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| FIGURE 1 Fire-water pump assemblies are typically skid-mounted to ease installation and operation. Shown here is an example of a skid-mounted, diesel-engine-driven, fire-water pump package |
Fire-water pumps are critical machines that save lives and prevent chemical process industries (CPI) facilities from damage more so than any other plant components. Fire-water pumps are nearly always centrifugal pumps with capacity from around 20–3,000 m3/h. Specific requirements for fire-water pumps are briefly noted in fire codes (such as NFPA 20 [ 1]), but this may not be sufficient for specifying high-performance fire-water pumps in a way that ensures good reliability and operation as well as optimum price. This article provides practical notes on fire-water pumps to expand upon the information that can be found in existing fire codes. Various styles and configurations of fire-water pumps are available at different prices. In addition to proper selection, fire-water pumps must be properly integrated into the overall fire-water system, as an integral part of the CPI facility.
Figure 1 shows an example of a skid-mounted, diesel-engine-driven fire-water pump package. The performance and reliability of a fire-water pumping system is an important issue, and details of the fire-water pumping system are usually a part of risk studies, HAZOP and inspection activities. Fire-water pumps are important to different stakeholders, including clients, investors in the CPI plant, and insurance providers. Usually around 20–35% of the insurance-deficiency rating points for a CPI plant are related to inadequate fire-water pumping systems. On average, 5–10% of all fire-water pumping systems in CPI plants have failed to provide satisfactory services at the time required (as evidenced during actual fire cases or drill-type exercises). Thus, it is critical to design, implement, operate and maintain these critical systems — beyond the minimum requirements set forth in published fire codes.
Fire-water system
Fire-protection efforts are categorized as passive or active. The primary passive measures for fire protection include efforts to ensure sufficient clearances, install protective barriers, limit and protect fuel sources, and other steps designed to reduce fire risks (such as the use of less hazardous materials, processes and equipment). By comparison, active fire-protection systems are designed to detect and apply fire-protection measures, which usually rely on some effort to actually extinguish the fire. Commonly used system components include fire hydrants, monitors, hose reels, water-spray systems, deluge-type fire-protection systems or water-exposure cooling systems. The fire-water pump plays an important role in most active fire-protection systems.
System design and sizing
There have been different sets of rules to define the required flow and head of fire-water pumps. In other words, a variety of different guidelines have been used to estimate the ideal water volume and flow requirements for CPI fire cases, depending on plant specifics, applicable codes, regulations and fire-fighting methods. As a result, these specifications will vary based on whether the plant is using fire control, fire suppression, exposure cooling and so on. Fire-water demands are usually calculated based on the maximum rate of water that will be required for a worst-case scenario — typically a potential scenario involving a large, single-fire incident. The most remote unit(s) from the fire-water pumps, or the largest unit(s), are typically examined to identify the worse possible fire scenario(s). The potential scenario of a vast fire in the largest unit should be used to define the capacity of fire-water system. The most-remote fire unit(s) should be used to define maximum rated pressure of the fire-water pump. CPI fire cases can be very different, considering the different types of materials handled and the types of operations carried out at different facilities. Today, computerized simulations play a critical role in identifying and modeling potential fire scenarios, validating fire-fighting methodologies, and estimating the required water capacity.
When evaluating a potential fire-fighting scenario, additional pressure (a safety margin to the calculated head) should be added to maintain the fire-water pressure in all remote units and critical fire-fighting systems; this is necessary to ensure that a fire-water stream with adequate pressure can be maintained to support all applicable fire-fighting systems should there be a fire in a unit. Fire-water and utility-water systems have sometimes been combined in non-critical plants. In the event of a fire, the connected utility water system would be tripped. However, these combined systems are always risky. Various fire codes recommend that no utility-water connections be made to the fire-water system. In some special cases, the fire-water system may be used for emergency process-cooling requirements, but only as the secondary (reserve) supply. Fresh (treated) water is always preferred (over seawater, brackish or untreated water, for instance) for fire-water systems in all onshore plants. Untreated or brackish water can cause many issues such as corrosion, which can potentially wreak havoc on the system components.
In general, engineers should purchase or construct the fire-water pumping system and the fire-water distribution system using proper materials (for instance, selecting suitable corrosion-resistant materials or proper protective coatings), because untreated raw water (such as seawater) could be used as the secondary source for extra fire-water capacity, in the case of an unexpected fire event. If this happens, the fire-water system should be flushed with treated water after the incident, to remove residual traces of untreated source water.
