(b) Drilling of main reservoirs.

1. (1) Only welded main reservoir originally constructed to withstand at least five times the MAWP may be drilled over its entire surface with telltale holes that are 3/16 of an inch in diameter. The holes shall be spaced not more than 12 inches apart, measured both longitudinally and circumferentially, and drilled from the outer surface to an extreme depth determined by the following formula:

   D = (.6PR/(S−.6P))

   Where: D = Extreme depth of telltale holes in inches but in no case less than one-sixteenth inch; P = certified working pressure in psi; S = 1/5 of the minimum specified tensile strength of the material in psi; and R = inside radius of the reservoir in inches.

2. (2) One row of holes shall be drilled lengthwise of the reservoir on a line intersecting the drain opening. When main reservoirs are drilled as described in paragraph (b)(1) of this section, the hydrostatic and hammer tests described in paragraph (a) of this section are not required during the annual inspection. Whenever any telltale hole shall have penetrated the interior of any reservoir, the reservoir shall be permanently withdrawn from service.

(c) Welded main reservoirs without longitudinal lap seams. For welded main reservoirs that do not have longitudinal lap seams, an appropriate NDE method that can measure the wall thickness of the reservoir may be used instead of the hammer test and hydrostatic test required in paragraph (a) of this section. The spacing of the sampling points for wall thickness shall not be greater than 12 inches longitudinally and circumferentially. The reservoir shall permanently be withdrawn from service where the NDE testing reveals wall thickness less than the value determined by the following formula:

t = (PR/(S−.6P)

Where: t = Minimum value for wall thickness; P = Certified working pressure in psi; S = 1/5 of the minimum specified tensile strength of the material in psi, or 10,000 psi if the tensile strength is unknown; and R = Inside radius of the reservoir in inches.

(d) Welded or riveted longitudinal lap seam main reservoirs.

1. (1) For welded or riveted longitudinal lap seam main reservoirs, an appropriate NDE method that can measure wall thickness of the reservoir shall be used instead of, or in addition to, the hammer test and hydrostatic test. The spacing of the sampling points for wall thickness shall not be greater than 12 inches longitudinally and circumferentially. Particular care shall be taken to measure along the longitudinal seam on both plates at an interval of no more than 6 inches longitudinally. The reservoir shall be withdrawn permanently from service where NDE testing reveals wall thickness less than the value determined by the following formula:

   t = (PR/(0.5S−0.6P))

   Where: t = Minimum value for wall thickness; P = Certified working pressure in psi; S = 1/5 of the minimum specified tensile strength of the material in psi, or 10,000 psi if the tensile strength of steel is unknown; and R = Inside radius of the reservoir in inches.

2. (2) Repairs of reservoirs with reduced wall thickness are prohibited.