Storing-up a load of trouble

Lee Forbes, managing director of Forbes, outlines the problems and solutions for the design of composite and thermoplastic tanks, particularly in the area of hazardous chemical storage

Despite increased regulations and directives, and more rigorous implementation, the storage of chemicals continues to present a health and safety and environmental risk. In response to these pressures, an increasing amount of advice is available on the safe design, construction, installation and operation of chemical storage facilities. Prosecutions brought by the Health & Safety Executive (HSE) and the Environmental Agency (EA) are carrying harsher penalties, with magistrate's courts able to levy fines of up to £20,000 per offense, many incidents give rise to more than one charge, with unlimited costs. The more serious occurrences are sent to crown court where the powers, including imprisonment, are unlimited.

Unfortunately a culture has developed in some industries of selecting a specific product purely on price, and often the purchaser is not fully aware of their chosen product's suitability for the safe, long-term storage of hazardous chemicals. This can be further compounded by the tank supplier incorrectly advising the user or taking short cuts in specification and build quality to reduce costs. This can put the supplier of quality products, built to the correct specifications, in a difficult position.

There are two basic types of plastic tanks used for chemical storage. All-thermoplastic tanks are excellent, simple units for small-volume storage. The thermoplastic acts as a chemical barrier as well as a load-bearing structure. Composite tanks are built to meet the rigorous demands of BS 4994-1987. The standard includes allowances for wind loadings, fill/empty cycles, snow, personnel and equipment loadings, negative and positive pressures induced during filling and emptying and the integrity of nozzles and their allowable loadings. Due to a lack of defined design methods and simplistic design codes for thermoplastic tanks, plus the overall fragility particularly of nozzle connections, applications for all-thermoplastic tanks are often limited and a capacity restriction is applied.

Another major consideration is access. All chemical storage tanks should be subject to strict scheduled inspections as outlined by the HSE. It is important these include internal inspections and testing where appropriate. Many companies, in making a risk assessment for entry into a confined space where hazardous chemicals have been present, discover access through a top-mounted manway is unacceptable. Low-level, side-mounted access is essential in these instances. The practicality and cost of fitting a side manway in a composite (GRP) tank presents little problem. A full design method for such an access is defined in BS 4994-1987. To fit a low-level manway in a large all-thermoplastic tank presents problems in terms of cost and verifying the design.

A safe storage facility consists of primary storage tank with secondary safe containment (bunding) to protect against leakage. Assuming the storage tank is correctly designed, manufactured and installed, there are pitfalls in defining the secondary containment system. Bunding can induce further hazards if not correctly specified.

It is important the bunded area incorporates tank connections and local ancillary items such as valves, pipework and pumps. Bunds must not have open drains unless piped to a containment/treatment tank and effluent disposal system. Secondary containment is achieved in a number of ways. The traditional method is to install the tank in a bunded area with 110% of the tank capacity. This can be a civil construction in brick and/or concrete with a suitable resistant coating or liner. Integral bunds in a similar material to the tank can be used. The tank is normally bolted into the plastic bund complete with outlet and inlet pipework and valves. These can be supplied with or without weather shrouds to prevent water ingress. Total containment is a new concept where a tank complete with all local pipework, valves and equipment is contained within a second larger tank.

Some consider double-skin (wall) tanks to be adequate secondary containment. This is only applicable to a tank with all connections above the contents level i.e. in the roof of the tank - underground petroleum garage forecourt tanks for example. With this type of double-wall structure the outer wall must be adequate to act as a containment tank in its own right and must have a fail-safe alarm between the skins. It is imperative the nozzles and ancillary equipment around the tank are within the secondary contained area, which is unfeasible in a double-skin tank.

Investigations have shown many leaks have occurred due to the immediate ancillary equipment failing or lack of operator procedure control, for example overfilling. Common causes of hazardous liquid escapes into the environment are:

  • tanks not designed or manufactured to the appropriate standard,
  • construction materials unable to provide long-term corrosion resistance, often because the manufacturer is only able to handle a limited range of materials,
  • contamination or incorrect specification of the contents,
  • floatation of tank in bunded area. Ideally a bund should be completely dry. If this is impractical it is important no rainwater accumulates in the bund area.

Accumulation can decrease the usable volume and create floatation or upthrust on the base plate of the storage tank, which will almost certainly lead to failure unless the tank is specifically designed to withstand this type of loading,

  • overfilling of the tank causing product to enter the bund. If product is corrosive, the valve/pipework bolting system can suffer rapid attack, causing joints to fail and the bund to fill. High-level fill warnings are a must. Overflow should be directed into a locally confined area within the bund,
  • sealing off the vent by overfilling causing a vacuum in the headspace,
  • pressurising the tank by inadequate venting. This is particularly relevant when using air pressure discharge from road tankers,
  • poor installation, the base supporting plinth not flat for example. The plinth must be perfectly clean and smooth. Unless otherwise specified, on vertical cylindrical tanks the base must be fully supported,
  • damaged by vacuum or pressure due to the vent being attached to an inadequate fume scrubber. The tank supplier should specify and provide the scrubber,
  • jetting of liquid over the top of a low bund from a leak high in the vertical wall of the tank. The bund must be of adequate height to prevent this happening,
  • bunds not being of adequate design strength or suitably corrosion resistant,
  • testing the bund and the tank with water then draining down the tank before the bund,
  • bolting heavy, unsupported valves and pipework to tank connections. Buyer beware

    There are a number of points tank purchasers need to take into account. It must be ensured the tank and secondary containment are designed to the correct standard and made from appropriate materials. Seek advice and obtain guarantees from the supplier. If in doubt seek third-party advice from insurers or design houses. Third-party inspection is a requisite if handling dangerous chemicals in any substantial volume. The parameters are laid down in BS 4994 -1987.

    It is best to stick to an experienced, credible supplier able to demonstrate applications by reference to satisfied long-term customers. Consider the range of materials the supplier can offer, this may limit their ability to offer the correct specification. Check the supplier's liability insurance cover and asset worth. There are manufacturers who will give guarantees which are worthless because the company has no assets. Remember that under Common Law, 'fit-for-purpose' is a long-term guarantee in itself.

    Carry out a risk assessment on the location, operation and what provisions are required in the event of a leakage and ensure all valves and pipework connections are adequately supported and protected from damage. Where practical make the supplier responsible for transport, off loading and installing the tank, and scrubber if specified. These areas are where the blame for a failure often falls. Ensure full operational procedures are issued, including an inspection schedule and calibration check on level control devices. Certain sectors of the industry must accept more responsibility in ensuring the design and operation of chemical storage and process facilities have total integrity through design, manufacture, installation and operation

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