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Water mist systems represent a relatively new technology and our understanding of them is still not complete. Here, Alex Palau outlines how they work and examines the guidelines available for their implementation.
What is the viability of water mist in providing a fire engineering solution in modern day active fire extinguishing and control? Pre-1987 methods dealing with active fire suppression fell into two areas: sprinklers, addressing the problem of fire control and containment; and Halon or Carbon Dioxide, dealing with fire extinguishing of specific areas or local application on equipment or processes.
After the restrictions on the use of Halon arising from the Montreal Protocol, and the subsequent concern on the safety criteria relating to the use of carbon dioxide in potentially occupied areas, there was a need to find alternative fire extinguishing agents. This had led to the wide range of different fire protection systems that are now found on the market but none of these systems are suitable for all applications, and not all of them can be designed as extinguishing systems.
Additionally, there has been a movement towards fire engineering to a ‘performance based code’ that requires consulting engineers, insurance companies and end users to define – at early stages of the project – the main goals expected from fire protection systems. In these cases, extinguishing the fire is not necessarily the key parameter, as the main guarantee is to provide:
– safe evacuation conditions
– building structural stability
– reduced damage to contents.
Once the main objectives have been defined, it is possible to decide which fire protection system is the best option. It is not unusual that the same application ends up with a different fire protection system, depending on the goals that have been defined.
Water mist is one possible engineering solution. It is environmentally friendly, suitable for occupied areas and uses a readily accessible and cheap extinguishing medium, i.e. water. It is probably true to say that despite great efforts in the last year by manufacturers, test laboratories and insurances companies, our understanding about the performance of these systems is not yet complete. Nevertheless, there is a general agreement that the primary extinguishing mechanism of water mist in total flooding designs, is one in which the droplets are vaporised by the heat of the fire and converted to steam. The generation of this steam extracts heat from the fire, and the reduced oxygen concentration through the increase in relative humidity leads to inerting of the fire. Test results prove that water mist systems readily extinguish larger ‘hot’ fires (relative to room size) in enclosed rooms, while small ‘hidden’ fires are considered a big challenge. The performance of the water mist system is influenced by drop size and velocity (momentum), flow rates, nozzle distribution and design discharge (run) time.
Engineers and end users need parameters with which to design these systems properly. Normally, prescriptive standards provide guidance in this respect. Unfortunately there are few standards for water mist systems and none of them will give any guidance on where nozzles should be placed and which is the correct flow rate. The NFPA 750 (Standard on Water Mist Systems, 2003), which is considered the main standard of reference, only describes existing water mist technologies, provides details of hydraulic calculation methods and gives minimum requirements for pipes, supports, and so on. NFPA 750 considers that water mist systems must be listed for specific hazards and protection objectives, due to the absence of a generalised design method. Included in the latest revision (2003) there is a list of worldwide published and recognised fire test protocols can be referenced (some of which are still under discussion).
Type testing protocols
These fire test protocols are published by different test houses, or standards or approval organisations (i.e. International Maritime Organisation, VdS, Factory Mutual and UL). The main purpose of these is to evaluate the performance of water mist under specific conditions. It is very important to understand that protocols are designed in accordance with the intended use of the fire protection system, as defined by the standard or by the design and installation manual approved by a recognised authority. As an example, CEN’s (European Committee for Standardisation) water mist task group has developed a standard (now at public enquiry stage) that includes a protocol to evaluate the total flooding water mist system performance to protect enclosed rooms where pool fires may develop. The draft standard is prEN14972: Fixed Fire Fighting Systems (Watermist Systems) Design and Installation (2004). Some of the fires on the protocol are small (around 1MW) and obstructed. The degree of obstruction has been a compromise between guaranteeing a fully developed fire (increasing the obstruction may have restricted air entrance to the flame) while providing a reasonable degree of obstruction to water droplets. It is believed that this protocol is severe enough, though it does not represent a real scenario. Because of this, the draft CEN standard specifies a pass/fail criterion that all fires described in the protocol have to be extinguished for the system to be installed as a control system (minimum discharge time 30 minutes). Nowadays there are three basic types of test protocols:
Total flooding protocols. These protocols evaluate the water mist system capability to extinguish fires in whichever place of an enclosed room. The pass/fail requirement on fire test is extinguishing, while the design approach is control. Example of total flooding protocols are:
– MSC/Circ. 668/728: Method for equivalent water-based fire-extinguishing systems for machinery spaces of category A and cargo pump-room. This protocol addresses liquid fuel fires in machinery compartments. Systems that have been able to extinguish all fires described (ranging from 1 MW to 6 MW) in less than 15 minutes are allowed to be designed as control systems with a continuous water supply of at least 30 minutes.
