LC in LNG Solutions

LNG

Stratification Prediction and
Roll-over alarm system

LC Engineers & Consultants BV (LC) is the sole developer and provider of the Stratification and Roll-over predictive software solution, so called “LNG Expert”, nowadays available in the new version 6.0. This software solution is developed in close co-operation by LC and the R&D department of Engie GDF-Suez in Saint-Denis in France, and is the only truly validated advanced control module based on 30 years of experience with LNG behavior in real processes.

“The only truly validated advanced control module based on 30 years of experience with LNG behavior in real processes.”

Engie GDF Suez

This software solution is developed in close
co-operation by LC and the R&D department of
Engie GDF-Suez in Saint-Denis, France.

What is LNG?

LNG is in fact a non-existing liquid. A natural, non-homogenous gas in a state of equilibrium, where every single component has its own boiling point and density.

VOL LNG

avg

Boiling point

degr. C

Density

kg/m3

  • 94,7
  • 4.8
  • 0.4
  • 0.06
  • 0.01
  • 0.02
  • 0.01
  • Methane
  • Ethane
  • Propane
  • Butane
  • Pentane
  • Nitrogen
  • Others
  • On average

CH4

C2H6

C3H8

C4H10

C5H12

N2

-162 °C

450 kg/m3

Temperature and density

The temperature and density equilibrium depends on the actual chemical composition
and vapor pressure.

Chemical composition

avg

Boiling point

degr. C

Pressure

bar

  • Pure water
  • Pure water
  • Pure water
  • Seawater
  • LNG source 1
  • LNG source
  • (rho = 1.00)
  •  
  •  
  • (rho = 1.025)
  • (rho = 465
  • (rho = 448)
  • 1 bar
  • vacuum
  • 2 bar
  • 1 bar
  • 1 bar
  • 1 bar

Influental factors on LNG

Average layer temperature and density are influenced by:

Factor 01
Chemical composition of
LNG and vapor pressure

Factor 02
Heat leaks via bottom,
side and roof

Factor 03
Ratio [tank volume]
to [actual LNG contents]

Factor 04
Ratio [tank height]
to [tank diameter]

Factor 05
Pressure in vapor
handling system

Phase 1

Phase 2

Phase 3

Phase 4

Stratification Development

01
Stratified LNG

  • The stored LNG will show a continuous boil-off at the surface changing the original chemical composition and their associated density and temperature.
    The lighter components will boil off first, followed by heavier components.
  • This boil-off process will maintain a relatively stable temperature, while the average density (corrected due to the changing composition) will increase as a consequence of both boil-off and heat leaks.
  • The upper layer will become “heavier”.

02
Interlayer Penetration

  • However, the lower layer is also subjected to heat leaks from the sides and bottom, and is isolated from the boil-off surface by the upper layer.
  • Consequently, the lower layer gradually warms up, causing the initial higher density to drop (“overheated”). The lower layer will become “lighter”.
  • Although initially the lower layer will be totally isolated from reaching the upper layer, a slow but steady penetration of lower layer molecules into the upper layer is occurring.

03
Roll-Over

  • “Rollover” itself is triggered by the evolution of the mentioned “stratification”.
  • The different initial grades of LNG (different temperature/density/chemical composition characteristics) eventually reach a state of density equalisation, where the lower layer will quickly be able to mix with the upper layer.
  • Thus allowing all of the accumulated overheat in the lower layer to be released via free surface evaporation.
  • This results in the generation of excessive boil-off, which can lead to overpressures in the tank’s gas phase, dependent on actual vapour handling equipment load and additionally released boil-off.

04
Homogeneous equilibrium

  • The LNG in the storage tank has reached a new, homogeneous equilibrium in temperature and density.
Tank01
Tank02
Tank 03
Tank 04

A roll-over occurs under certain conditions as densities of two layers of stratified LNG in a storage tank approach equality. This results in sudden mixing of these layers and any heat trapped in the system is released rapidly, generating a vapor which might easily exceed the venting capability of the storage tank or even the BOG compressor capacity

Roll Over BOG BOG BOG BOG BOG

phase 1 Heating up of the lower layer without mixing with the upper layer

phase 2 Interlayer penetration and gradual erosion of the interface

phase 3 Roll-over, hydrodynamic overturning

phase 4 Return to a thermodynamic equilibrium

LNG Expert

LNG Expert

Stratification Prediction & Roll-over alarm system

This software solution is developed in close
co-operation by LC and the R&D department of
GDF-Suez in Saint-Denis, France, and is the only truly
validated advanced control module based on 30 years of
experience with LNG behavior in real processes.

LNG Expert

Functions and Features

  1. Accurately predict the evolution of a stratification and the roll-over phenomenon
  2. Facilitate operations for LNG storage tanks
  3. Display tanks level, layer height, average density, temperature and gross calorific value per layer
  4. Calculate the in-tank chemical composition of LNG
  5. Indicate the status of plant vapor handling and safety devices
  6. Automatically generates alarms in case of an anticipated roll-over, without any operator intervention
  7. It handles all grades of LNG of which the chemical composition is saved in its database
LNG expert

The advantages are obvious

  • 1 Automatic and continuous monitoring of the stratified tank contents, calculation of its evolution to predict the occurrence of a roll-over
  • 2 Automatic alarm, without operator intervention,
    in case of a predicted roll-over
  • 3 Real time status of plant safety devices
  • 4 Real-time tank measurements
  • 5 OPC and/or Serial link communication with DCS
  • 6 User friendly graphic interface
  • 7 Easy configuration

“Installed successfully since 2006. In total protecting over
5.000.000 cbm of LNG storage tank capacity.”

World

LNG Expert now available in:

English, Japanese, Korean and Chinese

Contact us

At present time LC has installed many
installations on LNG sites all over the world
e.g. in China, South-Korea, Australia, Finland and the USA.

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