Creating an inert atmosphere for tank cleaning - which methods to choose from?
Anyone involved in cleaning oil tanks knows that tank explosions are a real risk and that safety is paramount. Inerting tank atmospheres with non-combustible gasses, such as nitrogen or carbon dioxide, is an effective method for reducing the risk. But which is the most cost-effective and efficient method? And is one method better than the others?
If done incorrectly, cleaning tanks that contain low flash point oil types, such as crude oil, can have devastating consequences. As recently as the past few decades, incidents involving static electricity build-up during cleaning have caused destructive tank explosions.
For an explosion to take place all three elements of the fire triangle need to be present: Flammable material, oxygen and source of ignition. By lowering the oxygen concentration inside the tank to a level below 8%, regardless of hydrocarbon content, the tank atmosphere is considered inert, i.e. unable to ignite and thus an explosion cannot occur.
The fire triangle
Five methods to choose from
Inerting a tank atmosphere requires replacing the oxygen content of the atmosphere with an inert gas. Two gasses, nitrogen and carbon dioxide are widely accepted for this purpose. However, there are several methods of producing and introducing an inert gas to tanks prior to cleaning:
1. Nitrogen Evaporator
A comparably easy way of creating an inert atmosphere is to use nitrogen. Nitrogen is delivered as a liquid and turned into gas (evaporated) on-site while being entered into a tank. This is generally seen as the easiest inertion method, but local conditions can occasionally make rental of the nitrogen tank and evaporator too expensive.
Several factors must be taken into account when calculating the cost of this method. These include cost per kilogram of nitrogen, transportation costs and cost of hiring an evaporator.
Pros
+ Dry gas with 100% purity
+ Easy to operate and adjust
Cons
– Dependent upon local pricing policies, availability and logistics.
– Expensive due to high delivery and transporation costs.
– Potential hazards due to handling of cryogenic nitrogen
Nitrogen evaporator. Here shown as mobile unit with cryogenic tank and evaporator.
2. CO2 evaporator
CO2 (carbon dioxide) can be delivered as a liquid and evaporated before being entered into the tank in the same way as with a nitrogen evaporator. However, a CO2 evaporator is approximately four times larger than a nitrogen evaporator. Also, because CO2 weighs 50% more than nitrogen, transportation tonnage is much higher. Although often the easiest way of producing an inert gas, local conditions can make tank and evaporator rental too expensive. Several factors must be taken into account when calculating the cost of this method. These include cost per kilogram of CO2, transportation costs and cost of hiring an evaporator and storage tank.
Pros
+ Dry gas with 100% purity
+ Easy to operate and adjust
Cons
– CO2 is heavy. This means that it stays near the ground and may cause increased risk of creating suffocating atmospheres
– Liquid CO2 can be rather expensive
– Rental costs of tank and evaporator
– Dependent upon local pricing policies, availability and logistics
3. Nitrogen generator using Pressure Swing Absorption (PSA)
Another method of creating nitrogen for tank inertion is by using compressed air and a molecular sieve. The sieve holds back oxygen and allows nitrogen to pass. The generator consists of two columns of molecular sieve. When one column is full with oxygen the generator switches to the other, hereby decreasing pressure and releasing oxygen.
This method generally uses 0.2 kWh to produce 1 Nm3 nitrogen. The purity (95-99%) depends on the load.
Pros
+ Independent of water for cooling
+ Capacity in relation to the size of the unit is high+ Low maintenance costs<+ On-demand 24/7
+ Available as stand-alone container
Nitrogen generator using Pressure Swing Adsorption (PSA). The generator is built into a 20” container.
Cons
– Electricity can be costly depending on region
4. Nitrogen generator using membranes
Nitrogen produced from compressed air using membranes is another way to make inert gas. This method is costly as it requires substantial electric power for the air compressor along with a heavy investment in membranes and later the maintenance of the membranes.
This method generally uses 0.3 – 0.4 kWh to produce 1 Nm3 nitrogen. The purity (95-99%) depends on the load.
Pros
+ Independent of water for cooling
+ Available as stand-alone container
+ Low maintenance costs + On-demand 24/7
Cons
– Electricity can be costly
– Investment in technology is very costly
– High maintenance costs
5. Inert gas from incineration of oil
This method is common onboard ships. Exhaust gas from the ship’s engine is cooled and sulphur is removed. This desulphurised gas can then safely be used to inert the tank. The same can be achieved by directly incinerating diesel oil and then cooling it before using it as an inert gas.
This method normally uses 0.08 kilograms of diesel and 60 litres of cooling water are required to produce 1 Nm3 of inert gas. The purity is 96-98% inert gas.
Pros
+ Investment in technology is low
Cons
– Require high amount of cooling water
– The gas has a high humidity
– Not possible to adjust capacity (on small units)
One method to avoid
Some people mistakenly believe that it is possible to produce inert gas from the exhaust gas from diesel engines. This sort of flue gas does not have a constant oxygen content below 8% (oxygen content is depending on load), and this gas is therefore not sufficiently inert to inhibit explosions.
Which method is the best?
There are pros and cons with each one of the five methods described above, so the most important factors to take into account are local conditions such as availability of inert gas, water for cooling and initial investment costs. Because of these many variables at individual refineries and tank farms, Oreco does not recommend one method over the other. However, like most suppliers of tank cleaning equipment, Oreco will provide assistance when you need to choose the most suitable method of inertion.
It’s all about safety
Whether one method is better than the others is still a matter of debate. What is certain is that all five methods are capable of creating an inert atmosphere of below 8% oxygen content in an oil tank. But no matter which method best meets your needs, the most important issue is to create a safe tank cleaning environment that complies with local safety procedures. It is also important to remember that inerting a tank atmosphere, although vital, should form only one part of your overall safety regimen.
Physical parameters
Atmospheric air
Atmospheric air consists of 20–21% Oxygen (O2), 79–80% Nitrogen (N2) and approximately 0.2% carbon dioxide (CO2). The oxygen content is what makes breathing, a fire or even an explosion possible. If the oxygen content is lowered below a certain level (below 11%) the risk of ignition is removed. As an extra precaution, international standards, such as ISGOTT (International Safety Guide for Oil Tanks and Terminals), are set at 8%.
Nitrogen
Kg Litre Nm³ (15ºC, 1 bar)
Kg 1 1.237 0.8547
Litre 0.6910 1 0.8055
Nm³ (15ºC, 1 bar) 1.170 1.447 1
CO2
Kg Litre Nm³ (15ºC, 1 bar)
Kg 1 1.214 0.541
Litre 0.824 1 0.466
Nm³ (15ºC, 1 bar) 1.8474 2.244 1
Carbon dioxide (CO2) is, at room temperature, a gas with no colour and no smell.
Sublimation point (°C): -79°C.
Published: 2004-12-27
Oreco A/S . Oldvej 1A . DK-3540 Lynge . Denmark . TEL. +45 4332 0200 . info@oreco.com