Fire fighting System An Overview

Firefighting

Ian Sutton, in  Plant Design and Operations (Second Edition), 2017

Firewater Systems generally has four main sections:

1.

A supply of firewater. This can come from storage tanks, a firewater lagoon, or a natural body of water such as the sea or a lake or river.

2.

A pumping system that provides a sufficient flow of water to extinguish the fire.

3.

A header network of pipes, often in the form of a ring main that transfers the water from the pumps to the fire.

4.

Hydrants, nozzles, sprinklers, or other local devices for directing the firewater to the location of the emergency.

Fig.1 Below can be used to describe some of the major components of a firefighting system.

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The facility is divided into zones. If a fire starts in a particular area, water will flow through the nozzles that protect that area.

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If a fire occurs in one of the zones, a fusible link will fail, causing the pressure control deluge valve (PCDV) to open and the main firewater pumps to start. Water will flow out of the sprinkler heads in that zone only. The PCDV can also be tripped manually.

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Individual sections can be isolated for maintenance. However, the isolation of one zone should not lead to the isolation of other parts of the system. For example, if Zone 2 in Fig. 12.1 has to be isolated, then Valves A and B will be closed. However, Valve C remains open so that firewater remains available to Zone 3.

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The only exception to this policy of having two routes to a zone is with regard to noncritical areas such as the fire training grounds. Such areas can be isolated with a single valve.

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There are two firefighting pumps, each with 100% capacity. If one is down for maintenance, the facility still has full firefighting capability.

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Pressure is maintained in the header through use of a jockey pump. If the pressure in the header falls—indicating that the firewater is being used somewhere —the main pumps turn on. In some facilities, the jockey pump is primed with cooling water.

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If freshwater is used, the main header will generally be liquid full. If it was dry, it would take a long time to fill it—something that would delay the emergency response. However, if seawater is used as the firefighting medium, the headers will normally be dry because they would otherwise be subject to corrosion. If during an emergency a normally freshwater-filled system has to be replenished with more corrosive water (such as seawater), the system can still be considered a freshwater system, assuming that prompt flushing takes place after emergency use to replace the corrosive water in the system.

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The sprinkler systems downstream of their own block valves  are generally dry. It will not take long to fill them and having water permanently present could lead to leaks and corrosion.

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Fig.1 shows the location of a “Critical Equipment Item.” This means that a high level of fire protection should be applied to that area, either because it is of high value or because a release could cause a major safety problem, maybe by making the fire worse. This area, therefore, is protected from both Zones 4 and 5. Consequently, were one of those zones to be out of service for any reason, the critical equipment would still be protected.

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Once the fire has been brought under control the system is reset. If seawater is used, then it is important to flush the zone headers and deluge nozzles with freshwater, otherwise  corrotion products will build up.

Problems with firewater systems can be overlooked for the following reasons:

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The system is rarely tested at full capacity so it is possible that it will not work as it should during an emergency. (There are less likely to be problems with hoses, hydrants, and monitors because these items are used on a more regular basis and can be tested more readily.)

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The firewater pumps are usually located remotely hence they may not be checked as regularly as process equipment.

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For onshore facilities, the firewater header is frequently located underground, thus protecting it from an explosion and vehicle impact. However, putting the header underground can create an “out of sight, out of mind” problem—buried systems may not be inspected and checked as thoroughly as those above ground, hence any problems are not easily seen during the routine external inspections discussed in Chapter 11, Inspection.

They may also be more subject to external corrosion  than if they were above ground. A compromise is to place the header in a below-grade trench. Doing so protects it from explosions but also allows for easy access for inspection and maintenance. This option may also reduce the cost of installation.

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Hot Bend and How it's Made

Induction Bending is a controlled means of bending pipes through the application of local heating using high frequency induced electrical power.

Originally used for the purpose of surface hardening steels, induction technology when used in pipe bending consists basically of an induction coil placed around the pipe to be bent. The induction coil heats a narrow, circumferential section of the pipe to a temperature of between 850 and 1100 degrees Celsius (dependant on the material to be formed). As the correct bending temperature range is reached, the pipe is moved slowly through the induction coil whilst the bending force is applied by a fixed radius arm arrangement.

Manufacture of Induction Bends

Induction bends are formed in a factory by passing a length of straight pipe through an induction bending machine. This machine uses an induction coil to heat a narrow band of the pipe material. The leading end of the pipe is clamped to a pivot arm.

As the pipe is pushed through the machine, a bend with the desired radius of curvature is produced. The heated material just beyond the induction coil is quenched with a water spray on the outside surface of the pipe. Thermal expansion of the narrow heated section of pipe is restrained due to the unheated pipe on either side, which causes diameter shrinkage upon cooling.

The induction bending process also causes wall thickening on the intrados and thinning on the extrados. The severity of thickening/thinning is dependant on the bending temperature, the speed at which the pipe is pushed through the induction coil, the placement of the induction coil relative to the pipe (closer to the intrados or extrados), and other factors.

Most induction bends are manufactured with tangent ends (straight sections) that are not affected by the induction bending process. Field welds are made or pipe pup sections are attached to the unaffected tangent ends, allowing for fitup similar to that found when welding straight sections of pipe together.

Induction bends come in standard bend angles (e.g. 45°, 90°, etc.) or can be custom made to specific bend angles. Compound bends (out-of-plane) bends in a single joint of pipe can also be produced. The bend radius is specified as a function of the diameter. For example, common bend radii for induction bends are 3D, 5D and 7D, where D is the nominal pipe diameter.

Benefits of Induction Bends

• Large radii for smooth flow of fluid.
• Cost efficiency, straight material is less costly than standard components (e.g. elbows) and bends can be produced faster than standard components can be welded.




• Elbows can be replaced by larger radius bends where applicable and subsequently friction, wear and pump energy can be reduced.
• Induction bending reduces the number of welds in a system. It removes welds at the critical points (the tangents) and improves the ability to absorb pressure and stress.




• Induction bends are stronger than elbows with uniform wall thickness.
• Less non-destructive testing of welds, such as X-ray examination will save cost.
• Stock of elbows and standard bends can be greatly reduced.
• Faster access to base materials. Straight pipes are more readily available than elbows or standard components and bends can almost always be produced cheaper and faster.


• A limited amount of tools is needed (no use of thorns or mandrels as required in cold bending).
• Induction bending is a clean process. No lubrication is needed for the process and water needed for the cooling is recycled.

ASME B16.49

ASME B16.49 Standard covers design, material, manufacturing, testing, marking, and inspection requirements for factory-made pipeline bends of carbon steel materials having controlled chemistry and mechanical properties, produced by the induction bending process, with or without tangents.

This standard covers induction bends for transportation and distribution piping applications (e.g., ASME B31.4, B31.8, and B31.11). Process and power piping have differing requirements and materials that may not be appropriate for the restrictions and examinations described herein, and therefore are not included in this Standard

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