Fire pumps are very important, but unfortunately, fire pumps did not start or could not work continuously during a fire, which led to catastrophic situations. Special attention should be paid to the design, selection, assembly, operation, monitoring and reliability of fire water pumps.
Centrifugal pumps with relatively flat characteristic curves (head and flow) are used as fire-fighting water pumps. In most cases, the fire that may occur in the largest unit of the factory indicates the capacity of the fire fighting water system. The furthest fire-fighting device can control the rated pressure of the fire-fighting pump. Many modern industrial plants require reliable and high-performance fire pumps.
The fire pump should be able to provide more than 150% of the rated flow at a head above 65% of the rated point. This needs to deal with the fire situation where the fire hydrant or active fire protection system is supposed to operate, therefore, the water consumed far exceeds the rated flow.
At the same time, some minimum level of pressure (water head) is required under such a large flow to make all these fire hydrants and fire fighting systems work. The correct choice of fire-fighting pumps usually exceeds these requirements. Excellent fire water pumps can usually provide more than 180% of the rated flow, and sometimes even exceed twice the rated flow when working on the right side of the performance curve. At 150% of the rated flow, this pump usually produces more than 70% of the rated head. At the end of the curve (usually the flow rate is 1.8 times the rated flow rate), a suitable fire pump can provide a head of about 55% to 60% of the rated head.
Many fire pumps are electric pumps. If a major fire or explosion occurs in the factory, the power grid will be disabled. Therefore, in addition to the electric-driven fire-fighting pumps, a set of independent pumps is required to ensure the maximum reliability and safety of the factory. In addition to motor-driven pumps, one or two diesel engines with independent diesel tanks and starting systems are usually provided to drive fire-fighting water pumps. Each diesel engine should be started by at least two independent starting systems.
The use of six fire water pump configurations is common in many key plants. In this configuration, two centrifugal pumps driven by electric motors, two centrifugal pumps driven by diesel engines, and two booster pumps are used. For key facilities such as oil refineries, large chemical plants and petrochemical plants, this is a very safe and reliable choice.
Many factories use other arrangements, such as two centrifugal pumps driven by electric motors, one centrifugal pump driven by diesel engines, and two joints to reduce costs. This decision should be made based on the overall safety and reliability of the plant or facility.
If the main system is short of water, some factories or units need a backup fire-fighting water system to supply fire-fighting water.
For example, some factories use treated water as fire-fighting water. They have water tanks and facilities that can store the treated water for 8 hours, 12 hours, or 18 hours. As a backup, some of these factories have special vertical pumps that can draw untreated water from nearby lakes, seas, wells or ponds when needed. The backup water is untreated water from the ocean or lake, so the entire fire fighting water system should be designed to use untreated water. This is important for material selection and other aspects.
Horizontal pumps are usually the first choice for firefighting water pumps, but vertical pumps are often used. For example, when pumping water from the sea, lake, well or pond, the suction flange of the pump is above the water level. When the pump is started, it needs to automatically bleed to expel air from the column and discharge head. As an indication, use an automatic exhaust valve with a pipe size of 1.5 inches (38 mm) or larger. The valve should also allow air to enter the column to dissipate the vacuum when the pump is stopped. This valve is located at the highest point of the discharge line between the fire water pump and the discharge check valve.
This case study involves two vertical centrifugal seawater firefighting pumps driven by diesel engines as backup pumps in a large industrial complex. The complex uses treated water as the main fire fighting water system, which has six pumps, two electric motor-driven centrifugal pumps, two diesel-driven centrifugal pumps and two regulating pumps.
If the treated water storage capacity (which can hold for 12 hours) is used up, seawater will be used as a secondary source (backup) for additional fire fighting water in case of a fire. Each sea water pump is equipped with a sea water submerged inlet, a right angle gear box and a diesel engine drive. Each diesel engine drive has two electric starters, dual-start motors (one working and one standby) and dual batteries.
Each fire pump has sufficient capacity and capacity to meet the highest expected demand while maintaining a residual pressure of 10 barg (gauge pressure) at the fire hydrant with the furthest hydraulic pressure. The capacity and discharge pressure are calculated as approximately 1,852 cubic meters per hour (m3/h) and approximately 15.1 barg.
A diesel engine of 1,500 kilowatts (kW) at 1,760 revolutions per minute (rpm) was selected. The 4-stage vertical pump has a speed of 1,180 rpm. Provides a 1.5 ratio right angle gear unit.
The pumps are large, so they are installed in accordance with National Fire Protection Association Standard 20, but are not listed on the Underwriters Laboratories list and are not approved by the Factory Mutual Aid Association. The pump is inspected and certified by a selected third-party inspection agency. Super duplex stainless steel is designated for all parts in contact with seawater.
The pump curve is relatively flat, with a closing pressure of 18 barg. From the rated point to closing, the water head has risen by 19%. At 150% of the rated flow (2,778 m3/h), the resulting discharge pressure is approximately 13 barg. The head of this point is more than 85% of the head of the rated point. The curve extends to 3,900 m3/h (over 210% of the rated flow), and the head at the rated point exceeds 48% of the head. The net positive suction head (NPSHr) required at 150% of the rated flow is 9.5 meters (m); NPSHa (available) is 11.5 m, and there is a margin of 2 m (worst case) at this time. The diesel engine is a 16-cylinder, 4-stroke cycle turbocharged, aftercooled diesel engine. This is a large engine, each with a bore of 170 mm and a stroke of 190 mm. The engine size is 4 m × 1.8 m × 1.9 m, and the engine weight is approximately 9,000 kilograms (kg).
Each pump is equipped with an automatic exhaust device and a vacuum circuit breaker. A 1.5-inch (38 mm) pipe size air relief valve is provided to vent air from the column and discharge head when the pump is started.
Amin Almasi is Australia's main machinery/mechanical consultant. He is a chartered professional engineer of the Institute of Engineers Australia (MIEAust CPEng–Mechanical) and IMechE (CEng MIMechE). He holds a Bachelor of Science and a Master of Science in Mechanical Engineering, and is RPEQ (Registered Professional Engineer Queensland). He specializes in machinery and mechanical equipment, including pumps, compressors, turbines, engines, motors, material handling systems, power generation, condition monitoring, maintenance, and reliability. He has authored more than 200 papers and articles on pumps, rotating equipment, mechanical equipment, condition monitoring and reliability.