Firewater Lines

If a firewater line needs to be temporarily isolated and an isolation means is not available on the immediate portion of the system needing work, a unique solution is to use a liquid nitrogen low temperature coil line freezing apparatus. This mechanism causes an ice plug to form in the line, effectively sealing the line from leakage, during the period the temporary isolation is needed. 

The type of MIC-related problems that may be expected in, for exanple, submersible and semi-submersible platforms are, more or less, similar to stagnant water problems caused in firewater lines or pipelines. Such platforms have pontoons and columns that, when flooded with seawater, cause the pontoons to submerge to a predetermined depth. Figure 7.10 shows an example of a semi-submersible offshore platform. 

Steel pipe should be used aboveground for firewater lines. Underground piping systems can be constructed of steel, cement-lined steel, or high-density... 

In the third e.vample, the line terminates at 53%. This means DO NOT run this pump at less than 53% of the BEP. 53% of 4500 gpm is 2385 gpm. Because this is a firewater pump and because firemen need to throttle the nozzles on their fire hoses, then we need to install a pressure relief valve on this system with a discharge bypass line so that the pump dumps the restricted water (less than 2400 gpm) back into the suction tank or lake. If not, this firewater pump is likely to suffer bearing failure during an emergency. 

For means of protection, the use of water based suppression systems may be a hazard due to the disposal of firewater water, which will freeze quite readily in exposed locations. This may also be the case with exposed hydrocarbon fluid lines that, if isolated, say for an ESD activation, may freeze up due to lack of circulation. This will hamper restart operations for the facility. Typical use in the past has been the reliance on gases fire suppression agents for enclosed area, particularly Halon. Other methods include fire water storage tanks that are kept warm, together with fire mains deeply buried and continually circulated. 

Safety Systems. Major expenditures here include the flare system (the flare structures and large lines extending throughout the plant) and the firewater system (high-capacity pumps and extensive piping). Safety systems, fortunately, are usually given particular attention. At this study phase, the main thrust should be to check the completeness of licensor equipment lists for cost estimation purposes. 

Sacrificial areas - the use of sacrificial areas involves the routing of firewater run-off to a designated, remote area, which is provided to allow infiltration of any contaminant and to prevent run-off from the site. The contaminant is often contained within a layer of permeable soil or other similar material and should be prevented from dispersing into other strata or ground water by an impermeable lining system which should be capable of containing both vertical and horizontal seepage. 

In climates where freezing does not occur, above-ground installation of steel firewater distribution lines has the advantages of low first cost and ease of inspection and repair. In cold climates, distribution lines should be buried below the frost line. 

An underground firewater system is any part of a firewater system that is underground, including storage, piping, pumps, valves, hydrants, and monitors. The underground firewater system should be a loop with branches off it. This means that, if the underground line is broken or blocked at any point, firewater can still reach the point where it is needed. 

Earthquakes (and tsunamis) pose a great danger to process plants. An earthquake can cause equipment items to collapse, lines to rupture, and tanks to fail. For this reason, the construction codes used in seismically active areas are very strict. One item to watch for in particular is a situation where an earthquake causes a major leak of flammable materials and also destroys the firewater header. 

A reverse flow incident involving a temporary hose connection led to an explosion and fire in which two people were seriously injured. A hydrocarbon system was to be cleaned using water. A hose was connected to the firewater system as shown in Figure 17.7. It was left overnight prior to starting work the next day. The hydrocarbon system was full of liquid at low pressure. The block valves shown were left open, even though they should have been closed. A temporary check valve was not placed in the hose line. 

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