1 Introduction:

Mechanical contaminants, grit, mud, heavy metal compounds, hydrocarbons, tar, and other heavy oil components contained in the oil product will naturally settle during the long storage process of the oil tank due to the specific gravity differential. These elements will pile up in the base of the oil tank, forming a thick and black gelatinous material layer that is storage tank sludge. The sludge will accumulate with the perpetual increase in time of oil transport and storage, influencing the transport and storage of storage tank over period, so it is critical to clean the oil tank on a regular basis, as required by international safety standards.

Chemical cleaning is one of the most effective and prevalent method of cleaning of oil tanks. In chemical cleaning, first of all, the oil is drained from the tank and the oil tank is adequately ventilated and the oil tank is measured to keep the oil and natural gas concentrations within a safe levels. Workers cleaning oil tanks enter the tank to remove oil, mud, and other debris. The tank cleaning machine is used to clean the structure and apparatus.

The most important part of the process is the application of de-rusting and pickling chemicals which removes all the impurities like inorganic compounds, depositions, stains and rust from the surface of the oil tank. This process generally takes around 2 hours but the exact time of cleaning a specific tank depends upon its condition and other factors. After this process, the acid is completely removed from the tank and it is washed with natural whishing liquid. The washing is done for around 30 minutes and it is ensured that the acid is completely removed from the tank.

After removing all the waste the passivation treatment is done in the oil tank for 2 to 3 times depending upon the conditions. The passivation treatment uses an acid solution in order to remove any free iron from the surface of the tank which prevents the rusting of the surface of the tank.

Figure 1 Passivation Process

After passivation the tank is washed with water at very high pressure of around 300 kpa for 10 minutes on average. After that, all the waste is removed from the tank and the inside of the tank is mopped to completely clean up the tank. Then the tank is ventilated adequately in order to remove any moisture or humidity. At the end, the tank is inspected by the professionals look for any faults and to ensure that the cleaning process is carried out properly.

After cleaning the oil tank, no corrosion, pollutants, moisture, or grease should be completely removed from the tank. Following the completion of the oil tank cleaning process, quality check and acceptance should be performed, and the acceptance report must be signed by the concerning parties. Manhole should be sealed and pipes should be connected and the tank should be restored to its original state as soon as possible once it has been accepted. Following the completion of the oil tank cleaning process, the site should be completely cleaned in accordance with rules, and the results should be recorded. Finally it should be ensured that whole process is both safe and effective.

2 5 major mistakes in chemical cleaning of oil tanks:

A variety of dangerous situations can arise during the chemical cleaning of oil tanks. This article discusses possible dangers which might take place due to oversight or negligence such as ignition, exposure to chemicals, low oxygen levels, injuries, and exposure to radiation, and offers advice on how to avoid them by taking precautionary measures. [1].

Tank cleaning is a very risky activity.  Working in confined spaces, workers are exposed to many hazards, often due to human error, some of which have resulted in injury or even death.

There are many interpretations of confined spaces, but there are also widely used descriptions. A location with no natural ventilation and a hard-to-reach place that prevents rapid evacuation in an emergency.

Indoor work is associated with health and wellbeing and is not recommended for those with claustrophobia, heart or breathing problems, or those physical disabilities. Identifying and eliminating potential hazards in the reservoir cleanup planning phase improves project performance, health, safety and environmental performance. The potential dangers when cleaning tanks are as diverse as the design and physical dimensions of the tanks themselves. We have listed some of the most important errors that lead to a disaster [2].

2.1 Failure to insulate flammable material

Cleaning the oil tank with flammable materials poses particular risks. If there is a mixture of vapor and combustible air in the tank, the occurrence of a spark could cause a fire or even an explosion.

The three components of a fire triangle must be related to the explosion. If the oxygen content is kept around 7% or less, the atmosphere is considered inert, and the fire cannot start regardless of the oil content. [3].

Eliminating any combustion sources or reducing the hydrocarbon vapor content below the combustion threshold has the same effect.

Figure 2 Combustion Triangle

When the access ports on a storage tank combustible materials are removed by mistake, the petroleum vapors inside the tank are mixed with air, and the combination reaches a flammable limit.  It is critical to remove any possible ignition sources completely.

