Hazards related to tank cleaning

Many dangerous situations can occur during tank cleaning and working in confined spaces. Our guest author, Mr. Steve Scott of Bro Nant International, discusses possible hazards like ignition, chemical exposure, oxygen deficiency, physical hazards etc. and gives his views on how they can be avoided.

Steve Scott has a highly specialised background in the field of oil tank cleaning, hydrocarbon recovery and waste minimisation.

Tank cleaning is an extremely hazardous activity. When working in a confined space personnel are exposed to a number of hazards that in some cases have led to injury or even death.

There are various definitions of a ‘confined space’ though two are consistently applied. “A place which does not have the benefit of natural ventilation” and, “a place which is difficult to enter and therefore presents a hindrance to rapid escape in case of an emergency”.

Working in a confined space requires a degree of good physical and mental health and is certainly not for the claustrophobic, those with heart or breathing problems or those with physical disabilities.

Identification and elimination of potential hazards at the tank cleaning planning stage will enhance the Health & Safety and Environmental performance of a tank cleaning project.

Hazards relating to tank cleaning are as many and varied as the designs and physical dimensions of the tanks themselves. We list below some of the most significant hazards, you will identify others by risk assessment.

Fire and explosion
The cleaning of a tank having contained a flammable material presents specific hazards. If a flammable mixture of vapour and air exists inside a tank, then the introduction of a source of ignition may cause a fire and/or explosion.

Working in confined spaces can be a dangerous activity and manhours spent inside a tank during cleaning should therefore be kept to a minimum.

How to avoid the hazard? 

For an explosion to occur all three elements of the fire triangle need to be present in the appropriate ratios. If the oxygen level can be maintained at below 8% then, regardless of hydrocarbon content, the atmosphere can be considered inert and a fire or explosion cannot occur. Excluding a source of ignition or reducing hydrocarbon vapour levels to below the LFL (Lower Flammable Limit) will have the same effect.

Flammable Vapours
Whenever we remove the manways on a tank containing flammable material, the hydrocarbon rich vapour within the tank becomes diluted with air and the mixture approaches the flammable zone. It is imperative that we eliminate any potential source of ignition.

How to avoid the hazard?
For a mixture of hydrocarbon vapour and air to ignite, the ratio has to be within certain limits. For most hydrocarbons the lower flammable limit (LFL) and the upper flammable limit (UFL) are typically 1% and 10% by volume respectively.

The requirement for man-entry into a tank containing flammable vapours is currently generally accepted as 10% of LFL. This means that in real terms hydrocarbon in air measurements are at or below 0.1%.

Beware that in tanks where sludge or scale is present, disturbing the sludge on man-entry can release trapped vapours. This may cause the atmosphere to re-enter the flammable zone.

Should this occur, there is no alternative but to remove personnel from the tank until the atmosphere is retested at below 10% LFL.

The tank atmosphere should be closely monitored, using intrinsically safe constant monitors installed inside the tank, until all of the sludge has been removed. This is particularly important during the early days of a tank clean

Sources of Ignition
Defined as any heat source having sufficient energy to ignite a flammable vapour/air mixture by locally raising it above its auto ignition temperature. Possible sources of ignition in and around tankage include: Sparks from electrical equipment, Lightning, Static Electricity, Hot Surfaces and Pyrophoric Scale.

There are many sources of ignition to be avoided

How to avoid the hazards?
Electrical equipment – The use of electrically driven equipment, lighting, internal combustion engines and battery operated devices should be strictly controlled.

Lightning – is well known for causing tank fires. All tank cleaning activities should be suspended during electrical storms. Personnel should follow emergency evacuation procedures and should subsequently leave the vicinity of the tank.

Static Electricity – is a surface phenomenon associated with the contact and separation of dissimilar materials. High pressure water jetting, steam cleaning, grit blasting, air driven lighting systems etc. are all potential generators of static electricity. To avoid accumulation of static charge, hoses, nozzles etc. should be made from electrically conductive material and should be bonded and earthed.

Hot Surfaces – smoking and fires must be strictly controlled and only allowed in restricted locations. Avoid the risk from hot exhaust systems by siting powered equipment at an appropriate distance from the open tank manways. Steam lines, temporary or otherwise, should be lagged. Look for other heat sources and eliminate.

Pyrophoric Scale – is iron sulphide which can be formed in tanks which store sulphur containing products e.g. sour crude oil, naphtha etc. due to a reaction between sulphur compounds and rust. The reaction between iron sulphide and air is exothermic and can generate sufficient heat to ignite any flammable vapours in the vicinity. Control by damping with water and removing from the tank as quickly as possible.

Chemical Hazards
Tanks which have contained hazardous chemicals require special precautions during the cleaning process. Exposure to hazardous chemicals can arise from skin contact, inhalation or accidental ingestion. Hazardous chemicals often encountered during tank cleaning include; Lead anti-knock compounds, Hydrogen Sulphide, Polycyclic Aromatic Hydrocarbons (PAH), and Benzene. Beware also toxic dusts from lagging, paint systems, grit blasting etc.

