Fixed Fire Detection

The fire detection grading approach is a coarse hazard assessment, which documents the credible fire hazards and defines and establishes acceptable fire detection performance requirements. The fire detection performance requirements are defined by determining what size of fire presents a credible escalation hazard. The systematic application ofthe grading process then develops an engineered detection system whose performance can be verified. The general process utilized to develop the detection layout is as follows  

Where the need for fire detection is identified, the required performance of the fire detection system is already specified as part of the grading process. Fixed fire detection is typically installed to protect equipment that is high value, long lead time, or likely to be significant fire escalation hazards. The performance specification defines fire size and response time thresholds for alarm and action(s). Fire hazards are defined by radiant heat output (RHO). RHO gives a reasonable indication of the potential damage and the probability that the fire will escalate or cause loss. The RHO should not be used to determine fire thermal loading onto equipment and structures. Table 8-3 compares RHO and flame area for some typical hydrocarbon fires. 

The decision to provide or not to provide fixed fire protection systems within process structures, areas, or on specific vessels or equipment is usually based on a qualitative assessment of the following factors such as  

Use of fire hazard analysis (FHA) approach will determine the size and expected duration of fires and allow selection of fire protection on a performance basis (see Chapter 5). 

Comparison of RHO and Typical Apparent Flame Area for Hydrocarbons 

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Table 17.2 Application of Fixed Fire Detection Devices ...

Fire-detection devices such as flame-sensing or ioni2ation-interference types operate much more rapidly than sprinkler heads and are used extensively both as alarms and to activate fixed fire-extinguishing systems. 

Provide safe separation distances Install fixed fire protection and alarms, water sprays (deluge), and/or foam systems activated by flammable gas, flame, and/or smoke detection devices... 

Table 6.13 General requirements for fixed fire-extinguishing systems (Activation may be automatic by a detection system, or manual)...

Switchgear and relay rooms are required to have smoke detection per NFPA 850, section 5.8.4 and IEEE 979, section 2.7. The activation of the fire alarm should shut down the air handling system. If the facility is especially critical to the continued hydrocarbon process consideration of a fixed fire protection system should be evaluated. 

Where a detection system is part of an automatic, fixed fire extinguishing system, complete compatibility between the systems is essential. 

It is important to select detection devices that are appropriate for the type of fire most likely to occur. Failure to do so will result in either a very slow response or the possibility of a large number of spurious alarms. The latter should be particularly avoided where the detection system is used to activate a fixed fire extinguishing system. 

Where water supplies are inadequate for conventional firefighting and foam making, automatically activated fixed clean agent or CO2 systems may be considered for seal fire protection. The agent should be discharged into the seal area below the secondary seal. Fire detection options for these automatic systems include reusable thermal wires and pneumatic tube devices. Thermal wire is typically the more economical choice. 

Fixed fire protection systems (detection and suppression) and portable fire suppression equipment... 

Radiation Protection (fixed area monitors) Fire Detection Water Purification. In operation In operation In operation... 

The fire scenario selected to be analyzed deals with the fire fighting means designated to be applied in a reference NPP once a fire occurs for any reason. One major assumption is the failure of the automatic actuation of the fixed fire extinguishing system in the compartment where the fire is supposed to start. This implies that fire fighting mainly depends on the operability of the fire detection and darm system, the performance of human actions as well as on the operability of active fire barrier elements (e.g. fire dampers and fire doors) and of the fire extinguishing systems which can be manually actuated. 

If equipment and procedures work as intended, fire fighting is a rather short process, because the compartment where the fire is assumed to occur is equipped with a fixed fire extinguishing system which suppresses the fire with a sufficiently large amount of water after actuation by the fire detection and alarm system. However, one major assumption of the andysis presented here was the failure of the automatic actuation of the fixed fire extinguishing system. This implies that the fire has to be extinguished by manual fire fighting means performed by the plant personnel in charge. 