– PrEN14972: Flammable liquids (control and suppression systems). This test procedure evaluates the fire performance of water mist fire protection systems for the protection of industrial flammable liquid hazards. The criterion of the fire test is extinguishing but the systems are classified as control/suppression systems because of the installation variables.
Opening areas are defined in the protocols. These are important so that they minimise the entry of fresh air, as this will significantly affect the water mist performance. Room integrity should be checked on real enclosures to guarantee that opening areas do not exceed those tested.
Local application protocols. These protocols evaluate water mist capability to extinguish localised fires. Protocol pass/fail requirement is extinguishing, while the design approach is usually extinguishing. Example of local application protocols are:
– MSC/Circ 913: Method for fixed water-based local application fire-extinguishing systems for use in category A machinery spaces. This protocol proves local water mist systems’ efficiency to deal with fires in machinery spaces. Pass/fail criteria is complete extinguishment of fires and designed for at least 20 minutes continuous water supply.
– VdS Mechanical Escalator Protection. This protocol evaluates performance of water mist systems to protect mechanical escalators (tracks and machinery spaces).
– prCEN Non-industrial kitchen fryers. This protocol has been developed to evaluate protection of fryers (fire extinguishing and oil cooling), filters and exhaust ducts. Water mist systems shall extinguish on test and are allowed to be installed as an extinguishing system (minimum discharge time five minutes).
Sprinkler alternative protocols. These protocols evaluate water mist performance to control a fire (to guarantee structural stability of buildings) as an alternative to traditional sprinklers. The pass/fail protocol requires performance at least as good as sprinklers and the design approach is control. The latest development in this direction is a working group looking at developing a water mist version similar to BS9251 (domestic & residential sprinkler protection).
– prCEN Fire test procedure for office occupancies of Ordinary Hazard Class 1 (OH1). This test method is intended for evaluating the fire performance of water mist systems equivalent to the fire performance of a sprinkler system for office and school occupancies belonging to Ordinary Hazard Group 1, as defined in BS EN12845. Under test conditions, a water mist system’s performance shall be compared to the performance of a standard sprinkler system. Those water mist systems that have proved to be at least as good as a sprinkler system will be allowed to be installed as control systems.
– Resolution A.800 (19) Guidelines for approval of sprinkler systems equivalent to that referred to in SOLAS Regulation II-2/12. This resolution describes several test protocols to evaluate performance of water mist systems to be designed as sprinkler equivalent systems (discharge time of at least 30 minutes).
– VdS False floors and false ceilings. This protocol evaluates water mist systems for false floors and false ceilings from 300mm to 800 mm with cable trays. The fire test lasts for 30 minutes. Pass/fail criteria of the approved system is the comparison of fire damage and temperature with a common sprinkler system according to VdS CEA 4001. On both accounts, the system under approval has to deliver results equal to, or better than, the sprinkler system.
– VdS Fire tests for stores (OH3). This protocol evaluates water mist systems for, on the one hand, rack storage (ST5/6) and on the other hand exhibition area compact storage (ST1). The fire load consists of a paper board container filled with plastic cups and the test lasts for 30 minutes. Pass/fail criteria of the system under approval is the comparison of fire damage and temperature with a common sprinkler system according to VdS CEA 4001. Again, the approved system has to deliver results at least equal to the sprinkler system.
– VdS Cable tunnels. Eight cable trays with different numbers of cables are put one upon the other within a certain distance. Pass fail criteria of the system is that the temperature in the burning room has to be less than 1000C after five minutes of water distribution; after 15 minutes of water distribution no flames or embers should be present at the cables and their surface must not be burnt one metre at the end of the cable trays.
– VdS Hotels (Based on IMO A 800, Figure 7, sofas). The fire load consists of four sofas like the IMO A 800 and the fire tests last for 30 minutes. Pass fail criteria of the system is a minimum of equal performance with a common sprinkler system regarding fire damage and temperature.
Water mist is a very promising fire engineering solution as an alternative fire protection system. It is usually designed as a control system for total flooding systems, and design for extinguishing is usually limited to local application systems, even though the water mist system can fulfil the main aims of protection in many cases. In order to design the system properly, it is very important to understand the test protocols and to know the use and limitations expressed in the standards or rules.