Till 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. As per OISD Standard -150 “Design & Layout for Mounded Storage Vessels”, coatings listed by NACE International Technical committee Report T-10D-17/T-6A-63 “Coatings For the Repair and Rehabilitation of the External Coatings of Buried Steel Pipelines” may be used. Performance criteria for buried pipeline and tanks are identical with the special consideration that the coating is applied “on site” rather than in a coating plant – hence the special relevance of the particular NACE standard. Additionally OISD Standard 150 requires that the coating system must be “suitable for the design conditions” i.e. service temperature of (-) 27^oC to (+) 55^oC and for LPG and (–) 43.5^oC to (+) 55^oC for Propane.

As per service temperature data of coatings provided in NACE T-10D-17/T-6A-63, conventional coatings like Epoxy, Coal Tar Epoxy and 100% Solids Tar Polyurethane (Pur-Tar) do not meet the requirements of design condition.

The coating of choice across the entire industry is 100% Solids (Non Tar Extended) Polyurethane Coatings. Stringent specifications for coating materials performance and application procedure including QA/QC have been drawn up by major users like Indian Oil Corp. Ltd (IOCL), Bharat Petroleum Corp. Ltd (BPCL), Hindustan Petroleum Corp. Ltd (HPCL) etc.

100% Solids (Non Tar Extended) Polyurethane Coatings meet all the criteria of NACE RP 0169, which form the basis of coating selection for NACE T-10D-17/T-6A-63:

• 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^® 386/9000, 100% Solids (Non Tar Extended) Polyurethane Coatings meet all the above criteria, OISD service temperature requirements as well as specifications drawn up by users like IOCL, BPCL, HPCL etc.

• Smaller Sized Bullets : The entire fabricated bullet is provided for coating after necessary NDT testing like X-Ray and after stress relieving. The bottom 120^o is positioned sideways for surface preparation, coating and testing. After completion of the bottom 120^o, the bullets are straightened and hydro-tested. Thereafter the top 240^o is coated.

• Larger Sized Bullets : Bottom 120^o of individual sections (normally 3 sections to a bullet) is coated after NDT / SR leaving an area extending approx. 1500 mm from the edges un-coated. The sections are fabricated together and hydro-tested. Thereafter the bottom 120^o of the weld seam area as well as the 240^o top areas is coated.

Amchem recently completed coating installation for IOCL, a Fortune 500 company, at four project locations - Chengelpet (TN), Coimbatore (TN), Una (Himachal Pradesh), and Quilon (Kerala). Several more projects are scheduled for different users. Detailed QA/QC Procedures and Field Quality Plans were drawn between Amchem, user and third party inspection agency (M/s BVIS for Una and M/s PDIL for other locations).

Surface was blast cleaned to SSPC SP-10 with an anchor profile of 75-100 Microns using granulated copper slag abrasive, primed using Purethane^® AR Primer and coated with Purethane^® 386/9000 to a thickness of 1,000 microns. Field tests were conducted for film thickness (DFT – ASTM E 376) using Positest / Elektrophysik gauges, for Adhesion (ASTM D-4541) using Positest AT-CM and for Hardness (ASTM D-2240) using Durometer with 100% results conforming to specification. The coating was tested for holidays in accordance with NACE 0188-90 using Tinker – Rasor high voltage holiday detector and a pinhole free coating was provided.

The projects at Quilon and Coimbatore were a race against time due to impending onset of the Indian monsoons. Amchem’s skilled application crew and state of the art high capacity equipment did an excellent job and met the target, despite high humidity levels and commencement of sporadic rainfall. Top 240^o area of the Quilon and Coimbatore projects were blast cleaned, primed, coated and tested in an amazing 7 and 9 days respectively!