Feature Enclosure materials
durable, but flexible, surface finish with a high resistance to weathering and hydrolysis loads (this kind of coating is also widely used in boat building). The gel coat is also much more resistant to UV than plastic materials, such as the acrylic paints used by many metal cabinet manufacturers. To maximise UV protection, Intertec applies a thick gel coat surface layer. Depending on the housing’s intended location and environment, it is typically between 400 and 800 micrometres (0.4 to 0.8mm) thick. In contrast, the typi- cal thickness of paint sprayed on steel enclosures is only 50 to 70 micrometres – an order of magnitude less. Even the acrylic paint finish of cars and trucks is generally only 70 to 80 micrometres thick!
Figure 2 shows a large cabinet from the Bayernoil refinery, containing fluid sighting tubes. Again, there is no sign of mater- ial degradation – note the absence of any corrosion around the door hinges or lock mechanism – and close-up examination reveals that the surface finish is much the same as when it was manufactured over 40 years ago, with no loose fibres.
Enhanced protection
Research into the ageing effect of weather on GRP has shown that, in the main, any long-term degradation is caused by mechanical forces and not by chemical interaction between the material and its environment. Over time, the combined effect of UV-induced surface embrittlement, water diffusion and erosion can cause bundles of glass fibres to be exposed, rendering them susceptible to direct water ingress. Repeated expansion and contraction of the water then causes cracks, which eventually penetrate deep into the material. The scanning electron microscope images in Figure 3 show the cross-section and surface of a nominally 1mm thick GRP sheet (of a type used by Intertec, but without the protective coating that Intertec applies) after 15 years of exposure to the weather. About 50 micrometres has been eroded from the surface, which is further damaged to a depth of 20 to 30 micrometres. These results can be linearly extrapolated; after 50 years approximately 250 micrometres will have been removed from the surface, with damage extending to a depth of about 80 micrometres. In other words, the sheet’s effective thickness will be reduced from 1mm to 670 micrometres, with a proportionate loss in strength of about 30%. However, virtually all Intertec protective housings – regardless of whether they are constructed with a single wall or a double wall containing a polyurethane foam core – utilise GRP sheeting that is at least 4mm thick. Even after 50 years, surface degradation will only reduce the effective thickness of the sheeting to 3.67mm and result in a strength loss of about 8% – a negligible amount that has no affect on the housing’s structural integrity. For maximum long-term protection, this marginal weather- induced damage can be prevented by applying a layer of UV resistant gel coat to the surfaces of GRP panels. The chemical composition and thickness of the gel coat are critically impor- tant to an enclosure’s long term stability. Any differences between thermal expansion, for example, could cause the coat to ‘creep’ and undergo excessive thinning.
Intertec uses a specially-developed pre-accelerated gel coat that fully matches the properties of GRP. Made of unsaturated polyester resin, it is applied as a spray before the polyester in the GRP panel is fully cured. The base component is a pure isophthalic acid resin, dissolved in styrene and HEMA (Hydroxyethylmethacrylate) monomer – the styrene content is about 30%. The gel coat chemically bonds with the polyester resin of the GRP panel. After curing, it provides an extremely
16 Figure 2:
The gel coat of this GRP cabinet is still intact and there are no loose fibres, 40 years after
installation at an oil refinery
The only degradation that occurs with this gel coat is a very slight thinning over time due to the effect of UV radiation. Typically, this only amounts to perhaps 100 micrometres (0.1mm) over 30 years and has no effect on the housing’s structural integrity, stability or function. Even under extremely hostile conditions that combine very high UV levels with sand or dust storms, which slightly increase gel coat loss rate through abrasion, there is no change in the performance of the underlying cabinet or shelter. For example, over the past few decades Intertec has supplied thousands of shelters for protecting instrumentation at oilfields in desert regions such as the Middle East. None of these has suffered material degradation, other than a slight chalking of surface finish.
Guaranteed performance
Having manufactured GRP-based field protection enclosures, cabinets and shelters for nearly 50 years, Intertec has acquired extensive empirical evidence to demonstrate its contention that GRP is a far more suitable material than metal for most outdoor field instrumentation protection applications, especially for projects with long life cycles, harsh media and where corrosion and UV exposure need to be considered. Continual research and development has enabled Intertec to progressively extend the life, performance and diversity of its products, and the company now offers one of the most extensive ranges of heating, cooling, fire-safe and explosion- proof field protection solutions in the industry. Improvements in materials, such as stronger GRP epoxy resins, foam insulation with lower heat transfer coefficients, and new formulations of gel coat with even higher UV resistance, have been combined with advances in production technology that ensure very tight control of the manufacturing process. Nowadays, for example, a patented CNC-based manufacturing process allows enclo- sures to be produced to application- specific sizes on demand. Using special software on Intertec’s website, instrumentation engineers can config- ure an application-specific housing to
Figure 3:
Cross section and surface of a GRP sheet after 15 years’ exposure to weather
millimetre accuracy and import the data into their CAD system for layout optimisation. The same data is subsequently used to control automated manufacture of the housing.
A long working life
As a result of its experience and expertise with GRP materials, Intertec guarantees that all its field cabinets and shelters have a working lifetime of at least 30 years under normal industrial conditions. These conditions include environments where salt, sour gas or sulphuric acid are present in the atmosphere – such as the offshore oil or coastal processing plants where many instrumentation engineers and metallurgists are cur- rently investing a lot of time and energy in specifying and adopting improved materials to extend equipment lifecycles.
Intertec Instrumentation T: 0800 756 1102
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www.intertec.info SUMMER 2013 Enclosure & Safety Solutions
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