There are many variables to consider in selecting an internal petroleum tank lining. Firstly, and most importantly, is the chemical resistance of the lining. This is heavily influenced by the temperature of the chemical (fluid) as well. In order to ensure a suitable lining is selected, the lining supplier should be given the exact chemical breakdown of the cargo by percentage of each chemical in the cargo. pH values are often given, but these are not as good an indicator as the concentration of the chemical. Commonly in hydrocarbon storage, a certain level of water is expected, and these details must also be provided to the linings supplier – the water is more corrosive to the tank and can also be more aggressive to the lining.
The operating temperatures should also be given, along with details of any anticipated variations in temperatures. Equipment will often have a design temperature and an operating temperature. Consideration should be given to both of these temperatures, as only using the design temperature can result in the lining being over-rated for the actual service and result in extra cost.
Following the chemicals and temperature, any other relevant factors should also be given – are there any anticipated contaminants (sulphur) or other chemicals (such as corrosion inhibitors) which are going to be added? Are there any solids which might cause abrasion of the lining or potential impact?
Conditions outside normal service should also be considered, such as how will the tanks be cleaned - high temperature steam, chemical cleaning agents, etc.
Tank linings can be broken down into numerous different groups. Commonly the first distinction is by the resin technology the lining is based on. For hydrocarbon storage this is typically epoxy resins, though other resin systems are used. Epoxies come in different grades from standard epoxy, through to generally higher performing epoxy phenolics and epoxy novolacs. The type of resin used is decided by the chemicals and their temperatures.
The next distinction is usually thin and thick film systems. Thin film systems tend to be solvent based and applied in multiple thin layers to allow the solvent to escape before the lining cures – a typical specification is 2 or 3 x 100µm (2 or 3 x 4 mils). These systems often have a low material cost per unit of area, but increased labor costs and extended application time. They are also really only suitable for new and good condition steel, as any pitting can lead to over thickness of the lining where the solvent cannot escape which results in solvent entrapment and potential defects in the lining. In using solvent based products to line a tank, another hazard is added to the job as the atmosphere inside the tank during application and cure will contain solvent from the lining.
Thick film systems to tend to be high volume solids (i.e. contain low levels of solvent) and therefore can be applied in a single thick film without the risk of solvent entrapment. This allows them to be applied more quickly with lower labor requirements. Due to the higher viscosity of the thick film systems, they may require more specialist spray equipment to apply. As they have ability to be applied more thickly, they can normally be applied over uneven surfaces, such as pitted steel in a maintenance turnaround filling the pits and lining the tank in a single process.
The fillers used in a lining also affect performance – glass flake is commonly used in high performance linings as it increases permeation resistance of the lining. Other fillers such as mica can be used to create good performing linings, but at a lower performance than glass flake.
Glass fibre can also be used as a filler in a lining and to increase the strength of the lining. Some lining products come with the glass flake already included in the lining, while other systems involve saturating separately supplied glass fibre in the form of mats or strands with the lining material. These glass fibre reinforced systems allow the linings to be applied more thickly (>1mm/40mils) without the build-up of stresses and loss of flexibility. They can also allow the lining to bridge holes created in the tank base by underside corrosion (the steel substrate corroding from the outside and destroying the steel to which the lining is adhered). Standards such as American Petroleum Institute (API) 652 and 653 allow the extension of the tank inspection interval if these types of reinforced linings are used.
Time taken to correctly select and specify a lining early in a project will be repaid by the lining performing as expected and offering the service life expected.