LNG Storage Tank Roll-over and prevention fundamentals

#LNG

# Operation

The primary objective of this article is to describe about the issues surrounding rollover in LNG storage tank.

Introduction

“Rollover” refers to the rapid release of LNG vapour that can occur as a result of the spontaneous mixing of layers of different densities of LNG in a storage or cargo tank. A pre-condition for rollover is that stratification has occurred, i.e. the existence in the tank of two separate layers of LNG of different density. The possibility of a sudden release of large amounts of vapour and the potential over-pressurization of the tank resulting in possible damage or failure is recognized by the major design codes. EN 1473 - "The design of onshore LNG terminals” and NFPA 59A - “Standard for the Production, Storage and Handling of LNG” both require this phenomenon to be taken into consideration when sizing relief devices. Whilst the relief valves may prevent damage to the tank, LNG vapour is not only flammable and heavier than air on release, but a valuable commodity and a potent greenhouse gas and therefore venting should be avoided whenever possible.

Fig-1

Basic Thermodynamics

Figure 2 shows an LNG tank without stratification. Methane evaporates from the surface, which cools due to loss of latent heat, causing the density of the surface layer to increase and the liquid to sink. Heat inleak through the tank bottom and wall insulation is sufficient to warm the lower side layers and a convection current is set up, ensuring mixing of the liquid. The lighter fractions will boil off first, resulting in the density of the remaining liquid gradually increasing, a process known as “weathering” or “aging”.

Rollover can only occur if stratification has taken place in the LNG. Stratification of LNG can occur when an LNG tank is filled with LNG of different densities. Stratification will occur readily if the LNG being introduced into the tank is either denser than that of the “heel” remaining in the tank and filling is at the bottom, or if the LNG introduced is lighter than the heel and filling is into the top of the tank. Studies undertaken in Japan in the late 1970s showed that a density difference of 1 kg/m3 (0.001 tonne/m3) could result in stratification if incoming LNG was introduced at a very slow rate.

Fig-2

Figure 3 shows a tank where stable stratification has taken place caused by filling a storage tank with liquids of different densities, the higher density layer being the lower layer. There is little heat or mass transfer between the layers and each layer establishes its own convection currents. A key indicator that stratification has occurred is a noticeable reduction in the normal boil-off rate.

Fig-3

Heat is lost from the upper layer by evaporation, but because of the density difference and very low thermal conductivity of LNG there is very little heat transfer from the lower layer to the upper layer. Instead, the heat, which is absorbed by the lower layer through the tank wall and floor, causes a rise in temperature and a decrease of density of the lower layer. When the densities are approximately equal, the lower superheated layer will rise through the upper layer, releasing its superheat and thereby generating large volumes of boil-off in a short period; this is rollover. Figures 4, 5 and 6 show the temperature, density and boil-off rate trends graphically.

Fig-4

Fig-5

Fig-6

Nitrogen presence in LNG

If the LNG contains significant quantities of nitrogen, it has been postulated that auto-stratification may occur, possibly resulting in “nitrogen induced rollover”. Four of the rollover cases in the GIIGNL study were attributed to this. Nitrogen has a boiling point of -196°C compared with -162°C for the average LNG. Furthermore, nitrogen has a molecular mass of 28, compared with that of 16 for methane, the main constituent of LNG. Therefore, as the nitrogen boils off, the density of the remaining LNG will decrease, unlike nitrogen-free LNG where the density will increase as it ages. If there is sufficient nitrogen present this can result in a layer of low density liquid which can remain on the surface, but will eventually mix with the lower layer resulting in rollover.

Detection of Stratification and Prevention

As mentioned earlier, a noticeable reduction in boil-off rate below the normal is a good indication that stratification has occurred. The following may give some indication that stratification has occurred:

1. Reduction in boil-off gas flow rate below normal
2. Tank level not decreasing at a normal rate; indeed, it may even increase
3. A careful examination of the CTS temperature probes shows the lower ones increasing in temperature whilst the upper ones are substantially constant.

The measurement of temperature and density throughout the liquid column will confirm this, but accuracy of measuring instruments is essential as Del. T of 0.1°C and density variations of 0.1% need to be detected. A reduction of 10% in boil-off rate should be taken as a warning of stratification.

Once stratification has been detected, the following means may be used to break up the layers:

• Transfer of the liquid from the tank either by exporting or transferring to another tank if possible
• Circulation of tank contents through jet nozzles or other mixing devices
• Recirculation of the liquid through a top fill line. It should be noted that the efficiency of this depends on the flow rate and it can result in high boil-off losses.

If a sophisticated tank management system is provided, the operator will have real time information available to enable break-up of the stratification before rollover occurs. Recently, intentionally induced density stratification has become routinely used by some operators to reduce high LNG boil-off rates, particularly when top filling is required for heavier LNGs. This means that boil-off gas compressor and pre-heater operating costs can be reduced both during and after unloading LNG tankers. These procedures require careful management, a sophisticated tank management system and a means to break up any stratification as referred to earlier.

Conclusion

To conclude about this article:

a) If grades of LNG with different compositions are going to be received and stored, the simplest counter measure option is to store them in separate tanks, if this is possible.

b) Stratification and thus rollover can be prevented by mixing LNG of different densities using top and bottom fill procedures and recirculation of the tank inventory through jet nozzles or other mixing devices.

c) If the incoming LNG is lighter than the heel in the tank, a bottom filling operation will generally ensure complete mixing of the two LNG grades, with little or no chance of stratification.

d) If the incoming LNG is heavier than the stored LNG in the tank, a top filling operation will avoid stratification and the risk of subsequent rollover. However, top filling usually results in excessive vapour generation, due to the flashing of the injected LNG into the tank’s vapour space and subsequent increase in tank pressure, which must be managed.

e) A jet nozzle fitted to a fill line located at the bottom of the tank can be very effective in preventing stratification, but there must be sufficient head in the filling line to ensure the jet can reach the surface of the liquid and sufficient time must be allowed to ensure the mixing process takes place throughout the tank.

f) Diffusers at the bottom of the fill line can also aid mixing. Perforated fill lines have also been fitted to some tanks, but these may result in excessive boil-off if any of the perforations are above the liquid surface during the filling operation.

References-

1. Sigtto Guidance for the Prevention of Rollover in LNG Ships
2. 296680306-LNG-Fundamental-Principles
3. Saeid-Mokhatab-John-Mak-Jaleel-Valappil-and-David-a-Wood-Auth-Handbook-of-Liquefied-Natural-Gas-2014-Gulf-Professional-Publishing-Libgen