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INDUSTRY, LABOR AND HUMAN RELATIONS 41 Ind 8 result of filling or emptying and atmospheric temperature changes,from exceeding the design pressure of the tank or vessel.Protection shall also be provided to prevent overpressure from any pump discharging into the tank or vessel when the pump discharge pressure can exceed the design pressure of the tank or vessel. (d) [2-2.4.4] If any tank or pressure vessel has more than one fill or withdrawal connection and simultaneous filling or withdrawal can be made, the vent size shall be based on the maximum anticipated simultaneous flow. (e) [2-2.4.5] The outlet of all vents and vent drains on tanks equipped with venting to permit pressures exceeding 2.5 psig shall be arranged to discharge in such a way as to prevent localized overheating of, or flame impingement on, any part of the tank, in the event vapors from such vents are ignited. (f) [2-2.4.6] Tanks and pressure vessels storing Class IA liquids shall be equipped with venting devices which shall be normally closed except when venting to pressure or vacuum conditions. Tanks and pressure vessels storing Class IB and IC liquids shall be equipped with venting devices which shall be normally closed except when venting under pressure or vacuum conditions, or with listed flame arresters. Tanks of 3,000 barrels capacity or less containing crude petroleum in crude- producing areas, and outside aboveground atmospheric tanks under 1,000 gallons capacity containing other than Class IA liquids may have open vents. (g) [2-2.4.7] Flame arresters or venting devices required in par. (f) may be omitted for Class IB and IC liquids where conditions are such that their use may,in case of obstruction,result in tank damage.Liquid properties justifying the omission of such devices include, but are not limited to,condensation, corrosiveness, crystallization,polymerization, freezing or plugging. When any of these conditions exist, consideration may be given to heating, use of devices employing special materials of construction, the use of liquid seals,or inerting. (6) [2-2.5] EMERGENCY RELIEF VENTING FOR FIRE EXPOSURE FOR ABOVEGROUND TANKS. (a) [2-2.5.1] Except as provided in par. (b),every aboveground storage tank shall have some form of construction or device that will relieve excessive internal pressure caused by exposure to fires. (b) [2-2.52] Tanks larger than 12,000 gallons capacity storing Class IIIB liquids and not within the diked area or the drainage path of Class I or Class II liquids do not require emergency relief venting. (c) [2-2.5.3] In a vertical tank the construction referred to in par. (a) may take the form of a floating roof, lifter roof, a weak roof-to-shell • seam,or other approved pressure relieving construction.The weak roof- to-shell seam shall be constructed to fail preferential to any other seam. (d) [2-2.5.4] Where entire dependence for emergency relief is placed upon pressure relieving devices, the total venting capacity of both normal and emergency vents shall be enough to prevent rupture of the shell or bottom of the tank if vertical, or of the shell or heads if horizontal. If unstable liquids are stored, the effects of heat or gas resulting from polymerization, decomposition, condensation, or self- reactivity shall be taken into account.The total capacity of both normal Register,March. 1982,No.315