Fire alarm Detector Placement

 

Spot-type detectors are usually installed on the ceiling, not less than 4 in. (0.1 m) from the wall. If their listing permits, they are also permitted to be installed on the wall, with their highest edge no less than 4 in. (0.1 m) and no more than 12 in. (0.3 m) from  the ceiling (Figure 14.2.16).

Where subject to mechanical damage, detectors must be protected. Any mechanical guard used with a smoke or heat detector must be listed for use with the detector. Otherwise, sensor performance may be degraded. Smoke detectors are often required to be set at higher sensitivity when used with plastic or perforated metal guards due to their effect on smoke entry. One manufacturer has a unique metal guard that looks like the series of smoke detectors it protects and that is engineered not to affect detector sensitivity. 

Smoke detectors should not be installed in an air stream from an HVAC supply grill, because that will inhibit smoke from a fire in the protected space from reaching the detector. (The smoke detector would be bathed in a clean air stream when the HVAC supply fan is running.) This can also affect heat detector performance, usually to a lesser degree. Locations adjacent to a return air grill should also be avoided, because returns can affect air circulation patterns in the room so as to inhibit the detection of smoke from low-energy fires.

FIRE ALARM SYSTEM BASICS

 

The basic components of each system are:

1. A system control unit (Figure 14.1.1) 

2. A primary, or main, power supply
3. A secondary, or standby, power supply
4. One or more initiating device circuits or signaling line circuits to which manual fire alarm boxes, sprinkler waterflow alarm initiating devices, automatic fire detectors, and other fire alarm initiating devices are connected
5. One or more fire alarm notification appliance circuits to which audible and visible fire alarm notification appliances, such as bells, horns, stroboscopic lamps, and speakers, are connected
6. Many systems also have an off-premises connection to a central station, proprietary supervising station, remote supervising station, or public fire service communication center by means of an auxiliary fire alarm system

Primary and Secondary Power Supplies

 The primary power is usually supplied by a connection to utility-generated electric power. The  connection must be from a branch circuit dedicated to the fire alarm system. The circuit and connections must be mechanically protected. The circuit disconnecting means must have a red marking, be accessible only to authorized personnel, and be identified as “Fire Alarm Circuit Control.” Inside the fire alarm system control unit, a permanent legend must identify the location of the electrical panel board that contains the circuit disconnecting means.

Secondary power supply for a fire alarm system is required to automatically supply the energy to the system within 30 seconds whenever the primary power supply is not capable
of providing the minimum voltage required for proper system operation.

The size of the secondary supply usually is measured in the amount of time that the secondary supply will operate the system, followed by a prescribed time period for the system to operate in an alarm condition. Local (protected premises), central station, remote station, proprietary, and auxiliary systems must have 24 hours of standby power, followed by 5 minutes of alarm. Emergency voice/alarm communication systems must have 24 hours of standby power, followed by 2 hours of emergency operation. To allow calculation of the power required for 2-hour emergency operation, NFPA 72 specifies that the 2 hours of emergency operation are the equivalent of 15 minutes of operation under full load (i.e., with all input devices and output appliances operating).

Design of Means of Egress

 

Designing a means of egress involves more than numbers, flow rates, and densities. Safe exit from a building requires a safe path of egress from the fire environment.

The path is arranged for ready use in case of emergency and should be sufficient to permit all occupants to reach a safe place before they are endangered by fire, smoke, or heat.

Proper egress design permits everyone to leave the fire-endangered areas in the shortest possible time with efficient exit use. If a fire is discovered in its incipient stage and the occupants are alerted promptly, effective evacuation may take place.

     Maximum permitted evacuation travel distances are related to the occupant characteristics, occupant alertness, and building fire protection. The less capable people are to move, the less alert they are (such as sleeping), and the less protected a building is (such as no automatic sprinkler protection), the shorter the permissible travel distance.

Depending on the physical environment of the structure, the characteristics of the occupants, and the fire detection and alarm facilities, fire or smoke may prevent the use of one means of egress. Therefore, at least one alternative means of egress remote from the first is essential. Provision of two separate means of egress is a fundamental safeguard, except where a building or room is small and arranged so that a second exit would not provide an appreciable increase in safety.

In some proposed egress designs, all the exits discharge through a single lobby at street level, even though this procedure results in egress travel through a common space. This design philosophy presumes that the lobby may be considered a safe area for all future egress needs during the life of the building. Where two remote means of egress are required, this type of egress design is not permitted by the Life Safety Code or by most model building codes.

SOURCE :  FIRE PROTECTIONHANDBOOK 

How many feed wide required for corridors in new hospital

 

the minimum width corridor required  in hospital ?

As per NFPA 101 The width of an exit access should be at least sufficient for the number of persons it must accommodate. In some occupancies, the width of the access is governed by the character of activity in the occupancy. One example is a new hospital, where patients may be moved in beds or in gurneys. The corridors in the patient areas of the hospital must be 8 ft (2.4 m) wide to allow for a bed to be wheeled out of a room and turned 90º.

