Walk-in shelters fabricated using composite glass-reinforced polyester (GRP) provide corrosion-resistant solutions for long life cycle, field-based instrumentation. Key to this application is the ease with which durable GRP sheets can be fabricated with other material layers to meet multiple performance targets including resistance to corrosion and ultraviolet (UV), and highly stable internal operating environments suitable for sensitive analyzers.
A current project on a natural gas pipeline in a semiarid American region illustrates this capability. The application requires four shelters to house analyzers that monitor gas quality at the start and end points of the pipeline. Each shelter houses a gas chromatograph and moisture and hydrogen sulfide analyzers. Because of the hazardous-area operation, the shelters are pressurized and purged to meet National Fire Protection Association (NFPA) 496 standards.
The physical locations of the shelters are in semidesert environments at altitudes of around 800 meters and 1,500 meters. The seasonal temperature extremes that have to be handled are -7°C and 45°C, along with high levels of UV radiation. With these minimum and maximum temperatures and five changes of air per hour, the load on the shelters’ HVAC systems is high, and a key goal for the client was minimizing power consumption since the local electricity supplies have limited capacity.
To meet these demands, the shelters were fabricated using a sandwich construction of inner and outer sheets of GRP enclosing a thick embedded layer of polyurethane foam insulation. This technique provides high insulation values of R-21 for shelter wall panels and R-28 for roof panels and bases.
On the hottest day for this pipeline monitoring application, the composite construction helps bring the internal shelter temperatures down to 22°C and 50 percent humidity — including the purging air changes — at the expense of approximately 1 ton or 3.66 kilowatts of air conditioning. The high levels of insulation of the composite GRP construction reduce the total power required by an estimated 15 percent compared with that used by a highly insulated steel shelter, according to an analytical calculation. The particular installation locations of these shelters have reliable electricity supplies available; however, passive cooling systems can be fitted for shelters that operate in remote arid regions to simplify installation.
The polyurethane insulation layer is bonded inside the GRP sheets. This fabrication approach eliminates "thermal shortcuts" between the interior and exterior that can result from the assembly fixings often used with traditional metal shelter constructions. Such conductive points often account for the majority of thermal losses — around 75 percent or more of the shelter totals in many instances. The design approach has particular benefits in avoiding cold spots (or hot spots) that lead to condensation problems. It also helps to deliver a stable operating environment for instrumentation, which can be crucial for process analyzers.
The outer GRP sheets of the composite construction provide the required environmental protection. GRP is an inherently inert material that is virtually immune to corrosion and atmospheric pollutants. It is also resistant to many petrochemical media.
GRP does not rust or degrade in any substantial way. High UV levels can induce some surface brittling of GRP, leading to marginal damage over long periods of time. However, in practice — and in this application — such degradation is prevented by a UV-resistant gel coat applied to the GRP panel surfaces. A specially developed gel coat of unsaturated polyester resin matches the properties of GRP and is applied as a spray before the panel is fully cured. The polyester resins of the gel coat and panel form a chemical bond, and after curing the coating provides an extremely durable, but flexible, surface finish with a high resistance to weathering and hydrolysis loads. It is also designed to be much more resistant to UV than plastic materials such as the acrylic paints used by many metal cabinet manufacturers.
The gel coat is applied as a 500 micrometer (mic)-thick layer. In contrast, the acrylic paint finishes of steel cabinets, and of cars and trucks, are generally around 70 mic to 80 mic.
The only degradation that occurs with such a gel coat surface layer is slight thinning over time because of UV radiation — which has been measured at around 100 mic over a period of 30 years — and which has no overall effect on the housing’s structural integrity, stability or function. Extremely hostile conditions combining extremely high UV levels with sand or dust storms can slightly increase the gel coat loss rate through abrasion, but the performance of the underlying enclosure has not been found to change.
In this application, advanced thermal insulation and UV protection are fundamental goals. However, the same composite layering technique can be varied to provide shelters with other properties such as advanced resistance to fire, blast, seismic or hurricane forces.
Mike Lucas-Carter is technical support manager for Intertec Instrumentation and works out of the company’s system integration facility in Houston, Texas. Prior to joining Intertec, Lucas-Carter had a long career in oilfield services as global technology manager for Weatherford and as a service engineer for Baker Hughes Inteq. Contact [email protected] or visit intertec.info for more information.