Selecting fire-water pumps
Centrifugal pumps with a relatively flat characteristic performance curve (a graph of head versus flowrate) are generally selected for fire-water pumps. Ideally, the head should rise continuously from the rated point to the shutoff point, with only a small increase of head (say, a 9–15% rise of the head from rated point to shutoff point). These pumps can provide a steady, stable flow of water at a relatively uniform pressure over a wide range of required fire-water flowrates. A relatively flat performance curve is always encouraged for centrifugal fire pumps for the following reasons:
- The control of a fire event usually requires variable amount of water at a relatively constant pressure
- Fire-water pumps are typically operated in parallel. A relatively flat curve ensures troublefree parallel operation
Sometimes, a large amount of water can be required by the fire-water system to battle a vast fire; in those cases, the required water could be considerably larger than the rated flow of the pump. In this regard, the fire-water pump overload point (the end operating point at the right side of the pump curve) should demonstrate a capacity of more that 150% of the rated capacity at a head more than 70% of the rated point. In other words, operation point could move to the far right side of the rated point and that point should offer sufficient flow and head. A steep pump curve should always be avoided. As a rough indication, the average slope of a fire-water pump curve should preferably be around 10–20% (for instance, an average slope of 1/10 up to 1/5). Fire-water pumps can idle against closed valves for a short period of time. In other words, for a short time, the pump should be able to operate in a closed water system without any fire-water application. Check valves should be provided at both the discharge and the suction. The rated pressure of a fire-water pump could be 4–30 barg. Single-impeller centrifugal pumps (for applications that require pressure below roughly 12 barg), and multi-impeller centrifugal pumps (for higher-pressure systems) are also commonly used. The differential pressure of a pump is proportional to both the square of the rotating speed and the square of the impeller diameter. A discharge pressure of around 10 barg can be obtained by a relatively large, single-impeller pump (with a suitable speed).
Overhung (OH) pumps have been used for small- and medium-sized fire-water pumps. Users should consider the between-bearing (BB) pump design when size, power rating and power-density exceed a certain level. As a rough indication, this limit could be 400 kW. As noted, the fire-water pumps installed at any given facility should be able to operate in parallel. However, there are some challenges and issues in ensuring parallel operation. Even in certain conditions, pumps designed to operate in parallel could be subject to overheating or damage. A well-known danger is one pump operating at higher flow, forcing another pump to operate at lower flow; operation at lower flow can be damaging to the pump.
When fire-water pumps are operated in parallel, the pump with the lowest head may work at a reduced flowrate. In this way, the pump could work far from the “best efficiency point” with a very low efficiency, high friction and heat generation, which can result in damage. Even in identical fire-water pumps, pumps that have been in use for more hours (and thus has probably been subjected to more wear), pumps with minor defects, and pumps with slightly lower speed could all be subjected to a reduced flow, which can create problems during an actual fire event. Because of this effect, operators should rotate pumps over time, so that each pump works as the main fire-water pump for some period of time; this can help to ensure even wear patterns among identical pumps in service. Individual protection against the minimum flow (to ensure a minimum flow for each pump) is recommended.
Monitoring of the differential temperature of each pump can provide valuable insight for estimating the parallel operation issue (the reduced-flow problem) and resulting inefficient operation (overheat). In case of reduced flow, the differential temperature (the discharge temperature minus the suction temperature) would rise and could indicate such a malfunction. Because of small leakage and small consumption of fire water, the pressure in a fire-water network could decrease slightly. Fire-water pump systems are usually designed in a way that spare pumps should be started if fire-water pressure dropped below a certain level. However, a slight pressure drop should not lead always to the startup of a large fire pump, as this could result in many unnecessary on-off operating cycles of the main fire water pump (Figure 2).
