Do you know how components that don't have a direct calculation for t(required) (Appendix 2 flanges, cone shell junctures nozzles etc) are handled?

In-service Inspection of Pressure Vessels Pare RB

RB-3236 REMAINING LIFE CALCULATION

Where the corrosion rate controls the life of the pressure vessel, the remaining life shall be calculated by the following:

Remaining life(years)= [t (actual)- t (required)]/
Corrosion rate

Where,

t (actual) = thickness in inches (mm) measured at the time of inspection for the
limiting section used in the determination of the t (required).

t (required) = minimum allowable thickness in inches (mm) for the limiting
section of the pressure vessel or zone. It shall be the higher of the following:

1. The calculated thickness, exclusive of the corrosion allowance, required for the pressure relieving device set pressure, static head or other loading and design temperature or

2. The minimum thick permitted by the provision of the applicable section of the original code of construction

Corrosion

Rate =inches (mm) per year of metal removal as a result of corrosion

(t Original - t actual)/years

Hello Mr Hill

That is a very interesting question. The straight line formula published by the National Board is excellent where corrosion rates are known, and 'trequired' is directly proportional to the pressure. However, in the case of flanges, there is no 'thickness' that governs the pressure retention property of the flange. A flange has a number of 'thicknesses'. If we have for example an integral flange, the bore is mostly susceptible to corrosion, but the thickness 't' of the actual flanges part might not corrode. In this case the relavent changing dimensions are the hub dimensions. The operating stresses (especially Sh) can be extremely sensitive to any changes in these dimensions. So, how do we proceed? Here is a suggested scheme: Let us suppose the corrosion is concentrated in the bore of the flange (the most likely scenario). Using modern pressure vessel software, it is a fairly simply process to quickly calculate the MAWP for different degrees of corrosion, and plot these MAWP values on a simple graph. We now have the relationship between pressure and corrosion. If the corrosion rate is known, the remaining life can be very easily read off from the chart. For those of us who are so inclined, it is possible to set it up on the spread sheet such as Excel generate the graph, and quickly find the remaining life.

Thank you for your excellent question.