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b) Heads: Measurements for tank heads shall divide the head into four equal sections by <br /> establishing a horizontal and a vertical diameter line as a axis. Each section shall be <br /> divided into three-foot by three-foot quadrants beginning at the axis center point and <br /> extending outward on each axis line. Any additional area of the tank head which is less <br /> than three-feet by three-feet shall be measured and treated as additional quadrants.(See <br /> Figure A2.2). <br /> A2.7.2 Quadrant Gauging:Thickness gauging measurements shall be taken in the center of each <br /> quadrant of the tank wall and heads.Thickness readings 75%or less than the original shell metal thickness <br /> shall require further gauging as prescribed for subdivided quadrant gauging.Thickness readings greater <br /> than 75%of the original shell metal thickness shall establish the average measurement reported for the <br /> quadrant. (See Figure A2.3). <br /> A2.7.3 Gauging Quadrant Subdivisions: Quadrants whose center gauge measurement is 75%or <br /> less than the original shell metal thickness shall be further divided into nine equal subdivisions, <br /> except for perimeter subdivisions of head quadrants,which may be less than equal in size. (See <br /> Figure A2.1).Thickness gauging measurements shall be taken in the center of each subdivision.The <br /> subdivision thickness readings shall be averaged, and the results shall establish the average <br /> measurement reported for the quadrant.Subdivision thickness readings of less than 50 percent of <br /> original shell metal thickness shall require further gauging as prescribed for target areas. (See Figure <br /> A2.3). <br /> A2.7.4 Thin Wall Target Area Gauging: Areas whose thickness gauging measurement is less than <br /> 50 percent of original shell metal thickness shall each receive eight additional readings made at 1.5-inch <br /> and three-inch radius around such identified areas. (See Figure A2.4).The average of the nine readings <br /> shall establish the average measurement reported for the subdivision,except for subdivisions containing <br /> perforations. (See Figure A2.3). <br /> a) Perforations: Perforations shall be identified and reamed to establish a minimum 1/8" <br /> edge thickness. Eight thickness measurements shall be taken around a perforation as <br /> follows: <br /> Four measurements taken 1'/2" from the perimeter of the perforation with measurements <br /> spaced equidistant. <br /> Four measurements taken 3" from the perimeter of the perforation with measurements <br /> spaced equidistant. (See Figure A2.5). <br /> The eight readings shall be averaged, and the results shall establish the average <br /> measurement reported for the subdivision within which the perforation is most closely <br /> centered. (See Figure A2.3). <br /> A2.7.5 Tank Shell Average: Tank shell average thickness shall be established from the computations on <br /> Figure A2.4. <br /> A2.8 Reports: <br /> A2.8.1 Thickness gauging readings shall be reported on a ultrasonic thickness gauging report. (See <br /> Figures A2.3). <br /> A2.8.2 Reports shall be maintained for reference and shall be made available to the chief.Computer <br /> recording, file maintenance, and reference retrieval may be utilized in lieu of the report forms in Figures <br /> A2.3. <br /> 2/99 <br /> Page 25 of 30 <br />