Page 16 - Caswell Firesafe Manual 2018

TECHNICAL MANUAL

FFRDL / TM / BS / 01 / 2018

BS 9999: 2017 identifies the design considerations in respect of fire resisting ductwork and also the
requirement for insulation; however, in basic terms, insulation should be specified in the following scenarios:

• If the duct is situated within 500mm (as a guide) of combustible materials that could be ignited
through radiant heat should there be a fire inside the duct

• Ductwork that runs in the void above protected corridors or shafts i.e. escape routes from buildings
• Fire resisting ducts that pass through rooms containing highly flammable materials and / or materials
that are easily combustible
• Ducts where the internal temperature needs to be protected from a fire outside the duct i.e. Kitchen
Extract ducts where there is a risk of grease deposits on the internal surfaces of the duct

All insulation materials used must be non-combustible and should have a Class 0 surface spread of flame.
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CASWELL FIRESAFE systems utilise Rockwool Slab which satisfies the requirements of BS 5422
“Method for Specifying Thermal Insulating Materials for Pipes, Tanks, Vessels, Ductwork and Equipment”.
The density and thickness of the insulation required differs depending on the type of duct, the orientation
(i.e. horizontal or vertical) and the period of time required before failure can occur.

In the event that a vapour barrier is required the insulation is specified with an aluminium foil face.

8 Ductwork Support Systems

The performance of a fire resisting ductwork system is, amongst other factors, determined by the method of
support and the fixings selected.
In designing the ductwork support system the following factors must be considered:

a) The specification of the ductwork in terms of stability, integrity and insulation. This determines the
construction of the duct, the thickness of the insulation and the combined weight of the system
b) The required pressure rating. This has significant bearing on the chosen duct construction

c) The orientation of the duct. There are different forces which need to be calculated where horizontal or
vertical (riser) is used.

d) The substrate to which the ductwork supports will be secured i.e. concrete, solid block, hollow block,
masonry. This dictates the fixing types to be selected

e) Site conditions i.e. can the ductwork supports be installed as standard or are there any obstructions?
Do the drop rods need to be spaced further away from the body of the duct than usual?
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The strength of steel at ambient temperatures is taken as 275N/mm ; however, at elevated temperatures steel
loses its tensile strength. The higher the temperature, the weaker the steel becomes until eventually it melts.

16 www.firesafeductwork.co.uk