Common Question & Answer about Storage Tank (part 7)

Question:Is the ASME Code, Section VIII, Div. 1 an acceptable alternative for the reinforcement of shell openings per Section 3.7.1.8

Answer:No

Question:Does note c of API 650 Table 3-6 allow the customer to locate nozzles lower than allowed by the weld spacing requirements of 3.7.3?

Answer:No

Question:Do notes 6 and 7 from API 650 Figure 3-4A require the use of nozzle flanges listed in Table 3-8 when substituting Table 3-6 nozzles for manways?

Answer:The API committee responsible for this standard has issued an agenda item to study this issue further to determine if a change in the standard is required. Any changes resulting from this agenda item will appear in a future edition or addendum to API 650.

Question:May a shell nozzle or manhole that otherwise complies with Figure 3-4B have its reinforcing plate welded to the nozzle neck with a full-penetration weld, in addition to the fillet weld in the same manner as API 620 Figure 3-8 Part 2 Panel m, if the purchaser has so approved as permitted by API 650 Section 3.7.1.8?

Answer:Yes

Question:The manhole or nozzle detail in Figure 3-4B of API 650 (details at the top of the page) show the fillet weld sizes as “A”. Where is the dimension for “A” found?

Answer:See Note 3 of Figure 3-4B and Table 3-7.

Question:Is API 620, Figure 5-8, Part 1, Panel d, an acceptable alternative reinforcement detail for larger than NPS 2 nozzles as allowed by API 650, Section 3.7.1.8?

Answer:No. Nozzle connections require full penetration welds between the shell and the nozzle neck. Refer to 3.7.2.1.

Question:Does API 650 allow reinforcement to be located on the ID of the shell?
Answer:Yes, if approved by the owner. Refer to 3.7.1.8.

Question:Does API 650 allow reinforcement to be located on the underside of the roof?
Answer:No. Refer to 3.8.5.1

Question:Is the thickness, T, in Table 3-4 the same as tshell for the thickness of the opening reinforcement?

Answer:Yes

Question:Is the thickness "tn" in Table 3-4 equal to T plus t?
Answer:No. In column 1, T and t are not considered to be a sum.

Question:Is a minimum repad required around all nozzles greater than 2" even if the area of the nozzle allowed as reinforcing (Section 3.7.2.3) is greater than the reinforcing required (Section 3.7.2.1)?

Answer:No, see Section 3.7.1.8.

Question:May shell opening reinforcement be determined either in accordance with Section 3.7.2.1 or, alternately, in accordance with Tables 3-8 and 3-9?

Answer:No, Section 3.7.2.1 requires that Tables 3-8 and 3-9 be used. However, alternate details satisfying the reinforcement requirements of Section 3.7.2.1 are allowed if the purchaser agrees to their use.

Question:Regarding Section 3.7.2 as it applies to Appendix F, when calculating the required shell thickness at the nozzle location is it necessary to use the joint efficiency factor that was used for calculating the required tank shell thickness?

Answer:No

Question:Referring to API 650, Section 3.7.3, is it correct to conclude that the minimum distance between the periphery of a nozzle reinforcing plate and the shell-to-bottom weld (measured as toe-to-toe) is as follows: 1) For shell plate ½ in. or less, or for any stress-relieved shell plate thickness, the required distance is the greater of 3 inches or 2.5 times the shell thickness? 2) For shell plate greater than ½ in., with or without stress relief, the required distance is 10 inches?

Answer:Question 1 is correct. Question 2 is incorrect. The requirement for the case of shell plate greater than ½ in. with no stress relief is the greater of 10 inches or 8 times the weld size. For the case of shell plate greater than ½ in. with stress relief, the distance is 3 inches or 2.5 times the shell thickness.

Question:Regarding reinforcement for shell openings, is the total area of provided reinforcing equal to the sum of the reinforcing areas above and below the opening?

Answer:Yes, the areas are measured vertically, along the diameter of the opening, and must be within a distance above or below the horizontal centerline equal to the vertical dimension of the hole.

