When a fire occurs at a refinery, offshore facility or petrochemical plant, the electrical systems which serve critical areas such as control rooms, process equipment, ventilation, sprinklers, alarms and other emergency systems must remain operational. In particular, cables which ensure the activation of emergency shut down valves (ESDVs) must be adequately protected in order to isolate the flow of fuel to the fire, preventing further escalation of fire and explosion.
The fire protection of electrical raceways or cable trays that act as conduits for cables supporting these process critical functions is therefore of vital importance to operators.
As well as mitigating against the financial implications that loss of property and disruption to ongoing production brings, a robust, well designed fire protection system can prevent catastrophic loss of life and devastating environmental impact.
A good starting point when considering the specification of such systems is the American Petroleum Industry (API) 2218, Fireproofing Practices in Petroleum and Petrochemical Processing Plants, which offers guidance for “. . selecting, applying, and maintaining fire proofing materials that are designed to limit the extent of fire-related property loss in the petroleum and petrochemical industries.”
According to API 2218, “The principal value of fireproofing is realised during the early stages of a fire when efforts are primarily directed at shutting down units, isolating fuel flow to the fire, actuating fixed suppression equipment, and setting up cooling water streams.”
A number of options are available to operators for providing hydrocarbon fire protection to cable trays including calcium silicate boards, intumescent and ablative coatings, ceramic fibre blankets and endothermic mats.
Each of these materials has different properties which operators should evaluate in order to select the most appropriate system for the requirements of their specific application.
When specifying fire protection systems for for cable tray applications, specifiers should consider a number of criteria, including:
- The proven ability of the system to maintain circuit integrity under fire
- Ampacity derating
- System flexibility requirements
- System weatherability requirements
- Weight load capacity
- Total system installation cost
All fire-resistant systems should be tested in accordance with industry standards in order to ascertain how the system will perform when subjected to a high intensity hydrocarbon fire. API 2218 recommends UL1709 (or functional equivalent) as the primary standard for hydrocarbon fire testing. UL1709 reaches 1100°C (2000°F) within 5 minutes, and maintains that temperature for the duration of the test.
API 2218 also recommends that critical wiring and control systems be protected for 15 to 30 minutes to UL1709 or equivalent testing If the control wiring is used to activate emergency systems during a fire. A reputable materials supplier will be able supply appropriate documentation to validate such testing.
Ampacity derating refers to the reduction of a cable’s ability to conduct electricity and can be tested through the use of IEEE 848 Standard Procedure for the Determination of the Ampacity Derating of Fire-Protected Cables.
The higher level of insulation provided to a cable, the less current it can conduct without risk of damage from overheating. Therefore, if a cable is derated by 40% it can only be used to conduct 60% of its ambient capacity. This can have major implications on design cost, footprint and overall system weight.
System flexibility: The choice of fire protection system should reflect the existing maintenance regime and possible requirements for future cable alterations. A more fixed system will be difficult to re-enter for inspection and upgrade of cable trays.
System weatherability: The location and level of exposure of a cable tray can vary greatly as can the weatherability performance of fire proofing materials. Systems exposed to high levels of liquid or vapour may require additional surface protection such as a top coat or surface cladding.
UL 1709 testing include a standard set of exposures for weatherability and chemical tolerance as part of its normal testing protocol.
Weight load capacity of cable trays will have an influence on the weight, and therefore type of fire proofing materials which can be applied.
Total system installation cost should be evaluated based on the total material costs, labour and installation costs, expected life cycle and the associated maintenance and replacement costs.
When upgrading existing facilities, disruption to ongoing operations as a result of system installation should also be a key consideration.
3M™ Interam™ Endothermic Mat
3M, the diversified technology company, has developed 3M Interam Endothermic Mat (or ‘E-Mat’) material that achieves hydrocarbon and jet fire protection for a range of applications.
E-Mat is a flexible material that provides a uniform covering which, when exposed to high temperatures, releases chemically-bound water to cool the outer surfaces of the wrap material and significantly retard heat transfer. 3M has advanced fire protection technology that offers outstanding performance in many fire scenarios including large hydrocarbon pool fires in accordance with UL 1709 (ASTM E 1529).
Using endothermic technology, E-Mat helps to prevent the transfer of heat from the exposed side to the protected items via a chemical reaction, which absorbs the heat energy.
E-Mat’s flexible, space-saving construction allows ease of installation for protection to critical areas of all types including cable trays, conduit, equipment shrouds and other electrical systems for up to three hours in intense heat. The endothermic nature of the product also allows heat from normal operations to dissipate, reducing any ampacity derating of the cables.
E-Mat can be used for both interior and exterior applications and is designed to endure harsh environments with virtually no maintenance. The product is also re-enterable and can be removed and reinstalled for quality inspections or future refits.
Because of its flexibility, engineers can use E-Mat to meet fire protection requirements in nearly any area and along virtually any wall, helping to reduce the need to make revisions to existing plans. This represents a significant cost-saving and time-saving benefit for both installers and designers.
For further information contact Sean Appleton (firstname.lastname@example.org).