Oil Tank & Storage Tank Cleaning: Kerosene Wash Method, Confined Space Safety and Acceptance

Abstract: Oil tank cleaning and storage tank cleaning is the safety precondition for hot work during maintenance, and a necessary step when the product grade changes or sludge build-up cuts the effective volume. Three methods are in common use: the kerosene wash method, water washing and closed mechanical cleaning (COW). Danyang Blue Star Cleaning mainly uses the kerosene wash route: drain and collect the bottom oil, open the manhole and ventilate naturally for at least 8 hours, force-ventilate with an explosion-proof fan, enter only after gas testing passes, wipe the tank bottom and the wall up to 2 m with cloth dampened in aviation kerosene, then collect the waste and carry out a final inspection. Cleaning cycles run 3-5 years for light refined product tanks, 2-3 years for heavy and fuel oil tanks, and once or twice a year for slop tanks. Throughout the job a confined space entry permit is handled under GB 30871-2022, oxygen, flammable gas and hydrogen sulphide are tested item by item, entry time is capped at 30 minutes per shift, waste is sorted and handed over to the client, and quality is accepted under GB/T 25146-2010 and HG/T 2387.

1. When Tank Cleaning Is Mandatory

Oil tank (storage tank) cleaning is not routine maintenance but a technical job with clear trigger conditions, so a written procedure has to separate "must clean" from "can be deferred".

Four cases are mandatory: before hot work on or around the tank the hydrocarbon vapour must be cleared, otherwise the hot work simply ignites it; when the product grade or specification changes, because residual old oil contaminates the new product; when sludge build-up reduces the effective volume and causes metering error; and when long-standing water at the tank bottom and bacterial corrosion cause pitting, thinning the shell with a risk of leakage.

Cases that can be deferred: a tank with no product change, no hot work, normal water draining, no obvious bottom sludge and normal gas readings may run a longer interval. The three criteria are whether hot work is involved, whether the product is changed, and the measured condition of bottom sludge and water.

2. Choosing Between the Three Tank Cleaning Methods

Tank cleaning splits into the kerosene wash method, water washing and closed mechanical cleaning (COW). Selection runs on two criteria - the cleanliness required inside the tank, and whether hot work or maintenance is involved.

Selection conclusion: for atmospheric tanks with thin sludge and no hot work, use water washing with circulation and manual wiping; where entry or hot work maintenance is involved, prefer the kerosene wash method and clean the tank bottom and the wall up to 2 m properly; COW suits large product oil tanks and is not our main recommendation.

3. The Seven-Step Kerosene Wash Method

The kerosene wash method is our main process. The seven steps run in order, and no step may start until the previous one has met its target.

(1) Tank transfer and bottom oil handling: drain the product, draw off the bottom oil and collect it separately - it must not return to the tank; bond the pump and the tank shell for static electricity.

(2) Open the manhole and ventilate naturally (≥8 h): open the manhole, gauging hatch and light hatch to form a cross flow, ventilating for at least 8 hours.

(3) Forced ventilation with an explosion-proof fan: an explosion-proof axial fan delivers air, with the duct run down to the tank bottom, until gas testing passes.

(4) Enter only after gas testing passes: test oxygen, flammable gas, hydrogen sulphide and carbon monoxide; enter only when the readings pass and the permit is complete.

(5) Wipe with kerosene-dampened cloth: wear anti-static clothing and shoes, and wipe the tank bottom and the wall up to 2 m with cloth dampened in aviation kerosene; metal tools are prohibited.

(6) Waste collection: cloth, sludge and bottom oil are collected in separate drums and placed in explosion-proof sealed containers.

(7) Final inspection and acceptance: re-test the gas and re-inspect visually, record against the three acceptance criteria, and sign off by both parties.

4. Confined Space Entry Safety

Entry cleaning is confined space work, and the safety measures are the technical core of this article - if they cannot be built into the procedure, the job must not start.

Explosion-proof equipment: the three explosion-proof essentials - explosion-proof fan, explosion-proof lighting and explosion-proof tools; lighting runs on safety extra-low voltage, and no non-explosion-proof tool or mobile phone goes into the tank.

Static control: wear anti-static workwear and anti-static shoes, and no synthetic clothing; metal tools are bonded to the tank shell at equal potential, and striking or rubbing inside the tank is strictly forbidden.

Permit and qualification: the confined space entry permit is approved by the client, and entry personnel hold the special operations certificate [hazardous chemical safety operations].

