Tank farm monitoring: meeting Australia's fuel reserve needs — Part 2


By Glenn Johnson, Editor
Wednesday, 05 October, 2016


Tank farm monitoring: meeting Australia's fuel reserve needs — Part 2

Australia is not meeting its IEA treaty obligations with regard to reserve fuel storage. In Part 1 we looked at the reasons why and the issues for tank farm operators. So how can modern tank farm monitoring technologies assist?

Australia’s high dependence on petroleum fuels for the transport of essential products such as food and pharmaceuticals makes a lack of fuel reserve a significant problem for the country as a whole. A Senate committee on energy resilience and sustainability found that while the IEA obligation is to maintain 90 days’ worth of fuel reserve, it has been estimated that Australia only has 34 days of fuel stocks:

“The 34 day figure is calculated on the average daily consumption of fuel in Australia divided by what is believed to be the volume of fuel available to the market.”1

Many existing tank farms use manual tank gauging methods, which do not support efficient use of available stock, nor meet recommendations for accurate monthly stock reporting. Along with better accuracy, upgrading to modern automated tank monitoring solutions will improve safety and therefore potential storage capacity, and help to maximise business returns by improving business processes.

Types of tank monitoring application

Bulk fuel storage terminals can be split into three types — pipeline, marketing and storage terminals:

  • Pipeline terminals are found at the beginning or end of a pipeline and receive products directly from a refinery or from tankers.
  • Marketing terminals are for temporary storage prior to distribution and usually store a small variety of products, such as gasoline and diesel.
  • Storage terminals may be used for storage of final product for a particular industry, such as jet fuel for an airport, or may store a wide variety of different products.

Measurement of product volume or mass is necessary for both inventory control and custody transfer.

Inventory control measurements are important for understanding exactly how much product is in stock, and reliability and repeatability are important considerations.

Safety overfill prevention systems use a point level sensing solution, since their only purpose is to detect the high level and prevent overfilling.

Accuracy challenges

The application of instrumentation creates opportunities to measure inventory far more accurately than any manual method. Manual methods essentially involve only measuring the level of the liquid surface — and possibly an oil/water interface — and as accurate as these measurements may be, they are not necessarily an accurate indicator of the actual quantity of product, for a number of reasons.

Various deformations and variations to tank dimensions can occur over time. The dimensions of a tank can change through deformation caused by the varying mass of liquid in the tank, and by temperature variations. Due to their weight, tanks can move or tilt over time, and both the bottom of the tank and the roof can move. All these deformations cause variation in the liquid level for a given volume of liquid. Some but not all of these variations can be compensated for by tank correction and capacity tables.

Hydrocarbons also vary in volume depending on temperature — a variation in temperature of 1°C typically causes a volume change of around 0.1%. Varying amounts of water are normally present as well, which need to be measured to calculate the correct quantity of the stored liquid.

It is the volume or the mass of the stored material that is of interest. There are two main methods of tank monitoring — a mass-based method and a volume-based method. The mass-based method is based on measuring the hydrostatic pressure of the liquid column using pressure instruments. The volume-based method combines a level measurement with a temperature measurement. In either case it is also necessary to measure the free water volume in the tank.

In addition to these two main methods, there has also been an increase in the growth of hybrid tank measuring systems (HTMS), which use highly accurate level measurement combined with hydrostatic pressure measurement for mass. This is often the preferred method, particularly for product that is often measured based on mass. Furthermore, for crude, ‘water bottom’ can be a very critical measurement as many or most crude tanks intentionally have water at the bottom of the tanks that will need to be deducted from overall volume.

Recommended technologies for volume measurement

Servo level gauges

Servo tank gauges operate on the principle of displacement measurement. A small displacer with a higher specific density than the liquid is suspended on a measuring wire that is unwound from a drum and positioned in the liquid medium using a servomotor. A resolver coupled with the wire drum is used to measure variations in the weight of the displacer, according to Archimedes Law.

When the displacer is lowered and touches the liquid, the weight of the displacer is reduced due to the buoyancy of the liquid. As a result, the torque in the drum is changed, and this change is measured by the resolver along with the distance the displacer has been dropped.

The displacer can also be lowered through the liquid until a new change in buoyancy is detected, enabling the servo gauge to detect an oil-water interface.

Servo gauges are one of the most accurate methods of level measurement, with an accuracy of within ±0.4 mm over a depth of 40 m. They also inherently measure the density of the fuel, since it is directly related to the buoyancy.

Radar level instruments

Radar level instruments are a non-contact method of measurement in which the instrument is mounted at the top of the tank and transmits microwave pulses down into the tank.

For high-accuracy liquid level measurement in storage and process applications, radar gauges operate based on the frequency-modulated continuous wave principle (FMCW). The radar emits a precise crystal-oscillated, continuously varying frequency wave from the antenna. The wave is reflected off the product surface and received again by the radar system.

