What is the difference between tank cleaning and tank degassing?

Tank cleaning and tank degassing are two distinct processes that serve different purposes in industrial maintenance. Tank cleaning removes physical residues, sludge, scale, coatings, and hydrocarbon deposits, from the interior surfaces of a tank. Tank degassing removes flammable, toxic, or otherwise hazardous vapours and gases from the tank atmosphere. Both are essential steps before any confined space entry or hot work can take place, and each carries its own safety requirements and equipment needs.

The two processes are closely related and often performed in sequence, but confusing one for the other can create serious safety risks. This article walks through each process in detail, explains their differences, and covers the equipment, sequencing, and safety considerations that technical teams need to understand before entering or working on any industrial storage tank.

What actually happens during tank cleaning?

Tank cleaning is the physical removal of solid and liquid contaminants from the interior surfaces of a storage tank. This includes sludge, scale, sediment, hydrocarbon residues, old coatings, and any other material that has accumulated on the tank floor, walls, or roof. The goal is to restore the tank to a condition suitable for inspection, maintenance, recoating, or a change of product.

In practice, tank cleaning involves several stages. An initial drain removes the bulk of any remaining liquid product. From there, technicians or robotic systems address the residual material that clings to surfaces. Depending on the tank contents and condition, this may involve:

• High-pressure water jetting to dislodge and flush away sludge and deposits
• Mechanical scraping or brushing for hardened scale
• Chemical cleaning agents to dissolve specific residues
• Vacuum recovery systems to collect waste without spreading contamination

For oil storage tanks in particular, the sludge layer at the bottom can be dense, waxy, and difficult to remove with water alone. High-pressure water jetting in the range of 500 to 3,000 bar (7,252 to 43,511 PSI) is widely used because it delivers the mechanical force needed to break up and flush away stubborn deposits without damaging the steel substrate when used correctly.

Once cleaning is complete, the tank surface must be dry and free of contaminants before any inspection, coating, or welding work begins. This is why closed-loop systems that collect both water and debris simultaneously are particularly valuable: they leave a clean, dry surface without requiring a secondary clean-up pass.

What is tank degassing and what does it remove?

Tank degassing is the controlled removal of hazardous vapours and gases from inside a storage tank to make the atmosphere safe for human entry or hot work. It primarily targets flammable hydrocarbon vapours, hydrogen sulphide (H2S), carbon monoxide, and other toxic or oxygen-displacing gases that accumulate in tanks previously used to store petroleum products, chemicals, or other volatile substances.

These gases do not disappear simply because a tank has been drained. Residues left on surfaces continue to off-gas, and vapours can stratify in layers within the tank, meaning the atmosphere at one height may read safe while a dangerous concentration exists at another level. Degassing addresses this by actively ventilating the tank atmosphere and, in some cases, using inert gases or controlled purging methods to displace the hazardous vapours.

The degassing process typically involves:

• Forced ventilation using fans or blowers to flush vapours out of the tank
• Continuous atmospheric monitoring at multiple points and heights
• Inert gas purging (nitrogen, for example) in situations where open ventilation would create an ignition risk
• Verification that vapour concentrations are below the Lower Explosive Limit (LEL) and within acceptable toxicity thresholds before entry is permitted

Degassing is not considered complete until atmospheric testing confirms that the tank meets the entry requirements defined by the applicable safety standards. A tank may look and smell clean but still contain dangerous vapour concentrations that are invisible and, in some cases, odourless.

What’s the difference between tank cleaning and tank degassing?

The key difference is this: tank cleaning targets physical contamination on surfaces, while tank degassing targets chemical contamination in the air. Cleaning deals with what is on the tank; degassing deals with what is in the tank’s atmosphere. Both are required before safe confined space entry, but they address entirely separate hazards through entirely separate methods.

To make the distinction concrete:

• Tank cleaning uses mechanical force, water, or chemicals to remove sludge, scale, and residues from tank surfaces. The result is measured by surface condition, cleanliness grade, dryness, and freedom from visible contamination.
• Tank degassing uses ventilation, purging, or inert gas displacement to reduce hazardous vapour concentrations in the tank atmosphere. The result is measured by atmospheric readings, LEL percentage, oxygen level, and toxic gas concentration.

Another practical distinction is who performs each task and with what equipment. Tank cleaning is typically carried out by specialist cleaning crews using high-pressure water jetting equipment, vacuum recovery units, and, in modern operations, robotic crawlers. Tank degassing is managed by safety officers and atmospheric monitoring specialists using ventilation equipment and gas detection instruments.

The two processes can overlap in time, ventilation may begin during cleaning, but they are governed by separate procedures and separate safety sign-offs.

Which process should come first, cleaning or degassing?

Degassing must begin before tank cleaning crews can safely enter the tank. However, in practice, the two processes often run in parallel during the early stages. Initial ventilation and atmospheric monitoring start as soon as the tank is opened and drained, while cleaning preparation takes place externally. Entry for hands-on cleaning work is only permitted once the atmosphere has been confirmed safe.

