Double Skinning A Double Skin

Project Information

At the end of last year Abfad were called back to a tank we had previously lined with our Fuelvac® double skin lining system as there was a drop in vacuum pressure on one of the gauges which monitor the tank. This particular above ground storage tank contained over 2 million litres of hazardous liquid and the double skin liner was installed in several overlapping zones each with their own individual monitoring gauge, which are attached at intervals around the outside of the tank in order to monitor the internal areas of the tank. The gauge which monitored half of the floor area was showing the loss of vacuum pressure, and is where the sump and internal suction pipe are located. We mobilized our personnel straight away to investigate the problem. Once on site all external fittings and seals were inspected around the gauge and valve assembly, to deduce whether the vacuum loss could be occurring from one of these paths. Once all joint fittings had been inspected they were re-sealed and vacuum pressure was pulled to around -800mbars, locked off at -600mbars and left to settle out.

Vacuum pressure continuing to fall
The vacuum pressure on the zone continued to fall and reached the level where it sent an alarm signal to the site control room. We returned to site and vacuum was pulled to a high level once again and checks were made to see if any stored fluid could be pulled through the double skin interstice as the tank was still over half full at this point, equating to over 1 million litres of hazardous product still being stored. No stored liquid could be detected within the interstitial space and the only smell that could be found was from the double skin lining materials themselves. The vacuum gauge was removed along with the valve assembly so that they could be tested in a laboratory environment to see if monitoring equipment was holding vacuum, as both the valve assembly and the gauge itself was proven to hold vacuum at 100% they were both refitted to the tank. As the vacuum levels on the floor area continued to drop and no external cause could be found on the monitoring equipment it was decided that the best course of action was to drain down the remaining stored fluid in order to begin internal inspections to the double skin liner. Whilst waiting for the liquid in the tank to be fully drained down we received a call from the client alerting us that one of the wall zones was now also beginning to lose vacuum pressure. Once again all external valves, gauges and equipment were inspected to ensure that the fault wasn’t with them, as no problem could be found externally with monitoring equipment the only course of action was to fully investigate both zones internally once the remaining stored product was drained and the tank was cleaned and de-gassed.

Detached suction pipe
Once we finally gained entry to the tank we were met with the revelation that the GRP suction pipe, approximately seven metres long, had detached itself from the internal steel pipe and flange near the tank wall, shearing its eight connecting bolts in the process. This pipe ran from the wall of the tank to the sump in the centre, which is all part of this floor zone area. An intense visual inspection was performed to the double skin liner in zones where there was vacuum loss, along with vacuum tests and holiday detection (Spark testing), it was clear from our inspections and tests that the detached pipe or debris from the pipe had damaged the liner whilst it was floating freely around the tank during the draining down process. Several deep scratches were found along with two pronounced slits to the coating and many other scratch marks which could only have been made by mechanical means. We believe that the detached GRP pipe or debris as it floated around the tank during draining down had been hitting off the side walls and floor area causing damage to the double skin liner. Damaged areas to floor were repaired and vacuum pulled again, however the same consistent drop in vacuum level remained, meaning another problem existed elsewhere. We also had the chance to inspect the parts of zone wall area which were visible, this particular wall zone runs from the termination point of floor zone on the side wall vertically up the wall of the tank to a height of 12 to 13 metres, and the same scratch marks and damage could be clearly seen to these areas, again we believe this damage was caused by the free floating GRP pipe/debris hitting off the side walls and damaging the coating of the liner as the tank was being drained. Repairs were made to the lower areas of wall zone, as a large portion of this zone stretches up high to 13 metres and is unreachable without the installation of scaffold, this was erected to allow us access to the full he. Repairs to lower areas of wall zone slowed the vacuum loss but didn’t eliminate it altogether, and clearly more damage existed further up. To ensure that the floor zone double skin was defect free another round of intense visual inspections and holiday detection tests commenced, including independent inspections carried out by the clients onsite NDT contractor, and no further damage was found to these areas. Discussions were held with the client as to whether over-coating both zones would solve the vacuum loss problem, as over-coating the zones would repair the mechanically affected areas.

What we recommended
Abfad’s recommendation at this stage was to continue with the inspection of wall zone once the scaffolding was erected as we were confident that we could locate all problem areas and bring the zone back to holding vacuum. However, the client’s preference was to overcoat both zones, and Abfad geared up to perform this work. Over-coating of the existing wall zone and floor zone was achieved by sanding back the existing coating and applying another layer of the protective finish coat, once dried this was inspected for any pin holes. Wall zone vacuum was pulled after the above steps were completed and held its vacuum levels as it had before the mechanical damage had occurred, the over-coating had been a success. Floor zone, however, was a different matter, as once the same over-coating process was completed this zone still continued to show the same continuous 1 to 2mbars vacuum pressure loss every 24 hours. The problem with this zone clearly did not exist with the double skin liner, and we started to suspect whatever caused the pipe to detach from flange had done further damage to tank floor plate weld(s) underneath the double skin lining system. After discussions with the client it was agreed to attempt something we had never before tried, to double skin the existing double skin on the floor zone. We knew that the existing floor zone double skin was sound from earlier inspections, this factor was critical if a vacuum was to be held between the existing double skin and the new double skin being installed, so in theory double skinning a new zone on top of the existing one should solve whatever the problem with the loss of vacuum and we would have a fully vacuumed double skin lined tank in operation again. This new area was also extended to incorporate an overlap onto wall zone on the side wall. The area was prepared and double skin materials installed, all the while thorough quality checks and inspections were carried out. Once the double skin materials were installed, which is a very quick process, the sealer coat was applied and once dry inspected for pin holes then abraded back to prepare it for the finish acid resistant protective resin coating. Once this second double skin had been fully installed and the coating dry, all quality checks and inspections were carried out and the new zone was commissioned. We now pulled the vacuum for new floor zone which held vacuum around -630mbars. Now all gauges and valve assemblies on the storage tank were holding vacuum and operating without any loss of vacuum pressure, and only original floor plate zone remains with a vacuum loss, which is assumed to be caused by a micro-crack in the floor plate weld area of that zone.

Fuelvac® doing what it was designed to do
Whilst we were eager to fix this issue for our client and have their asset back in operation as swiftly as possible, we were delighted with how the Fuelvac® double skin lining system reacted to the sustained mechanical attack which caused damage inside the tank, floor area double skin even with all the damage endured did not allow stored liquid into interstice, main cause of vacuum loss in this area we suspect was down to floor plate weld seam crack. The double skin liner operated exactly as it was designed to do and alerted the client that there was a breach in the tank after the incident occurred with tank suction pipe, allowing client to initiate action and investigate, and subsequently to authorise advanced control measures to ensure that all stored product remained safely contained. During this investigation and all repairs including new double skin installation there was no loss of product from the tank into the environment at all, either during or after this incident, demonstrating the effectiveness and success of the Fuelvac® double skin lining system.