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A large warehouse fire can be particlulary devastating for a business and the community around it. Joe McCafferty highlights the role of sprinkler protection in these buildings.
Warehouses present a particular fire risk to local communities and a major fire could have a devastating effect both economically and environmentally. These buildings can be extremely large with a variety of highly flammable and polluting goods stored in them. This storage is usually in the form of multi level, multi row racks so getting water to the seat of the fire quickly is vital if a major incident is to be avoided.
These buildings can be located right in the heart of residential areas which makes them an added risk to life and health. Fighting any fire has the added problem of containment of firewater runoff and keeping it to a minimum is vital to protect the environment. With the advances in automated storage and retrieval of goods, the number of employees required to run these premises is reducing. This means that there are less people who could notice a fire at the early stages.
There are various options on how to prevent and control fires in warehouses. These can be passive fire protection like using robust fire rated products to construct the building or active fire prevention like sprinklers to control and extinguish the fire. Using these options in conjunction with one another can greatly improve the probability that lives, property, the environment and the business can be saved. The benefit of early warning systems – like smoke and heat detectors – in the prevention of fire deaths should not be overlooked.
The materials used to construct the building should be selected from products that can give up to two hours’ fire rating. As warehouses are normally one large open space, the higher fire rated structure would contain – but would not stop – the fire spreading within the building and the stored goods. When the fire brigade arrives at the incident they put their lives at risk by entering the building to fight the fire. The best they could hope for is that the building and some stock is saved but will the business survive? Installing a fire alarm system, and giving an early warning, would improve the firefighters’ chances of reducing the spread of the fire, by getting them there earlier. In this day and age we should not be asking firefighters to enter these buildings in a serious fire situation.
By far the best way to tackle a warehouse fire is with the installation of a fixed, automatic, water based control or suppression system. This will not only give an early warning but will immediately attack the seat of the fire. With their excellent reliability record, the probability is that well designed and installed sprinkler systems will have contained or extinguished a fire prior to the arrival of the fire service.
There are four main sets of rules for the design, installation and maintenance of automatic sprinkler systems commonly used in the UK. They are: LPC Rules for Automatic Sprinklers 1990 which includes BS 5306 Part 2; LPC Rules for Automatic Sprinklers incorporating BS EN 12845; and two American sets of standards – NFPA and Factory Mutual. The first two have been used as reference in the following paragraphs.
The following are some of the types of sprinkler systems that can be installed in a warehouse. A roof only sprinkler system consists of a network of pipes installed just below the roof sheeting. The sprinklers are spaced at intervals of about 3 metres in all directions. They have an operating temperature of 68 degrees C and a ‘standard response’ thermal sensitivity rating. They operate at a minimum pressure of 0.5 bars with a flow of 56 l/min., and at least 29 sprinklers will operate.
In high volume warehouses, the storage arrangement and type of goods stored are a determining factor on the limitations of installing a roof only sprinkler system. The maximum heights of storage allowed vary from 3.0 metres to 7.6 metres. Water application can be as low as 7.5 l/m2/min and as high as 30 l/m2/min. The total water demand for the fire area can be from 2,300 l/min to 11,000 l/min. with pressures of 6 to 8 bars. As it is unlikely the local water supply will cater for these demands, a water storage tank and pumps will be required as a minimum. The preferred system for warehouses is a wet type which is permanently charged with water and can deliver the water to the fire source quickly. Alternate wet/dry type systems can be used for colder buildings where water might freeze – these systems usually demand at least 25% more water application. Roof only systems are a cost effective and reliable means of protecting low height warehouses which have lower fire risk storage.
Roof and in-rack
In-rack systems are installed when the height of racking exceeds that which can be protected by roof only sprinklers. In lower height warehouses, they can also be used in conjunction with roof systems to reduce the water demand at the roof and reduce the roof system pipe diameters. The roof system is as described above but is enhanced by having sprinklers installed within the storage racks. The in-rack sprinkler pipes are located behind the horizontal beams that support the storage pallets and centrally on the rack flues. The sprinklers are usually located no closer that 150mm above the stored goods with their water deflector located about 20mm below the horizontal beam. This places the sprinklers in the ideal location to detect and attack a fire at the earliest opportunity.
Depending on the severity of the risk a range pipes may be fitted at every level or at vertical intervals up to 3.5 metres apart. Sprinkler heads will be installed from 1.4 metres to 2.8 metres apart on the horizontal range pipes. The sprinklers are designed to operate at 68 degrees C with a ‘quick response’ thermal sensitivity rating, operating at a minimum pressure of 2 bars with a flow of 113 l/min. The number of sprinklers required to operate within the racks depends on which rules are used. LPC/BS5306 rules could have three sprinklers operating on each range, at every level and up to three racks operating. This could require 45 in-rack sprinklers operating simultaneously with the roof sprinklers, though BSEN 12845 rules only require up to a maximum of nine sprinklers operating. The width of the aisle between the racks is a major factor in determining the number of in-rack sprinklers operating. The wider the aisle the fewer sprinklers need to operate.
The total water demand for a fire area can be from 2,700 l/min to 9,000 l/min. with pressures of 6 to 8 bars. Again a water storage tank and pumps will be required as a minimum and duplicate tanks would further enhance the reliability of the system – wet type systems are preferred. As an alternative for colder buildings, pre-action systems can be used, with a heat detection system to operate the control valve. Roof and in-rack systems are particularly effective in protecting against fires in ‘high bay’ type warehouses. In-rack sprinklers are usually protected from damage by a wire guard and are fitted with water shields to protect them from water if sprinklers at higher levels operate.
ESFR (Early Suppression Fast Response) sprinklers are suppression mode sprinklers, designed to reduce a fire as fast as possible using an aggressive large drop spray. The were developed by FM Global in the USA 1980s and more recently have been included in the LPC and BS EN 12845 sprinkler rules.
Ceiling heights of up to 13.7 meters with a maximum slope of 9.5 degrees and storage heights of 12.2 meters can be protected by an ESFR system. These sprinklers have a much larger orifice size than those used in normal sprinkler systems and can discharge flows of 670 l/min when operating at a pressure of 3.5 bars. The benefits of ESFR systems are that in-rack storage can be as high as 12.2 without the need for in-rack sprinklers.
ESFR protection performance depends on rapid fire detection and response, and is designed for the operation of fewer sprinkler heads than a normal sprinkler system. Successful ESFR sprinkler performance depends on open frame storage that includes enough space for the water to penetrate. It is essential that the first few ESFR sprinklers that operate are able to deliver enough water to suppress a growing fire. They are, however, vulnerable to obstructions from building structures, lights, ducts, electrical conduits and cable trays.
The total water demand for the fire area can be from 5,300 l/min to 9,600 l/min. with pressures of about 8 bars. Two half capacity water storage tanks and pumps must be installed as a minimum and only wet type systems are allowed.
Joe McCafferty is sprinkler product manager at Tyco Fire and Integrated Systems. For further information on sprinkler thermal sensitivity ratings see LPC Technical Bulletin 20 or BSEN 12845 Technical Bulletin 207.