The ratio of oil vapor to air must be within certain threshold values ​​for combustion to take place. The low flammability limit (LFL) and high flammability limit (UFL) are typically between 1 and 9% for most petroleum products. The general requirement for access to the tank with combustible vapors is 9% LFL. This means that the oil level in the air is actually below 0.2%. Be aware that tanks containing mud, which stir the mud when entered, may release trapped vapors. As a result, the atmosphere can penetrate flammability restrictions.

If this occurs, the only option is to remove workers from the tank until the atmosphere is properly tested at 9% LFL or even lower. Till all the sludge has been stripped away, the tank atmosphere must be monitored closely using appropriate safety continuous monitors installed inside the oil tank. This is especially useful in the early stages of a tank cleaning process.

Figure 3 Upper and Lower Flammable Limits

Any heat source with enough heat to ignite a combustible vapor mixture by bringing it up above its self-ignition temperature. Sparks from electronic systems, thunder, static energy and hot surfaces are all potential combustion sources in and around oil tanks.

The use of electrical appliances, lighting, internal combustion engines and battery systems must be strictly controlled to avoid accidents. Lightning is a great way to start a tank fire. In a lightning strike, all cleaning work must be stopped. Workers must use emergency evacuation methods and then evacuate adjacent tanks.

Static charge is a surface phenomenon that occurs when markedly different materials come into contact and separate. Static electricity can be generated by high-pressure water jetting, vapor cleaning, grit jetting, and air-driven lighting systems, among other things. Nozzles, valves, and other electrically conductive materials should be bound and connected to the ground to prevent static charge concentration.

Smoking and fires must be tightly monitored and permitted only in designated areas. The dangers of hot ventilation systems by placing fueled hardware at a respectable distance from open tank manholes. Steam lines, whether temporary or permanent, should be tended to lag.  Other sources of heat should also be identified and eliminated.

2.2 Exposure to hazardous chemicals:

Oil tanks containing hazardous chemicals necessitate precautionary measures during the cleaning procedure. Dangerous chemical exposure can occur through contact with skin, inhalation, or accidental intake.

Lead compounds, Sulphides, Polycyclic Aromatic Hydrocarbons, and Formaldehyde are all dangerous chemicals frequently encountered during tank cleaning. Toxic sand particles tend to lag; paint structures, grit blasting, and other processes should also be avoided.

A few of the substances have an instantaneous harmful effect on human health, while others have long-term cancer, causing an effect on worker’s health [4].

The Material Safety Data Sheet should be frequently referred for the product kept when cleaning a tank. Because refined petroleum tanks are not prone to severe sludge formation, the exposure time should be kept to a minimum. It should always be made that anyone accessing the oil tank wears the proper personal protective equipment (PPE).

3.2 Failure to maintain oxygen levels:

The safe oxygen level in the tank environment without hazardous and flammable gases is 20%. Below this level, oxygen has to be transported. Oxygen starvation can occur for several reasons, particularly during gas delivery when the tank environment is filled with products.

Some things require an inert gas blanket, such as B. Nitrogen, which is much higher than the liquid level, leading to a lack of oxygen after opening the oil tank. Due to the oxygen consumption in the internal corrosive environment, the oxygen concentration in the washed and then resealed tank can decrease and remain inactive for some time [5].

In any event, oxygen levels should be checked before workers enter the tank without a respirator, and gas sensors should be used to monitor oxygen levels during activity frequently. Failure to follow instructions can be fatal to both workers and the thorough cleaning process.

3.3 Lack of attention towards physical conditions leading to injuries:

Workers accessing an oil tank for chemical cleaning face various physical dangers like climate conditions, tripping and obstructions hazards that are unique to the location of the tank and physical design. Any mistake or negligence could result in a catastrophe.  To reduce the danger of harm, it is critical to undertake a complete risk evaluation before entry and implement a safe work environment.

Oil Tank cleaners can be exposed to dangerous situations due to the weather. It isn’t easy to control the internal temperature too much, so employees should take regular breaks and drink water and a grain of salt. Loud noise and freezing weather can also be stressful [6].

Because tripping risks and other obstructions abound in oil tanks, workers must fully know the tanks inside architecture before entering. This problem is exacerbated by poor lighting conditions, which are usually difficult to fix due to the flammable atmosphere and the physical size of the tanks.