Ventilation of Restaurant-Type Cooking Equipment (CFPS)

 

Exhaust systems for restaurant equipment require careful design, because grease condenses in the interior of the ducts. Grease accumulations may be ignited by sparks from the cooking appliance or by a small fire on the cooking appliance caused by overheated cooking oil or fat in a deep-fat fryer or on a grill.
If the duct did not have a grease accumulation, cooking appliance fires could often be extinguished before causing appreciable damage.

Fire risk is especially high in frying because cooking oils and fats are heated nearly to their flashpoints

Ducts. The following should be considered when designing a duct system for commercial cooking equipment:

1. The system should be designed to minimize grease accumulations, with a minimum air velocity of 1500 ft/min (458 m/min) through any duct.
2. Ducts should be arranged with ample clearance from combustible materials to minimize the danger of ignition, incase of fire in the duct.
   
3. Ducts of substantial construction (not lighter than No. 16
   Manufacturers Standard Gauge steel or No. 18 Manufacturers Standard Gauge stainless steel) should be used with all seams and joints having a liquid-tight, continuous weld.
   
4. Systems should be separated to ensure there is no connection with any other ventilating or exhaust system.
   
5. Ducts should be led directly outside the building, without dips or traps, unless automatic grease removers are employed at the dips and traps.
   
6. Openings should be provided for inspection and cleaning. Dampers should not be installed in any duct system, unless required as part of a grease extractor or extinguishing system.
   

DRY CHEMICAL EXTINGUISHING

 

OVERVIEW OF DRY CHEMICAL EXTINGUISHING AGENTS

 Dry chemical extinguishing agents are commonly listed for use on Class B and Class C fires. Dry chemical extinguishing agents are not effective on Class D fires. Multipurpose dry chemical because they are listed for use on Class A, B, and C fires.

The principal base chemicals used in the production of currently dry chemical extinguishing agents are sodium bicarbonate, potassium chloride, urea potassium bicarbonate, and mono-ammonium phosphate.

Dry extinguishing chemical agents are stable at both low and normal temperatures and are considered to be non toxic. However, other physiological conditions such as minor skin and respiratory irritations may occur on exposure to the agents.

Toxicity.

Dry extinguishing chemical agents are stable at both low and normal temperatures and are considered to be nontoxic and noncarcinogenic. However, other physiological conditions such as minor skin and respiratory irritations.

Extinguishing Properties

Smothering, cooling and radiation shielding contribute to the extinguishing efficiency of dry chemicals .but studies suggest that a chain-breaking reaction in the flame is the principal cause of extinguishment.

Cooling

Cooling action of the dry chemical cannot be substantiated as an important reason fot its ability to promptly extinguish fires. To be effective, any dry chemical must be heat sensitive and, as such, absorb heat in order to become chemically active.

Radiation Shielding.

Discharge of dry chemical produces a cloud of powder between the flame and the fuel; this cloud shields the fuel from some of the heat radiated by the flame.

Smothering Action

For special applications, such as kitchen range, hood, duct and fryer fire protection, the extinguishing mechanism for dry chemical is based on the process of saponification. Saponification is the process of chemically converting the fatty acid contained in the cooking medium to soap, or foam, and it accomplishes extinguishment by forming a surface coating that smoothers the fire.

Chain-Breaking Reaction.

The discharge of dry chemical into the flames prevents reactive particles from coming together and continuing the combustion chain reaction. The explanation is referred to as the chain-breaking mechanism of extinguishment.

Fixed systems are of two types:

• Total Flooding
• Hand Hose line/ Local Application

Uses and Limitations of Dry Chemical Systems

• Due to the rapidity with which dry chemical extinguishes flame, dry chemical is used on surface fires involving ordinary combustible materials (Class A Fires).
• There are several areas in the textile industry, notably opener-picker rooms and carding rooms in cotton mills, where regular dry chemical has been used effectively.
• Multipurpose dry chemicals  becomes sticky when heated, it is not recommended for textile card rooms or other locations where removal of the residue from fine machine parts may be difficult.
• Dry chemicals should not be used in installations where relays and delicate electrical contacts are located, as the insulating properties of dry chemical might render such equipment inoperative.
• Dry chemical extinguishing systems can be used in those situations where quick extinguishment is desired and where re-ignition sources are not present.
• Regular dry chemical will not extinguish fires that penetrate beneath the surface or fires in materials that supply their own oxygen for combustion.

In total flooding applications, a predetermined amount of dry chemical is discharged through fixed piping and nozzles into an enclosed space or enclosure around the hazard.

Hand hose line systems containing regular or ordinary dry chemical have been used to a limited extent for quick spreading surface fires on ordinary combustible material.

Required Delivered Density

 

Required delivered density (RDD) is the minimum rate of water application that, if delivered to the top of the fuel package, is capable of providing early suppression.

Suppression can be considered to occur when the heat release rate of the fire is quickly knocked down and is prevented from resurging.

Actual delivered density (ADD) is the actual rate of water application that a particular configuration of flowing sprinklers is capable of delivering to the top of a fuel package, depending on the strength of the upward fire plume.