Composites tackle tank repair

Tank and vessel repair has traditionally relied on welding techniques, the alternative being complete replacement. Both options are undeniably costly and require the plant to be shutdown.

Now, with the development of its advanced composites technology, FD Alliance (FDA) is offering a fully validated and warranted repair and structural strengthening option that can be applied on-line, potentially saving customers millions of pounds.

Tanks and vessels may not, strictly speaking, be plant machinery, but they remain fundamental plant assets. A leak could have significant effect on plant efficiency. In the past this has been a costly problem since the only solution available has been to either replace the container or shut down and weld.

Either way, the inconvenience and cost is considerable. Weld repairs to tanks and vessels generally require post-weld heat treatments in order to reduce the residual stresses in the welds and avoid embrittlement - no small task as it will usually involve draining the entire tank before the heat is applied. In addition, the cleaning requirements before the tank can be refilled are stringent and time-consuming, incurring substantial cost and loss of production. Damage to internal linings can also be caused during weld applications, and - where an internal weld is required - health and safety demands thorough cleaning of the container.

A recent example at a Glaxo SmithKline pharmaceuticals plant illustrates just how crucial avoiding this cycle of shutdowns can be. Internal erosion was discovered in the upper and lower jackets of a glass-lined chemical reactor vessel at the plant in Montrose, Scotland. The exacting operating conditions of the vessel - which is subject to frequent changes of temperature - meant that repair options available were limited.

David Ward, Plant Engineer at GSK, explains: 'To replace the vessel would have been extremely costly because it required revalidation of the whole process. And there would be no guarantee that a new vessel would achieve the same exacting conditions required by the batch process.'

It's in situations such as this that FD Alliance (FDA)'s composites technology comes into its own. The FD Alliance was formed last year as a collaboration between engineering solutions company Furmanite International and DML Composites, with the primary objective of offering bespoke, engineered composites solutions to the offshore industry to keep assets earning. However, as confidence in composites has grown, application methods have been developed and further research has been undertaken, composites materials' benefits are proving to have far reaching potential.

The Glaxo SmithKline reactor vessel and its jackets are classed under the Pressure Systems Regulations, so the plant's insurer had to give complete agreement to the design and methodology agreement of the repair before any engineering work could be contemplated. In an industry where validation of the manufacturing process is critical, and replacement of plant components can necessitate complete revalidation, on-line rehabilitation (restoring full structural and pressure integrity to existing plant) was a valuable alternative to shutdown and replacement.

A particular feature of this vessel is the rapidity of thermal cycling - the temperature can switch between -26 degrees C and +134 degrees C in as little as five minutes. These factors, coupled with the vessel's complex geometry, posed considerable problems. FDA's advanced carbon fibre composites repair system not only overcame these, but also provided a repair that was designed to restore full structural and pressure integrity, even given total loss of the jacket steel.

The materials used by FDA for such composites repairs are generally carbon fibre and epoxy resin, selected for their exceptional strength - up to ten times that of steel with twice the stiffness, yet less than one quarter the density - and corrosion resistance. Application requires no hot work, materials are lightweight and easy to handle and no prefabrication is required. Furthermore, carbon fibre will not allow corrosion to progress beneath the repair and does not suffer UV disintegration (as glass fibre and Kevlar are known to), making the finished repair as good as full replacement, but without the downtime or safety risks inherent with other options.

These benefits all had a role to play in the recent implementation of a repair to a mild steel storage tank (at the Aughinish alumina refinery, Ireland) containing caustic soda, which was found to be suffering severe corrosion. The 56-plate, eight-level tank showed severe wall thinning and numerous through-wall defects, mainly around the top three levels of the 21 metre high structure.

In this instance FDA put forward a composites solution package, ensuring full structural strengthening and avoiding the need for replacement or hotwork, with additional allowance calculated for the vibration caused when the tank is filled since the repairs would have to cope with any resulting fatigue.

FDA developments allow composites to be applied as a patch to the damaged area, rather than being wrapped around the circumference of the tank/vessel - a method used to effect the Aughinish repairs.

In undertaking the initial design and engineering for the repair there are also a number of additional criteria to consider when working with tanks and vessels. Flexing, roof weight and liquid content are each precisely calculated, allowing the resin matrix, repair thickness, and minimum bond lengths to be accurately planned for maximum strength on application.

In all applications surface preparation is key to achieving optimum bond strength for the finished repair and is generally carried out using grit-blasting, requiring a finish of SA2.5 (75 µm). A glass fibre tie-coat is then applied, creating a high quality interface between the carbon fibre to be applied and the substrate and providing electrical insulation to avoid the possibility of galvanic reaction.

With these preparations in place, the resin-impregnated carbon fibre is applied in layers. The resin matrix is pre-calculated and the layers are built up according to a specific design thickness. Usually about 5mm is enough to restore full structural strength and pressure containment but this varies according to the level of damage and strength required. In the case of Aughinish the number of layers varied from just two (approx 2.2 mm) to 19 (approx 20.9mm). In addition, each area is repaired with a pre-determined area of overlap from the circumference of the defect to again ensure optimum bonding.

Before curing a sacrificial peel ply layer is applied to remove excess resin and provide a surface finish that will accept further treatments should they be required.To date several large applications have taken place in industries from chemical to oil and gas, and so far it has been possible to carry out work on-line in every one, avoiding lost production in every instance.

The speed of response and mobilisation for this technology is a further benefit, as illustrated by an application at a chemical installation last year when a gas container was found to be suffering through-wall corrosion. Work was actually completed over a weekend without any need for shutdown.

Following a visit from an engineer for an assessment of the job a design and specification was immediately prepared, enabling the all-clear to be given by site operators in just 24 hours. FDA technicians were mobilised to carry out the repair just three days after the initial site visit.

Although an order had already been placed for a new flare drum, replacement has since been put on hold as the repair has been validated for a six year life span.

Jim Cuthill is business development manager at FDA