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TO AVOID COMMON FAILURE INCIDENTS, MAIN, RESERVE AND SUPPORTING FIRE WATER PUMPS SHOULD NOT BE LOCATED IMMEDIATELY NEXT TO EACH OTHER
Fire water systems usually require varied amounts of water at a relatively constant pressure. And fire water pumps usually operate in parallel. A relatively flat performance curve (head vs. capacity) eases parallel operation and is encouraged for centrifugal fire water pumps. Sometimes a large amount of water can be required by the fire water system (considerably larger than the rated point of the pump). Fire water pumps should be able to provide more than 150% of rated capacity at more than 65% of total rated head. A steep curve should always be avoided for fire water pumps.
A properly selected pump should preferably exceed the above values and show good performance at the far right side. When the pump is operating near the end of the curve, it can provide a water flow of more than 165% of the rated flow while having sufficient head (more than 70% of the head of the rated point to make sure the pump can deal with extreme situations).
The main fire water pumps are generally electrically driven while backup or reserve fire water pumps are generally diesel-engine driven. Small pressure changes due to variations in fire water consumption during a fire can result in unstable operation of the main fire water pumps. Therefore, unnecessarily rapid changing of the operating point of a large pump should be avoided as this could result in performance and reliability issues. Small capacity pumps, known as jockey pumps, are usually employed to maintain a relatively constant fire water pressure.
Jockey pumps usually start operation after a relatively small pressure drop. A commonly used arrangement for industrial plants is six fire water pumps: Two electric motor-driven pumps, two diesel engine-driven pumps and two jockey pumps. They are nearly always provided on a prefabricated skid. Fire code NFPA 20 provides specifications for pump tests, the performance curve, accessories and auxiliaries and packaging. This should be considered minimum requirements.
Fire water pumps are usually manufactured in standard capacities. One manufacturer offers 10 models, each with about 40% to 50% more capacity than the previous one. Fire water pump location should be carefully selected. Explosions or high hazard fires are the major concerns which can disable pumps. Ideally, there should be 40 m to 80 m clearance between pumps and a process unit or hazardous storage area.
This limit should also be respected for areas such as a power generation unit, a gas compression unit and an oxygen generation unit. The diesel fuel tank capacity should preferably last 10 hours as a minimum requirement. Some critical plants require 24 hour fuel tanks for each diesel engine. Each diesel engine should be provided with independent auxiliaries and accessories, including a dedicated fuel system and fuel tank. Diesel engine failure is usually traced to auxiliary systems such as the fuel system, lubrication system, a starting issue, wiring or component fatigue.
Overhauls and repairs are required, just like any other properly designed combustion engine. A suitable enclosure or building should be provided for fire water pumps. Sufficient reinforcement should be considered in case of a major earthquake or other incident. An open sided shelter is not desirable, although this can be observed in many plants. Fire water pumps should also be located upwind and at a higher elevation than the rest of the facility. To avoid common failure incidents, main, reserve and supporting fire water pumps should not be located immediately next to each other. This improves reliability and overall hydraulic behavior.
Fire Pumps for offshore
Fire water pumps for offshore applications such as oil & gas platforms should usually be submerged in the water. In some locations, sea water levels may fluctuate from -6 m to +14 m. As offshore platforms and other floating units are anywhere from 20 m to 50 m above the water level, offshore fire water pumps should be capable of producing a relatively high head. Offshore fire pumps are usually multi-impeller pumps. Properly designed highly reliable pumps are always specified. Electric motor-driven submersible pumps, downhole vertical turbine line shaft pumps or hydraulic-driven pumps are used for offshore applications.
The fire water pumping system is often located on a separate utility or accommodation offshore platform (or on a separate section). This saves them from major explosions (or other forms of damage) by gas or oil offshore modules. In addition, unmanned offshore facilities are becoming common and the trend is toward more compact design. In some offshore designs, the fire water pumping system is located at or near a non-process or utility module (not adjacent to hydrocarbon or hazardous units). Some modern specifications require 3 × 100% fire water pumps for offshore applications. A concern here is life safety during a major fire and to make provisions for a safe evacuation.