Steam Drum Minimum allowable thickness

For such problems the best is to use the codes since it is not a problem of personal opinion but a problem of responsibility in case of failure.

I think that the previous reply covers it.

Additionally: Did you measure a significant drop in thickness? Mind you that an as-fabricated wall thickness of X and a corroded wall thickness of X are NOT the same thing strengthwise. I wouldn't normally trust the latter. There may also be other structural failures in development (did you check for cracks or only for thickness?)

As the previous respondent said, if there are codes dealing explicitly with this sort of evaluation, use them. If not, be extremely conservative (it's ok to say "I can't tell for sure" if you really can't) and check for cracks. i.e. If wall thickness reduction is so significant as to alter the outcome of standard calculations, I would be very mindfull of the condition of the welds as well...

Hi inspectorjoe,

Without a detailed drum dimensions drawing and stating all design conditions (temperature, superimposed loads, etc.) it's impossible to calculate the required thickness.

Anyway, I've been involved in power plants a lot of years and steam drums and piping are usually thickness gauged periodically more when saturated steam (two phase flow) can be present and produce erosion-corrosion phenomena.

If it's a power boiler, had to be designed to some code (ASME I, TRD,...) Any code used in the design must have specify after calculation, the minimum thickness. You need to choose the minimum calculated thickness included the corrosion allowance.

If the results of gauging are lower than that minimum calculated thickness, you must reject the drum. It will need repair or a new one.

If the remaining thickness is greater than the minimum calculated, you need to know the initial real thickness and the service time, to estimate the mean rate of corrosion.

Then according to that rate calculate the time needed to reach the minimum value. This will be the maximum time before checking again the thickness. Take a safety factor!!!!

Hope this would help you.

Kind regards

Do you have a U-1 manufacturer's form that has the original thickness of the drum? You need to get the ASME sec. one code book and have someone that is familiar with this code to do the calculations. It is spelled out very well as to how to perform the calculations in this book.You can do a fitness for service and remaining life calculations as well. I would suggest that you get someone very familiar with performing these calculation to help as they can be very complicated. Steam drums usually have very little inside wear and we are usually concerned more with cold side corrosion or ash erosion on the outside of drums. The boilers we have in the paper mill run at 850 PSI and the drums are close to 5" thick. I can't imagine how thick a drum designed for 2000 PSI would be.

The best way is to call the Engineer/Surveyor for the company providing burst/collapse indemnity insurance, and discuss. After all, this <...steam drum...> is on the facility's General Register for periodic insurance assessment, isn't it? ISN'T IT?