Dec 2011

Variable-Speed Fire Pumps

Booster pumps are often needed to boost the pressure of the city water supply to make it adequate for sprinkler system designs. However, an unintended result could be obtaining pressures that are too high. This can occur because sprinkler system components – especially sprinklers – are often designed for 175 psi.

You may be thinking that 175 psi is a lot of pressure and that surpassing this pressure would be difficult to do, but this occurs more often than you may think. To understand how this can occur, you must first understand some basic hydraulics. City water pressures are highest when there is no water flowing. The more water that flows, the lower the pressure gets. This is due to frictional losses in piping and fittings along with the supply diminishing somewhat with usage. Additionally, pumps do not linearly increase pressures. A pump can boost pressure at no flow up to 40% more than at 100% rated flow and as little as 65% of the rated pressure at 150% of rated flow.

As an example, say a city water system can supply 1,000 gpm @30 psi with a static water pressure of 55 psi and a 1,000 gpm @ 100 psi pump is needed for the sprinkler system. This pump churns at 130% of rated pressure and the 150% flow is rated at 80 psi. Thus, the pressure at pump churn is the 55 psi static pressure from the city supply plus the 130 psi churn pressure from the pump for a total of 185 psi. This pressure does diminish to 130 psi when the pump is flowing at 1,000 gpm. This system pressure would exceed the 175 psi allowed.

Another example where system pressures can often exceed 175 psi is in multiple story buildings as each foot in elevation results in an additional 0.433 psi of water pressure needed at the pump. Thus, a 100 ft. high ceiling would need an additional 100 ft. *0.433 psi/ft. or 43.3 psi in water pressure than the same system would at an elevation that was at the pump level.

There are several ways to ensure that system pressures do not exceed the pressure ratings for the components. One is to use components that are rated for higher pressures. These are available, but they are generally more expensive and some agencies do not recognize sprinkler system component ratings above 175 psi. Another method to handle this is to use pressure reducing valves (PRV’s). PRV’s can be expensive to install and to maintain and have not proven to be very reliable. In fact, some cities, such as the City of Chicago do not allow PRV’s.

This lead to the use of main relief valves to help control overpressure conditions. These relief valves work by dumping the rated flow of water outside the building – this can waste a lot of water during normal testing. In the example above, the 1,000 gpm pump would dump 10,000 gallons of water during a 10 minute churn test for an electric pump and 30,000 gallons of water during the 30 minute churn test for a diesel pump. Additionally, if this water is introduced in areas where personnel walk during winter months, they could be injured from slipping on the ice. In 2003, NFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection, prohibited the use of main relief valves as a primary means to control the discharge pressure of pumps, and allowed the use of variable-speed pressure-limiting control to control overpressure.

In 2003, the first UL Listed variable-speed electric fire pump controller was introduced. In 2004, the first pressure-limiting device (PLD) diesel engine was produced. In 2008, FM Global started approving these devices. There are currently three manufacturers with FM Approved variable-speed electric pump controller models – Metron Fire Pump Controls, Master Control Systems Inc, and ASCO Power Technologies LP. Additionally, Clarke Fire Protection Products Inc. has multiple variable speed diesel engines with pressure limiting devices that are FM Approved.

The variable-speed electric controller works by varying the frequency of the power supplied to a variable-frequency drive (VFD) – the lower the frequency, the slower the motor speed, and the lower the water pressure from the pump. The PLD for diesel engines is set for a certain pressure. If this pressure is reached, the engine throttle is adjusted to reduce the speed of the motor, which lowers the water pressure from the pump.

These systems have proven to be rather reliable and safeties have been introduced to ensure that failure could not occur that results in too little or too much pressure to the sprinkler system. In the rare event that one of these fails, an automatic bypass is required to allow the pump driver to run at full speed. If this should occur, a main relief valve is also required on the system as a back-up so overpressure of the system does not occur.

If you have a potential overpressure problem at your facility, then a variable-speed fire protection pump may be the best choice for you. There are other instances where overpressure could be an issue that are not listed in this article – locations with multiple water supplies, foam-water systems, complex fire protection systems, etc. Please contact Risk Logic if you would like help determining if a variable-speed pump would be a good option for your facility.