Other sources of danger include falling debris, slippery surfaces, poor hygiene, head-level obstacles in floating deck tanks, poor maintenance or inadequate equipment, etc., all of which can be reduced through risk assessment and proper training.

Many risks occur when manually cleaning a tank owing to its location and structure, and many more are created by tank cleaning equipment and workers. In an emergency, it should be made sure there is always an unrestrained exit.

The management and workers often ignore these details, and such negligence results in injury and production loss. Therefore, the conditions must not be ignored and should be considered while performing the chemical cleaning of oil tanks.

Figure 4 Sludge Removal Process

3.4 Lack of protection from radiations:

A natural radioactive material (NORM) or a specific low activity scale (LSA) can be specified in some crude oil, sludge, and degassing systems. Radioactivity emanates from the formation fluids in the borehole and appears as a radiometer in pipelines transported to reservoirs. The cleaning of tanks with radioactive substances requires special precautionary measures. If you suspect a problem with radioactivity, contact a specialist.

Microbial growth is widespread in the bottom of the tank and sludge where water is stagnant. Infections or allergic reactions might occur in extreme situations. A knowledgeable specialist should be consulted if considerable microbial growth is detected.

The radioactive and other harmful materials must be identified and properly disposed of; otherwise, the failure to do so may result in a catastrophe.

Figure 5 Chemical Cleaning Process

3 Special instructions and things to avoid during chemical cleaning of crude oil tanks:

The enormous size of the crude oil deposit (its circumference reaches 130 meters) and the tendency of the crude oil to deposit sludge (approx. 10,000 cubic meters) deserve special attention.

A topographical survey should ensure that the floating roof lands on the mud banks and remove human footprints.

Using conventional methods with a pump, it takes around 400 working days to remove the sludge from a 90-meter long crude oil tank with 2,500 cubic meters of hydrocarbon sludge. These are the 400 working days exposed to many of the risks listed above.

Furthermore, crude oil sludge usually contains more than 90% useful hydrocarbons. This important resource must be retrieved in a usable state.

Dedicated business practices reduce import requirements for humans while extraction of soluble hydrocarbons is good for processing or export. Flood tanks and heavy furnace oil can have similar problems.

4 Concluding remarks:

Chemical cleaning of an oil tank is a dangerous job with many difficult factors to quantify. Workers should spend as little time inside a tank as a general rule. Workers and professionals often ignore the safety guidelines and proper procedures and make mistakes due to lax attitudes, resulting in injury and financial loss for the organization. Workers participating in such operations should be properly informed and trained in all safety processes and practices. The organization has to ensure that the workers are provided adequate training and proper equipment to carry out chemical cleaning of oil tanks. Training is the key to working safely in confined spaces. Choosing the right cleaning method is an important part of reducing risk. The business methods available include semi-automated and fully automated options. In any case, there is no denying that when automated entry cleaning technologies are used, many risks are drastically reduced, if not completely mitigated. To conduct safe operations, the mentioned mistakes should not be made.

5 References:

[1] B. Knegtering and H. Pasman, “Safety of the process industries in the 21st century: A changing need of process safety management for a changing industry,” Journal of Loss Prevention in the Process Industries, vol. 22, no. 2, pp. 162-168, 2009.

[2] R. Moradi and K. M. Groth, “Hydrogen storage and delivery: Review of the state of the art technologies and risk and reliability analysis,” International Journal of Hydrogen Energy, vol. 44, no. 23, pp. 12254-12269, 2019.

[3] J. Carter, T. Harrigan, and S. Punwani, “Computer Simulation and Validation of Fire Hazards in Fuel Tanks,” in ASME/IEEE Joint Rail Conference, 2007, vol. 4787, pp. 151-159.

[4] F. I. Khan and S. Abbasi, “Major accidents in process industries and an analysis of causes and consequences,” Journal of Loss Prevention in the process Industries, vol. 12, no. 5, pp. 361-378, 1999.

[5] A. T. Simpson, M. A. Hemingway, and C. Seymour, “Dangerous (toxic) atmospheres in UK wood pellet and wood chip fuel storage,” Journal of occupational and environmental hygiene, vol. 13, no. 9, pp. 699-707, 2016.

[6] J. E. Robinson, D. W. Scott, W. R. Knocke, and W. D. Conn, “Underground storage tank disposal: alternatives, economics, and environmental costs,” 1988.