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Class 1 Leak Detection - The ultimate protection

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Author: Thompson Jamie
I spent much of my life as a regulator controlling service stations and trying to prevent leaks, spills and accidents at filling stations.

A big part of this work in the past was trying to determine when single wall tanks would leak.

Although I have looked at many systems which were tried and tested no one system was able to inform us when a tank wall was breached - before product entered the ground.

I have had far too much experience of failures of systems and the consequences of leaking installations to have great faith in some forms of so called leak detection

While the loss of product from a tank or pipes from service stations has always been of concern that concern today is higher than ever before.

We should remember

  • The risk of fire and explosion is still with us from any leak
  • The consequences of a leak to the environment is considered by all those involved as of much higher concern now, with the subsequent claim against pollution damages caused by leaking product that can be catastrophic to a business and also negatively impact the reputation as well as harmful to the environment
  • The very high cost of fuel also means the monetary loss should be of concern to businesses, as a 10,000 litre loss =13,500 Euros


The biggest change in the UK came in 1990 when we decided that no more single wall tanks would be installed and double wall tanks with leak detection would only be permitted.
The UK were behind other European countries such as Germany who had already made such a decision many years before to have all underground tanks double wall with leak detection. It was somewhat ironic that after the reunification of Germany, one half of the country had double wall tanks, the other did not.
While each country made decisions on the regulations for underground tanks, in Europe the EU had formulated a number of Directives (Laws) which has to be made mandatory by each EU country. These Directives covered mainly environmental issues such as prevention of pollution, protection of groundwater and drinking water, and the disposal of waste water. One Directive also covers the storage and use of dangerous substances.

These actions by the EU now mean that all member countries have been encouraged by Directives to comply with these laws. The industry has moved forward quickly and embraced the European standards for double wall tanks, and double wall pipes and of course leak detection. These standards are all designed to support the various Directives.

This standard EN 13160 -1 to 7 provided industry with several options [SEE DIRECTIVES]

  • For those who chose the modern safe route of installing double wall tanks and pipes, then they should install Class 1 system which in fact is leak prevention (see images 1 to 4)
  • Class 2 and 3 are also defined to only be used on double wall systems (see images 5 and 6)
  • For those who already have single wall installations with no interstitial space then the options would be to install leak detection and classes 4, 5 & 6 are the options
  • EN13160-7 even covers the lining of existing single wall tanks so that a class 1 leak detection can be fitted to old installations

What do the classes mean?

They are designed to guide the designer or owner of the service station as to the effectiveness of the system they are installing. Class 1 being the highest and class 6 being the lowest level.

Class 1
Pressure or vacuum
(see images 1 to 4)
The pressure system operates by putting a positive pressure in the interstitial space between the two walls of the double wall tank. This pressure will be above any external pressures from groundwater or the fuel itself. Any breach of the two walls will be detected as a loss of pressure indicating a leak and an alarm sounded enabling the owner to remove product from the tank before there is a threat to the environment.
The vacuum system operates in a similar way but draws a vacuum in the interstitial space. Any breach of the skins allows air, water or fuel to enter the interstitial space, this rise of pressure indicates a leak and an alarm will sound.

Class 2
Liquid systems
This system has the interstitial space filled with a liquid, this liquid is a glycol water mix, and the installation requires a header tank which is installed high enough to provide the pressure to the interstitial space. Any breach of the walls will cause the liquid level to drop and the resultant loss of the liquid is detected in the header tank and an alarm will sound.

Class 3
Liquid, level or discriminating sensors
This system uses sensors, level sensors, discriminating sensors or vapour sensors. The sensors are used in double wall installations and the positioning of the sensors is critical to their effectiveness. They are normally placed at the lowest point in a system and once a leak is detected an alarm will sound.

Class 4
Tank gauge leak detection systems

These systems are divided into category A and category B, providing a leak detection system to the tank and to the pipes connected to the tank.

Category A systems measure what is sold through the dispensers against what goes out of the tank according to the tank gauge. When exceeding defined limits, it will release a sound warning to the operator that there is a discrepancy which should be investigated.

Category B is for leak detection of tanks only and is capable of detecting a leak when the liquid level is decreasing while the tank is not dispensing fuel.

Note:-Fuel will always enter the environment if a leak is found when operating on a single wall system.

Class 5
Monitoring wells
This class is shortly to be removed from the new standard when it is published next year. It defines the installation of a monitoring well around a tank or pipe or service station with sensors (according to class 3) installed in the wells which will detect a leak of product from the installation. The positioning, depth and construction of the well is an important factor to its effectiveness as a leak detector.

Note: Fuel will always enter the environment if a leak is found when operating on a single wall system.

Global differences
It is clear around the World there are differing requirements. A few countries I have visited have no regulations at all but rely on the people storing or using the product to provide a standard acceptable to the industry. Most have some regulations and it will also depend upon the enforcement of those regulations as to how effective they are.
One country I visited still uses single wall steel tanks with a Glass Fibre coating process and they claim to have had reasonable success with their tanks. But all single wall systems have the uncertainty that you do not know when the tank will leak. Last month I was told of one of these tanks which had been leaking for several months. The clean-up costs are being very high and providing bad publicity. The oil company concerned is now asking about specifications for double wall tanks with leak detection.

