Weak-seam roof tanks

A flammable liquid is to be stored in a large storage vessel. Two vessels are available. One vessel is called a weak seam roof tank, with the weakest part of the vessel being the welded seam between the roof and the vertical wall of the tank. The other vessel is a domed roof tank, with the weakest part being the seam along the bottom of the tank. Which tank is the best choice for storing this material ... 

Since discharges of vapors from highly hazardous toxic materials cannot simply be released to the atmosphere, the use of a weak seam roof is not normally acceptable. It is best that tanks be designed and stamped for 15 psig to provide maximum safety, and pressure relief systems must be provided to vent to equipment that can collect, contain, and treat the effluent. 

If the liquid had been stored in an ordinary low-pressure storage tank with a weak seam roof, then the roof w ould have come off, and the burning liquid would have been retained in the rest of the tank. 

By avoiding knock-on effects for example, if storage tanks have weak seam roofs, an explosion or overpressuring may blow the roof off, but the contents will not be spilled (see Section 5.2). 

Fixed cone roof vertical cylindrical tanks in diameters larger than 30-50 feet (9-15 m), should have a weak seam or weak seam roof. These are intended to respond to an internal overpressure by being blown off and allowingthetankto retain its integrity of content containment. Design of an effective weak seam roof becomes increasingly difficult as the diameter decreases. 

The tank must not lift up at the bottom under any conditions.. . . Thus tiedown bolts may be required.. . the primary objective of a weak seam roof is to ensure that the contents remain in the tank.. . . ... 

You might ask why Tank No. 393 ruptured at the floor plate and the cylinder and roof blew off. The CSB reported that the acid tank lacked a weak seam roof, which would have prevented it from separating at the floor. 

Tank roof Weak seam Strong seam... 

Weak roof-to-shell attachment for fixed roof tanks, as per API-Std-650 for Welded Steel Tanks for Oil Storage. The joint fails and excess pressure can be relieved (above the normal design provided). Such tanks do not require additional emergency vent equipment however, it can be provided in order to prevent the roof seam failure with its attendant replacement/ maintenance requirements. This type can only be used outside of a building, not confined. 

Atmospheric fixed roof tanks built according to API requirements will have a weak seam at the junction of the roof with tank side. If there is internal overpressure, such as an explosion, the seam will part or the roof blows off, leaving the shell in place to retain its contents. The resulting fire will therefore only initially involve the exposure surface of the liquids still in the tank. 

Many vertical low-pressure storage tanks containing flammable or combustible liquids are designed with a weak weld seam on the wall-to-roof connection. This loss prevention feature allows the roof to separate and peel back if an internal fire, an internal explosion, or just a pneumatic overpressure situation occurs within the tank. If the roof relieves or is blown off, the liquid is still contained within the walls if these tanks are properly designed and fabricated. These conical roof tanks are often built to the American Petroleum Institute (API) Standard 650 Welded Steel Storage Tanks. ... 

Consider a weak roof seam for API tanks if the tank is going to split under internal pressure, the roof seam should fail, not the bottom seam. The weak roof seam must be specifically included in the specifications and the mechanical design must address the issue. This emphasis is made because smaller tanks (less than 50 feet in diameter) manufactured under API 650 will not automatically have a weak roof seam. 

Where enclosed spaces may produce overpressures blow out panels or walls are provided to relieve the pressure forces. The connections of the panel are specified at a lower strength that normal panels so it will fail at the lower level and relieve the pressures. Similarly, combustible or flammable liquid storage tanks are provided with weak roof to shell seams so that in case of an internal explosion, the built-up pressure is relieved by blowing off the roof and the entire tank does not collapse. 

B) In a vertical tank the construction referred to in paragraph (i)(2)(v)(A) of this section may take the form of a floating roof, lifter roof, a weak room-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. 

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