Ullage space

The heat weakens the tank shell around the ullage space. Thermally induced stresses are created in the tank shell near the vapor/Iiquid interface, and the heat-weakened tank plus the high internal pressure combine to cause a sudden, violent tank rupture. Fragments of the tank are propelled away from the tank at great force. Most of the remaining superheated liquid vaporizes rapidly due to the, pressure release. The rest is mechanically atomized to small drops due to the force of the explosion. A fireball is created by the burning vapor and liquid. 

When it is assumed that the gas in the ullage space is completely mixed and that there is always a layer of saturated liquid at the vapor-liquid interface, the heat transferred from the gas to the interface can be represented by... 

Two different mechanisms affect the top of tanks depending on cargo or emptiness. When the tank is filled with crude, the ullage space is filled with inert gas that should limit corrosion. The crude oil will emit corrosive gases such as hydrogen sulfide that can combine with moisture and oxygen to form sulfuric and sulfurous acids that can attack the steel and cause general corrosion. 

A resin based on rosin was manufactured by an en masse reaction. Only a small exotherm was involved. In an attempt to keep the product mobile for ease of transfer and handling, the reactor was heated above the (unknown) autoignition temperature of the flammable vapours in the ullage space. Autoignition occurred, there was an explosion and a fire. 

Figure 12 shows the temperature gradient in the gas ullage space at different times during the run. This plot clearly shows a temperature stratification which was maintained during the entire run. All other runs showed a similar stable stratification pattern. 

Changes in liquid level and ullage-space temperature gradient during tests were read out on a strip-chart recorder. The signals for this recorder were generated by seven thermocouples mounted at equal intervals of height on a "tree" mounted within the tank. 

All cryogenic storage vessels, transportable and stationary, are designed for a liquid volume of 90% and a vapor volume of 10%. The latter, known as the ullage space, is designed to extend the time that the inner vessel can be sealed off before the pressure level in the inner vessel due to heat inleak requires venting of some of the vapor formed above the liquid. 

One result of stratification is that a limited portion of the fluid is heated and vaporized. This can rapidly raise the tank pressure to the saturation pressure of the warm layer. The severity of stratification can be illustrated using hydrogen. A 0.6 K increase in fluid surface temperature raises the vapor pressure in a tank from 0.03 to 0.07 MPa since a difference in temperature of 8-22 K can develop between the bulk and the stratified layer, the pressure in the ullage space may increase by as much as 2.8 MPa. 

For removing liquid hydrogen or helium by gas pressurization froni a dewar, a more detailed analysis is required. In this analysis consider the ullage space of the storage vessel to be the system as shown in Fig. 7.27. If an energy balance is written around the system, the heat transferred to the system minus the work done by the system equals the increase in internal energy of the system plus the heat transferred to the liquid minus the heat transferred from the inlet gas. In equation form this can be represented by... 

This will lead to (a) sub-atmospheric pressure in ullage space... 

In practice, such a device introduced into the neck or ullage space of a containing vessel or tank has never been observed to reduce the liquid boil-off and is not recommended. Indeed, removal of the flow isolator, and replacement with a set of horizontal radiation baffles has led to significant (i.e. 50 % or more) reduction in BOR for several applications. 

Laboratory studies with freely evaporating cryogenic Uquids have witnessed on many occasions the switching off of evaporative mass flows for periods of many minutes, and the subsequent rapid rise in evaporation, or vapour explosions, beyond the upper limit of the flow meter. However, the introduction of carbon dioxide, either into solution, or into the ullage space, in order to stimulate, or simulate, the formation of a film and switch off the evaporation, proved to be inconclusive. 

Fig. 6.2 Autostratification by addition of sub-cooled liquid. For LNG, when density difference between layers >1.0 %, or sub-cooling of lower layer >3 K. For LPG, sub-cooling of lower layer >5 K. This will lead to (a) sub-atmospheric pressure in ullage space and (b) rollover. Conclusion avoid, particularly in LNG and LPG sea tankers...

Fig. 7.1 Two layer density stratification in LNG under pressure within VI tank. Upper layer 44 % average, at saturation temperature T and ullage space vapour pressure P. Lower layer 56 % average, remaining at filling temperature T but sub-cooled under ullage pressure P+hydraulic head...

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