The USA, one of the world’s largest users of motor fuel, has gradually changed over the years to double wall in their EPA regulations and although many forms of leak detection such as SIR were accepted by regulators. The move towards more effective leak prevention systems is seen by those who operate stations as a positive way forward. The US has many double wall GRP tanks and the trend towards leak detection system being similar to the European Class 2 system, but using Brine instead of the glycol water mix.
The US regulators I talk to are also coming around to the preference of having both walls of the tank being under test the whole of its life as a good move.

European preference is changing

The system installed in tanks for many years in Europe was the class 2 liquid system but there has been a move in recent years by environmentalists and responsibles at oil companies to the Class 1 system.

This change is based on a variety of reasons

  • In environmentally sensitive areas, even the leak detection fluid is considered a hazard to the groundwater
  • When leaks occur the detection fluid in the interstice has to be disposed of
  • Economy - the cost of leak detection fluid adds to the cost of tank manufacture
  • Operators are taking environmental issues seriously and are changing to the more environmentally friendly systems

Pressure and vacuum leak detection systems for double wall tanks and pipes have been operating in Europe for over 35 years. There are over 700,000 such installations in use monitoring tank and pipes. They detect a small number of leaks each a year, around 1% of those being monitored. 75% of those leaking have a leak in the outer wall and 25% in the inner wall.

Class 1
Pressure system
This is a system for new tanks and pipes.
The leaks detected allow enough time to take action without the risk of contamination. There is no record of contamination of a double skin tank with Class 1 leak detection fitted.
The pressure system creates a fixed operational pressure in the interstice. The alarm pressure and the operational pressure are always higher than the pressure (weight) of the stored fuel or the groundwater in its deepest point. In the event of a leak the compressed air prevents the stored product or groundwater entering the interstitial space.
The compressed air in the interstice is dried by a dry filter preventing condensation forming in the interstitial space and the pump operates and adjusts for variations in temperature and atmospheric pressure:

  • It is important to remember that the two walls of the tank are under test the whole of the working life of the tank
  • If there is a hole in the inner skin then the pressure in the interstice backed up by the small compressor in the leak detector has been known to keep product from entering the interstice enabling the tank to be emptied and the breach repaired.

Class 1
Vacuum system
Class 1 vacuum systems are normally installed where tank linings or jackets are used and the pressures generated by a pressure system are not a suitable choice.
The general requirements and test methods for leak protecting linings and jackets are covered in Part 7 of EN 13160.
With the vacuum system, the operational vacuum in the interstice is below atmospheric pressure and is adjusted against the pressure of the stored product. The vacuum is high enough to suck the stored fuel/air or groundwater into the interstitial space and up to the liquid stop valve. The alarm then sounds providing the operator with enough time to remove the stored product.

With a vacuum system:

  • The pump provided is ATEX approved as flammable vapour may pass through it.
  • The two walls of the tank or pipe are under test the whole of the working life of the tank.
  • Any leak from the inner wall below liquid level will result in the interstice being filled with the stored product.

Pipe systems

The history of leaks from service stations have shown that in the past the majority of leaks occurred in single wall steel pipe where corrosion played a major part. Although in recent years the move towards non-metallic systems has improved the situation. The industry has also moved towards installing more pressurised pumping systems where the submersible pump pushes the fuel to the dispenser. In such systems a leak can be very damaging with up to 200 litres per minute flowing from a breach.

The use of “double wall” or “secondary containment” technology is of course good engineering practices which are sometimes a legal requirement and at other times are subjected to a risk assessment. Publications and codes such as the APEA/EI publication “Design construction modification maintenance and decommissioning of filling stations” recommends double wall pipe with leak detection on pressure systems. The classes of leak detection recommended in the publication depends on the risk from a class 3 system in lower risks to a class 1 system in high risk situations.
Such systems are in use in London in high risk areas close to Underground railway stations and have been successfully operated for a number of years providing the travelling public with good protection.

The one note of caution I should add is that the leak detection systems should not be fitted and forgotten. They need, like all good security systems, maintenance, servicing and checking to ensure they are in working order.

Personnel should receive training on how the systems operates and what to do if the alarm sounds. The alarms should be in a position where someone responsible can hear them; they should not be silenced without action.

The ultimate deterrent

My advice to designers and operators of service stations is to specify the best method of providing secure leak detection and meeting ethical environmental concerns by ensuring the double wall tank is under a pressure test the whole of its working life by installing a Class 1 pressure system.

It is reassuring to have the knowledge that there have been no recorded losses of fuel in Europe from a double wall underground tank which has class 1 leak detection system in around 50 years of experience in this field.

With underground piping systems there are alternative leak detection systems according to the level of risk from a Class 3 system to a class 1 system in higher risk areas.

Jamie Thompson joined the London County Council in 1961 and trained as a Petroleum Inspector and ended up as Principal Petroleum Inspector for the London Fire Brigade the largest petroleum authority in Europe. He has specialised in petroleum standards, construction, legal enforcement, equipment approval and new design of Petrol Filling Stations for well over 40 years. He is currently chairman of European Standards committee (CEN TC 393) dealing with equipment for service stations, which has produced 23 European standards relating to filling stations. He also chairs CEN TC 265 WG8 on underground and above ground storage tanks, and sits as a European contributor to the Underwriter Laboratory standards for fuel tanks and fuel lines in the USA. He was Editor of the APEA Technical Journal “The Bulletin” for 23 years and as Chairman of the technical committee of the APEA he is involved in the publication of the APEA/EI Guidance on design and construction of filling stations known as the Blue Book.