L O S S C O N T R O L

Transcription

1 L O S S C O N T R O L GAS-FIRED SUSPENDED UNIT HEATER technical guide

2 Gas-Fired Suspended Unit Heaters Permanently mounted, gas-fired unit heaters are self-contained units that consist of a burner, combustion chamber, heat exchanger and fan. Gas control equipment will typically consists of a 24 volt combination gas valve consisting of main and pilot gas cocks, main and pilot gas pressure regulator, pilot solenoid gas valve and safety shut-off valve. The most common type is an indirect-fired unit that has an automatic or semiautomatic ignition. This and all units should be equipped with a build-in safety control circuit. Widely used in industry, gas-fired unit heater often introduce fuel hazards, open flames and hot surfaces in areas built of combustible construction or containing combustible contents. Unit heaters should not be installed in hazardous areas where there are combustible dusts, flammable liquids, vapors or gases, or lint-producing fibers unless specifically listed/classified for the occupancy. Standard Equipment Gas-fired suspended unit heaters will consists of the following basic components: Cabinet usually constructed of cold rolled steel Heating Section constructed of aluminized steel or stainless steel. Gas Components Gas manual shutoff valves, gas pressure regulators, pilot and main gas burners, supply gas piping. Electrical Components Electrical ignition system, solenoid valves, gas safety shutoff valve, high-limit control, fan control. Installation Procedures Clearance A listed gas unit heater should be installed with its sides, back and bottom at least 18 from any combustible material. If the heater has an internal draft hood, the clearance at the top should be at least 6 feet. A 1-inch clearance is required above the top of the sloping side of a vertical draft hood. A front clearance of at least 24 inches is recommended for all unit heaters. This clearance should be increased to 48 inches for externally mounted burners. Unlisted or listed oil and gas oil-fired unit heaters must be installed with the minimum clearances indicated on the heater or in the manufacturer s installation instructions. Additional servicing clearances may be needed. Refer to the manufacture s installation instructions.

3 Venting Unit heaters should be connected to noncombustible flues, chimneys or vent connectors, which discharge from the building. Listed Type B vents, usually constructed of double-walled, corrosion resistant metal piping, are commonly used. The venting system must include a listed cap and roof assembly. The manufacturer's minimum clearance requirements should be followed, although a 1-inch clearance is usual. Ordinary single-wall metal pipes must have a minimum clearance of at least 9 inches. Type B vents and single-wall metal pipe must: Terminate not less than 5 feet above the heater; and Extend 2 feet above the roof or any portion of the building or roof that is within a 10-foot horizontal radius. Only listed Type B vents are permitted to pass through an attic, concealed space or combustible floor or ceiling. However, a single-wall metal pipe vent may pass through a roof constructed of combustible material provided that either of the following conditions is met: 1. A non-combustible, non-ventilating thimble is installed around the vent through the roof opening. The thimble s diameter should be at least 4 inches greater than that of the vent pipe, and it should extend at least 18 inches above and 6 inches below the roof. The annular space should be open at the bottom and closed at the top. 2. A non-combustible, ventilated thimble is installed around the vent through the roof opening. 3. If the unit heater is listed and the vent pipe is less than 6 feet long, the thimble s diameter must be at least 4 inches larger than that of the vent pipe, If the vent pipe is longer than 6 feet, the thimble s diameter must be at least 2 inches larger than the vent pipe s diameter. 4. If the unit heater is unlisted, the thimble s diameter must be at least 6 inches larger than the vent pipe s diameter. Seismic Bracing & Flexible Gas Piping Provide the following safeguards for space heaters located in geographic areas subject to seismic activity: Provide flexible gas line between gas supply piping and the heater. Brace all four sides of the heater as illustrated below.

4 Adequate bracing will prevent the unit from twisting and/or dislodging from hanging supports particularly when hangers and hanger connections are not strong. Strong connections and bracing will allow the unit to move with the roof diaphragm during seismic events resisting damage by seismic forces. The more ridged the bracing the better. Flexible connections will allow movement between the unit and the gas supply, which may not be well secured to the roof diaphragm. When properly braced and provided with flexible gas connections it is expected that there will be no damage to the heater and no leakage of gas when subjected to seismic forces. If not properly braced and with only moderate shaking intensity there can be expected mechanical damage and and/or gas leak which may result in fire. With severe shaking intensity the unit can be expected to fall with gas connections broken resulting in significant gas leakage with potential of explosion and fire. Automatic Sprinkler Protection Considerations National Fire Protection Association Standard 13, Standard for the Installation of Sprinkler Systems, Article addresses the need for the provision of high temperature and intermediate temperature rated automatic sprinkler heads within the high temperature zone (7ft from heater all directions) and intermediate temperature zone (7ft to 20ft cone shaped dimension from unit discharge) of unit heaters. Non-adherence to this standard may result in the premature operation of sprinkler heads if not of the proper temperature rating.

5 Suspended Space Heaters and Storage The following safeguards should be adhered to when space heaters are used in occupancies storing combustible materials such as warehouse and distribution facilities: Maintain combustible storage a minimum of three feet from heater units. It is preferred to locate space heaters over storage aisles in lieu of directly over storage. If possible, limit the lift height of forklifts to less than that height with pallet load that could cause impact with space heaters. Forklift operators should be instructed to immediately report any incident of hitting a space heater to facilitate immediate inspection of the unit for damage. Forklift operator training should include observance of the above safeguards. During in-house fire inspections, all space heaters should be inspected for damage, proper operation and compliance of the above safeguards. Any unsatisfactory conditions should be repaired immediately. This information has been developed from sources believed to be reliable. However, since it is a general guide to safety, compliance with all federal, state or local laws and regulations is the policyholder s responsibility.