Subpart L is concerned with fire protection and fire prevention. This subpart contains requirements for fire brigades, aU portable extinguishers, fixed fire suppression systems and fire detection systems, and alarm systans. It contains training requirements for the organization and personnel. It describes requirements for training and protective equipment for fire brigades. 

Properly designed and installed fixed fire suppression systems enhance fire safety in the workplace. Automatic sprinkler systems throughout the workplace are among the most reliable fire fighting means. The fire sprinkler system detects the fire, sounds an alarm, and puts the water where the fire and heat are located. 

Other types of fire protection equipment or systems can be broken into four categories (1) portable fire suppression equipment including standpipe and hose systems (2) fixed fire suppression equipment including automatic sprinkler systems and fixed extinguishing systems (3) fire detection systems and (4) employee alarm systems. In addition, certain equipment is often installed in workplaces to control heat sources or to detect fuel leaks. An example is a temperature limit switch. Alarm systems may include, but are not limited to, manual pull box alarms, public address systems, radio, or telephone.)... 

Fixed Extinguishing Systems -1910.160 Fire Detection Systems -1910.164 Grain Handling - 1910.272 Vinyl Chloride - 1910.1017(i)... 

OSHA has many requirements concerning fire protection in the workplace. These requirements are found in Subpart L and in some other specific standards. Among the items addressed by OSHA standards is fire brigades, all portable and fixed fire suppression equipment, fire detection systems, and fire or employee alarm systems. 

The usual protection for large installations is to provide a water-spray system. For small bulk storage, fire hoses or monitors are often adequate. However, for installations over 50 tons of storage (and all major cylinder-filling plants) it is accepted that a fixed water-spray system needs to be provided which is automatically initiated by a system capable of detecting a fire threatening the vessels... 

There are two common types of heat detectors - fixed temperature and rate of rise. Both rely on the heat of a fire incident to activate a signal device. Fixed temperature detectors signal when the detection element is heated to a predetermined temperature point. Rate of rise detectors signal when the temperature rises at a rate exceeding a pre-determined amount. Rate of rise devices can be set to operate rapidly, are effective across a wide range of ambient temperatures, usually recycle rapidly and can tolerate a slow increase in ambient temperatures without providing an alarm. Combination fixed temperature detectors and rate of rise will respond directly to a rapid rise in ambient temperatures caused by fire, will tolerate a slow increase in ambient temperatures without effecting an alarm, and recycle automatically on a drop in ambient temperature. 

There are few models with automatic test capability. Testing is usually limited to hand held devices only 2 meters (7 ft.) from the detector or directly on the lens test unit. It can be ineffective if ice forms on the lens. It is sensitive to modulated emissions from hot black body sources. Most of the detectors have fixed sensitivities. The standard being under five seconds to a petroleum fire of 0.1 square meter (1.08 sq. ft.) located 20 meters (66 ft.) from the device. Response times increase as the distance increases. It cannot be used in locations where the ambient temperatures could reach up to 75 °C (167 °F). It is resistant to contaminants that could affect a UV detector. Its response is dependent on fires possessing a flicker characteristic so that detection of high pressure gas flames may be difficult. 

Fixed installations, such as water spray systems, halon systems, sprinkler systems, carbon dioxide extinguishing systems, explosion suppression systems, and other fire protection installations are often provided with flow and trouble detection switches connected to transmitters. A signal indicating the condition of the system should be sent to the attended location(s). 

Fixed temperature detectors are preferred because they require less calibration and maintenance. Heat detectors are normally more reliable than other types of detectors because of the simple nature of their operation and ease of maintenance. These factors tend to lead to fewer false alarms. The main disadvantage of heat detectors is that they are unlikely to detect fires in the incipient stage, where little heat is generated, but much smoke is likely.. Since heat detectors are inherently slower in operation than other types of detectors, they should be considered for installation in areas where high speed detection is not required. 

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