Metrology for LNG

Improving LNG density and calorific value calculations

The aim of this work package is to reduce uncertainties in the calculation of LNG density. This will be achieved by validating density calculation methods and by assessing the impact of non-ideal mixing in ship tanks on the calculation of transferred energy. A primary LNG densitometer will be developed in task 4.1. with an order of magnitude better uncertainty down to 0.02% compared with 0.2-0.3% for systems that have been used in the past decades to produce significantly improved density reference data in task 4.2. The validity of these results will be checked with additional reference data that is produced with an alternative cryogenic pycnometer method in task 4.3. The thus obtained consistency checked set of new reference data will be used to validate or improve the many existing LNG density equations of state in task 4.4. The many possible implementations of density calculation methods, the non-homogeneous conditions in LNG tankers and variable reference conditions have a large impact on uncertainty and financial risks. This will be quantified by conducting a thorough study in tasks 4.5 and 4.6.

The density calculation from known composition can be calculated using many different equations which results in uncertainties between 0.3% and 1% depending on the composition

Description of Work

Task 4.1: Development of a primary LNG densitometer (RUB, E.ON Ruhrgas, VSL, PTB)

The aim of this task is to develop a primary LNG densitometer based on preceding work that resulted in a conceptual design of an accurate LNG densitometer. Detailed feasibility studies have shown that this new densitometer enables measurements in the whole liquid phase (saturated and homogeneous liquid) of LNG and LNG-like mixtures with excellent accuracy (+/-0.02% in density) and without phase equilibrium related decomposition of the liquid in the measuring cell. For details on the concept and related references see the included REG application.
Based on this conceptual design and other preceding work, the detail engineering including the detailed thermal layout and the control and measurement concepts will be developed. A report illustrating the design of the new densitometer in detail will be delivered [D4.1.1]. Some essential parts with long delivery times will be ordered in advance (in parallel to the detailed design) [D4.1.2]. The new densitometer will be completed within 12 months, including test measurements on methane. Operability of the main components of the densitometer will be reported by month 20 including a validation by VSL of the liquefaction of natural gas reference material [D4.1.3]. Traceability to SI will be established at the highest level by very accurate calibration of the sinkers at the beginning of the measurements and end of the project [D4.1.4]. A report on the complete validation of the new densitometer will be delivered by the end of month 24 of the JRP (month 18 of the REG) [D4.1.5.]

This task will be active from month 6 until month 24.

Activity summary:

• Report the design of the new densitometer (RUB, PTB, E.ON, Ruhrgas)
  
• Order essential parts with long delivery times (RUB)
  
• Complete densitometer tests and report on its operation including method of liquefaction of natural gas reference material (RUB, VSL, PTB)
  
• Establish traceability to SI at the highest level by accurate calibration of the sinkers (PTB)
  
• Report the complete validation of the new densitometer (RUB, PTB, E.ON, Ruhrgas)

Task 4.2: Produce experimental reference data with LNG densitometer (RUB, E.ON Ruhrgas)

The aim of this task is to use the new LNG densitometer to measure reference densities for at least four LNG-like mixtures [D4.2.1 and D4.2.2], gravimetrically prepared with the highest achievable accuracy and best traceability by VSL. This way, a set of traceable reference data for the density of LNG will become available. The uncertainty of the new data will be at least one order of magnitude smaller (down to 0.02%) than the uncertainty of the existing data (0.2-0.3%.) For the first time, densities of the homogeneous liquid will be described consistently to the density of the saturated liquid. To date only density data for the saturated liquid are available in the literature. For typical LNG compositions the new data set will form a whole new basis for the evaluation and development of density calculations and for the calibration of secondary density measurement devices.

This task will be active from month 25 until month 36.

Activity summary:

• Generate experimental reference data for first two LNG-like mixtures (RUB, E.ON, Ruhrgas)
  
• Generate experimental reference data for two other LNG-like mixtures (RUB, E.ON, Ruhrgas)

Task 4.3: Produce experimental reference data with LNG pycnometer system (INRIM, VSL, RUB)

