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ASME SECTION VI-2008 pdf download

A. Gage Range. The gage range should be selected so that the gage will normally operate in the middle of the scale. For example, if the operating pressure is 50 psi (350 kPa), then a 100 psi (700 kPa) gage should be used. For steam heating boilers, the gage should have a range of not less than 30 psi (200 kPa) nor more than 60 psi (400 kPa); and for hot water boilers, not less than 1 1 ⁄ 2 times nor more than 3 1 ⁄ 2 times the safety relief valve setting. B. Accuracy. The gage accuracy is expressed in percent of full scale reading. For example, if a 100 psi (700 kPa) gage is 2% accurate, then it will be within ±2 psi (±14 kPa) of the actual pressure. A gage is usually more accurate at mid-scale and shouldbe calibratedatthatpoint. Mostgages used on boilers have an accuracy of 1% to 1 1 ⁄ 2 %. An inspector gage is usually 1 ⁄ 2 % accurate and a laboratory gage may have an accuracy of 1 ⁄ 4 %. C. Calibration. The gage used on a boiler should be calibrated at least once per year. This can be accomplished by comparing it to an inspector gage or using a deadweight tester. If an inspector gage is used, the accuracy of that gage should be verified with a deadweight tester at least once every 2 years. If the gage is damaged or cannot be calibrated to provide consistent readings, it should be discarded and replaced with a new gage. D. Siphon Tube. On a steam boiler, a siphon tube (pig- tail) is required to protect the gage from steam. A valve is also provided to facilitate demand and servicing of the gage. INSTALLATION NOTE: The following is taken from the mandatory rules of Section IV that apply to the boiler when manufactured and initially installed. The use of the word “shall” throughout reflects the mandatory nature of the Section IV requirements. The reader should consult the latest edition of Section IV for the current requirements.
(2) No safety valve for a steam boiler shall be smaller than NPS 1 ⁄ 2 (DN 15). No safety valve shall be larger than NPS 4 (DN 100). (3) The minimum relieving capacity of valve or valves shall be governed by the capacity marking on the boiler called for in HG-530 of Section IV. (4) The minimum valve capacity in pounds per hour shall be the greater of that determined by dividing the maximum Btu output at the boiler nozzle obtained by the firing of any fuel for which the unit is installed by 1000, or shall be determined on the basis of the pounds of steam generated per hour per square foot ofboiler heating surface as given in Table 3.20. For cast iron boilers the minimum valve capacity shall be determined by the maximum output method. In many cases a greater relieving capacity of valves will have to be providedthan the minimumspecified by these rules. In every case, the requirement of 3.20C(5) below shall be met. (5) The safety valve capacity for each steam boiler shall be suchthatwith the fuel burning equipmentinstalled, and operated at maximum capacity, the pressure cannot rise morethan5 psi(35 kPa) abovethe maximumallowable working pressure. (6) When operating conditions are changed, or addi- tional boiler heating surface is installed, the valve capacity shall be increased, ifnecessary, to meet the new conditions and be in accordance with 3.20C(5) above. The additional valves required, on account of changed conditions, may be installed on the outlet piping provided there is no intervening valve. B. Heating Surface. The heating surface shall be com- puted as follows. (1) Heating surface as part of a circulating system in contact on one side with water or wet steam being heated on the other side with gas or refractory being cooled, shall be measured on the side receiving heat. (2) Boiler heating surface and other equivalent sur- face outside the furnace shall be measured circumferen- tially plus any extended surface.

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