California Code of Regulations › Title 8 - Industrial Relations › Division 1 - Department of Industrial Relations › Chapter 4 - Division of Industrial Safety › Subchapter 2 - Boiler and Fired Pressure Vessel Safety Orders › Article 7 - Repairs › Section 797 - Example of the Application and Explanation of Patch Tables and Charts

Current through Register 2024 Notice Reg. No. 28, July 12, 2024
(a) A patch is to
					 be placed in the fire sheet of a horizontal return tubular boiler having a
					 shell plate seven-sixteenths-inch (7/16") thick, a longitudinal seam efficiency
					 of seventy-four percent (74%), and a length of patch of thirty-six inches
					 (36"). Find the width, W, of patch to be applied so that there will be no
					 reduction in pressure, using a single-riveted seam of normal design.
Referring to Table 1, it is found that
						seven-sixteenths-inch (7/16") plate with fifteen-sixteenths-inch (15/16")
						diameter rivet holes, pitched two and one-fourth (2 ¼ ) gives a seam
						efficiency of fifty-six percent (56%).
Referring to Table 2,
E
= 0.74 and
e
= 0.56 gives a constant
C
= 1.75; then
						width
W
=
L
x
C = 36
x
1.75
= sixty-three inches (63").
(b) Pressure Allowed on an Existing Patch.
A crescent-shaped patch has already been installed on a
						horizontal tubular boiler. It is thirty inches (30") long and forty-eight
						inches (48") wide. The seam is single riveted with thirteen-sixteenths-inch
						(13/16") rivet holes pitched one and fifteen-sixteenths inches (1 15/16"). The
						boiler shell plate is three-eighths-inch (3/8") thick. The longitudinal seam is
						of the double-riveted butt-strap type having an efficiency of eighty-two
						percent (82%). The safety valve is set for one hundred twenty-five pounds
						(125#) pressure. What maximum pressure should be allowed on the boiler?
						Reference to Table 1, shows that the normal efficiency of the patch seam is
						fifty-seven percent (57%). (If the efficiency is not found in the Table, refer
						to any other available table or determine it in the manner described in
						paragraphs A-1, A-2 and A-3 of the Appendix of the A.S.M.E. Boiler
						Code.)
Divide the width of the patch, W = forty-eight inches
						(48") by the length,
L
= thirty inches (30") to find the
						constant,
C
= 48/30 = 1.60. Follow down column 0.57 of Table 2
						until 1.60 is found. It will be noted that this is somewhere between 1.56 and
						1.62 representing E somewhere between 0.72 and 0.73. As the difference between
						1.56 and 1.62 is 6, and the difference between 1.56 and 1.60 is 4,
E
will be 0.72 plus 
						4/6 of 0.01
						which is 0.7266.
The maximum allowable working pressure varies directly as
						the seam efficiency. Accordingly,
P
= (0.7266/0.82) x 125 =
						110#. If this pressure allowance interferes with the operation of the plant,
						the patch shall be replaced by a new one with the proper dimensions giving a
						diagonal efficiency of eighty-two percent (82%).
(c) Design of Patch for Water-tube Boiler.
Reference to Table 1 shows that a single-riveted lap seam
						with seven-sixteenths-inch (7/16") plate, fifteen-sixteenths-inch (15/16")
						diameter rivet holes, and two-and-one-half-inch (2 1/2") pitch has a normal
						efficiency of fifty-six percent (56%).
Reference to Table 3, shows
E
= 0.82 and
						e = 0.56, the constant C is 3.16.
Then width
W = C
x 1 / 2.
W = 3.16 x 36 / 2 = 56.88 or practically fifty-seven
						inches (57").
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1.
						Repealer and new subsections (a) and (b) filed 8-12-76; effective thirtieth day
						thereafter (Register 76, No. 33).
2. Change without regulatory
						effect repealing and adopting new Figures 1-6 within subsection (b) filed
						5-19-2008 pursuant to section
						100, title 1, California Code of
						Regulations (Register 2008, No. 21).