The sequence in a typical tank maintenance operation looks like this:

1. The tank is taken out of service and the bulk product is drained or pumped out
2. Ventilation begins immediately to start reducing vapour concentrations
3. Atmospheric testing is conducted at intervals to track progress
4. Once entry conditions are met, cleaning crews or robotic equipment enter
5. Ventilation continues throughout the cleaning operation to manage vapours released by disturbing residues
6. Final atmospheric verification is conducted before any hot work or coating application begins

It is worth noting that cleaning itself can temporarily increase vapour concentrations. When high-pressure water jetting breaks up sludge and hydrocarbon residues, it can release trapped gases back into the tank atmosphere. This is why continuous atmospheric monitoring during cleaning is not optional: it is a critical safety control that must remain active throughout the entire operation.

What safety risks are specific to each process?

Tank cleaning and tank degassing each carry distinct safety risks that require targeted controls. Understanding which hazards belong to which process is essential for building an effective risk management plan.

Safety risks in tank cleaning

The primary hazards during tank cleaning include high-pressure injection injuries, confined space risks, slips and falls on wet surfaces, and exposure to chemical residues. High-pressure water jetting at pressures from 500 to 3,000 bar (7,252 to 43,511 PSI) can cause severe injection injuries if a jet contacts skin, a risk that is not always immediately visible but can be life-threatening. Robotic systems significantly reduce this exposure by removing operators from the direct jetting zone.

Confined space entry during cleaning also introduces risks of engulfment, entrapment, and sudden changes in atmospheric conditions. Proper permit-to-work systems, rescue planning, and continuous atmospheric monitoring must be in place before and during entry. Industry bodies including SIR (Netherlands), EWJI (Europe), WJA (UK), WJTA (USA), and DIRV (Germany) publish guidelines and certification standards for safe high-pressure water jetting operations, and their frameworks should be consulted when developing cleaning procedures.

Safety risks in tank degassing

The primary hazards during degassing are fire and explosion from flammable vapours, toxic gas exposure, and oxygen deficiency. Hydrocarbon vapours are invisible and can accumulate to explosive concentrations without warning. Hydrogen sulphide, commonly found in crude oil tanks, is acutely toxic at very low concentrations and can cause rapid incapacitation before an operator realises they have been exposed.

Oxygen deficiency is another serious risk: inert gas purging methods, while effective at displacing flammable vapours, can reduce oxygen levels to life-threatening concentrations if not managed carefully. All personnel involved in degassing operations must be trained in atmospheric monitoring, understand the readings they are taking, and know the action levels that require evacuation. The same industry bodies, SIR, EWJI, WJA, WJTA, and DIRV, provide relevant safety standards that apply to confined space and hazardous atmosphere work associated with tank operations.

What equipment is used for tank cleaning versus degassing?

Tank cleaning and tank degassing rely on fundamentally different equipment categories. Cleaning equipment applies mechanical or chemical force to surfaces; degassing equipment manages and monitors the atmosphere.

For tank cleaning, the core equipment includes:

• High-pressure water jetting units operating in the 500 to 3,000 bar (7,252 to 43,511 PSI) range, delivering the force needed to dislodge stubborn sludge and scale
• Vacuum recovery units to collect water and debris simultaneously, leaving a clean, dry surface
• Robotic crawlers that can operate in confined or hazardous spaces without requiring human entry into the most dangerous zones
• Rotating tank cleaning heads for interior washing of tank walls and roofs
• Abrasive blasting equipment for surface preparation prior to recoating

For tank degassing, the equipment is focused on atmosphere management:

• Forced ventilation fans and blowers to exchange the tank atmosphere with fresh air
• Multi-gas detectors measuring LEL, oxygen, H2S, CO, and other relevant gases simultaneously
• Nitrogen purging systems for situations where open ventilation is not safe due to ignition risk
• Vapour combustion units for controlled destruction of recovered vapours in some operations

How MagTrack supports tank cleaning operations

When it comes to practical tank cleaning solutions, robotic technology has changed what is achievable in terms of both safety and efficiency. Our MagTrack robotic crawler system is specifically engineered for demanding environments like oil storage tanks, where the combination of confined spaces, hazardous residues, and high-pressure water jetting creates significant risks for human operators.

Here is what MagTrack brings to tank cleaning operations:

• Remote-controlled operation: keeps operators out of the most hazardous zones during active jetting
• High-pressure capability up to 3,000 bar (43,511 PSI): delivers the mechanical force needed to remove heavy sludge, scale, and hydrocarbon deposits effectively
• Closed vacuum blast can: collects water and debris simultaneously, leaving a clean, dry surface without secondary clean-up and without releasing contaminants into the tank atmosphere
• Modular tool system: the same carrier can be fitted with different tools for waterblasting, abrasive blasting, or inspection, reducing equipment changeovers
• ATEX Zone 2 certified option: the MagTrack Mini is available as a certified ATEX unit, making it suitable for use in explosive atmospheres where flammable vapours may still be present
• Compact design for confined spaces: the MagTrack Mini is lightweight and easy to transport, suitable for narrow access points including offshore tank environments

Whether you are planning a routine tank maintenance programme or facing a complex cleaning job with tight safety requirements, we are ready to help you find the right configuration for your application. Contact our team to discuss your tank cleaning requirements and discover how MagTrack can reduce risk, cut cleaning time, and deliver the surface quality your next inspection or recoating project demands.