Proceedings:

16th Australasian Tunnelling Society Conference

Publication Date:

Oct 2017

Authors:

Nick Agnew and Ben Tuckwell

The concept of forcing airflow along a tunnel at some ‘critical velocity’ to prevent the upstream movement of smoke has been known to tunnel fire and life safety and tunnel ventilation practitioners for several decades. For much of that time, the industry has relied on formulae forestimating critical velocity that are based on over-simplified semi-empirical models with little to no fullscale experimental validation covering the possible ranges of fire and tunnel parameters. Despite the advances in our knowledge of fire and smoke behaviour in tunnels, designers still need to take care when interpreting the critical velocity concept. This paper seeks to provide practical guidance on the application of this design input which often receives more focus than perhaps is necessary.
Fire dynamics research from Sweden indicates that the critical Froude number is not constant, but varies with heat release rate, so that Danziger and Kennedy’s industry-trusted equation grossly underpredicts critical velocity at low heat release rate values. The latest 2017 edition of NFPA 502 (Standard for Road Tunnels, Bridges, and Other Limited Access Highways) incorporates this latest thinking, and when compared to the previous 2014 edition, suggests much higher critical velocity values for low to moderate heat release rates (e.g. 5 to 30 MW, a range covering commonly assumed car, bus, train, and small truck fires). On a recent rail project in Australia, the mandated application of the new NFPA 502 methodology resulted in a large increase in the proposed ventilation plant capacity. As the authors understand, there are also plans to incorporate similar guidance into NFPA 130 (Standard for Fixed Guideway Transit and Passenger Rail Systems).
In many tunnel fire situations, there are valid reasons for minimising the longitudinal velocity. Tunnel ventilation plant and power requirements can be minimised by taking a pragmatic approach to tunnel fire and life safety that allows some minor backlayering during a fire. That is also consistent with a ‘So Far As Is Reasonably Practicable’ (SFAIRP) approach to safety in tunnels which requires us to do our best, but not at any cost.
With all objectives considered holistically, it is not so critical to achieve critical velocity.

Key Words

Tunnel ventilation, backlayering, critical velocity, NFPA 502, fire and life safety.