Thermal overfill

Thermal expan- Drum at proper temperature sion due to liquid, Keep drum away from heat source overfill leading to loss of reaction is complete before drumming containment. Allow adequate freeboard for each material CCPS G-3 CCPS G-14 CCPS G-22 CCPS G-29... 

It is still necessary to have a small relief system to allow for thermal expansion of a liquid-full system. This relief system is also necessary for handling hydraulic overfill and fire conditions, but the system is usually relatively simple. 

The normai fiii ievei shouid be ciose enough to the LAH to enabie overfilling to be rapidly detected (and to maximise the usabie capacity of the tank), but should be set an adequate margin beiow the LAH to prevent spurious operation of the aiarm, eg due to liquid surge or thermal expansion at the end of an otherwise correctiy conducted transfer. 

If the BOR drops below the average normal figure, then some of the heat inflow is being stored in the liquid as thermal OVERFILL. The subsequent uncontrolled release of this stored energy by sudden, increased evaporation above the normal BOR, is called a ROLLOVER. 

Density equilibration between two stratified layers, by mechanical mixing, will yield significant evaporation from the heat of mixing of the two compositions, as well as from energy release of thermal overfill. 

If Stratification is detected or auto-stratification is suspected, then urgent action is needed to mix the contents of each tank Allowing the stratification to continue wiU allow more thermal overfill to bmld up in the lower layer, and increase the amount of vapour produced subsequently by a mixing operation or a rollover. 

Thermal overfill is the additional heat energy taken up by a superheated hquid and... 

If some of the heat is stored by heating the liquid, the BOR will be reduced. This stored heat or thermal overfill , can with time lead to unstable rises in BOR... 

The A heat flows are absorbed by evaporating some liquid as bod-off gas and superheating part of the bulk liquid as thermal overfill. On the other hand, most or all of the B heat flows can generally be absorbed by heating the vapour only, i.e. 

At the centre, the liquid motion becomes focussed into a strong downward jet. This central jet carries excess superheat, which has not been released by surface evaporation, into the liquid core as thermal overfill. Secondary convective processes now produce mixing and distribution of the excess superheat either throughout the core, or alternatively in a stratified layer just below the surface. A large depth/diam-eter ratio will tend to encourage this type of stratification. 

The overall amount of superheat energy, or thermal overfill (TO) , is therefore an important parameter for describing the overall thermodynamic state of the stored liquid. 

In formal terms, thermal overfill is the sum of the excess enthalpy (H-Hq) of the stored liquid in relation to the value of Hq defined for the surface of a homogeneous liquid in thermodynamic equilibrium at To, with its saturated vapour at a prescribed pressure Pq. For normal isobaric storage under atmospheric pressure at sea level, the prescribed reference pressure will be close to, but not exactly equal to, 1 bar, and will depend on operational and environmental conditions. 

The important part, (TO)+, is the positive excess energy in the vessel which, if released by the uncontrolled vaporisation of liquid, could lead to an overpressure in the vessel. It should therefore be borne in mind that, when the term thermal overfill is used, the relevant quantity is usually (TO)+ because of the poor mixing. 

Before discussing the concept of thermal overfill, let us consider the basic energy equation relating the heat absorbed by evaporation, m ev) (where m(ev> is the evaporation mass flow and k is the latent heat of vaporisation), to the total heat inflow Q through the insulation into the liquid. 

Thermal overfill is largely a characteristic describing the liquid core which does not take part in the primary convection flow, the latter being driven by heat absorption at the wall and floor, and evaporation at the surface. 

Thermal overfill is also time dependent. If the total heat flow into the stored liquid exceeds the heat absorbed by the boil-ofP vapour mass flow rate, m(ev> via the latent heat of vaporisation X, then the rate of change of thermal overfill with time is ... 

If the left-hand side of (2.4) only is zero, then a meta-stable state with constant (TO)av, or constant superheat, exists. This is the normal storage situation because, as will be discussed in Chap. 4, a finite, constant superheat or thermal overfill is a necessary condition to drive the equihbrium evaporation of a cryogenic hquid. 

However, when the rate of change of thermal overfill d(TO)av/dt is positive over a period of time, then a hazardous storage situation is building up part of the heat inflow is being stored in the hquid and is not being absorbed by evaporation and removed from the hquid. It should be noted that d(TO)av/dt is positive when the atmospheric pressure (and the reference pressure Po) is fading. 

The magnitude of the thermal overfill can be very considerable. For example, consider LNG in a large storage tank of 100,000 m liquid capacity with bulk superheats of 0.1,0.2 and 0.4 K. Then the associated thermal overfills are 14,700,29,400 and 58,800 MJ respectively large quantities of energy to dissipate. 

Compared with the chemical energy stored in the liquid as heat of combustion, these thermal overfill energies are, however, relatively small only a few per cent. On the other hand, they are physical energies which may be more easily released than chemical energy via triggering mechanisms. These triggers may not be so easily identified. 

In the example, the LNG has bulk liquid superheats of 0.1,0.2 and 0.4 K, which together with corresponding evaporative mass fluxes of 0.12,0.26 and 0.57 g/m s (from (2.4)) are equivalent to boil-off figures of 0.05, 0.1 and 0.23 %/day. These boil-off rates will dissipate thermal overfill at the rates of about 10,500,22,750 and 49,800 MJ/day, daily figures which are similar in magnitude to the thermal overfills of the bulk superheated liquid. 

It therefore follows that the excess thermal overfill from bulk superheats of 0.1, 0.2 and 0.4 K, will be dissipated by excess boil-off, falling from peaks of about 1.6, 3.5 and 7.6 times the normal rate respectively, back to the normal rate over a period of about 1 or 2 days. 

Beduz, C., Rebiai, R., Scurlock, R.G. Thermal overfill and the surface evaporation of cryogenic liquids under storage. Adv. Cryog. Eng. 29,795 (1983)... 

If for any reason, the BOR falls below the total heat in-leak, the thermal energy flow into and out of the liquid is not balanced and the surplus energy flow is stored in the liquid as superheat or thermal overfill. This surplus energy remains in the liquid and builds up with time. It can only be released by evaporation of some of the now superheated liquid via one of the unstable evaporation phenomena described below. These uncontrolled evaporation phenomena constitute a hazard which increases with time, and with the scale of the storage container and with the volume of liquid being stored. 

Surface Evaporation, the Only Way of Dissipating Superheat and Thermal Overfill 47... 

The high evaporation rate will then be maintained until the thermal overfill energy, or superheat, of the whole volume of bulk liquid is dissipated by the latent heat of the evaporated mass flow. 

Release of thermal overfill and heat of mixing during rollover via increased BOR. 

This mixing, or so-called rollovef , is uncontrolled and may cause a rapid rise in evaporation rate, and tank pressure, associated with the uncontrolled release of the thermal overfill energy from the lower layer, and the heat of compositional mixing of the two layers, within the tank. 

The BOR starts to fall below normal BOR, as thermal overfill becomes locked in the bottom layer. The liquid surface in the tank starts to rise, as the bottom layer heats up and expands—only visible in spherical tank filled to 98.5 % or more. 

We now have a convective mechanism whereby heat entering the lower liquid layer through the walls and floor of the storage tank cannot be released by the normal process of surface evaporation, Instead, this heat is mixed convectively throughout the lower layer, via the central down ward jet, as thermal overfill. The mean temperature of the lower layer rises, and the mean density falls with time. 

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