Leak detection methods for underground storage tanks

Underground storage tanks (USTs) have been used since the early stages of industrialisation. Although they are most often associated with the storage of petroleum products, USTs are also used to store other hazardous substances, including waste products.

USTs are unobtrusive, suited to confined sites and not at risk of damage from vehicle collision or vandals. Underground storage also removes the aesthetic concern associated with above ground tanks. However, underground storage systems are considered to be one of the major sources of soil and groundwater contamination, due to the potential of product leakage. The resulting contamination usually represents risks to human health and the environment.

For many years, the ‘out of sight, out of mind’ attitude was in practice and underground leaks were only a concern if the quantity of product lost would have a significant financial impact. Today, with increasing environmental awareness, those practices are no longer acceptable. Around the world, governments under pressure to enforce increased environmental responsibility have formulated new laws and guidelines. The implementation of leak detection methods and installation of sensors has become mandatory.

Figure 1: A double-walled tank

Various types of underground tanks Traditionally, underground tanks were made of steel. Steel tanks are strong and relatively easy to install but are subject to corrosion. In the 1960s, tank manufacturers introduced new tanks made of fibreglass. Fibreglass tanks, however, had the reputation of being difficult to install, and thus didn’t become widely accepted until the 1980s. The advantage of fibreglass is its resistance to corrosion, while the advantage of steel is its strength and broad chemical compatibility. Steel tank manufacturers addressed the problem of corrosion with various methods, including cathodic protection, protective coating and electrical isolation. Cathodic protection systems, however, need extra maintenance, which is not necessary with fibreglass. With the advantages and disadvantages of both technologies, the use of both materials in a composite form has emerged. Double-wall steel tanks covered with fibreglass laminate provide the most protective UST available.

Underground storage systems (USSs) are defined in Australian Standard AS1940-1993 as being those systems where the storage vessel is either totally or partially installed below ground level. The tank system includes the tank, underground connected piping, underground ancillary equipment and any containment system.

Hazardous substance USTs have to meet the same requirements as petroleum USTs concerning correct installation, spill, overfill and corrosion protection, corrective action and closure. In addition, hazardous substance USTs must have a secondary containment and interstitial monitoring for leak detection. A single-walled tank is the first or ‘primary’ containment. By using only the primary containment, a leak can escape into the environment. By enclosing a UST within a second wall, leaks can be contained and detected quickly before they reach the environment.

There are several ways to construct a secondary containment:

• Designing double-walled systems by placing one tank inside another tank or one pipe inside another pipe.
• Placing the USS inside a concrete vault.
• Lining the excavation zone around the USS with a liner so the hazardous substance cannot penetrate the environment.

Leak detection methods

In recent years, the advent of new and ever-changing rules and guidelines gave rise to a new industry — companies offering leak detection services or automatic leak detection systems emerged around the world. Some detection methods, called continuous monitoring, include monitoring the USS during normal operation. Other methods require a complete shutdown of the system for a period of time that can range from a few hours to a few days, depending on the size and complexity of the system, as well as the method used. Old installations, for example, cannot easily be fitted with effective continuous monitoring. In such cases, a periodic tank tightness test will be required.

Figure 2 shows the possible location of sensors to detect leaks from a double-walled petroleum tank.

Figure 2: Leak detection sensors

Secondary containment and interstitial monitoring

The secondary containment provides a barrier between the tank and the outside world. The barrier holds the leak between the tank and the environment so that the leak can be detected. The barrier is shaped so that a leak will be directed towards the interstitial sensor. Sensors are used to check the area between the tank and the barrier for leaks and alert the operator if a leak is suspected.

Some sensors indicate the physical presence of the leaked product, either liquid or gaseous. Other sensors check for a change in condition that indicates a hole in the tank, such as a loss of vacuum or a change in the liquid level between the walls of a double-walled tank.

Monitoring can be as simple as periodically using a dipstick at the lowest point of the containment to check if the product has leaked and pooled there. Or it can be a sophisticated automated system continuously checking for leaks.

Automatic tank gauging (ATG) systems

ATG systems use level sensors permanently installed in the tank. These sensors are linked electronically to a nearby control device to provide information on product level and temperature. The gauging system can automatically calculate the changes in product volume that might indicate a leaking tank.