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| FIGURE 2. Shown here are several examples of fire-water pumps; an identical spare pump is commonly used to increase the reliability of fire-water pumping systems |
On the other hand, small pressure changes resulting from variations in fire-water consumption during a fire incident can result in an unstable operation of the main fire-water pump(s). For instance, this may lead to unnecessary fast changing of the operating point of a large pump, which can result in performance and reliability issues. Smaller-capacity pumps (known as “jockey” pumps) are usually employed in conjunction with the main pump(s) to maintain a relatively constant fire-water pressure. Jockey pumps usually initiate operation after a relatively small pressure drop (say 0.5–1 bar) in a fire-water system. Main fire-water pumps are typically electrically driven and the spare (backup or reserve) fire-water pumps are typically driven by diesel engine. A commonly used arrangement for critical CPI facilities is to install 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 help to ease the alignment and installation issues and ensure high reliability. As noted earlier, the fire code NFPA 20 is dedicated to fire-water pumps. It specifies proper requirements for pump tests, pump performance curves, pump accessories and auxiliaries, and some packaging details. However, in this author’s view, NFPA 20 should be considered as a minimum requirement for a fire-water pump. For critical fire pumps, the well-known API-610 pump standard [ 2] is additionally applied.
API-610 fire-water pumps
The API-610 pump standard is used to ensure the reliable operation of high-performance pumps, mainly in the oil-and-gas, petroleum refinery and petrochemical sectors. The API-610 is usually considered to be the minimum specification for pumps that handle hazardous, flammable, toxic and explosive liquids since any reliability issue associated with these pumps could result in a potential disaster. API-610 pumps are also very popular in applications with extreme temperatures, including pumps for both high-temperature service (such as boiler-feed-water, or BFW, pumps) and low-temperature applications (for example, pumps used in liquid petroleum gas (LPG) liquefied oxygen, and liquefied natural gas (LNG) service). For critical (high-risk) CPI units, fire-water pumps are usually specified to comply with the API-610, to ensure that they are able to achieve a high reliability level — the same as other pumps in the unit.
Engineers often struggle with whether or not to use API-610-compliant fire-water pumps for a CPI plant. This decision would depend on the application, pump head, power rating, capacity, pump speed and expected reliability. The main variable is CPI service (the CPI plant and expected reliability). For instance, for critical units handling flammable liquids and gases, API-610-compliant fire-water pumps are often preferred. For a fire-water pump with differential pressure more than 20 bar, API-610 is usually specified. The pump power rating is a bit tricky, since there are many non-API fire-water pumps available (with successful references) that are intended for high power ranges in a wide array of industry applications. As a general rule of thumb for many CPI plants, API-610 can be considered for pumps above the 350-kW range.
Fire-water pump arrangement
The location for fire-water pumps should be selected carefully to minimize various risks and potential hazard situations. Explosions or high-hazard fires are major concerns, which can disable fire-water pumps. Ideally, there should be 40–80 m of clearance between fire-water pumps and a hydrocarbon or chemical process unit or storage area. This limit should also be respected for some utility areas, such as a power-generation units, gas-compression units, oxygen-generation units, and similar. The possibility of an unconfined vapor-cloud explosion is one of the main concerns, as this could disrupt utilities, damage major support facilities, and damage the fire-water pumping system. Generally, there is a great possibility of the electrical network or the steam-distribution system failing in the event of a major explosion or extensive fire event. This underscores the critical role of independent, diesel-engine-driven fire-water pumps. Fire-water diesel engines should generally comply with NFPA 37 [ 3]. Regarding the diesel fuel-tank capacity, typically, a 12-h duration is specified as the minimum requirement. However, some critical CPI plants require 24-h-capacity fuel tanks for each fire-water pump-diesel engine. Meanwhile, each diesel engine should be provided with independent auxiliaries and accessories, including a dedicated fuel system and fuel tank. The startup of the engine is commonly managed by a battery system (with two independent barriers). The failure of a diesel engine is usually the result of a problem with one of the auxiliary systems. Major reasons for such a failure include fuel-system issues, a lubrication-system problem, a starting issue, a wiring problem or component fatigue. Only clean, high-quality diesel fuel should be used, and special attention is required for the lubrication oil selection and supply.
Proper overhauls and repairs are required, just like for any other properly designed combustion engines. Experience has shown that the diesel-engine-driven, fire-water pump is the most reliable option currently available for severe loss incidents in a CPI plant. The reliability and availability of micro-turbines (small gas turbines in the 50–400 kW range) could be higher than diesel engines, but their efficiencies are relatively lower (in terms of lower operating duration with the same amount of fuel). Currently they are not popular for fire-water pump systems. Fire-water pumps are typically arranged for both manual and automatic startup. Automatic startup is expected to happen rapidly, in a very reliable manner, once a fire event has been detected. Fire-water pumps are usually stopped manually at the pump’s local control panel. In other words, operator intervention is usually used to turn off the pump, once the situation has stabilized and the fire is out. A suitable enclosure (or building) should be provided for fire-water pumps.
