Oil Gas Storage Tanks Safety

Tank Safety

The following are a guide for fixed, floating, cylindrical and spherical storage tanks and apply to the operation, isolation, gas freeing, cleaning and repairing of such tanks.

5.1       Bunds

Tank bunds and retaining walls should be maintained in good condition.  Vegetation inside bunds must be kept cut short.  A tank bund must not be opened unless the proper permit has been issued. This permit is to specify the method of opening, the machines and tools that may be used and any precautions that need to be taken if buried cables are present including their location.

5.2       Handrails, Stairs and Ladders

These should be kept in good condition and any defects reported and repaired.  Stairs and ladders are to be kept free of any tripping hazards or obstructions.

5.3       Relieving Devices/Relief Valves

All tanks must be protected by atmospheric vents or pressure/vacuum relieving devices according to duty, which are regularly inspected and maintained in good working condition

5.4       Dip Hatches

The covers of tank dip holes, sampling hatches and manholes must be kept closed when not required for operational reasons.

5.5       Manholes

Manholes used for entry to a tank should not be less than 450 mm long and 400 mm wide or, if circular, not less than 450 mm in diameter.  Safe access and method of entry and exit should be provided at all manholes.

5.6       Access to Tank Roofs

Access to the normal dip point, which should be adjacent to the access ladder or steps, would not normally involve any problems and is therefore allowed during normal operating conditions.  Access passed the normal point should be restricted as follows:

With fixed roof tanks the condition of the tank roof is unknown and access onto the roof of a fixed roof tank is authorized by a permit.  The conditions for safe access must include the wearing of a safety belt and lifeline with two people on standby at the normal dip point.

5.7       Sampling Containers and Dip Tapes

Sample containers and the plummets of dip tapes should be made from a non-sparking, low resistivity materials.  Due to the possibility of static electrical buildup, nylon or similar synthetic ropes should not be used for sampling.  Manila or sisal ropes are permissible.

5.8       Oil Spillage

Oil spillage on tank roofs and dipping platforms should be avoided.  If spillages occur, they must be cleaned up immediately as they pose a safety hazard.  Cleaning materials should be removed from the roof as soon as practical.  Rags and other cleaning materials are used for wiping tapes, cleaning sample bottles, etc., and must not be left on tank roofs or in the tank bund but be properly disposed of.  This is to keep the area clean and prevent fire from the spontaneous ignition of hydrocarbon soaked material.  The use of nylon or other synthetic material can lead to static electrical buildup and should be prohibited.

5.10     Sumps and Pits

All sumps and pits should be completely covered with an adequate type of flooring (steel or chequer plate) or be fenced of to restrict entry.

5.11     Emptying Tanks

Tanks must, as far as possible, be emptied of their contents by means of normal connections or drain lines.  A flexible hose can be used to pump away the contents providing they are at a temperature below the spontaneous ignition temperature of the product.  If the manway is to be removed to allow product removal, hand dip the tank first to check that the product level is below the bottom of the manway.

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5.12     Isolation

By removing a valve or a section from each connecting line and its junction with the tank as near to the shell or wall as possible and sealing off the line with a standard blank flange.

By inserting standard blank flanges as close to the tank as possible, with the line block valve isolated, depressurized and drained.

Sumps can be isolated using expandable rubber plugs.  If the line is pressured some other form of blanking is required.

Under no circumstances is it permissible to rely on block valve isolation when personnel have to enter the tank.

When opening manholes or breaking lines on a tank, the possibility of pyrophoric scale being present must be considered.  Steam or water hoses must be available if pyrophoric scale is suspected to dampen it down.  Care needs to be exercised in the case of tanks that have contained crude oil, gas, and some distillates as sulphur compounds present form an easily oxidizable that is often pyrophoric.  When breaking any line, the line must be isolated, drained, depressured, with toxic materials identified so that appropriate safety gear can be used.  Then the permit can be issued to carry out the work.

5.13     Gas Freeing

Following the isolation of a tank entry is only permitted under conditions clearly defined by a permit.  To gain free access, the tank must be gas freed.  The following methods can be used to remove vapours, flammable gases or liquids, either singly or in combination.

This method is employed when the use of steam or water is impracticable or undesirable.  Where ever possible, forced ventilation should be used so that flammable vapours can be cleared in the shortest possible time.  Fans and extractors should be of flameproof construction.  A blower fan can be positioned at the manway and an extractor positioned on the roof manhole so that extracted air is not discharged at ground level posing a possible safety hazard to people moving around the base of the tank. 

5.14     Entry

Before any person is allowed to enter a tank, a permit must be issued showing the conditions of entry with these conditions being clearly understood.  Where breathing apparatus is stipulated on the permit the following conditions apply:

Breathing apparatus and lifelines are to be worn by all personnel entering the tank.  Canister respirators are not acceptable in any circumstance.

A tank is not certified as safe for entry without breathing apparatus unless:

It is completely isolated and effective steps have been taken to prevent the ingress of vapour.

It has been adequately ventilated  with an adequate supply of air and negative gas test with 20.8% oxygen.

Any sludge or other deposits that could give off vapours have been removed.

In tanks that have contained leaded spirit, the recommendations of Octel/Ethyl are observed

The tank must be opened at the top and the bottom to promote a natural draught, preventing the accumulation toxic vapours and providing an alternative exit in cases of emergency.

5.15     Removal of Sludge and Scale

After a tank has been opened up, as much oil, sludge and scale as possible must be removed by means of bailers, squeegees, etc., aided if convenient by adequately earthed water hoses.

All pyrophoric scale must be removed quickly offsite in suitable containers.  During the time that it is exposed to air while in drums, the scale must be wet with water.  Following the removal of sludge and scale it must be tested to confirm that it is gas free.

5.16     Entry After Gas Freeing

Before personnel are allowed to enter a tank after gas freeing to perform inspection or maintenance work, the permit writer must be satisfied that the tank is completely isolated, that no pockets of gas remain and that no sludge is left that is likely to give off significant quantities of gas or vapour.  When these conditions have been met, the permit can be issued for entry.

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