Duct sprinklers

When ducts requiring sprinklers are used, a great deal of thought has to be given to the installation. The sprinkler heads must be able to stand up to the hostile atmosphere in the fume exhaust duct. There are numerous heads which are listed for use in these areas. They include lead, stainless steel, turn metal, wax and Teflon coated heads. The most common head used is the field-modified ordinary brass head. A 212°F intermediate temperature head can be coated with multiple layers of beeswax to effectively prevent it from the hostile atmosphere. In addition, the heads are normally encapsulated in a plastic bag. Based on the high challenge fire expected in plastic duct work, this will not significantly slow down the operation of the heads. 

Recently, we have seen installations where the heads were modified by dipping 212°F heads in paper cups filled with beeswax and allowed to congeal. These are very questionable because of the long exposure to the elevated temperature of the wax and the insulating properties of the massive wax. These field-modified heads are presently being tested to determine how they will perform in real life situations. They are delayed when using the plunge test. 

The model building codes recommend sprinklers in duct at 10 intervals when the duct is 10 or larger. The testing of the duct work would support sprinklers when ducts are 8 or larger. 

The sprinkler heads should penetrate the duct from the top and have a union for easy removal. It s critical where heat producing machinery vents into the system. This would prevent drainage from the sprinkler operating from flowing down the duct to the heat producing equipment and creating a steam explosion. The best solution is a separate metal duct for heat removal. 

Where ducts are sprinklered, a drainage system to a safe location must be provided. Where inadequate drainage was provided, the systems collapsed with sprinkler operation. 

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Although hoods are most commonly used to control concentrations of toxic vapors, they can also serve to dilute and exhaust flammable vapors. Although theoretically possible, it is extremely unlikely (even under most worst-case scenarios) that the concentration of flammable vapors will reach the lower explosive limit (LEL) in the exhaust duct. However, somewhere between the source and the exhaust outlet of the hood, the concentration will pass through the upper explosive limit (UEL) and the LEL before being fully diluted at the outlet. Both the hood designer and the user should recognize this hazard and eliminate possible sources of ignition within the hood and its ductwork if there is a potential for explosion. The use of duct sprinklers or other suppression methods in laboratory fume ductwork is not necessary, or desirable, in the vast majority of situations. 

NFPA 318 took an interesting approach for hydraulics on duct sprinklers. They allow the use of /8" heads. This is desirable in smaller ducts. It will reduce the amount of airflow restriction the operating sprinkler would create in the duct. By using less water and finer spray, the spray from the 3/8" head is more effective in extinguishing the duct fire. By putting less water into the ducts, there are less problems with drainage. 

Recirculating ventilation systems should be designed for shutdown in the event of a fire or chemical spill. Exhaust systems should continue to run during the incident to facilitate the removal/ treatment of potentially toxic materials. Caution must be exercised when considering the placement of automatic sprinklers inside exhaust ducting. The flowing sprinklers have been shown to greatly reduce the capacity of the exhaust system. 

Sprinklers are available in various temperature ratings. It is important to check for proper temperature rating of sprinklers in close proximity to hot surfaces, such as steam pipes, ducts, unit heaters, or hot equipment. For example, high temperature sprinklers should be used when located within 30 in (76 cm) vertically and 12 in (30.4 cm) to the side of a steam pipe or under skylights. 

Since water is more heat conductive than air because of its higher specific heat, these distribution systems require only 25 percent of the energy that would be required for an air-filled duct distribution system, and because water is denser than air, water-filled pipes take up less space inside a building than ducts. For these reasons, water-source systems are typically preferred in large, multistory buildings. Because many such buildings also have sprinkler systems, the distribution pipes may be integrated into the sprinkler system to reduce overall costs. 

Dedicated exhausts used for flammable vapors should be of ferrous construction. Flammable liquids do not have an adverse effect on materials so there is no reason to use plastic ducts. Flammable vapors could condense and the duct should be sprinklered. This condensation of the material could occur where the duct is subject to change in temperature or the solvent stream is subject to a change in velocity. 

Sprinklers for Combustible Tools. Heads should be loeated at the transition pieces of any tool of combustible construction or handling flammable or combustible liquids. This is regardless of the size of the duct. 

Any water discharge into the duct has to be removed. Sprinklers discharging into the duct have backed up into equipment. When the sprinklers tie into the trunk duct at the top, there have been cases where drains were omitted and the duct collapsed. Drains can be calculated in a manner similar to drains to prevent ponding on a roof A rule of thumb would be to use a 4" duct drain for every 50 of duct work. These drains need to be ducted to a safe location. 

Workstations. All workstations must be installed with fire sprinkler protection. Workstations constructed of combustible materials containing HPMs must contain a sprinkler head within four feet of the exhaust duct connection. Sprinkler coverage of the horizontal surface at any workstation shall not be obstructed. NFPA 318, in addition to requiring automatic sprinkler protection of the horizontal surface, requires sprinkler protection in the exhaust transition piece of combustible workstations (Paragraph 2-1.2.8.1). A sprinkler shall be installed within the exhaust duct cormection of workstations of combustible construction. 

Exhaust ducts with a cross-sectional diameter of 10 inches or more, which contain flammable vapors, must contain fire protection per the Uniform Mechanical Code Section 609.7 and UFC Article 5104.3.2.4. UMC 609.7 and NFPA 318 2-1.2 cover design criteria for sprinkler systems for such ductwork (see reference below). 

Sprinklers installed in duct systems shall be located at 12-foot intervals, and at changes in direction. The sprinklers shall be hydraulically designed to provide 0.5 GPM over an area derived by multiplying the distance between the sprinklers in a horizontal duct by the width of the duct. A separate indicating control valve shall be provided for sprinklers installed in ductwork. Drainage shall be provided to remove all sprinkler water discharged in ductwork. The sprinklers shall be accessible for periodic inspection and maintenance. (UMC 609.7, NFPA 318 2-1.2.6.1, NFPA 318 2-1.2.6.2, and NFPA 318 2-1.2.6.5). 

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