The reliability of a plant's fire water pumps is rarely considered on a consistent basis—until that plant needs those pumps to work. Many fire water pumping systems have failed during exercises or even actual fires. This oversight can have significant consequences for plant owners and operators. Based on my experience, a fire water pumping system can account for 15 to 35 percent of insurance deficiency rating points for a plant. Without careful planning and the right equipment, facilities could see their processes slowed or halted by uncontrolled fires and leave themselves vulnerable to high insurance costs.
Fire water system pressure can drop because of small leakages or intermittent consumptions of water. While these minor dips in pressure should not be enough to start the main fire water pump—resulting in unnecessary on/off operating cycles—the pressure changes could lead to unstable operation during an actual fire. Unnecessary fast-changing of a large pump's operating point could also result in performance and reliability problems.
Small-capacity, or "jockey," pumps maintain a relatively constant fire water pressure. Jockey pumps usually start the operation after a relatively small pressure drop—about 0.5 to 1 bar—in a fire water system.
Electric motors typically drive main fire water pumps, while diesel engines drive reserve pumps. Critical industrial plants, such as large oil and petrochemical refineries, commonly use six fire water pumps—two electrically driven pumps, two diesel-engine driven and two jockey. Fire water pumps are nearly always provided on a prefabricated skid, easing alignment concerns and offering high reliability.
API 610 Pumps
Oil and gas facilities often rely on the America Petroleum Institute (API) 610 standard when selecting reliable, high-performance pumps. The API 610 standard is usually considered the minimum specification for pumps handling hazardous, flammable, toxic and explosive liquids. API 610 pumps are also popular in high-temperature services, such as boiler feedwater pumps, and low-temperature services, such as liquefied natural gas applications.
The American National Standards Institute (ANSI) is another important pump code frequently used in chemical and utility services. API and ANSI pumps have some essential differences. For example, horizontal API pumps are often centerline mounted, while horizontal ANSI pumps are often foot-mounted.
The thermal movement of a horizontal ANSI pump can be two to three times more than the thermal movement of a horizontal API pump. The difference in thermal expansion and contraction has a direct effect on pump train alignment and reliability. Because API pumps move less than ANSI pumps, API pumps are more appropriate for critical applications.
In critical units, fire water pumps must often comply with the API 610 standard, same as other pumps in the unit. When deciding to use API 610 pumps, operators must consider the critical applications in their facility, pump head, power rating, capacity, pump speed and expected reliability. An API 610 pump is usually specified for differential pressures more than 25 bar, power ratings above 500 kilowatts and speeds above 4,000 rotations per minute.
Diesel Engines for Pumps
Electrical systems can fail after a major explosion or during an extensive fire. In case of failure, independent diesel-engine driven fire water pumps must ensure the system's reliability. Diesel engines often fail because of poorly maintained auxiliary systems.
A well-stocked fuel system is essential to diesel engine reliability. Diesel tanks often hold 12 hours of fuel, but some critical plants require 24 hours of fuel for each fire water pump diesel engine. Only clean, high-quality diesel should be used. A dedicated fuel system with additional fuel tanks—even two independent fuel tanks, in some cases—should be provided.
To prevent explosions from disabling fire water systems, between 40 and 70 meters of clearance should separate the fire water pumps from a hydrocarbon or chemical process unit. Main and reserve fire water pumps should not be located next to each other. Utility areas, such as power generation, gas compression and oxygen generation units, should also respect this clearance. An unconfined vapor cloud explosion could disrupt these utility services and damage the fire water pumping system.
Fire water pumps are arranged for both manual and automatic startup. Fire water pumps automatically start after a fire is detected. The pumps are usually stopped manually at the pump's local panel.
Fire water pumps should be housed in a suitable enclosure or building. The enclosure should be sufficiently reinforced in case of an earthquake. Fire water pumping systems must be fully operational to handle the fires that frequently occur after seismic events. The pumps must sit at a higher elevation than the facility and upwind.
Fire water pumps are often submerged in seawater for offshore applications, such as oil and gas platforms. For some locations, the water level may fluctuate between minus 6 below to about 17 m above mean sea level. In addition, offshore fire water pumps must often produce a high head to overcome the height of the platforms, which varies between 10 to 30 m.
On offshore platforms, fire water pumping systems should be located on a separate utility to protect them from a fire or major explosion. In some offshore designs, fire water pumping systems are located at or near a non-process or utility module—away from hydrocarbon or hazardous units. Unmanned offshore facilities are becoming popular, making compact design essential to the platform's safety.