Vapour monitoring

Vapour sensors sense and measure product vapour in the soil around the tank and piping to determine the presence of a leak. This method requires the installation of carefully placed monitoring wells. Vapour monitoring can be performed periodically using manual devices or continuously using permanently installed equipment.

Groundwater monitoring

Groundwater monitoring devices sense the presence of liquid product floating on the groundwater. This method requires installation of monitoring wells at strategic locations in the ground near the tank and along the piping runs. To discover if leaked product has reached groundwater, these wells can be checked periodically manually or continuously with permanently installed equipment. This method is effective only at sites where groundwater is within six metres of the surface.

Statistical inventory reconciliation (SIR)

Based on the ‘what goes in must go out’ principle, SIR uses sophisticated computer software to decide whether a tank system is leaking. The computer conducts a statistical analysis of inventory, delivery and dispensing data collected over a period of time and provided by the operator to a vendor.

Tank tightness testing

Tank tightness testing consists of periodic tests conducted by vendors who temporarily install special equipment to test the soundness of the tank. Some methods consist of creating a vacuum in the tank’s ‘ullage’ (tank headspace above the liquid level within the tank, including fill and vents) and monitoring pressure changes over a period of time. Other methods involve noise measurement inside the liquid using sophisticated acoustic equipment. Newly installed tanks must be tested for tightness before they are put into operation to make sure that the tank was not damaged during installation.

Manual tank gauging

Manual tank gauging is only effective for tanks of 7000 litres or smaller. This method requires taking the tank out of service for at least 36 hours each week to take measurements of the tank’s contents.

The power of the law

In recent years, Australian governments have brought in new laws for environmental protection and waste management. Those laws currently operating in Australian states and territories include the following acts:

• Queensland: Environmental Protection Act 1994
• Northern Territory: Waste Management and Pollution Control Act 1998
• Western Australia: Environmental Protection Act 1986
• South Australia: Environmental Protection Act 1993
• Victoria: Environment Protection Act 1970
• Tasmania: Environmental Management and Pollution Control Act 1994
• New South Wales: Pollution Control Act 1970. Since amended by various acts, including the Pollution Control Amendment (Load-Based Licensing) Act 1997
• Australian Capital Territory: Environmental Protection Act 1997.

With different wording but the same meaning these environmental protection acts make pollution prevention everyone’s responsibility by establishing a general environmental duty. Environmental protection (EP) legislation requires people and corporations to take all reasonable and practical steps to avoid causing environmental harm. They also make it an offence to place contaminants where they might reasonably be expected to cause environmental harm. EP laws also prescribe significant penalties for failing to comply with these requirements. Penalties for corporate offenders are up to five times larger than for equivalent offences by individuals.

Under the environment protection acts, each state also created a statutory authority called an Environment Protection Agency or Environment Protection Authority, well known as EPA. The EPA provides advice to the Minister for the Environment through the preparation of environmental protection policies, the assessment of development proposals and management plans, as well as providing public statements about matters of environmental importance. Some of the EPAs are quite recent; for example, the Tasmanian EPA is due to start operating from 1 July 2008!

Conclusion

The hidden hazard threatening underground storage systems cannot be ignored anymore. Leaks caused by corrosion, poor installation practices or mechanical damages because of poorly compacted soil or on sites subject to seismic movement must be detected and fixed. Governments all over the world, under increasing environmental awareness pressure, are taking action to monitor and remedy this problem. Rules and regulations, as well as leak detection methods, developed quickly, initially aimed at the petroleum industry, but now including a wide range of industries. The subject is complex and requires professional advice as well as consultation with the relevant authorities.

Leak detection starts at the system design level. One must make sure that chances for the USS to leak are reduced to a strict minimum and then, if against all odds it happens, leaks must be detected immediately. This is an area where prevention is cost effective because correction can be very expensive!

*Gerald Musy has held a number of positions around the world teaching and coaching field service engineers from various industries, ranging from industrial electronics to telecommunications. Since moving to Australia in 1990, Musy has specialised in writing technical documentation and training manuals.