3 replies to this topic

[fahrudin]

Dear All,

We have two tank in our Waste water treatment unit for DAF system.
1. Alum (aluminium sulfate) dilution Tank -->Mt'l carbon steel with epoxy
2. Casutic Tank ---> Mt'l SUS304.

we have frequent leak problem with the Alum tank, even already reapair by rewelding.

And now we consider to switch of above tank.
but, as we now the caustic also corrosive substance.
another alternatif is we not switch above tank but make rubber lining of inside surface alum tank.
my question is the second alternatif is better solution?

Please share to me your expereince.

Regards,

Fahrudin

[Erwin APRIANDI]

Dear Fahrudin,

Switching the tank will be the best solution as Caustic Soda will be less corrosive than Aluminium Sulfate.
I can prove it to you where in my current project we only use CS + PWHT + 1.5 mm CA for storage of 99% caustic soda (PWHT is to prevent Alkaline Stress Corrosion Cracking)

but, before swithcing the tank you have to make sure that the welding and etc has been repair and you also have to see whether PWHT is required for the re-welding.

My advice is to switch the tank

1. Caustic Tank --> Mt'l carbon steel with epoxy
2. Alum (aluminium sulfate) ---> Mt'l SUS304.

As mostly what I know that solution with less pH will have more corrosion rate than a solution which having higher pH

[Adam_K]

Hi Fahrudin,

Looking at a number of sources, storing the aluminium sulfate in the SUS304 tank will be fine - this type of metal has excellent resistance to both acid and alkaline solutions. On the other hand, both the carbon steel and the epoxy have fair resistance to caustic soda. As your tank has previously leaked, you should check the welding (as Erwin has already mentioned) and also ensure that the epoxy has been re-applied in the welded area.

Have you checked the maintenance records for the carbon steel storage vessel? It may be that it has been in service for too long and simply corroded both the lining and the tank walls through regular use.

I would first recommend consulting with a senior engineer on your plant (if there is one) or talking to an experienced engineer first, as leaking acid (or caustic if the tank swap fails) is a very serious safety risk. At the very least, check both tanks regularly for any signs of stress or damage, wear appropriate PPE around the tanks, and repair any leaks by restoring that part of the tank to its original condition.

[Erwin APRIANDI]

Hi Fahrudin,

One thing that I missed out is, since aluminium sulfate solution and caustic soda solution will have difference in density therefore you may have to check the hydrostatic pressure of each liquid to confirm the possibility to ecxhange the tank. since the hydrostatic pressure of fluid inside the tank at the maximum liquid level shall not exceed the design pressure of the tank.

And yes Adam K has point out some good thing, if this is the first time you are trying to optimised a plant, try to understand first regarding to the plant means since this is a existing plant I presume. It is better for you to look on the mechanical datasheet of the tank and also the design calculation.

There's a lot of things that you have to taken into consideration on plant optimization, and discussing with senior engineer in your plant is also a good idea.