SMOKE MANAGEMENT ISSUES IN BUILDINGS WITH LARGE ENCLOSED SPACES


SYSTEM RELIABILITY

Smoke management systems can be complex and involve the operation of many interacting components, including detection systems, exhaust fans, natural ventilators, automatic smoke curtains, dampers, fresh air intakes, etc. Experience of actual installed systems in real buildings has led to concerns on the efficacy of some smoke management systems, especially over the lifetime of a building.

Reliability

There is limited data on the reliability of smoke management systems. Klote and Milke [1] provide data based on five different system designs as shown in Table 1. The level of complexity was increased for each design.

System No of HVAC No of other Reliability of new Mean life of system fans components system before commissioned

Table 1: Reliability data given by Klote and Milke [1]

Table 1 shows that as the number of components used in the system increases, the reliability of the system decreases significantly, such that the mean life of the commissioned system with 5 fans and 54 other components is just 3 months. However, it must be stressed that the analysis assumed that failure of any one component would lead to failure of the complete system, which may not be the case in reality.

Moran [2] carried out a survey of smoke exhaust systems in shopping centres around the Brisbane region. 32 centres responded to the survey of which 15 had some form of smoke exhaust system. 5 out of the 15 centres reported problems, with various incidents of failure, such as: motor (1), contractor (1), fan bearing (3), fan motor blade (3), fire detection (2) and water ingress (1).

Moore and Timms [3] provided data on the efficacy of systems from less complex designs that are appropriate for low-rise shopping centres (see Table 2).

Probability of attaining the efficacy range: Design 1, Design 2 Efficacy range Maintenance, Installation and Commissioning quality.

Table 2: Reliability data from Moore and Timms [3]

These data demonstrate the benefits of a robust maintenance, installation and commissioning regime in increasing the probability of the efficacy of a smoke management system over the lifetime of a building.

Sources of failure

The efficiency of a smoke management system can be dependent on a number of factors.

Morgan [4] describes some common sources of error in the design, construction and implementation of a smoke management system. The following list of errors is not definitive [25]:

  • Inadequate theories - All calculation methods have theories which have been developed for relatively idealised geometries. Real buildings can depart from those ideal conditions. Typical examples include: spill plumes rising past non-straight spill edges; where the smoke flow approaches the straight spill edge at other than a right angle; and plumes which are partially adhered and partially free.

  • Unknown input data - The building geometry is usually known when designing, but there are several necessary input parameters which are more subjective in many cases (i.e. specifying the design fire).

  • Poor communications - There can be poor communications between the system designer and the installer/s of the system; and between both and the architect; and between all of them and the regulators. Examples include:

  • a system which had smoke curtains installed 1.5 metres too short.

  • channelling screens fitted half-way across a shop’s open front, completely negating the reason for fitting them.

  • a hole in fixed smoke curtains at the reservoir boundary in order to allow a smoke detector’s light beam to pass through.

Problems can arise when late detail design changes are made without telling either the designer or the regulator because the changes are (wrongly) thought to be trivial.

Changes to the building structure over the lifetime of the building may also reduce the efficacy the smoke management system, if these changes are not brought to the attention of the designer.

  • Poor construction/installation – Installation on site is often done by workers who do not know what the equipment is for, or why it is being fitted, and where the workers are under severe time pressure to finish the job. Some consequences have included:

  • a wheelbarrow and bricks left inside a smoke exhaust duct partially blocking it.

  • a fan installed backwards.

  • restraints intended to protect equipment during transit to the site not being removed before fitting.

  • Forgetfulness - A common example is the omission of smoke dampers in HVAC ducting, allowing siphoning of smoke when the HVAC is shut down.

  • Simple incompetence - Although this is rare, examples have included:

  • designing ducts not strong enough to withstand the internal drop in pressure when fans are activated.

  • failure to design expansion joints into ducts held rigidly at the ends while immersed in the hot buoyant smoke layer.

The examples given in the list above are real, however, they are not intended to be identifiable, for legal reasons.

REFERENCES

  1. Klote J H and Milke J A. Principles of Smoke Management. American Society of Heating, Refrigerating and Air-conditioning Engineers, Atlanta, GA, 2002.

  2. Moran T. When I enter a large shopping centre, am I safe from the affects of fire?Smoke exhaust systems designed for life safety and fire fighter intervention. IFE Australia website.

  3. Moore I and Timms G. Reliability of smoke control systems. Fire Code Reform Centre, Project 6. Scientific Services Laboratory, XR0122/R2, June 1997.

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