Vacuum vessels external pressure

Tanks that could be subjected to vacuum should be provided with vacuum-breaking valves or be designed for vacuum (external pressure). The ASME Pressure Vessel Code contains design procedures. 

Causes of Vacuum Hazards The design for the internal pressure condition of vessels is usually straightforward and well understood. The design for external pressures is more difficult. The devious ways in which external pressure can be applied can often be overlooked. 

Maintenance and testing. It is not a good idea to apply vacuum on a vessel during maintenance or testing without full knowledge of the external pressure rating, unless a suitable vacuum relief device is in place and operable. 

Vessels subject to external pressure should be designed to resist the maximum differential pressure that is likely to occur in service. Vessels likely to be subjected to vacuum should be designed for a full negative pressure of 1 bar, unless fitted with an effective, and reliable, vacuum breaker. 

Two types of process vessel are likely to be subjected to external pressure those operated under vacuum, where the maximum pressure will be 1 bar (atm) and jacketed vessels, where the inner vessel will be under the jacket pressure. For jacketed vessels, the maximum pressure difference should be taken as the full jacket pressure, as a situation may arise in which the pressure in the inner vessel is lost. Thin-walled vessels subject to external pressure are liable to failure through elastic instability (buckling) and it is this mode of failure that determines the wall thickness required. 

External pressure implies that the pressure on the outside of the tank or vessel is greater than that in its interior. For atmospheric tanks, the development of an interior vacuum results in external pressure. External pressure can be extremely damaging to tanks because the surface area of tanks is usually large, generating bigb forces. The result of excessive external pressure is a buckling of the shell walls or total collapse. In some cases wind velocities during hurricanes have been sufficient to knock down and collapse tanks. 

Unless designed to withstand external pressure (see Section 13.7), a vessel must be protected against the hazard of underpressure as well as overpressure. Underpressure will normally mean vacuum on the inside with atmospheric pressure on the outside. It requires only a slight drop in pressure below atmospheric pressure to collapse a storage tank. Though the pressure differential may be small, the force on the tank roof... 

Two fundamentally different types of failure may occur in vessels operated under vacuum (as opposed to high pressure). The problem in vacuum operation is the elastic stability of the vessel shell when it is under an external pressure loading. In general, elastic instability is a problem that must be considered in all structures having limited rigidity when subjected to bending, torsion, compression or a combination of these loadings. In failure by elastic instability, the structure buckles or collapses like an evacuated thin-shelled vessel. 

This calculation leaves out the extra allowance for corrosion, etc., but illustrates the basic approach used in designing pipelines and pressure vessels for internal pressures of as much as 3000 psig (design for external pressures or vacuums is more complicated). For more details see the ASME code [2]. 

Coils inside pressure vessels may be subjected to the internal pressure of the vessel acting as an external pressure on the coil. In addition, steam coils should be designed for full vacuum or the worst combination of external loads as well as the internal pressure condition. The coil must either be designed for the vessel hydrotest, externally, or be pressurized during the test to prevent collapse. 

Vacuum Stiffener Rings Rings made of flat bar or plate, or structural shapes welded around the circumference of the vessel. These rings are installed on vessels operating under external pressure to prevent collapse of the vessel. Also used as insulation support rings. 

In the absence of corrosion, wind, and earthquake considerations and for internal pressures greater than the external pressure (i.e., excluding vacuum operation), the cylindrical shell wall thickness is computed from the ASME pressure-vessel code formula ... 

For full or partial vacuum conditions, the design pressure is applied externally and is the maximum pressure difference that can occur between the atmosphere and the inside of the pressure vessel. This applied external pressure is normally 760 Torr for full vacuum conditions. 

Furnaces are constructed using an induction heated crucible of graphite or silicon carbide, contained within a refractory lined vacuum vessel. The zinc condenser may be external, in the vacuum train, or may be contained within the lid of the containment vessel. Seals are by rubber O rings in water-cooled flanges and the ability to maintain vacuum is a critical aspect of efficient operation. Target operating pressure is around 80 mbar absolute. The final temperature of the retort bullion (impure silver) formed within the retort at the end of a batch is in the range of 1100 to 1200°C. The detection of the end point of the batch distillation is also an important aspect of operation and usually is determined by the vacuum pressure and the temperature of the residual metal. Typical performance is shown in Table 12.2. 

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