Corrosion allowance on a lube oil tank 2

[Talanca63]

We have a customer (paper mill industry) asking us to indicate the corrosion allowance of a 20 year old carbon steel lube oil tank.
The tank is not under pressure, like a boiler or condensate separator - it's vented to the atmosphere.
Because there is no record of any consideration for that in our files, I wonder if there is any rule-of-thumb to guide me on that.

 

[JStephen]

It doesn't necessarily have any corrosion allowance. IE, it may be zero. For example, on API-650 tanks, the corrosion allowance is always zero unless the customer specifies a CA.

What size/ thickness is it?

If a small tank, it probably has a fair bit of extra material inherent in it's construction. On small tanks, the required thickness from calculations may be paper-thin, but welding and assembly requirements will dictate heavier sheet or plate.

 

[LSThill]

Talanca63 (Mechanical)

At the Gluf Oil Tank farm Port Author the tank was built in 1896 and 1907 the bottom were replace and some patch work. Over 100 years of services still little or no problems.

 

[JStephen]

Let me elaborate a bit since I have some time now.

If this tank is a flat-bottom cylindrical vertical tank, especially if field erected, then check into API-653. It won't tell you specifically what corrosion allowance the tank was built with, but tells you how you should calculate the minimum thickness for it to remain in service, how to inspect and monitor thickness, etc. You'll probably need API-650 to go with it, and these will set you back a couple of hundred dollars each. You can download them from API. You may fall under state or federal requirements that mandate you have to monitor corrosion on the tank, etc- check if that has been done lately.

If it's a very small tank, perhaps a rectangular tank, it may not have been built to any particular specification. If it has a UL label, check with UL to see what their specifications required for thickness when it was constructed. If it's rectangular, but not built to any code, check required thickness per the plate bending formulas in "Formulas for Stress and Strain".