Extending fire survival time for critical equipment
30 September 2013
Following a number of headline incidents, such as the fires on offshore oil platforms in the Gulf of Mexico, there is growing demand for even higher levels of safety in these types of process operations. As a result, many oil, gas and petrochemical companies are now seeking means of further improving their emergency management systems and plant infrastructure, including the use of fire-resistant enclosures.
Fire-resistant enclosures in a petrochemical environment
One area of safety that is receiving particular attention is the protection of critical safety equipment against high temperature hydrocarbon fires. The longer that valves and actuators in emergency shutdown and fire suppression systems remain operational in the event of a fire, the greater the likelihood of averting disaster. However, the ferocity of hydrocarbon pool or jet fires – the latter resulting from the continuous escape of pressurised flammable media – poses major design challenges. These types of fires are characterised by very rapid rises in temperature, typically reaching 1,100°C within the first five minutes.
Passive fire protection offers the simplest and most inherently reliable means of guarding against premature equipment failure due to fire. Various forms of close-fitting passive protection such as flexible fire-resistant bags and blankets, and intumescent coatings, have been on the market for years, but can have disadvantages when used with field equipment in hostile environments. Tailored fittings, in particular, can be susceptible to weather damage; over time they can absorb moisture and sag, making them difficult to reseal properly after maintenance of the valve or actuator. Thick film intumescent coatings for resistance against hydrocarbon fires generally need to be applied to equipment at the factory. These types of coatings can hinder access for maintenance and corrosion monitoring, and can require periodic repainting to prevent them becoming hygroscopic when exposed to weather.
Rigid passive fire protection cabinets are designed to overcome these disadvantages by providing a maintenance-free solution that can be retrofitted to existing process equipment in the field and which does not restrict service access. However, the choice of construction materials is key. Some manufacturers use metal for the outer walls of the enclosures, which can corrode due to the presence of salt or other aggressive chemicals in the atmosphere. Furthermore, the weight of such enclosures can preclude direct mounting on process pipes, making installation more difficult by demanding use of load-bearing support brackets.
There are no specific international standards for this application area, so process plant safety designers tend to refer to the ANSI/UL 1709 standard which contains a rapid rise temperature/time curve for hydrocarbon fires. The problem with this approach is that the standard is intended for the protection of structural steel, which can withstand temperatures as high as 550°C without losing its integrity. Valves, actuators and electronics are another matter entirely – they often have a maximum operating temperature of 60°C.
One solution is for manufacturers of fire protection shelters to define the test procedures and conditions for their own products. This is the approach adopted by enclosure specialist Intertec, which produces two types of fire shelter. Its regular fire shelter is designed to withstand ‘normal’ fires with a standard temperature/time curve, such as that defined in the ISO 834 fire resistance tests for buildings intended for residential or business purposes, and is available with protection times of 30, 60, 90 or 120 minutes. A second 1709 fire shelter is tested against the rapid rise temperature/time curve for hydrocarbon fires used in the UL 1709 standard. The design has been verified by an independent agency, which found that the surface temperature of a sample electrical actuator inside the shelter remained below 55°C throughout a 90 minute simulated hydrocarbon fire.
These GRP shelters retain integrity after 90 minutes in a hydrocarbon fire
This level of protection is believed to be unique for passive fire shelters and is achieved through a patented combination of materials and construction techniques, including multiple close-fitting composite walls that incorporate mineral wool and other fire resistant insulators, together with thermal seals and a special fire retardant mineral coating. The entire structure is designed for graceful degradation to ensure that it maintains its mechanical integrity and thermal insulation properties for as long as possible in the event of a fire.
Although a number of rigid fire shelters on the market use metal for the ‘skin’ of their insulating walls, glass reinforced polyester (GRP) can be a better alternative. GRP has an excellent strength/weight ratio – it is almost as strong as stainless steel, but four times lighter – and is highly resistant to weathering and the corrosive effect of salt and other aggressive chemicals, as well as being immune to most common types of petrochemicals. It also has a much higher thermal resistance than metal, which makes it easy to avoid condensation, and is an electrical insulator.
Passive fire shelters constructed from GRP-based composites are inherently low in weight and extremely rigid. They can be mounted directly around a valve or actuator and supported by the process pipework – with no need for disconnection during installation – and can incorporate easy-access panels for routine maintenance and inspection.
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