The reflected energy is dependent on the fluid’s dielectric constant, which is significantly different from air for both water and hydrocarbon liquids. Due to the further difference in dielectric constant between the fluid and water, the interface level can also be detected.

Radar level instruments typically provide an accuracy of ±0.5 mm and have the added advantage of low maintenance, having no moving parts as servo gauges do. However, they do not measure density, and this will need to be determined by additional instrumentation.

Vibrating fork level switches

For safety overfill detection, the recommended level-switching instrument is a vibrating fork instrument. Such instruments consist of a fork with tines that are vibrated by a piezoelectric crystal oscillator at a resonant frequency of about 1 kHz in air. When immersed in a liquid, the vibration rate will slow down by about 20%.

The advantage of vibrating fork level switches is that they are maintenance-free and highly reliable — essential qualities for a safety application. They are not affected by material build-up on the tines, nor by turbulence, bubbles or other liquid phenomena.

In addition, there is now an industry trend towards favouring continuous radar, since it allows ramp alerts prior to an overfill ‘panic’. This means the operator is given an early warning that if the tank continues filling at its current rate it will overfill in a predicted time.

Tank safety systems

To meet safety requirements and at the same time maximise tank capacity, it is essential to implement an independent Safety Instrumented System for this purpose.

Automated IEC 61511-certified systems are available that make the detection, indication and prevention of overfill simple to implement. Such systems offer complete functional safety loops covering safety integrity levels SIL2 and SIL3.

Figure 1: Example of an automated overfill prevention system. Image Source: Endress+Hauser.

Figure 1: Example of an automated overfill prevention system. Image source: Endress+Hauser. For a larger image, click here.

Such a system (Figure 1) takes its inputs from point level switches at the top of the tank and acts as a system independent of all other controls, automatically closing a safety shutdown valve if required to prevent overfill.

Networking

Implementing or extending a tank farm inventory monitoring system will of course require implementing an infrastructure to integrate the tank instruments into a control system, and many legacy tank farms have obsolete or non-existing signal wiring from the tank storage area. Traditional methods of running cables or optical fibre over large tank farms would normally form the largest part of the cost of deployment, and in many cases may be cost-prohibitive.

In recent years, industrial wireless technologies have all but eliminated the wiring cost, replacing cable runs with wireless instruments. A secure, wireless infrastructure can halve the cost of deployment over a wired solution. Where existing instrumented tank monitoring exists, but improved connectivity is required, wireless adapters or gateways are an option. Wireless tank monitoring means that precise inventory data for tanks that was previously out of reach can be made available.

Automated inventory management

A precise calculation of net volumes is key for accurate business accounting purposes. A 5 mm level measurement error plus a temperature error of 0.6°C in a large fuel tank can cost tens of thousands of dollars per tank per annum. But accurate measurement is not the only thing to consider — it is also important to get best business value possible from the tank monitoring system.

By replacing a manual or ageing tank farm monitoring system with an up-to-date automated one that runs closer to constraints, an optimised system can generate higher returns. Closing the gap between planned and actual schedules is a key objective, since deviations between the availability of product and the product delivery schedules not only impact the tank farm process but can also have cost implications for downstream operations.

Automated processes are better able to monitor what’s going on in the field to help improve the management of stock and of all activities and workflows — making interoperability important. Fortunately, by networking the tank monitoring systems, data can be integrated with SCADA, DCS and ERP systems via commonly available technologies such as OPC.

Software for non-refinery storage terminals

Where a tank farm is used for only storage and terminal purposes, a separate inventory management system may be appropriate.

An inventory management platform can support users in collaborative demand planning, event-driven replenishment planning and scheduling as well as the reconciliation and consolidation of geographically distributed inventories. It is also possible to involve partner organisations for further improved supply chain operations.

Today, organisations can choose to implement an inventory system on their own computing infrastructure — suitable for larger operations — or to use cloud services (software-as-a-service) where the operation is smaller, further reducing business costs. Cloud services also make the management of multiple sites easier and more cost-effective.

System commissioning and integration

In a tank farm operation that was previously manually operated, the implementation and integration of the monitoring and safety systems may present challenges — all of which can be overcome with the assistance of an experienced vendor that can also provide engineering and integration services.

By involving a competent partner right from the start, fuel refinery, terminal and storage operators can be sure of smooth project handling and the seamless handover of a fully operational plant, with increased safety, reliability and availability.

A partner should be selected that has the expertise to ensure the overall performance of the tank monitoring network, and integrate it into any existing DCS, SCADA or ERP system, as well as provide all necessary training and ongoing support.

References
  1. Senate Standing Committee on Rural and Regional Affairs and Transport 2015, Australia’s transport energy resilience and sustainability, Commonwealth of Australia.

Image credit: ©iStockphoto.com/HAYKIRDI

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