The aim of this task is to extend the pycnometer density standard at INRIM (currently operating between -15°C and 150°C) for operation at LNG temperature (-163°C) with a target uncertainty of between 0.1% - 0.01%. INRIM will design and set up an apparatus in which a sample cell of known volume (pycnometer) is connected to a reservoir of LNG by means of a valve [D4.3.1]. The whole circuit will be housed in a temperature-controlled cryostat. Before assembly of the circuit, the cell volume will be measured exactly at the operating temperature and its weight including the valve will be determined [D4.3.2]. After assembly, a low vacuum is drawn inside the cell and in the line that connects it to the reservoir. The cryostat is set at the working temperature and the reservoir is gently filled to ensure that the fluid remains in liquid phase. The process of liquefaction of natural gas reference material will be validated by VSL. After the filling of the measuring cell, the cell and the valve will be weighed again. Experimental reference data will be produced for at least four LNG-like mixtures similar to the ones used for the LNG densitometer [D4.3.3]. The pycnometer reference data will be compared with the densitometer reference data [D4.3.4].

This task will be active from month 13 until month 36.

Activity summary:

• Complete development of Cryogenic Pycnometer including functionality tests (INRIM)
  
• Accurate measurement of Pycnometer volume and its weight [incl. The valve] (INRIM)
  
• Generate experimental reference date for at least four LNG-like mixtures (INRIM, VSL)
  
• Established degree of equivalence of LNG reference data between
  densitometer and pycnometer (INRIM, RUB)

Task 4.4: Validating and comparing Equations of State used for density calculation (RUB, VSL, TUV NEL, Elengy, E.ON Ruhrgas, INRIM)

The aim of this task is to use the results from task 4.2 and 4.3 to validate and improve the methods for calculating the LNG density based on composition [D4.4.1]. The results of the validation will be used as input for the development of guidelines and standards (WP5)
An overview of all relevant equations and methods for density calculation will be produced by Elengy. The financial implications of different methods being applied to a cargo will be evaluated. By analysing the information compiled, the most accurate method will be recommended for custody transfer. This will be based on agreement with available experimental measurements from task 4.2 and 4.3. [D4.4.2].

This task will first be active from month 3 until month 36.

Activity summary:

• Develop and validate Equation of State to better match reference density data (RUB, E.on, Ruhrgas, VSL)
  
• Compare multiple density calculation methods with experimental
  reference data (TUVNEL, Elengy, INRIM)

Task 4.5: Calculation of LNG Calorific Value at different reference conditions and the impact on total energy transferred (TUV NEL, PTB)

The aim of this task is to compare calculations of LNG Calorific Value at various reference conditions (Temperatures: 0, 15, 20 and 25°C) used by available standards and clearly demonstrate the consequence of using one reference condition against the other on the calculated energy transfer. Real examples of LNG cargoes will be used for these calculations [D4.5.1]. An accurate calculation of the energy content of LNG in the real fluid state is not described in any standard or guideline. The correction related to the enthalpy difference between the liquid state and standard conditions is significant, contrary to the case of pipeline gas. To calculate this correction accurate information on the composition of the LNG and on enthalpy differences between the actual state of the liquid and the gaseous reference state of the pure component calorific values is needed. Based on the work in task 3.1 and task 4.4 the combined uncertainty budget for calorific values and enthalpies of formation under the real LNG pipeline conditions will be developed and made available to WP5 [D4.5.2].

This task will be active from month 6 until month 26.

Activity summary:

• Report differences between LNG calorific value at various applied reference conditions (TUVNEL)
  
• Report uncertainty budget for calorific values and enthalpies
  of formation under real liquid conditions (PTB)

Task 4.6: Assessing impact of temperature, composition and density gradients in tanks on the total measurement uncertainty (TUV NEL, Elengy, ENAGAS)

The following real data will be collected from the field [D4.6.1]:

• Data sets for temperature from four probes in the liquid in each of the LNG tanks (normally 4 or 5 tanks per LNG carrier). Data will be collected before unloading at receiving terminals or after loading at LNG production plant). This will be repeated for various cargoes to obtain a representative matrix of data for uncertainty analysis.
  
• Same approach as above: data sets for the composition of LNG. Composition data will be used to study the impact on LNG density and calorific value calculations.

The two data sets will be used to perform an analysis of impact of temperature and composition gradients in tankers on the density and gross heating value calculations. The corresponding contribution to the overall uncertainty of LNG energy transfer will be estimated [D4.6.2].

This task will be active from month 10 until month 22.

Activity summary:

• Collect data on temperature & composition under real circumstances (Elengy, TUV NEL, ENAGAS)
  
• Complete uncertainty estimation for LNG energy transferred
  based on data from 4.6.1 (TUVNEL, Elengy, ENAGAS)

Summary of Deliverables for WP4