During tank cleaning, alongside confined space hazards such as poisoning and asphyxiation, the fire and deflagration risk created by residual oil vapour is equally significant.
Even where a tank has been taken out of service and emptied, flammable material may remain in bottom residues, oil sludge and internal fittings. If flammable vapour continues to evaporate during cleaning and forms an explosive atmosphere with air, a spark, static electricity or tools and equipment that do not meet explosion protection requirements can rapidly trigger a serious accident.
1. Why did a deflagration occur during a tank cleaning job?
On 29 June 2010, a deflagration accident occurred during tank cleaning at the crude oil transfer station of a refinery belonging to a petrochemical company in Liaoyang, Liaoning Province.
Outcome: a 30,000 m³ crude oil storage tank | 5 fatalities and 5 injuries.
According to material published by the emergency management authorities, oil vapour generated during the work mixed with air and formed an explosive gas atmosphere. Sparking from non-explosion-proof lighting, or sparks generated when ferrous cleaning tools struck the tank bottom, then triggered the deflagration.
The chain of risk exposed by the accident is not complex:
Residual material keeps evaporating → flammable vapour accumulates → an explosive atmosphere forms → an ignition source appears → deflagration occurs
Many years have passed since the accident, but the emergency management authorities still cite it as a typical historical case. For tank cleaning it highlights a point that is easily overlooked:
The fact that the oil has been drained does not mean the deflagration risk inside the tank has gone.
2. Why can a tank still be dangerous after it has been emptied?
After long-term storage of crude oil, refined products and other flammable media, a certain amount of residue may remain in bottom sludge, on the tank shell, in internal fittings and in dead corners.
As manual cleaning, sludge disturbance and washing proceed, volatile components in that residue may be released further.
Emptying a tank therefore only means that most of the liquid medium has been removed; it cannot be equated directly with a safe working environment inside the tank.
A deflagration normally requires several basic conditions at the same time: a flammable substance at a certain concentration, oxygen, and sufficient ignition energy.
That is why tank cleaning cannot rely on rules of thumb such as "the oil has all been pumped out" or "there is no obvious smell inside the tank" to judge whether the site is safe.
AQ 3042—2025, Safety specification for cleaning operations of onshore oil and gas field storage tanks, requires at least two gas detectors on site for comparative measurement, and sets corresponding requirements for oxygen, toxic substances and flammable gas concentrations. Where a tank is cleaned manually, the gas environment inside the tank must also be monitored continuously while personnel are inside.
What really matters is not only the visible residual oil, but also the invisible gas environment inside the tank.
3. No naked flame does not mean no ignition source
Once an explosive atmosphere has formed, the trigger is not necessarily an obvious open flame.
In the accident described above, possible ignition sources included electrical sparks from non-explosion-proof lighting and sparks generated when ferrous cleaning tools struck the tank bottom.
Explosion protection management on a tank cleaning site therefore cannot stop at "no smoking, no naked flames".
Whether electrical installations meet the applicable explosion protection requirements, whether the tools used are suitable for the current working environment, and whether static electricity can accumulate all have to form part of risk control.
AQ 3042—2025 explicitly requires that electrical equipment and installations used in flammable and explosive hazardous areas have an explosion protection rating complying with GB 50058. For manual cleaning, sounding rods, ullage tapes and similar items must also be made of anti-static material or meet explosion protection requirements.
The underlying logic can be summarised as two lines of defence: controlling the formation of a hazardous flammable vapour environment, and eliminating as far as possible the ignition sources that could set it off. Neither line of defence can be omitted.
4. Manual tank entry means personnel are placed directly in a high-risk environment
Manual cleaning does not inevitably lead to an accident, and it is not prohibited outright by the current standard.
But once personnel enter a tank they face confined space conditions, a changing gas environment, explosion and static electricity hazards and emergency evacuation requirements at the same time, which places higher demands on site safety management.
AQ 3042—2025 requires gas testing within 30 minutes before personnel enter a tank. During the work, flammable gas, toxic gas and oxygen concentrations must be monitored continuously and recorded at least once every 2 hours. If a concentration alarm exceeds the limit, work must stop immediately and personnel must be evacuated; if work is interrupted for more than 60 minutes, testing must be repeated and passed before work resumes.
Safety in manual tank cleaning therefore cannot rest on a single item of personal protective equipment or a single test before entry. It depends on a complete set of risk control measures covering gas detection, ventilation, explosion protection, site supervision, personal protection and emergency response.
If any one of these links fails, the personnel working inside the tank may be exposed directly to a hazardous environment.
5. Reducing tank entry is another approach to risk control
Faced with the multiple risks involved in tank cleaning, another option, alongside strengthening protection once personnel are inside, is to look at the working method itself: can direct entry into the tank be reduced as far as possible?
AQ 3042—2025 states that for tanks with a capacity of not less than 1,000 m³ where mechanical cleaning is feasible, mechanical cleaning shall be selected. The standard also sets specific requirements for the installation of mechanical cleaning equipment, inert gas protection and related aspects.
Mechanised cleaning does not mean that gas detection, explosion protection and site management can be relaxed. By changing the working method, it reduces the need for personnel to enter the high-risk environment inside the tank to carry out cleaning, lowering direct exposure at the front end of risk control.
Moving from "how to protect personnel once they are inside" towards "how to minimise the need for them to go inside" is a direction worth attention in tank cleaning safety management today.
Tecway Industrial | Tank cleaning equipment rental and cleaning services
For companies with periodic tank overhaul and cleaning requirements, Tecway Industrial can provide mechanised tank cleaning equipment rental together with associated cleaning services, with cleaning arrangements configured flexibly according to tank type, project schedule and site requirements.
For companies that do not need to own a complete cleaning system on a long-term basis, the rental model reduces one-off equipment investment. At the same time, mechanised cleaning reduces the need for personnel to enter tanks for high-risk work, improving efficiency while giving better regard to safety management and project delivery requirements.
References
- Ministry of Emergency Management: Hazardous chemical accidents that occurred in June in past years
- Ministry of Emergency Management: AQ 3042—2025, Safety specification for cleaning operations of onshore oil and gas field storage tanks
About Tecway Industrial
Tecway Industrial Systems Limited, formerly Tecway International (Petrochemical) Limited, was established in Hong Kong in 1995. Beijing Tecway Xuyang Industrial Equipment Co., Ltd. is its Beijing-registered subsidiary.