Corrosion and Minimum Thickness Evaluation:

Corrosion occurs in two ways, general (a fairly uniform wasting away of a surface area) or pitting (the surface may have isolated or numerous pits, or may have a washboard like appearance in severe cases).  Uniform wasting may be difficult to detect visually and ultrasonic thickness measurements are normally done for that reason.  A pit may be deeper than it appears and should be investigated thoroughly to determine its depth.  The minimum actual thickness and maximum corrosion rate may be adjusted at any inspection for any part of a vessel.  When there is a doubt about the extent of corrosion the following should be considered for adjusting the corrosion rates.

a. Nondestructive examination such ultrasonics or radiography.  If after these examinations considerable uncertainty still exists the drilling of test holes may be required.

b. If suitable openings exist readings may be taken through them.

c. The depth of corrosion can be gauged from uncorroded surfaces adjacent to the area of interest. 

d. For an area of considerable size where circumferential stress governs the least thickness along the most critical element of the area may be averaged over a length not exceeding the following:

1. For vessels with an inside diameter of 60 inches or less one-half the vessel diameter or 20 inches whichever is less.

2. For vessels with an inside diameter greater than 60 inches one third the vessel diameter or 40 inches whichever is less.

e. Widely scattered pits may be ignored if the following are true:

1. No pit is greater than half the vessel wall thickness without adding corrosion allowance into the wall thickness.

2. The total area of the pits does not exceed 7 square inches. in any 8 inch diameter circle.

3. The sum of their dimensions along any straight line with in the circle does not exceed 2 inches.

f.  As an alternative to the above the thinning components may be evaluated using the rules of Section VIII Division 2 Appendix 4 of the ASME Code.  If this approach is used consulting with a engineer experienced in pressure vessel design is required.

g.  When corrosion is located at a weld with a joint efficiency less than 1.0 and    also in the area adjacent to the weld special consideration must be given to the calculations for minimum thickness.  Two sets of calculations must be performed to determine the maximum allowable working pressure;  one for the weld using its joint efficiency and one for the remote area using E equals 1.0 .  For purposes of these calculations the surface at the weld includes one (1) inch on either side of the weld or twice the minimum thickness whichever is greater.

h.  When measuring a ellipsoidal or torispherical head the governing thickness may be as follows:

1. The thickness of the knuckle region with the head rating calculated using the appropriate head formula.

2. The thickness of the central portion of the dished region, in which case the dished region may be considered a spherical segment whose allowable pressure is calculated using the Code formula for spherical shells.

The spherical segment of both ellipsoidal and torispherical heads shall be considered to be in an area located entirely in with a circle whose center coincides with the center of the head and whose diameter is equal to 80 percent of the shell diameter. The radius of the dish of torispherical heads is to be used as the radius of the spherical segment. The radius of the spherical segment of ellipsoidal heads shall be considered to be the equivalent spherical radius K₁D, where D is the shell diameter (equal to the major axis) and K₁ is as given in Table 1.