Sufficient reinforcement should be considered for the fire-pump enclosure. This is very important. For example, in the case of a major earthquake, fire-water pumping systems need to be fully operational to respond to fire events resulting from the earthquake. An open-sided shelter is not recommended. And, fire-water pumps should be located at a higher elevation than the majority of the CPI facility and upwind of it. To provide another layer of protection (in order to avoid common failures), the main fire-water pumps, and any other reserve or supporting fire-water pumps should 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.
Special fire-water pumps. For critical CPI plants, additional emergency (or reserve) firewater pumps should be provided — in addition to the conventional fire-water pumps — to supply seawater (for CPI plants located at the coastal regions) or other sources of untreated raw water (such as untreated water from a lake or water wells), to quickly supply additional capacity to the plant’s fire-water network. For a seawater-based emergency fire-water pumping system, the pumps are usually submerged in seawater. For some locations, the seawater level may fluctuate from –7 m to +16 m. Considering that there is often a long distance from the sea to the CPI facility, these fire-water pumps should be designed to produce a relatively high head. These special fire-water pumps are usually multi-impeller pumps. Properly designed highly reliable pumps are always specified for such complex service.
Electric-motor-driven submersible pumps, or sometimes hydraulic-driven pumps are used for these special applications. These are usually down-hole, vertical turbine-type pumps. In some cases, local regulations or plant specifications require three dedicated fire-water pumps (as the minimum) for special CPI applications (such as CPI plants that handle highly explosive or highly flammable materials). In such cases, one of the main concerns is personnel safety during a major fire case and provisions must be made to ensure a safe personnel evacuation.
Installation and commissioning
The piping installation and connection to a fire-water pump can lead to relatively high loads on the pump nozzles and to the pump train’s sensitive components, such as bearings, coupling and rotating assemblies. The fire-water pumps are often left on standby, therefore any high nozzle loads or misalignment might be left unchecked. This can potentially wreck the pump in the first hours of operation in the event of fire. Periodic checks are important. A well-known method to monitor the movements and deformations of critical machinery components during the piping connection (piping flange and machinery flange bolt tightening) is to install dial indicators (or other types of indicators) that monitor movements in critical parts of the machinery train. Usually, two dial indicators are used to observe movement in each machinery component (such as the driver, the fire-water pump and the gear unit, if any) compared to the base-plate or foundation (the main purpose is to identify improper support of machinery, called “soft-foot”). Two dial indicators can be used to monitor critical bearing housing movements in the fire-water pump (usually in x and y directions). Acceptable movements should be below 0.04 mm (40 micrometers) to ensure a proper piping-pump connection. A similar limit should be applied to movements in all critical pump train locations (such as bearing housing, coupling, machinery support and others), and suction and discharge nozzle flanges (in terms of limiting deformations in all directions). For special fire-water pumps, depending on the machinery design, speed, power rating and applications, a limit higher or lower than the above-mentioned (0.04 mm) may be specified.
♦ Edited by Suzanne Shelley
References
1 NFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection, National Fire Protection Assn. (NFPA), 2013.2 API 610, Centrifugal Pumps, American Petroleum Inst. (API), 2009.3 NFPA 37, Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines, National Fire Protection Assn. (NFPA), 2014.
Author
Amin Almasi is a rotating-equipment consultant in Australia (Email: [email protected]). He previously worked at Worley Parsons Services Pty Ltd. (Brisbane, Australia), Technicas Reunidas (Madrid, Spain) and Fluor Corp. (various offices). He holds a chartered professional engineer license from Engineers Australia (MIEAust CPEng – Mechanical), a chartered engineer certificate from IMechE (CEng MIMechE), RPEQ (registered professional engineer in Queensland) and he also holds M.S. and B.S. degrees in mechanical engineering. He specializes in rotating machines including centrifugal, screw and reciprocating compressors, gas and steam turbines, pumps, condition monitoring and reliability. Almasi is an active member of Engineers Australia, IMechE, ASME, Vibration Institute, SPE, IEEE, and IDGTE. He has authored more than 60 papers and articles dealing with rotating machines.