Fire protection and fire alarm system

The fire protection approach as defined in Ref. [6] is generally adopted as a guideline for the design and construction of fire protection systems. Creation of multiple fire barriers has been adopted as a general convention.

However, this system poses some problems during construction. Emphasis is now placed on a simplified fire barrier system with early detection and active fire protection as advancements. The fire water pump house is conventionally constructed as a part of cooling water pump houses of portable water. Main fire water headers are conventionally laid as buried piping to various buildings and locations. The water risers are duplicated and sourced by redundant water headers at various floors of the plant in order to avoid single cause failure.

Some fire water pump houses are now constructed as safety related structures with their own water reservoir, and the fire water piping is laid in the tunnels with adequate space for maintenance and in-service inspection. The redundancy in power supply to the fire water pumps, additional diesel-driven fire water pumps (shown in Fig. 46) working on independent source of power supply, and redundancy in main fire water headers and piping are laid out and constructed such that any adverse situation can be handled.

It is to be ensured during construction that at least one ring header and one fire water riser of the important areas are fed from two independent sources of water supply. Such areas have sufficient combustible material or fire load for firefighting in any of the NPP locations, in spite of the single cause failure as postulated in the design.

FIG. 41. Insulation spacer rings.

FIG. 42. Insulation details for flanged joints/Grayloc joint.

FIG. 43. Details for insulation areas prone to inadvertent foot stepping load.

FIG. 44. Typical insulation details for valves.

Manual firefighting systems were conventionally chosen over the automatic sprinkler system (shown in Fig. 47) to minimize spurious alarms leading to operation of the automatic system inside the containment building.

However, automatic sprinkler systems inside the containment are now effectively being adopted as a result of robust advancements that have taken place in fire detection and fire alarm systems.

FIG. 45. Insulation details for welded joints.

FIG. 46. Fire water diesel generator pump (credit: NPCIL).

The operation of fire protection systems requires positive fire detection. With the advancement of electrical cable insulation material, use of non-volatile oil and other material with slow burning capabilities, the fire load can be further reduced.

Major areas vulnerable to fire hazards in an NPP are:

— Turbine oil systems;

— Transformer oil systems;

— Oil hydraulic systems being used as instrumented medium;

— Polymer material;

— Diesel oil;

— Hydrogen used in the turbogenerator cooling system;

— Cable spreading room;

— Other combustion material such as insulation, wood and cotton rags.

Fire water systems with a sufficient number of hydrants are networked in all the buildings of NPPs, except for diesel generator rooms, main oil tanks and turbine oil tanks, which are provided with an automatic carbon dioxide system. Use of a carbon dioxide system in these areas is more appropriate and safer than the fire water system, because of energized equipment.

A high velocity automatic sprinkler system (shown in operation in Fig. 48), commonly known as an automatic deluge system or mulsifire system, is provided to protect all different types of transformers installed in the plant.

FIG. 47. Automatic sprinkler system (credit: NPCIL).

FIG. 48. Automatic sprinkler system in operation (credit: NPCIL).

Fire water and carbon dioxide are used as the main fire extinguisher materials. The major advancement has been in early fire detection and effective actuation of fire protection systems in a positive manner. Figure 49 shows some field fire detectors.

Fire alarm system

The fire alarm system is designed and constructed for the detection of fire in its early stages so that it can be extinguished effectively and rapidly. The fire detectors may be smoke detectors (shown in Fig. 50), thermal detectors, flame detectors or manual push button call points installed at various locations in the plant. Ionization type smoke detectors with a dual ionization chamber have a higher sensitivity to relatively smaller smoke particles than the optical type of smoke detectors.

Thermal detectors sense the temperature by means of thermisters. The thermisters provide a fast response to rapid increase in temperature, and are capable of actuating an alarm when the surrounding temperature reaches a preset point. The beam detector works on the light obscuration principle. These advanced detectors are used most effectively inside the containment as supplements to other types of detectors.

Advanced flame detectors that work on the principle of detecting the infrared radiation emanating from the flame are also used to supplement smoke detectors provided in the various rooms and locations of the containment.

Linear heat sensors are provided in the cable trays for detecting the fire in electrical carrier systems at regular intervals. However, utmost care must be exercised during the installation of these cables. The location of the fire

FIG. 49. Field fire detectors (credit: NPCIL).

FIG. 50. Smoke detector (credit: NPCIL).

detectors is decided after considering the possible air movement as contemplated in normal ventilation, air-conditioning and contaminated air purge systems. Detectors are also located right above the main equipment and at other vulnerable points, as shown in Fig. 51.

Guidelines given in Ref. [6] are generally followed for the placement of detectors.

Fire control panels are conventionally kept inside the main control room. As an improvement, local area control panels are also provided so that the field operator can operate the fire protection system in case of emergency. There are various methods for looping the detectors, but the class A mode is preferred. The central control unit of the fire alarm system has undergone major advancements. A software controlled microprocessor based central control panel has been provided (Fig. 52), as well as an alarm display on supervisory computers (Fig. 53) and logging on printers.

The control panels are entirely independent of each other so that the failure of one control panel does not affect the standby control panels. Each control panel provides local indication with a response time of less than one minute. Each control panel is generally configured with a dual redundant power supply unit, one of them being through the station power battery system. The self-diagnostic feature of the control unit continuously monitors the functioning of the detectors and health of the loop wiring, etc.

With the advancements now available in the software for fire alarm detection, all problems are reported to the control unit in such a manner that quick and easy rectification is possible. Supervisory computers have a colour graphic facility to store plant floor plans.

FIG. 51. Fire detector on equipment (credit: NPCIL).

As shown in Fig. 54, the operator is able to see the corresponding floor plan should any of the fire detectors be actuated in plant in a particular zone. This also actuates automatic stoppage of intake ventilation fans and actuation of various fire dampers.

Conventionally, mineral insulated cables used for interconnecting various fire detectors, and multi-pair fire retardant low smoke (FRLS) cables were used for interconnecting the various buildings. Use of poly-ether-ether-ketone (PEEK) is now being preferred as an improvement. The fire alarm system also had audiovisual annunciations both in the control room and in the affected areas as an effective warning to take immediate remedial action. The entire fire alarm system, including detectors and control unit, is qualified as per noise immunity standards. All types of detectors and field modules are also qualified to meet the background gamma radiation requirements as per applicable codes. Many advancements are incorporated in the fire detection system from time to time by providing effective sensors on rotating equipment such as bearing temperature control equipment and field wiring temperature equipment; on the cable carrier system at regular intervals; and in the overall volumetric area under surveillance.

FIG. 53. Supervisory computer (credit: NPCIL).

FIG. 54. Graphics of plant layout (credit: NPCIL).