Mounded Bullets & Underground Steel Tanks External Coating.Purethane coatings are the industry standard for external coating of buried tanks such as Mounded LPG Bullets and buried fuel tanks. They have all the protection properties of an outstanding pipeline coating, combined with the ease of onsite spray application to large tanks.
Criteria for selection of coatings for Mounded Bullets / Buried are essentially the same as for Pipeline coatings as enumerated in NACE RP 0169 with the special consideration that the coating is applied “on site” rather than in a coating plant.
- Effective electrical insulator.
- Effective moisture barrier.
- Can be applied with minimum defects.
- Good adhesion.
- Ability to resist development of holidays with passage of time.
- Ability to resist damage during handling, storage and installation.
- Ability to maintain substantially constant electrical resistivity with passage of time.
- Resistance to disbondment.
- Resistance to chemical degradation.
- Ease of repair.
- Retention of physical properties.
- Non-toxicity to the environment.
Purethane coatings meet all these criteria and are the industry’s coating of choice for these applications. More than 500,000 Buried Fuel Tanks have been coated in North America under the STI-P3 programme with 100% Solids Polyurethane Coatings.
Mounded BulletsTill recent years, bulk storage of highly inflammable Liquefied Petroleum Gas (LPG) was being done in above ground storage tanks (Horton Spheres). However some major fire / explosions underlined the need to review the design, procedure, maintenance, fire fighting and safety aspects of LPG handling.
A safer option was introduced in the form of Mounded LPG Storage Bullets since it provides intrinsically passive and safe environment and eliminates the possibility of BLEVE (Boiling Liquid Expanding Vapor Explosion). Mounded LPG Bullets are large, buried, horizontal cylindrical steel tanks with dished ends of size ranging between 3.5 to 7.0 meter diameter and lengths of 35 to 70 meters or more.
Mounded bullets allow storage of large quantities of LPG - up to 2,000 MT or more in a single location! The cover of the mound protects the vessel from fire engulfment, radiation from a fire in close proximity and acts of sabotage or vandalism. The area of land required to locate a mounded system is minimal compared to conventional storage.
Corrosion protection of mounded LPG bullets is critical and the highest performing coatings combined with impressed current cathodic protection system are employed. Additionally safety standards require that the coating system must be “suitable for the design conditions” i.e. service temperature of (-) 27 °C to (+) 55 °C and for LPG and (–) 43.5 °C to (+) 55 °C for Propane.
As per listed service temperature data, conventional coatings like Epoxy, Coal Tar Epoxy and 100% Tar Polyurethane (Pur-Tar) do not meet the requirements of design condition (low temperature).