Requalification of Type IV Pressure Vessels in CNG Service

FIBA was granted special approval by the Department of Transportation to requalify Type IV pressure vessels manufactured by Hexagon Lincoln using hydrostatic testing under Special Permit 14951.

This approval allows owners of Hexagon Lincoln CNG Bulk Hauling Titan Modules to comply with the manufacturer’s 5 year requalification requirements.

Hydrostatic testing is a proof pressure test method that demonstrates a vessel’s ability to safely perform at its specified operating pressure. For Titan Module’s this involves individually testing each vessel at a pressure of 5439 psig using water.

Although hydrostatic testing is a key component of requalification, many steps are taken in the process to ensure safe and proper handling of the vessels along with visual inspection of the product and subassemblies.

Below is a detailed overview of the steps taken to re-qualify a Titan Module:

The module(s) are received in to a quarantined area of the facility until it is inspected for residual content and pressure before processing.

Excess product must be removed to safely handle and test each vessel. The unit is first inspected to see if pressure is present within the unit. If pressure is found to be present, it is assumed to be flammable and arranged for proper removal. Once the majority of the flammable product is removed and vessels are drained to atmospheric pressure, purging with nitrogen will then take place to remove all residual flammable product. The vessels and piping will continually be purged with nitrogen gas as needed until they are deemed to be in a non flammable and non hazardous state.

Visual inspection is thoroughly performed by a qualified technician to identify any concern(s) surrounding the module frame, vessels and components. The frame is inspected to confirm there are no present issues with welds, structural anomalies or corrosion. The plumbing is inspected for leaks and any signs of damage. The fire safety system is evaluated to confirm that it is in proper working condition with no signs of damage. Prior to removal, the exposed areas of the vessels are also inspected for damage.

All components are properly marked to identify orientation prior to disassembly and removal. Valves, piping and instrumentation are then removed along with top panels and upper support members of the module.

The vessels’ orientation is recorded and documented prior to removal.  Each vessel is then lifted using nylon slings with an overhead crane with sufficient capacity.

Prior to positioning vessels being removed from the Module, proper padded supports are arranged to evenly distribute the load of each unsupported vessel. This is particularly important throughout water filling, hydrostatic testing and water draining phases.

External inspection of the vessel is performed by trained technicians to identify cuts, scratches or abrasions of significance. The technicians also look for any signs of fire, impacts or potential leakage on the exterior. The full surface of the vessel is analyzed in a well lit area outside of the module and aided with hand held lighting as necessary. The polar bosses are inspected for cleanliness, wear, scratches, gouges and any broken mounting fasteners. The external inspection is well documented and included in the quality package.

The internal surface of the vessel is inspected with the aid of high lumen LED lighting. Trained technicians look for any signs of damage such as blistering, cracks, pin holes, inward buckling, creases or any other type flaw as well as contaminants or debris on the internal surface. The “O” ring boss at each end of the vessel is inspected for any scratches or gouges and are wiped clean of any oil or debris. The internal inspection is well documented and included in the quality package.

Prior to hydrostatic inspection, any residual oil and debris is removed from the vessels. Each vessel is then filled with filtered water. Careful effort is made to ensure no air is left remaining in the vessel prior to hydraulic pressurization. In addition, precaution is taken to ensure that each vessel is only depressurized below 150 PSI for under 24 hours.

A certified hydrostatic test operator then performs the hydrostatic test. Using suitable hydrostatic testing equipment the vessels are then pressurized with water to the required proof test pressure. Proof test pressure, once attained must be maintained for a minimum of 3 minutes without pressure decaying more than 20 psig within the 3 minute hold time. After hydrostatic testing is completed the pressure is then slowly drained until the pressure within the vessel(s) is 0 psig. The hydrostatic inspection is well documented and included in the quality package. During hydrostatic testing, pressure increase speed must not exceed 200 psi/minute.

After hydrostatic testing is complete, all water is drained and recycled. The vessel is then inverted and all residual water and contaminates are removed. The vessel is then dried using dry, oil free compressed air to achieve the appropriate moisture level in the vessel.

Re-qualified vessels are reassembled into the module to the same stacking orientation that was documented prior to disassembly. New rear tube mounting fasteners are installed and tightened to the manufacturers recommended torque specifications and tightening sequence.

All tank head valves are fully dissembled, cleaned and rebuilt. All valve soft goods as well as the balls and stems are automatically replaced as most valves at time of tube requalification show significant signs of wear and leakage.

All components are then reassembled on the vessels and module in accordance with the manufacturers recommended installation specifications.

The assembled modules valves and manifold components are pneumatically pressure tested to the MAWP of the vessels up to the tank head valves only and again thoroughly inspected for leaks. The tank head valve to pressure vessel interface is leak tested at time of initial first fill by the customer.

Documentation is prepared by the Quality Assurance Department, which records all critical steps, inspections and test measurements to confirm comprehensive and qualified requalification of the module. The unit’s pressure vessels and data plates are then marked to record the date of requalification.