corrosion allowance 3

[engbassbosa]

in API 650 indesign section of bottom and shell plate
CA = corrosion allowance the Purchaser must define it
i don't know how to define it i know storage liquid , steel plate material
so please i want to know how to calculate corrosion allowance
thanks

 

[IFRs]

Steel corrodes (oxidizes). In some environments it corrodes faster, in others slower. If you have zero corrosion allowance then when the steel corrodes and becomes appreciably thinner it may not be able to perform it's design task. Your job is to evaluate the corrosion rate and determine the expected life of the tank. With those two numbers you can determine a corrosion allowance that's right for this tank. Corrosion rate x tank life = corrosion allowance. It can be based partly on your historical data at the site and the effect that the environment, stored liquid and operations will have on the steel. If you have a tank in similar service, use the corrosion rate for that vessel and with the desired life of the new tank determine a corrosion allowance that will work. If you have no information, start looking and doing research. Good luck...

 

[MJCronin]

bassbosa....

As stated above, corrosion allowances for both tanks and piping systems depend on the user's experience. This is why consultant engineering firms are necessary and valuable.

My experience has been that most corrosion allowance run from 0.0 to 0.125 inches.

If you know and understand the liquid, temperature range, velocity, and component life, you can come up with a reasonable CA.

Most stainless steel tanks and piping systems use a CA of 0.0....... This is because the purchaser of the tank/system decided to spend the extra money for a corrosion resistant material and, if the purchaser knows what he is doing, the material should perform for the expected life with no corrosion.

There are many rules of thumb:

Most carbon steel systems are commonly specified with a CA of 0.0625 inches.....

93% sulfuric acid stored in carbon steel tanks have a CA of 0.125 inches.....

Raw (oxygenated)water systems should have 0.0 CA and a well selected coating....

etc...etc

 

[engbassbosa]

the tank material are carbon steel , fluid gasoline , the tank in avery moisture enviroment (beside the sea)so i wish to tell me what is the refrence to get the corrosion range depend on these data

 

[engbassbosa]

eng MJCronin please tell me what is the standard (refrence)for these values
thanks for your valuable comment

 

[fegenbush]

Perry's Chemical Engineers' Handbook Sixth Edition has fairly extensive corrosion tables. The Seventh Edition would not be acceptable since the information changed significantly between versions. The Pressure Vessel Design Handbook Second Edition by Bednar also has some tables, but they are less informative on corrosion rates than Perry's.

 

[darthsoilsguy2]

automobile gas is not very corrosive for the inside of a steel tank. You will see the majority of inside corrosion at the bottom if water is not sumped out occasionally as a maintenance item and at the tank roof from the condensation cycles. The exterior side could have a corrosive environment from what you stated. Having a well-applied coating (2-part epoxy type for example)should be priced. If you do those things, the tank should have a very low corrosion rate and 0.005 inches/year should be a safe number. Although, i would expect the corrosion rate to be negligible between 5-year cycle readings until there was a maintenance problem or coating failure.

All that being said, tanks have problems with localized corrosion and the corrosion allowance can give a false sense of security. The best way to prevent localized corrosion is to understand what corrosive/erosive elements that are in play from an operational and environmental basis, and to incorporate that in the design.

 

[lpfaustini]

My suggestion is to take a look at NACE Corrosion Engineer's Reference Handbook.

Regards,