Smoke ventilation in power plants: smoke clearance or fire safety?
Fires do not often break out in power stations, but they do happen. And when they do, it can prove very challenging to bring them under control, as the recent fire at the Tilbury Power Station in Essex showed.
The norm: smoke clearance only
Smoke ventilation is usually installed in power stations to help protect the equipment and to clear the smoke quickly so that the system can be started up again in the shortest possible time. It is specified as a smoke clearance system and sized on a percentage of the floor or roof area. Smoke clearance systems do not necessarily provide conditions which will aid escape or assist fire fighters to find and control the fire.
Why building size and height matter
Power plants are different from other types of facilities in terms of the size and height of the areas where the power generation process takes place, such as the boiler hall or turbine hall. This makes it impractical to design a smoke control system to provide a clear layer high enough to fully protect equipment from full or partial immersion in the smoke layer.
It is uncommon to install a smoke control system designed to create smoke-free areas specifically to help staff escape and give better access to the fire service, because the overall dimensions of this type of facility are often considered to provide sufficient protection.
Implications on fire safety
However, if the ventilation system doesn’t have the ability to remove the smoke at high level, the area could become fully smoke logged. This would make it difficult for people to evacuate the building and for the fire service to gain access and attack the fire. In addition, if the smoke is not allowed to escape, the heat at high level can increase dangerously and help spread the fire through the building. In the worst-case scenario, it can result in the roof collapsing.
Designing for escape or fire fighting requires a smoke control strategy, where the ventilation rate is determined by quantifying the anticipated smoke production rate for a given fire size and clear layer height.
Protection of offices
A power station also has offices. In many cases, these are located at high levels with stair cores over 18m in height. This means that a fire fighting stair core needs to be provided and protected under the requirements of BS 9999. If the fire fighting lobbies have an external wall, protection is easily achieved with 1m2 openable windows at each level. If fully enclosed, a stairwell pressurisation system or a lobby extract system is needed. The stairs also require 1m2 of ventilator area at the head of the stairs.
What do you think?
There are no regulations covering the design of smoke ventilation at power plants, so usually either the developer, the insurer or the Fire Officer make the decision whether to install a smoke clearance system for business continuity or one providing comprehensive fire safety for occupants and fire fighters. As a smoke control system provider, when the client doesn’t give specific guidelines to us at Colt, we prepare a proposal for a smoke clearance system based on a percentage of the floor area. We do make sure that our proposal includes the request for confirmation from our client and the Fire Officer involved that this meets their requirements.
What do you think? Is there a case for designing ventilation systems in power stations to provide fire safety, rather than just focusing on clearing smoke once the fire has been extinguished? If you have any thoughts on the matter, do please leave a comment here.
Bradley Smith is a Business Development Manager at Colt UK with experience in the design and application of HVAC and smoke ventilation systems.