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To motorise or not to motorise – that is the question!

David Stevenson takes an in-depth look at the role of fire dampers in reducing smoke and heat from spreading from one fire compartment to another through HVAC ductwork and believes that motorized actuators ensure greater efficiency and safety.

| | Jun 13, 2012 | 5:10 pm
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David Stevenson takes an in-depth look at the role of fire dampers in reducing smoke and heat from spreading from one fire compartment to another through HVAC ductwork and believes that motorized actuators ensure greater efficiency and safety.

The use of fire dampers in compliance with various codes of practice is generally considered to be a mitigating factor which prevents spreading of fire through the ductwork. The prevalent codes recognise various qualities of dampers and their method of operation, but regulatory guidance states that all alternatives are equally acceptable. This obviously leaves the specifier to go for the cheapest option, such as the thermally operated fire curtain damper, for regulatory compliance. However, this is not an effective method of preventing the spread of cold or even warm smoke, and certainly would not always be 100% effective, even against the spread of hot smoke, depending upon the efficiency of any thermal triggering device.

Furthermore, the efficiency of any fusible link could be seriously reduced in a sprinklered environment, where the smoke temperature may be lowered to below the device’s operating temperature. Despite a sprinkler suppressed fire having the potential to generate more than a normal amount of smoke if the thermal devices fail to operate, there is a greater risk of uncontrolled smoke spread.

Contradictory maintenance guidance

There is a further potential conflict between the regulatory authority, whether it is a local authority or a building control officer and the risk assessor. This once again brings the issue of maintenance into sharp focus, thanks to the wooliness in the guidance. The guides state that in “dust-laden” and similar atmospheres, the operation of the damper should be confirmed by testing at intervals suited to the degree of pollution. It has long been recognised that checking of damper operation is more easily carried out with motorised actuators than with manual operation. The risk assessor will normally take frequency of checking into account and may well require an enhanced level of checking over and above that which is sensibly achieved by manually checking fusible link-operated dampers. However, if the minimum specification has been accepted by the building control authorities, the risk assessor’s expectations may be deemed incompatible.

Regulatory guidance and risk assessment

The important question to ask is: Does regulatory guidance satisfy a life safety-based risk assessment? BS5588: Part 9, for example, states that “all fire dampers should be simple in their operation with the minimum number or moving parts”. In fire performance terms, the code only requires the dampers to satisfy the integrity requirements of BS476: Part 20: 1987, and whilst it recognises the enhanced insulation that an intumescent damper can provide, and even acknowledges the existence of intumescent dampers with integral, detector-operated smoke blades, the basic recommendation is for fusible-link operated “integrity only” dampers. Such dampers are recognised as not generally being regarded as fully effective in resisting smoke. But it does state that the “integrity only” damper will “assist” in reducing or retarding the movement of smoke. Without qualification, such a statement could be seen as highly optimistic.

A greater level of guidance is given on the activation of fire/smoke dampers by smoke detectors. Indeed, there are positive advantages in life safety terms in actuating fire/smoke dampers by smoke detectors (in addition to) thermally activated devices, particularly in buildings presenting a high or special life hazard, such as hotels, hospitals and other non-domestic buildings like entertainment premises, involving a sleeping risk. The catch 22 situation is that building control has no mandate to ask for a higher standard, and the risk assessor is not duty-bound to accept a lower standard.

Are the concerns supported by fact?

It is a fact that in these litigious days, society is no longer permitted the luxury of an accident. Invariably, on the occurrence of death or injury, somebody will be looking for compensation. The principle of “no-win-no-fee” engagement of solicitors makes it almost risk-free to take an action against anybody who may be at fault. In the event of an employee being injured, or even worse, dying as a result of a fire incident in a building that has recently been identified as being safe, it would become a very high profile target for such an action.

Taking that to be the case, one needs to look at the guidance to see the area where the spread of smoke is most compromised. Air handling ducts that link enclosures and, indeed, compartments together for the purpose of creating tolerable working environments must be an obvious potential route for the spread of fire and smoke.

Codes of practice, such as, BS5588: Part 9 heavily promote the concept of dampers for isolating the duct between the separate compartments which handles the risk of fire spread. However, very little guidance is given on their specification or performance. Dampers dividing compartments will invariably be fairly simple, robust, mechanical dampers that are operated by a fusible link if the regulatory guidance is followed. Such dampers do not react to smoke ingress into the duct until the smoke has a temperature sufficiently elevated to operate the fusible link. As a consequence, in a slowly developing fire, significant volumes of smoke can spread through the ductwork, potentially putting people at risk in remote positions before the damper begins to operate.

In the light of this, the fact that dampers are not generally designed to provide smoke resistance is of a lesser concern than the fact that they will not operate early in the event of a fire. The important question that needs to be asked is: How can a damper be installed, in such a way that it does get activated early enough to make a contribution to the control of smoke spread? The only sure way to ensure this is to have the damper so that it operates from a smoke detector, preferably closing in a controlled manner using motorised actuators.

The writer is the Managing Director of Belimo Automation. He can be contacted at: david.stevenson@ belimo.ae

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