Tank Gauging Systems

As the price of oil continues to rise, increasing attention is being paid to the accuracy of petroleum product transfers. Richard Siereveld examines some measurement options.

In the past, when the price of gas was relatively low, ensuring the accuracy of LNG (liquefied natural gas) product transfers was not that important. Gas was a waste product in oil exploration; losses or gains were negligible from a financial point of view.

But today, the price of gas has risen as a result of two key factors: demand is outstripping supply; and the close link between gas and oil prices have also helped push the price of gas even higher. As a result, losses and gains during transfers now have significant financial consequences.

Long-term Take-Or-Pay (TOP) contracts have also been superceded by flexible contracts, pushed by the liberalization of the energy market. Energy producers are no longer state-owned and statefunded businesses; they have become commercial operations that compete with each other.

Having flexible contracts allows the producers to squeeze suppliers in order to gain competitive advantage, and they are only prepared to pay for what is received. In turn, suppliers are also only willing to deliver what is being paid for. This means that both producers and suppliers require more accurate and reliable figures on LNG transfers. This article outlines the different measuring principles for accurate level measurements on light hydrocarbon liquids like LNG, to ensure the accuracy of transfers.

Achieving accuracy

In general, the quantity of LNG is determined and recorded in volume, and/or mass, with energy content calculated in calorific units. Irrespective of the place where custody transfer is carried out, i.e., either on board a ship or onshore, the liquid level in the tank is one of the most significant parameters to determine the quantity of connecting lines if the static measurement method is used.

Other factors include tank calibration tables, composition and/or physical data of the liquefied gases and vapor, vapor pressure, vapor and liquid temperature, volume of liquid in the pipelines and status of flush valves.

Automatic tank gauges are mandatory for the accurate determination of the liquid level inside bulk storage tanks. On shore, servo or radar technology is mainly used to determine the approved product level measurement in storage tanks for accurate custody transfer and legal metrology.

The choice between using servo or radar technology depends on the customer’s requirements. Factors such as accurate inventory measurement, optimization of the process and service requirements of the instruments play an important role in this selection process. The user should also be familiar with the approval issues around servo and radar technologies when measuring light hydrocarbons, such as LNG.

Servo technology

To comply with installation standards and recognized installation practices like ISO and API, the installation of a servo gauge on LNG tanks is in accordance with the following procedure.

A ball valve is used for process separation on a standard stilling well, mostly reduced at the top towards a 6-inch connection. This requires removing the gauge without taking the tank out of operation. On top of that is the so-called “calibration chamber” for the inspection of the gauge position and installation of the displacer.

It is possible to verify the gauge without taking the tank out of operation. This is done with several gauge commands:

• TEST – an action performed around the actual product surface performed action

• LOCK, BLOCK and UNLOCK – can be used to lift the displacer to any desired location, hold it and lower it again

Furthermore, users have the option of lowering the displacer towards the tank datum plate for bottom verification, or lifting the displacer towards the tank’s top position inside the calibration chamber at a pre-determined commissioning time.

The total weight of the unwound measuring wire should be compensated for, as well as the mechanical expansion/ contraction of the wire due to temperature differences.

With an installed accuracy within +/-1mm, the Honeywell Enraf 854 ATG servo gauge series is a targeted solution for measuring LNG, and has been approved for use on legal metrology and custody transfer applications, including measurement of LNG/ LPG.

Radar technology

In a radar gauge installation on LNG tanks, the horn antenna is mostly seen as a fixed mechanical part of the tank structure, and allows users to remove the gauge without taking the tank out of operation. The 1-inch full bore ball valve (depicted) leaves the antenna installed in an operational tank.

When the horn antenna is considered as part of the gauge, the 4-inch full bore ball valve separates the entire radar gauge from the process (see picture). The horn antenna is now installed on top of the ball valve, with an additional spool piece.

When using radar technology for level measurement in closed bulk storage tanks containing light hydrocarbons, there are two issues to consider:

1. The low dielectric constant difference between the vapor above the liquid and the liquid itself

2. The effect the vapor has on the radar signal propagation speed

When measuring products with low dielectric constants like light hydrocarbons such as LPG and LNG, the signal strength reflected back from the product level is quite low. A 4-inch stilling well should be used to focus the radar signal onto a small product surface to ensure a reliable reading. To cope with the pressure inside the tank, a 4-inch horn antenna is used to adjust the diameter of the feeder electronics towards the inner diameter of the stilling well.

A full bore ball valve is required to guide the radar signal towards the product level smoothly and act as a process seal whilst complying with international regulations. Inside the stilling well, verification pins are installed to enable the authentication of the performance of the gauge without taking the tank out of the operation.

The vapor above the liquid slows down the propagation speed of the radar signal. As every radar system is calibrated in the air and the exact content of the vapor is hardly stable, compensation for this effect should be done by using vapor pressure and temperature measurement.

Getting approved

Weights & Measures authorities recognize the effect vapor might have on the accuracy of the radar gauge, and have made a distinction in the range of products to be measured.For petrochemical products where the molecular structure contains less than 8 C-atoms, the accuracy has to be calculated and compensated for where possible.

Radar technology used on LNG/LPG tanks is normally not approved by the authorities due to the unpredictable vapor composition which affects the overall accuracy. Moreover, measurement on LNG/LPG is not authorized due to the untraceable vapor composition and absence of the possibility to verify with a manual hand dip.

Level measurement on light hydrocarbon fluids such as LNG and LPG can be done with either servo or radar technology. However, certain aspects of the radar technology installation have a crucial impact on the performance of the radar gauge, which leads W & M authorities to not allow radar technology for fiscal measurements even though radar gauges have an advantage in being maintenance free. Thus, servo gauges are preferred as they ensure best level measurement performance, as well as assured compliance with international regulations.

Level gauge based on servo technology

radar gauge installation

Trading Up

One of the world’s largest independent oil terminals, and the biggest in Asia, is set to start operations on Singapore’s Jurong Island in the first quarter of 2008. Known as Universal Terminal, the S$750 million (US$523 million) facility is the first oil terminal in the region with two very large crude carrier berths (VLCC). The terminal houses 73 storage tanks with sizes ranging from 2,000 cubic meter (cbm) to 100,000 cbm for crude and petroleum products. It will comprise 12 berths, including the two VLCC berths built side by side that can accommodate vessels as large as 320,000 deadweight tons (dwt), a unique feature that allows for active bulk-breaking of larger cargoes into smaller parcels. The facility, along with new investments in storage facilities by the likes of Chemoil, Royal Vopak and Horizon Terminal, is set to boost Singapore’s position as a global oil trading and transshipment hub. After New York and London, Singapore is the world’s third largest oil trading hub, with an annual turnover exceeding S$260 billion in physical oil trade and S$300 billion in derivative trade.

Richard Siereveld is Business Development Manager, Honeywell Enraf.