Ssk the process engineer and the materials engineer. Also, you can find it in the basis of mechanical design or process design or in the material selection document. The corrosion allowance depends on the properties of fluids contained and the selected MOC of your vessel.
Cheers,
gr2vessels
It's a function of the expected corrosion rate and how long you want the vessel to last. If the thickness difference between the minimum required thickness per Code and supplied thickness is say 3/16", then at 1 mil/yr general corrosion rate, the vessel theoretically would last for nearly 200 years before you would be down to the minimum required for your design pressure and temperature. At 10 mil/year, you are looking at about 19 years. At 100 mil/yr, you are down to less than 2 years. Note this is for general corrosion, pitting is another issue and corrosion allowance really provides no protection for pitting.
If the expected life of the vessel isn't sufficient, you can increase the corrosion allowance. Going from 1/8" to 1/4" theoretically doubles the vessel life. Personally, if I think I need to use a 1/4" corrosion allowance, I'd be considering alloying up the vessel to a more corrosion resistant material or cladding/lining the vessel with a more corrosion resistance material. For example, for a project I'm working on, the tower in the top is being lined with Hastelloy. The bottom part is being lined with 317LL and the middle portion, where the client hasn't seen corrosion issues in the past, is just carbon steel.
Note that a lot of that comes from past experience with similar service. If you're designing a vessel from scratch for a product/service you know nothing about, you have no good basis for picking one value over the other. If you're the fabricator, the corrosion allowance would generally be specified by the purchaser.
As per ASME section VIII division 1, UG-25, " the user or designated agent shall specify corrosion allowance other than required by rules of this division.where corrosion allowance is not specified, this fact shall be indicated in data report."
For corrosion allowance of vessel, this is the responsibility of purchaser to specify with help of process design engineer or corrosion engineer.
For detail , kindly study
ASME section VIII division 1, UG-25, Corrosion
ASME section VIII division 1, UG-26, Linings
ASME section VIII division 1, Appendix E , suggested good practice regarding corrosion allowance.
ASME section v111 division 1 UG-25 says the users of his designated agent shall specify corrosion allowances other than required by the rules of his division it futher says where corrosion allowances are not provide,thsi fact shall be indicated on the data report.those this mean that its the designated agent that will specify the corrosion allowance?
