Feature Enclosure materials The challenge: avoid corrosion


Most designers protect field process instrumentation and control equipment against the effects of its environment by using some form of enclosure. But if, as is often the case, the environment contains


corrosive chemicals, the long-term integrity of the enclosure can be threatened. For any application with a long lifecycle, it is therefore essential that the enclosure fabrication material does not contribute to, or suffer from, corrosion. Klaus-Dieter Meyer, vice president of Intertec, reports


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irtually all field instrumentation requires protection against environmental factors such as extreme heat or cold, wind, rain, snow, dust or sand. The specific requirements are dictated by the type of instrumentation and the climatic conditions at its location. If the atmosphere is corrosive due to the presence of natural or manmade chemi- cals, the equipment housing needs to be impervious to their effect, otherwise its protective integrity will be compromised. Perhaps the most obvious example concerns offshore oil and gas exploration and production platforms, where the salt in seawater spray – which is predominantly sodium chloride (NaCI) – accelerates corrosion. The effect is exacer- bated by galvanic corrosion, which occurs where dissimilar metals come into contact, such as the pipework entry ports in steel enclosures. If a sour or acid gas such as hydrogen


sulphide (H2S) or carbon dioxide (CO2) is present, this further compounds the problem.


As companies strive to recover oil and gas from depleted resources there is a trend to design offshore platforms and coastal processing plants with a longer service life, making corrosion prevention even more of an issue. Experience of metal-based instrumentation housings in salt laden environ- ments, which can suffer severe degradation, is leading many designers to investigate use of better enclosure fabrication materials for long life-cycle projects.


GRP material benefits


Despite increasingly stringent fugitive emissions standards, many petrochemical plants and oil refineries still create localised corrosive atmospheres. An inevitable by-product of ‘cracking’ heavy oil by heating it to produce lighter hydrocar- bons is the production of large quantities of sulphur dioxide


(SO2), which combines readily with water vapour and oxygen to form highly corrosive sulphuric acid (H2SO4). Even though this is a well-known problem, many refineries still house field instrumentation in painted steel enclosures,


which are particularly susceptible to rusting and H2SO4 corrosion, and can have a lifetime of just a few years. It was recognition of the limitations of using metal materi- als for field equipment housings that led to the founding of enclosure specialist Intertec in 1965. From the outset, the company chose to base its products on glass fibre reinforced polyester (GRP), which is inherently inert and virtually immune to corrosion and the effect of atmospheric pollutants


such as NaCI or H2SO4. It is also resistant to a wide range of petrochemicals, including benzene, butylene, ethylene, napthalene, propylene, toluene and xylene. As a building material for environmental protection enclosures, it has numerous advantages over metal. Almost as strong as stainless steel but with only a quarter of its weight, GRP has a much higher thermal resistance – making it easier to insulate against heat and cold – and because the material is an electrical insulator, it does not contribute to galvanic corrosion.


Proven longevity


The longevity of GRP enclosures, cabinets and shelters is well proven. To date, the company has produced over a million


Enclosure & Safety Solutions SUMMER 2013


Figure 1:


Despite 40 years of exposure to an atmosphere containing


sulphuric acid, after cleaning (bottom right), the surface of the GRP enclosure was undamaged


protective housings, many of which are sited in areas where corrosive chemicals are present in the atmosphere. As an example, there are more than 4,000 Intertec GRP enclosures and cabinets installed at the Al Taweelah seawater desalina- tion and power plant in Abu Dhabi. Many of these have protected instrumentation for more than 20 years and show no sign of degradation, despite prolonged exposure to salt and very high levels of UV radiation from the sun. Another example of their resistance to salt comes from the Irving Oil refinery in Saint John, New Brunswick. This is Canada’s largest oil refinery, and Intertec supplied nearly all its instrumentation enclosures. Many of these have been in service for as long as 30 years and show little evidence of ageing, other than a slight roughening of surface finish – which is often referred to as ‘chalking’ or ‘frosting’. This is despite a particularly hostile climate, combining salt air with very hot summers, extremely cold winters and rapid changes in temperature – during spring, the temperature can change by as much as 40˚C in a single day.


The long-term immunity of GRP enclosures to damage from petrochemical media is demonstrated at the Bayernoil refinery complex in southern Germany. When the oil refinery was first constructed in the mid 1960s, Intertec supplied a large quantity of two-part enclosures to protect process transmitters distributed across the site, as well as cabinets for larger instrumentation clusters. Following a recent instrumentation upgrade, a number of these original enclo- sures became available for detailed inspection and analysis, allowing Intertec to assess the effect of long-term exposure to SO2 and H2SO4 on GRP.


Figure 1 shows a Multibox enclosure containing pressure regulation and temperature monitoring instruments that had been in continual service at the refinery’s cracker furnace for more than 40 years. No maintenance had been carried out on the enclosure – nor was it needed – during this time. Although there is surface rust on some of the pipework connections and mountings, the instruments are clearly still functional and the lid seal is intact. Aside from requiring a new window seal, the enclosure itself has suffered no degradation. The close-up view shows that, after cleaning, its surface is free of blemishes. Although these field protection housings are constructed almost entirely from GRP, they inevitably include some metal components, such as internal instrumentation support rails, door hinges and clasps, and external mounting clamps. For corrosion resistance, Intertec manufactures all these compo- nents from 316 stainless steel or hot-dip galvanized metal.


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