Above Ground Oil Storage Tanks (PPG2)

5. Oil storage tanks

Before choosing a new or replacement tank, you should check the following:

  • if there are any legal requirements you must comply with;
  • that it meets minimum manufacturing standards;
  • the type of tank is suited to your requirements.

Also consider the:

  • type of oil to be stored
  • need for secondary containment
  • location of the tank
  • safe filling and dispensing
  • safe installation and routine maintenance
  • intended use.

If you’re replacing a tank you’ll also have to consider what to do with your old one.

a) Manufacturing and Quality Standards
There are recognised British Standard and trade association tank standards for different types of tank. Your tank manufacturer, supplier or installer should advise you on the minimum design and manufacturing standards under the appropriate accredited quality assurance scheme. It should comply with BS EN ISO 9001 (Reference 11). Plastic and steel tanks and tank systems manufactured to OFTEC Standards, OFS T100 and OFS T200 respectively, meet these requirements.

Please note that compliance with standards for construction and manufacture doesn’t guarantee compliance with OSR. Other aspects such as pipework, location and deliveries, also affect OSR compliance.

Choose a tank that:

  • is expected to last at least 20 years, with proper maintenance, before it needs to be replaced;
  • is made of a material that is suitable for the type of oil stored;
  • is of sufficient strength and structural integrity to ensure that it won’t burst or leak in ordinary use;
  • has a way of preventing drain down by gravity e.g. top off-take and / or isolating check valves.

Check the recommended maintenance needed for your tank with the manufacturer.

b) Types of oil storage tank
There are a number of different types of oil storage tanks available. Above ground oil tanks are usually made from steel or polyethylene (plastic). They can be single skinned, double skinned or have a built in bund (integrally bunded) as a complete containment system.

Single skinned tanks are tanks made from one layer of steel or plastic. Single skinned tanks must be put into a secondary containment system, often referred to as a bund.

Double skinned tanks have two layers of steel or plastic with a very small space between them; any ancillary equipment is positioned outside the second skin. The space between the two layers is not enough to contain 110% of the oil your tank can hold. If you choose a double skinned tank, it will not be compliant with the OSR England or OSR Scotland unless it’s installed in a constructed secondary containment system.

Integrally bunded tanks have a primary container manufactured with integral secondary containment (see Section 5c and Figure 2) that can hold a minimum of 110% of the volume of oil the inner tank is designed for (Reference 12). Ancillary equipment will also be positioned within the secondary containment.

Figure 1. shows a diagrammatic example of a single skinned oil tank in an open bund that is constructed in situ. The diagram shows both fixed and flexible draw-off pipes. For tanks in open bunds, we recommend there is a minimum distance of 750 mm between the tank and the bund wall and 600 mm between the tank and the base to allow access for external inspection and maintenance.

 

Click to enlarge
Figure 1. Example of a single skinned oil tank within an open bund

Figure 2. Shows a diagrammatic example of an integrally bunded oil tank. These tanks are manufactured off site and installed onto a suitably constructed base. The diagram shows a fixed draw-off pipe, overfill prevention device and oil monitoring between the tank and secondary containment.

Click to enlarge
Figure 2. Example of integrally bunded tank (could be constructed of metal or polyethylene and may not be this shape and design)

Figures 1 and 2 aren’t drawn to scale or engineering drawings. They are for illustration only. Different configurations exist depending on the manufacturer, type of tank and installation.

c) Secondary Containment (also known as bunds)
Secondary containment is an area around a tank and its ancillary equipment designed to contain any loss of oil and to prevent it from escaping to the environment. It can be manufactured as part of an integrally bunded tank system or built on site ready for the tank to be put into it. Your tank may need to have secondary containment by law depending on where it is and what it’s used for. But to protect the environment we recommend you should bund all tanks as it is good environmental practice.

Secondary containment must hold at least 110% of the volume of oil the tank is designed to contain.

The extra 10% margin is intended to take into account a range of factors, including:

  • loss of the total tank contents, for example due to vandalism or an accident;
  • sudden tank failure or leaks;
  • overfilling;
  • containment of fire-fighting agents;
  • dynamic factors such as overtopping caused by surge and wave action following tank failure;
  • an allowance for rainfall during an oil spill incident.

If you have more than one oil storage tank in the system, the secondary containment must be capable of storing 110% of the biggest tank’s capacity or 25% of the total capacity, whichever is the greater.

Secondary containment must be impermeable to oil and water with no direct outlet:

  • connecting it to any drain, sewer or watercourse;
  • discharging onto a yard or unmade ground.

Ideally, any pipework to fill or empty your tank shouldn’t pass through the secondary containment floor or walls (the bund). If this is unavoidable, the joint between pipe and bund should be sealed with a material that is resistant to attack by the oil stored, so that the containment remains leak-proof.

Don’t store anything in the secondary containment as this will reduce the volume available in the event of a spill and can cause a fire risk if it becomes soaked in oil.

For agricultural oil storage in England, Northern Ireland and Wales it is a legal requirement that bunds are expected to remain impermeable, with proper maintenance, for at least 20 years, (see Reference 9).

If you’re considering an integrally bunded tank system, check with the manufacturer to make sure that it is designed to provide at least 110% secondary containment capacity. Some tank systems may not provide enough capacity if your tank is overfilled. Be aware that it isn’t easy to see into the secondary containment section of an integrally bunded tank. We recommend your tank is fitted with an overfill prevention device and secondary containment sensors that detect if oil has collected in the bund from an incorrect delivery, overfill or inner tank problem and to warn you if additional maintenance is needed.

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