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For many process pumps, there have been requirements that head vs. capacity curves should be steep. There have been notes and warnings in different codes and specifications to avoid flat curves for many process or utility pumps. However, the case is totally different for a fire water pump. A relatively flat performance curve (head vs. capacity) is always encouraged for centrifugal fire water pumps because:
1- The operation of a fire water system (in a fire case) usually requires varied amount of water at a relatively constant pressure.
2- Usually, fire water pumps should operate in parallel. A relatively flat curve eases the parallel operation.
(A centrifugal fire water pump)
Sometimes, a large amount of water can be required by the fire water system (considerably larger than the rated point of the pump). In this regard, the fire water pump overload point (the end operating point at the right hand side of the pump curve) should demonstrate a capacity more that 150% of the rated capacity at a head preferably more than 70% of the rated point. A steep curve should always be avoided for fire water pumps.
The main fire water pumps are electrically driven and the spare (the “backup” or the “reserve”) fire water pumps are diesel-engine driven. Because of small leakages and small consumption of fire water, the pressure in a fire water network could be lowered. However, this small consumption should not lead to the start-up of a large fire pump, which could result in many unnecessary on/off operating cycles of the main fire water pump. Small pressure changes due to variations in fire water consumption during a fire incident can result in an unstable operation of the main fire water pumps. In other words, unnecessary fast 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 the operation after a relatively small pressure drop (say 0.5 - 1 Bar).
A commonly used arrangement for industrial plants includes six fire water pumps including two electric motor-driven pumps, two diesel engine-driven pumps, and two jockey pumps. Fire water pumps are nearly always provided on a prefabricated skid. This packaging concept can also help to ease alignment issues.
The fire code “NFPA 20” specifies fire water pumps. Good specifications are required for pump tests, pump performance curve, pump accessories/auxiliaries and some packaging details. In the author’s view, NFPA 20 could be considered as a minimum requirement for a fire water pump. These tests and specifications cover pump shaft details, shaft stresses, bolts, bearing life, materials of construction, performance tests, and other critical items.
The vendor usually manufactures fire water pumps in standard capacities. For example, one well-known manufacturer offers 10 models, each model with 50% capacity more than the previous one. The location for fire water pumps should be carefully selected to minimize various risks and hazardous situations. Explosions or high hazard fires are the major concerns which can disable the fire water pumps. Ideally, there should be 40-80 m clearance between fire water pumps and a hydrocarbon or chemical process unit or a storage area. This limit should also be respected for some utility areas such as a power generation unit, a gas compression unit, an oxygen generation unit.
The possibility of an unconfined vapor cloud explosion is a key concern, which could disrupt utilities, major supports facilities, or damage the fire water pumping system. Generally, there is a great possibility of failure of the electrical network or the steam distribution system in case of a major explosion or an extensive fire event. It shows the critical role of independent diesel engine-driven fire water pumps. The fire water diesel engines should generally comply with “NFPA 37”.
Regarding the diesel fuel tank capacity, typically, 12-hour duration is specified as the minimum requirement. Some critical plants require 24-hour fuel tank for each fire water pump diesel engine. Each diesel engine should be provided with independent auxiliaries and accessories, including a dedicated fuel system and a fuel tank. The start-up of the engine by the battery system (with two independent barriers) is common.
The failure of a diesel engine is usually because of auxiliary systems. Major reasons for failure are fuel system issues, lubrication system problems, starting problems, wiring problems and component fatigue. Overhauls and repairs are required like in any other properly designed combustion engine. Experiences have shown the diesel engine-driven fire water pump is the most reliable option currently available for severe loss incidents in a plant. There are studies on using liquid-fuel microturbine-driven fire water pumps for some critical applications. Reliability and availability of microturbines (small gas turbines) could be higher than diesel-engines, but their efficiencies are relatively lower.
Fire water pumps are arranged for both manual and automatic start-up. Usually, automatic start-up is expected in a very reliable manner and in a very short time after a fire case is detected. Fire water pumps should only stop manually at the pump local panel. A suitable enclosure (or building) should be provided for fire water pumps. Sufficient reinforcement should be considered for the fire pump enclosure for an earthquake incident. In case of a major earthquake, the fire water pumping systems should be fully operational for fire events after the earthquake. An open-sided shelter is not desirable (although can be seen in many plants).
Fire water pumps should be located at a higher elevation of the facilities and upwind. To avoid common failure incidents in critical plants, main, reserve and other supporting fire water pumps may not be located immediately next to each other. Locating fire water pumps at two separate locations can improve both the fire water system reliability and overall fire water network hydraulic behavior.
Fire Pumps for Offshore
Firewater pumps for offshore applications (such as oil/gas platforms) should usually be submerged in the water. For some locations, the sea water level may fluctuate from -6 m to +14 m. Considering a relatively high level of offshore platforms and other floating units (say 20 m to 50 m), offshore fire water pumps should produce a relatively high head. Offshore fire pumps are usually multi-impeller pumps. Properly designed, highly reliable pumps should always be specified. Electric motor-driven submersible pumps, downhole vertical turbine line shaft pumps or sometimes hydraulic driven pumps are used for offshore applications.
Often, the fire water pumping system is located on a separate utility/accommodation offshore platform (or a separate section) to save them from any major explosion (or other forms of damage) in gas/oil offshore module(s). Nowadays, unmanned offshore facilities are becoming very 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 regulations require 3×100% (three 100%) fire water pumps for offshore applications. The main concern is safety of life during a major fire case and provisions for a safe evacuation.
Amin Almasi is a Chartered Professional Engineer in Australia, Queensland and U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.