In an ever changing world, service stations are constantly being built, demolished or redeveloped, which means that redundant underground tanks have to be excavated and the steel recycled; a simple enough process, but the trick is to do it without blowing yourself up.
When a petrol/gasoline tank has most of the fuel pumped out, there is always some product left in the bottom, along with sludge that has settled out of the fuel and accumulated over the years. The rest of the tank will contain a volatile fuel/air mix. The whole thing is a bomb just waiting for a spark, so meticulous precautions have to be taken to ensure it does not receive that spark.
The tank only becomes safe to excavate or work on when this fuel/air mixture has been expelled; a process known as gas freeing. This process has to be carried out by experts who are properly trained and equipped to issue a gas-free certificate.
The problem is that, to issue a gas-free certificate, the provider has to be certain that all vapour has been expelled, any remaining product and sludge removed and the interior thoroughly cleaned. However, this certificate will only be current for 24 hours in normal circumstances and tanks have to be made safe and kept safe from the moment they become redundant to when they are removed from the ground and recycled.
As few service stations have their tanks excavated within 24 hours of the site closing, other steps have to be taken to keep the tanks gas-free.
If a redundant tank has to be left in the ground for a long period, it is generally filled with a concrete slurry or RG22 foam. These are both liquids that are designed to fill the tank interior completely and set solid, thus preventing vapour building up again. Their relative effectiveness is discussed in my article on RG22 foam.
For shorter periods of up to six months, water can be used. Over longer periods, water filled tanks can rust and leak contaminated water into the surrounding soil and leave a void behind, where vapour can build up again.
The problem with using water for a short-term filling is that if you have a 30,000ltr tank, when you get around to excavating it, you will have 30,000ltrs of contaminated water to dispose of safely; an extremely time consuming and expensive undertaking these days.
This is where Nitro-foam comes into its own.
This uses a tiny amount of water, a small amount of detergent and nitrogen gas, to produce inert, nitrogen filled foam that can be pushed into the tank under pressure, expelling any vapour as it goes.
This type of inerting is useful when rendering tanks safe for temporary inerting prior to cleaning and gas-freeing, breaking down manhole chambers and exposure of tank tops and removal of manhole covers and associated pipework.
This foam is guaranteed to retain its integrity for at least 72 hours. When the foam bubbles eventually break down, all that is left behind is a small amount of white liquid and a tank full of inert nitrogen that can be vented to air; a considerable saving on time and the cost of the eventual waste disposal.
When the tanks are excavated, they are simply opened and the foam is either allowed to dissipate naturally or compressed air can be used to burst the bubbles.
If carried out with the right equipment and by trained personnel, it is a simple and quick process, but there are a number of technical issues that have to be dealt with correctly.
The first thing is to ensure the recipe for the foam itself produces stable foam that will hold together long enough, without using too much water or detergent. This is not just a matter of getting the proportions right, water quality is equally important. As with soap, the best results are achieved with soft water. As water hardness can differ hugely within just a few miles, it is important to test for this, and take steps to soften the water, before trying to produce foam.
Water pressure is also important and can vary from region to region and how far along the mains supply you are. If the pressure is too low, a booster pump will need to be used.
The other important issue is the size of the nitrogen bubbles produced; too small and you would end up with shaving foam, that would use too much water and chemicals; too large and it would leave too many small pockets of fuel/air to certify that the tank is safe and the bubbles would be less stable. The size of the bubbles is controlled by varying the water pressure, nitrogen pressure and amount of detergent.
The number of nitrogen bottles required will vary depending on the volume of the tank or tanks to be inerted. For example, inerting a single 22,000ltr tank would require approximately 54,000ltr of nitrogen.
If a working service station, or a recently closed one, is being worked on, the foam can go in through the fill pipe and the vapour can be vented via the vapour recovery connection to a high level, from where foam will eventually issue when the tank is full.
In conclusion, Nitro-foam inerting is a much more cost effective way of temporarily gas freeing a tank than water filling. It is also much more environmentally friendly. However, it does need to be carried out by personnel with the right equipment and training.