Gas testing criteria: oxygen 19.5-21% by volume (not more than 23.5% in oxygen-enriched atmospheres; if exceeded, keep ventilating); flammable gas within the GB 30871-2022 limits (≤0.5% by volume where the lower explosive limit is ≥4%, ≤0.2% by volume where it is below 4%); hydrogen sulphide and carbon monoxide not detected. Entry is prohibited if any item fails, and the readings are recorded on the permit.

Attendant and rotation: a dedicated attendant stays outside the tank with a safety line and checks in with the entry crew every 10 minutes; entry time does not exceed 30 minutes per shift, and the crew rotates after that.

Fire equipment: dry powder extinguishers, fire sand and a fire blanket are provided; water must never be used on an oil fire, because it floats the oil, spreads it across a wider area and lets the fire grow.

5. Waste and Environmental Disposal

Tank cleaning waste is collected in three streams - waste cloth, waste liquid and bottom oil - each placed in explosion-proof sealed containers, labelled, lidded and kept away from ignition sources.

Disposal follows the contract: we sort and collect on site and hand over, and the client commissions a licensed contractor under local environmental requirements, with no residue left on site. Where the oily sludge counts as hazardous waste, the documents are retained on file as acceptance records.

6. Cleaning Cycle and Acceptance Criteria

Cleaning cycle: light refined product tanks with normal water draining and no hot work are cleaned once every 3-5 years; heavy oil and fuel oil tanks every 2-3 years; sulphur-bearing and water-contaminated slop tanks once or twice a year. The final trigger is the measured sludge thickness at the tank bottom.

Three acceptance criteria: (1) by visual check, no oil film and no residual bottom oil on the tank bottom and the wall up to 2 m; (2) flammable gas within the GB 30871-2022 limits (≤0.5% by volume where the lower explosive limit is ≥4%, ≤0.2% by volume where it is below 4%); (3) all waste handed over, with both parties signing the acceptance record.

Quality acceptance follows GB/T 25146-2010 and HG/T 2387; confined space entry and pre-hot-work gas testing follow GB 30871-2022, oxygen-deficiency protection refers to GB 8958-2006, and occupational hazard protection in confined spaces refers to GBZ/T 205-2007.

7. Frequently Asked Questions

What level of cleanliness makes an oil tank acceptable?

All three acceptance criteria must be met: no oil film and no residual bottom oil on the tank bottom and the wall up to 2 m by visual check; flammable gas within the GB 30871-2022 limits (≤0.5% by volume where the lower explosive limit is ≥4%, ≤0.2% by volume where it is below 4%), meeting the requirement for hot work during maintenance; and all waste handed over and the acceptance record signed.

What preparations does the client need to make before tank cleaning?

The client first drains the tank and draws off the bottom oil, blanks off the connected lines, and locks and tags the power supply, then approves the entry permit and assigns an attendant. For a cleaning assessment, call +86 18952832843.

Kerosene wash or water wash - which should our tank use?

Decide on two criteria: where the tank interior needs a lower level of cleanliness and no entry for maintenance is involved, water washing with circulation and wiping is usually enough; wherever hot work or entry is involved, the bottom and the wall up to 2 m must be cleaned free of residue, so the kerosene wash method is preferred.

How many days does it take to clean a 1,000 m³ oil tank?

For a 1,000 m³ vertical oil tank the working time is about 2 days per tank, and the overall tank downtime - including tank transfer, natural ventilation for more than 8 hours, forced ventilation, testing and acceptance - is generally 2-4 days; a 50 m³ horizontal tank takes about 1.5-2 days. Both depend on sludge thickness and the number of manholes. Call +86 18952832843.

Who disposes of the oily sludge produced by tank cleaning?

Waste cloth, waste liquid and bottom oil are placed in explosion-proof sealed containers, stored on site by type and handed over to the client, who commissions a licensed contractor under local environmental requirements; the documents are retained as acceptance records.

Related Reading

Further reading on comparable equipment and the same acceptance route:

• Reactor Cleaning Technology - comparable practice for confined space entry and material restrictions
• 304/316L Stainless Steel Tank and Reactor Surface Passivation
• Industrial Steam Boiler Chemical Descaling Technology
• Equipment Scaling Diagnosis Tool · Maintenance Cycle Calculator

All tank cleaning processes and confined space controls in this article follow GB 30871-2022 Safety Code of Special Operations in Hazardous Chemical Enterprises, GB 8958-2006 Safety Regulation for Working under Hazardous Oxygen Deficiency and GBZ/T 205-2007 Specification of Prevention and Control on Occupational Hazards in Confined Space; quality is accepted under GB/T 25146-2010 Quality acceptance specifications of chemical cleaning for industrial equipment and HG/T 2387 Quality standard for chemical cleaning of industrial equipment.

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