our wastewater pretreatment system. The lining (it appears to be black rubber) developed a hole(s) or a rip and the liquid degreaser migrated between the liner and the steel walls, forcing the liner to balloon out.

The tank is 16 years old and holds 750 gallons. It cannot be removed/ replaced due to its location in the system.

The raw, powdered degreaser contains the following chemicals: 40-70% Sodium carbonate, 10-30% Sodium Hydroxide, 10-30% Sodium Metasilicate, 3-7% Sodium Tripolyphosphate and less than 5% each of Nonylphenol and Phosphate ester. The concentration in our process tank is 400 pounds of the powder per 600 gallons of water.
This tank is used to clean stamping/lubricating oils from our production parts.
The pH ranges from 10.5 to 12.5 depending on how old it is when it is pumped over to the system. The temperature is approximately 140 degrees when it is pumped over and then it cools down to room temp.

We plan to remove the liner so the steel tank can be inspected. We have been discussing two repair options. The first is to install a new epoxy type liner and the second is to “build” a new tank from 1/4 inch 316 Stainless Steel inside of the old one.

Any thoughts on these two options? Will the 316 SS offer enough corrosion resistance?

Thanks

Kim

 

[unclesyd]

316 SS will make a fine tank.

We use similar composition degreasers in CS tanks for heavy duty service. Where we want to kept down the rust and subsequent carry over we use 304L for the large tanks, a total of about 10,000 gals use up to boiling. Our smaller purchased degreasers are 316 SS.
We chose to use 304L SS because at times we use HNO3 as part of the cleaning system and we had a lot of plate that didn't meet our revised standards.
I always tried to stay away from linings in chemical cleaning service due to the fact that these tanks tend to see rough service and if someone uses the wrong chemicals so goes the lining.

 

[vipereyes]

Do you know if 304L would be less expensive than 316?

Kim

 

[unclesyd]

Generally, yes the 304L would be less expensive than 316 SS.
You will probably see a dual certified plate material 304/304L, this is OK for your service.

 

[kenvlach]

Although I'm not sure of the effects of the "Nonylphenol and Phosphate ester" ingredients, I don't see the need for either a liner or an SS tank.

The metal finishing industry uses mild steel tanks for alkaline cleaners, caustic etches and caustic electrocleaners, typically operated at 140-165^oF. Alkalinity, phosphate, & silicate prevent rusting.

 

[0707]

Kim

There are epoxy resins reinforced with fibber glass clothe linings which can be suitable for your application, you have to balance repairing versus new construction costs. For your service is not clear that changing to SS would increase corrosion resistance against your chemical wastes.

Regards

Luis Marques

 

[vipereyes]

Thanks for all the replies.

I am surprised at the mild steel comment. Our process tanks in our finishing room for degreasing, zinc phosphating and rinsing are made from 316 stainless steel.

At one point we had a 1/2" thick mounting plate installed on this tank to support the metering pump and in less than two months we had to replace it with PVC plate due to the amount of corrosion that we seen. Also, we have now removed the bottom portion of the liner and we see pretty severe rusting on the tank where contact with the degreaser occurred.

We did have one company come in yesterday to quote a new epoxy liner. It was about 1/2 the cost of the stainless steel option. They did have concerns about their epoxy adhering to the old. See, this tank is the middle of a three tank system welded together. In other words, they made a large rectangle tank and welded two steel dividers in to make three separate tanks and then applied the coating over all of them. The new liner would not only have to adhere to the steel but also to the old liner on the other tanks.

Our other concern about a liner is primarily our lack of knowledge/expertise to choose the correct substance. Our first attempt at a tank re-liner went very bad. We had our 6,000 gallon clarifier re-line two years ago. That contractor used a tar based coating (guaranteed it would be fine) and it caused havoc with our wastewater system for months until it finally cured. Our system uses chemical precipitation to remove metals and the coating leached whatever into our system rendering our system useless.

Thanks everyone.

 

[unclesyd]

I would stay away from a lining if at all possible. The only things that can happen to a lining are bad, especially in a situation as you describe were the possibility exist that extraneous materials could get into the system. A lined vat always limits it's end use in a metal finishing and cleaning line such as your describe. A coating is no better than the applicator.

A SS tank offers almost a unlimited possibility of uses in a system such as yours.

All exterior surfaces of a CS tank will have to painted and maintained as you probably know.

Apparently you are not having any trouble with SS in your existing finishing line, I definately try to get SS.