Some of the compounds have an immediate toxic effect, others have a long-term effect (carcinogenic, mutagenic) on your health.

How to avoid the hazard?
When planning any tank clean always consult the Material Safety Data Sheet for the product stored. Fortunately, refined product tanks are not prone to significant build up of sludge therefore the duration of exposure should be limited.

Always ensure that personnel entering the tank are wearing appropriate Personal Protective Equipment (P.P.E.).

Oxygen Deficiency
In the absence of toxic and flammable gases, the safe level for oxygen concentration in the tank atmosphere is 20%. Below this, breathing apparatus must be worn. Oxygen deficiency can arise in a number of ways, particularly during the gas freeing phase when the tank atmosphere is product rich.

Certain products require a blanket of inert gas e.g. nitrogen above the liquid level which can result in oxygen deficiency once the tank is opened. Oxygen levels in a tank that has been cleaned and then resealed and left unused for a period of time may be depleted due to the oxygen being consumed by the process of internal corrosion.

How to avoid the hazard?
In any event, oxygen levels should be checked before allowing personnel to enter a tank without breathing apparatus and monitored frequently thereafter for the duration of the exercise.

Physical Hazards
Personnel entering a tank will be exposed to a number of physical hazards (climatic conditions, tripping/obstruction hazards etc.) particular to the location and physical design of the tank. It is imperative that a thorough risk assessment is conducted prior to entry and that a safe system of work is initiated to minimise the risk of injury.

How to avoid the hazard?
Climatic conditions can create unsafe conditions for those engaged in tank cleaning. Excessive internal temperatures are particularly difficult to manage and personnel must be allowed regular breaks for rest and for water and salt tablets to be consumed. Extreme cold and excessive noise can be equally debilitating.

Tripping hazards and other obstructions proliferate in storage tanks, it is essential that personnel be given a thorough understanding of the tank internal layout prior to entry. This problem is exacerbated due to generally poor lighting conditions which are often difficult to improve due to flammable atmospheres and/or the physical dimensions of the tank.

Falling objects, slippery surfaces, poor housekeeping, obstructions at head height in floating roof tanks, poorly maintained or inappropriate equipment etc. are other sources of danger which can be minimised by adequate risk assessment and training.

When performing manual cleaning, many hazards exist due to the location and design of the tank, and many more are introduced by way of tank cleaning equipment and personnel. Always ensure that an unrestricted exit is maintained for evacuation in case of emergency.

Many hazards exist when performing manual cleaning.

Radiation hazards
Naturally occurring radioactive material (NORM) or low specific activity (LSA) scale can be detected in certain crude oil, ballast and slop systems. The radioactivity originates in the formation water at the well and results in radioactive scale in pipelines, which can be carried through to storage.

How to avoid the hazard?
The cleaning of tanks containing radioactive material requires special controls to be observed. Where radiation is considered a potential problem, the advice of an expert in this field should be sought.

Microbiological Hazards
Where water is present in tank bottoms and sludge, microbial proliferation is not uncommon. In severe cases, infection or allergic reaction can result.

How to avoid the hazard?
Where significant microbial proliferation is suspected, the advice of a qualified expert should be sought.

Special precautions when cleaning crude oil storage tanks
The sheer physical dimensions of a crude oil storage tank (up to 120 metres diameter) and the propensity for crude oil to deposit sludge (can be more than 10,000 m³) make these tanks worthy of special consideration. Always determine by Topographic Survey that it is safe to land the floating roof on the sludge banks and/or that it is safe to remove the manways. Using traditional, pump assisted desludging techniques on an 80 metre diameter crude oil storage tank containing c. 2,500 m³ hydrocarbon sludge will require c. 300 mandays. That is 300 mandays during which personnel are at risk from many of the above hazards.

Perspective view of tank from manway (143 deg).

Additionally, crude oil sludge will typically contain more than 90% valuable hydrocarbon. It is imperative that this valuable commodity is recovered in a useable form.

Specialist proprietary techniques are commercially available which minimise the need for man-entry while recovering the soluble hydrocarbons in a form suitable for processing or export. Ballast water tanks and heavy fuel oil tanks may present similar problems.

Tank cleaning is an extremely hazardous activity with many difficult to quantify variables. As a general rule, time spent by personnel inside a tank should be kept to a minimum.

Training is the key to safe working in a confined space and personnel engaged in such activities should be carefully instructed and trained in all safety systems and procedures.

A key element in minimising the risk is choosing an appropriate cleaning method. A number of semi and fully automated methods are commercially available. In any event, there is no doubt that when implementing non-man entry cleaning systems many hazards are dramatically reduced and some even eliminated.

Published: 2003-01-22

Oreco A/S . Oldvej 1A . DK-3540 Lynge . Denmark . TEL. +45 4332 0200 . info@oreco.com

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