Fire-water pumps are important and, unfortunately, there have been cases where fire-water pumps were not started or would not work continuously during a fire, resulting in disastrous situations. Great care should be taken for the design, selection, assembling, operation, monitoring and reliability of fire-water pumps.
Fire-Water Pumps
Centrifugal pumps with relatively flat characteristic curves (head versus flow) are used as fire-water pumps. Most often, a fire that could occur in the largest unit of a plant indicates the capacity of the fire-water system. The most remote fire unit(s) could govern the rated pressure of a fire-water pump. Reliable and high-performance fire-water pumps are needed for many modern industrial plants.
A fire-water pump should be able to provide more than 150% of the rated flow at more than 65% of the head at rated point. This is needed to deal with fire cases where hydrants or active fire protection systems should operate and, as a result, consumed water is far more than the rated flow.
At the same time, some minimum level of pressure (head) is required at this large flow to let all these hydrants and fire systems work. Properly selected fire-water pumps usually exceed these requirements. Excellent fire-water pumps usually can supply more than 180% of the rated flow, even in some cases more than two times the rated flow when working at the right side of the performance curve. At 150% of the rated flow, such a pump most often produces more than 70% of the rated head. At the end of the curve (often at flow 1.8 times the rated flow), a proper fire-water pump can provide head around 55% to 60% of the rated head.
Many fire-water pumps are electric motor-driven pumps. In case of a major fire or explosion in the plant, the electrical network would be disabled. Therefore, a set of independent pumps in addition to electrical-driven fire-water pumps are needed for the utmost reliability and safety of the plant. One or two diesel engine-driven fire-water pumps with independent diesel tanks and starting systems are often provided in addition to electric motor-driven pumps. Each diesel engine should be started by at least two independent starting systems.
How Many Pumps Are Needed?
Using a six fire-water pump configuration is common in many critical plants. In this configuration, two electric motor-driven centrifugal pumps, two diesel engine-driven centrifugal pumps, and two jockey pumps are used. This is a very safe and reliable option for critical facilities such as refineries, large chemical and petrochemical plants, etc.
Many plants use other arrangements such as two electric motor-driven centrifugal pumps, one diesel engine-driven centrifugal pump and two jockeys to reduce the cost. Such a decision should be made with respect to overall safety and reliability of the plant or facility.
Some plants or units need backup fire-water systems to supply fire water if the main system runs out of water.
For instance, some plants use treated water as the fire water, and they have tanks and facilities to store treated water for 8 hours, 12 hours or 18 hours. As a backup, some of these plants have special vertical pumps to pump untreated water from a nearby lake, sea, well or pond in case of need. The backup water is untreated water from a sea or lake, therefore the whole fire-water system should be designed to use such untreated water. This is important for material selection and other aspects.
Horizontal vs. Vertical
Horizontal pumps are often preferred for fire-water pumps, but there are many cases when vertical pumps are used, such as when water is drawn from the sea, lake, well or pond and the pump suction flange is above the water level. Automatic air release is needed to vent air from the column and the discharge head upon the startup of the pump. As an indication, a 1.5-inch (38-millimeter) pipe size or larger automatic air release valve is used. This valve should also admit air to the column to dissipate the vacuum upon stopping the pump. This valve is located at the highest point in the discharge line between the fire-water pump and the discharge check valve.
Diesel-Driven Fire-Water Pumps
This case study involves two diesel engine-driven vertical centrifugal seawater fire-water pumps as backup pumps for a large industrial complex. This complex uses treated water for the main fire-water system, which has six pumps, two electric motor-driven centrifugal pumps, two diesel engine-driven centrifugal pumps and two jockey pumps.
Seawater is used as the secondary source (backup) for extra fire water in case of a large fire if the treated water storage, which was sized for 12 hours, runs out. Each seawater pump is furnished with a seawater submerged inlet, right angle gear box and diesel engine driver. Each diesel engine driver has two electric starters, dual starter motors (one duty, one standby) and dual batteries.
Each fire-water pump has sufficient capacity and capability to supply the highest anticipated demand while maintaining a residual pressure of 10 barg (gauge pressure) at the hydraulically most remote hydrant. Capacity and discharge pressure were calculated around 1,852 cubic meters per hour (m3/h) and about 15.1 barg, respectively.
A 1,500-kilowatts (kW) 1,760-revolutions per minute (rpm) diesel engine was selected. The 4-stage vertical pump speed was 1,180 rpm. The right angle gear unit with 1.5 ratio was provided.
The pumps were massive, thus they were installed as per the National Fire Protection Association standard 20 but non-Underwriters Laboratories listed and non-Factory Mutual approved. Pumps were inspected and certified by a selected third-party inspection agency. Super duplex stainless steels are specified and used for all components in contact with seawater.
The pump curve was relatively flat and shutoff pressure was 18 barg. There was 19% head rise from the rated point to the shutoff. At 150% of the rated flow (2,778 m3/h), the produced discharge pressure was around 13 barg. The head of this point was more than 85% of the head at rated point. The curve was extended to 3,900 m3/h (more than 210% of rated flow) with the head more than 48% of the head at rated point. Net positive suction head required (NPSHr) at 150% of rated flow was 9.5 meters (m); NPSHa (available) of 11.5 m was provided with 2 m of margin at this point (worst case). The diesel engine was a 16-cylinder, 4-stroke-cycle turbocharged, after-cooled diesel engine. This was a large engine with big cylinders each 170 mm bore and 190 mm stroke. Engine dimensions are 4 m × 1.8 m × 1.9 m, and the engine weight was about 9,000 kilograms (kg).
Each pump was provided with an automatic air release and vacuum breaker. A 1.5-inch (38-mm) piping size air release valve was provided to vent air from the column and the discharge head upon startup of the pump.