Question:Is the total area of required reinforcing equal to the product of the vertical diameter of the hole and the nominal plate thickness?

Answer:Yes, however, when calculations are made for the required thickness considering all design and hydrostatic test load conditions, the required thickness may be used in lieu of the nominal thickness.

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SAFETY PRECAUTIONS FOR GAS SHIELDED ARC WELDING

When any of the welding processes are used, the shielded from the air in order to obtain a high molten puddle of metal should be quality weld deposit. In shielded metal arc welding, shielding from the air is accomplished by gases produced by the disintegration of the coating in the arc. With gas shielded arc welding, shielding from the air is accomplished by surrounding the arc area with a localized gaseous atmosphere throughout the welding operation at the molten puddle area.
Gas shielded arc welding processes have certain dangers associated with them. These hazards, which are either peculiar to or increased by gas shielded arc welding, include arc gases, radiant energy, radioactivity from thoriated tungsten electrodes, and metal fumes.

PROTECTIVE MEASURES
a. Gases.

(1) Ozone. Ozone concentration increases with the type of electrodes used, amperage, extension of arc tine, and increased argon flow. If welding is carried out in confined spaces and poorly ventilated areas, the ozone concentration may increase to harmful levels. The exposure level to ozone is reduced through good welding practices and properly designed ventilation systems.

(2) Nitrogen Oxides. Natural ventilation may be sufficient to reduce the hazard of exposure to nitrogen oxides during welding operations, provided all three ventilation criteria are satisfied. Nitrogen oxide concentrations will be very high when performing gas tungsten-arc cutting of stainless steel using a 90 percent nitrogen-10 percent argon mixture. Also, high concentrations have been found during experimental. use of nitrogen as a shield gas. Good industrial hygiene practices dictate that mechanical ventilation, be used during welding or cutting of metals.

(3) Carbon Dioxide and Carbon Monoxide. Carbon dioxide is disassociated by the heat of the arc to form carbon monoxide. The hazard from inhalation of these gases will be minimal if ventilation requirements are satisfied.

(4) Vapors of Chlorinated Solvents. Ultraviolet radiation from the welding or cutting arc can decompose the vapors of chlorinated hydrocarbons, such as perchloroethylene , carbon tetrachloride, and trichloroethylene, to form highly toxic substances. Eye, nose, and throat irritation can result when the welder is exposed to these substances. Sources of the vapors can be wiping rags, vapor degreasers, or open containers of the solvent. Since this decompsition can occur even at a considerable distance from the arc, the source of the chlorinated solvents should be located so that no solvent vapor will reach the welding or cutting area.

b. Radiant Energy.

Electric arcs, as well as gas flames, produce ultraviolet and infrared rays which have a harmful effect on the eyes and skin upon continued or repeated exposure. The usual effect of ultraviolet is to “sunburn” the surface of the eye, which is painful and disabling but generally temporary. Ultraviolet radiation may also produce the same effects on the skin as a severe sunburn. The production of ultraviolet radiation doubles when gas-shielded arc welding is performed . Infrared radiation has the effect of heating the tissue with which it comes in contact. Therefore, if the heat is not sufficient to cause an ordinary thermal burn, the exposure is minimal. Leather and WoOl clothing is preferable to cotton clothing during gas-shielded arc welding. Cotton clothing disintegrates in one day to two weeks, presumably because of the high ultraviolet radiation from arc welding and cutting.

c. Radioactivity from Thoriated Tungsten Electrodes.

Gas tungsten-arc welding using these electrodes may be employed with no significant hazard to the welder or other room occupants. Generally, special ventilation or protective equipment is not needed for protection from exposure hazards associated with welding with thoriated tungsten electrodes.

d. Metal Fumes.

The physiological response from exposure to metal fumes varies depending upon the metal being welded. Ventilation and personal protective equipment requirements shall be employed to prevent hazardous exposure.

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