Electrical power is important part of our day-to-day life that’s sometime taken for granted. Almost everything needs power; we sometimes forget how it is quietly transmitted across thousands of kilometers from generating stations to homes, schools, hospitals, refineries, through essential infrastructure: the conductors. To enable efficient transmission over long distances, the power is sent at high voltage and the conductors must be covered with quality insulation material to become a capable cable.
Cable has come a long way since the first insulation material used was vulcanized rubber in the New York City power distribution system established 1882. Ever since, power insulation material has evolved into more advanced materials like Water-TreeRetardant Cross-linked Polyethylene (TR-XLPE) protecting cables carrying several hundred kilovolts. Cables are also used in fire alarm systems to trigger warning or alert system. The proper insulation materials are necessary to deploy power and signal through cables effectively.
When a fire breaks out, especially at refineries where temperatures can go up to 1100C within five minutes, the electrical insulation cable jacketing materials are vulnerable to burning away, leading to the cable losing its ability to transmit power and signal. Within minutes, the electrical circuit critical to life safety and process controls will fail, potentially endangering lives and leading to property loss and impacting business continuity.
Case Study: Mid West Refinery Fire
In the late 1990’s, at an oil refinery in the Midwest, a pipe containing combustible material burst, resulting in a fire plume 15 feet above ground. The fire’s intense heat threatened to breech a nearby tank within the refinery that contained hundreds of gallons of hydrofluoric acid.
Immediately recognizing the seriousness of the situation, plant employees sought to move the acid to another tank away from the fire. A cable tray containing critical control circuitry required to send diverting signals to valves had been exposed to the fire. This circuitry needed to stay operational to drain out the tank and 3M Interam Endothermic Mat (commonly known as 3M E-mat) had been installed.
In such critical scenario that required lightning speed actions, it still took 30 minutes to drain out the tank.
How did E-mat help the cables to survive such severe conditions for 30 minutes, which was expected to fail within minutes as the insulation materials burns away?
3M™ Interam™ E-5A-4 Endothermic Mat “E-mat” is a flexible material that uses endothermic technology to release chemically bound water to cool the protected element and retard heat transmission in the event of fire. The product has been tested in schemes that protect power cables against 1100C degree hydrocarbon for 60+ minutes. Many oil and gas corporate fireproofing specifications call for 15-30 minutes of protection. . It is often by risk assessment that the user determines the balance of business continuity, cost and life safety. The 3M system can be tuned to address these requirement by lessening the layers of material that are applied.
What is the art behind this hour of circuit protection where many expect to fail within minutes?
The Power Cable
Wires in the power cables can work at very high temperatures and sometimes at intermittent temperatures of up to 250C until oxidation would occur. But in the industry, rarely one would rate their cables at 250C but rather at a maximum of 90C normal operating temperature and up to 40 years of longevity at this normal operating temperature. This is driven by the insulation material’s characteristics that actually age over time with temperature. Overtime 40 years the cables jacket can become brittle and more prone to breakage.
As always, there is bound to be a time, where power cables may have to be overloaded and this emergency overload temperature is generally recognized by industry as 130C. But what happens to the cable insulation lifespan?
As a rule of thumb, for every 10C rise in temperature, the cable lifespan drop by a factor of 2. This mathematical model between cable lifespan and temperature is called Arrhenius relationship.
At 130C, the cable lifespan is shortened considerably to just 1000hrs (42 days). This is why 130C is known as the ‘emergency operating temperature’.
Applied Fireproofing of Cables
To ensure cables are to function in the event of cellulosic or hydrocarbon fire, the user shall determine its own cable emergency operating temperature and the required time to control the fire. In the mentioned case study, the cutoff temperature was as a maximum rise of 140C.
The threshold average temperature rise of 140C comes from the ASTM E 1725 test standard which bears the title “Standard Test Methods for Fire Tests of Fire-Resistive Barrier Systems for Electrical System Components”.
In the standard, the cable functionality is dependent on the insulation material and its ability to retain its function upon fire exposure. There are various fire test studies on electrical functionality pointing out for the same composition and classification of insulation materials, the failure temperature ranged from 149C to 427C. Independent oven testing of the cables also pointed out varying temperature functionality. This discovery led to the use of temperature as an end point criteria rather than electrical functionality.
E-mat offers great flexibility and thickness consistency that can be easily applied in either 1, 2, 3 or even 10 layers depending on the requirement, and can be installed in a very quick manner. This gives tunable fire barrier protection levels. The layers can be removed easily and re-deployed to other critical protection services when needed.
E-mat offers low ampacity derating of power cable because it allows heat to be dissipated away from power cables during normal operation, unlike other insulation materials that trap heat and could lead to overheating situations.
For telecommunication industry, fiber optics cables are often re-routed. If wrapped in E-mat access ports can incorporated into the cable trays envelope and allow for easy service and addition of cables; the cables are still identifiable. If the cables were covered in a coating the ability to identify the cables is lost.
Does the application stop here?
Structural steel supports are now more commonly found in buildings and refineries. They are typically lighter than concrete and help to speed up construction of a building or storage vessels in a refinery and also taking less space. Though these are the merits of using steel, there are also areas of concerns: fire protection and corrosion.
Concrete and structural steel can both be designed to fulfill load bearing requirements at ambient conditions, but when exposed to fire steel is more vulnerable and will yield at approximately 538C. To prevent such yield and collapse scenarios from happening in an event of fire, structural steel must be protected. Older conventional methods of protection are applying cementitious or intumescent coatings. These traditional methods using cementitious and intumescent coating are sometimes called ‘wet methods’ of fire protection. Their installation can be labor-intensive and may include the need for a water supply, careful surface preparation, sophisticated pump systems, and have limited installation environments (humidity and temperature). There are also concerns on whether environmental exposure and live loading of a structural beam could possibly affect the adhesion or cracking of these coatings. In addition, coating is permanent and short of doing destructive testing, corrosion under insulation is very difficult to detect.
E-mat as a dry method for fire protection can be applied easily over beams and columns giving flexibility of removal and inspection for corrosion. E-mat is also tested to environmental conditions and its properties are relatively unaffected and it is mechanically attached to beams/columns. Any detachment can be easily detected and easily tightened back. Further merits would be service cables that can be installed and protected along an I-beam wrapped with E-mat, whereas using wet methods this may not be possible.
In any fire protection design, it is important to understand the type of elements to be protected. Be it power cables, structural steel or pipes, a user has to understand the material characteristics. E-mat provides true flexibility in protecting any of these elements as a single go-to fire protection material. The art of fireproofing can be simple when the science of endothermic technology is applied.
3M E-Mat has been used in various applications ranging from power cables, structural steel and pipes. It is the same product used for over 30 years and provides versatile and effective fire barrier protection in building and construction, oil and gas and nuclear industries.
For more information, go to www.3m.com/emat