Major Maintenance of Storage Tanks

A major maintenance takes an asset out of service to return it to its design condition. For a storage tank, that means emptying, cleaning, inspecting, repairing and protecting it, so that it goes back into operation with its structural integrity, its measurement systems and its corrosion protection restored.

The service brings together tank cleaning and tank rehabilitation in a single multidisciplinary effort, so that every discipline works toward the full operational recovery of the tank.

What the service covers

• Industrial mechanical cleaning: removal of sediments and residues, with safe waste handling
• Technical inspection of the tank and of its structural bases
• Civil works
• Mechanical works and metalworking
• Surface treatment by blasting, and coating
• Electrical works, cathodic protection and lightning protection
• Instrumentation works
• In plant shutdowns: separators, furnaces and boilers, heat exchangers and rotating equipment

• When the scheduled internal inspection falls due, which requires the tank to be emptied and cleaned
• When corrosion, loss of thickness or leaks are detected in the floor, the shell or the roof
• When accumulated sludge has reduced the usable capacity
• When the stored product or the service of the tank changes
• When the roof, its seals, the foundation or the accessories are damaged
• When the tank must be brought up to current technical standards

At the bottom of oil tanks, sediments, water, heavy hydrocarbons, scale and rust settle and form sludge. Part of that material can be recovered as product; the rest is waste and must be handled as such.

1. Planning

   Review of the tank history and previous inspections; scope, schedule, materials and safety plan.

2. Taking out of service

   The tank is emptied and isolated: lines blinded or disconnected and energy sources locked out.

3. Cleaning and degassing

   Removal of the remaining product and sludge, and ventilation until the atmosphere is safe.

4. Inspection and diagnosis

   The condition of the floor, shell, roof, foundation and accessories is assessed, and the repairs are defined.

5. Rehabilitation

   Civil, mechanical, electrical and instrumentation works, according to the diagnosis.

6. Surface protection

   Surface preparation and application of the coating system, inside and outside.

7. Tests

   Inspection of the repairs, tightness tests and verification of the instruments and of the cathodic protection.

8. Return to service

   Final documentation and handover of the tank to operations.

1. Product removal

   Using explosion-proof equipment, with grounding of equipment and hoses.

2. Manway opening

   With spark-proof tools.

3. Visual inspection

4. Degassing

   Forced ventilation until a detector verifies that the atmosphere is not explosive (0% LEL).

5. Mechanical and manual cleaning

   Personnel entry for mechanical and manual cleaning and removal of sludge and sediment from the bottom.

6. Final verification

1. Segregation

   Separation of recoverable hydrocarbon, water and solid waste.

2. Recovery

   Recovery of the hydrocarbon that can return to the process.

3. Containment and transport

   Storage in suitable containers and transport by authorized vehicles.

4. Treatment and disposal

   Treatment and final disposal in accordance with the applicable environmental regulations, with records of each step.

A clean tank makes the internal inspection possible. Its findings define the scope of the rehabilitation:

• Thickness measurement of the floor, the shell and the roof
• Review of welds and detection of corrosion and pitting
• Technical inspection of the structural bases, the foundation and the settlement of the tank
• Condition of the coatings, inside and outside
• Roof, seals, nozzles, manways, stairs and platforms
• Inspection report with the repairs that are needed

The work is organized in four disciplines that run together as one project:

Surface protection

• Surface treatment by blasting
• Anticorrosive coating systems for the interior and the exterior of the tank
• Thermal insulation where the service requires it

See Fabrication & Welding Works

See Instrumentation & Automation

A tank that stores flammable liquids is exposed to lightning: a direct strike, or the currents that a nearby strike induces, can produce a spark where there are vapors, typically at the rim seal of a floating roof or at the vents. The protection gives that current a continuous, low-resistance path to ground, so that it does not jump across a gap.

• Grounding system of the tank and measurement of its resistance
• Bonding of the shell, the roof, stairs, piping and accessories, so that all metal parts are at the same potential
• In floating roofs: shunts and bypass conductors between the roof and the shell
• Air terminals, masts or overhead wires where the protection study calls for them
• Surge protection for the instrumentation and the electrical systems of the tank
• Inspection of connections and records of the measurements

A plant shutdown is the planned window in which a facility stops to inspect, repair and replace what cannot be touched while it operates. The major maintenance of tanks is usually scheduled within that window, and the same stop is used for the rest of the static and rotating equipment.

How a shutdown is organized

1. Scope

   List of jobs for each piece of equipment, defined and closed in advance.

2. Preparation

   Materials, spares, tools, scaffolding and personnel on site before the plant stops.

3. Isolation

   Equipment emptied, cleaned, blinded and locked out, with its work permits.

4. Execution

   Work by fronts and shifts, with daily follow-up of the schedule and of the findings.

5. Tests and start-up

   Inspection, pressure tests, alignment and loop checks; start-up and handover to operations.

See maintenance of compressors and motor-generators

See pressure vessels under the ASME Code

• Non-destructive testing of the repair welds
• Tightness tests of the floor welds
• Hydrostatic test when the scope of the repairs requires it
• Inspection of the coating: thickness and continuity
• Calibration of the level measurement and verification of the cathodic protection
• Measurement of the grounding and verification of the lightning protection
• Final dossier: inspection records, certificates of materials and tests

Degassing is the highest-risk stage. Safety perimeters are established, certified equipment for explosive atmospheres is used, and a procedure is required for:

• Confined spaces
• Hazardous substances
• Work at height

Controls during the work

• Work permit and isolation of the tank: lines blinded or disconnected, and equipment grounded and bonded
• Continuous monitoring of the atmosphere (explosivity, oxygen and toxic gases) while people are inside
• An attendant at the manway at all times, in communication with the people inside
• A rescue plan and rescue equipment ready before entry
• Emergency plan and briefing of the whole crew before starting

See the active project: Tank 05T-102

See major maintenance of compressors and motor-generators

Reference standards commonly applied in the sector. They are listed for orientation and do not replace the specification of each project.

API 653

Tank inspection, repair, alteration and reconstruction.

API 650

Welded tanks for oil storage.

API 651

Cathodic protection of aboveground petroleum storage tanks.

API 652

Linings of aboveground petroleum storage tank bottoms.

API 2015

Safe entry and cleaning of petroleum storage tanks.

NFPA 30

Flammable and combustible liquids code.

API RP 545

Lightning protection of aboveground storage tanks for flammable or combustible liquids.

NFPA 780

Installation of lightning protection systems.

API 510

Pressure vessel inspection code: in-service inspection, rating, repair and alteration.