Variable speed fire pump controller

A variable speed (VFD) fire pump controller is a fire pump controller that is capable of maintaining a constant discharge pressure by varying the speed of the motor according to the demand of the fire pump.It differs from other industrial variable speed control equipment in that it is now considered life safety equipment and therefore must comply with the stricter standards set forth by NFPA-20.

How variable speed fire pump controllers work

Since the output pressure of a centrifugal pump varies with the square of the speed, the pressure can be precisely controlled by controlling the speed of the pump.Example:Pump running at 1785 RPM during off period with output pressure of 204 PSI and suction pressure of 0 PSI.If the motor speed is reduced by 10% to 1607 RPM, the new output pressure becomes 165 PSI.This in turn enables the variable speed controller to maintain a perfectly flat pump discharge curve until full pump rating is reached. Plus, the variable speed controller adjusts for varying suction pressures as in the example below: 65 PSI of city water drops to 26 PSI as it flows. The variable speed drive will speed up to accommodate the sudden drop in pressure and slow down again when not needed while maintaining a constant total discharge pressure.

Advantages

• Significantly reduces genset sizing for variable speed controllers,NFPA-20 2010 paragraph 9.4.3 allows a voltage drop of no less than 15% at start, or lower at bypass start (depending on manufacturer's ability).For this reason, for a typical application with a 150 hp motor using a soft start, a genset manufacturer's sizing program would select a 250 kW genset.For the same application, when using a variable frequency drive and primary reactor bypass, the program would select a 125 kW genset, drop the starting voltage to 30%, and halve the genset.

• Energy saving “Most motors turn at a nearly constant speed.However, the equipment they drive runs below the maximum design speed most of the time.This reduction can be achieved with an adjustable speed drive (ASD), This drive changes the shaft speed to reduce the speed of the pump or motor in this way more efficiently than letting the motor run at a constant speed and then restricting or bypassing the flow with a valve or damper,if the shaft speed 10% reduction, the flow rate is reduced by  10%, and the power consumption is reduced by 27%.However, due to the limited running time of the fire pump, the initial investment.

• Smooth Start  “ASD provides a soft start where the motor starts slowly and then accelerates.This reduces mechanical stress on the motor and motor-driven equipment.”Variable speed also provides an initial inrush current of 125–150%,making it a The softest starting method available.

• PRV Replacement Variable Speed Fire Pump Controllers can serve as a viable replacement for PRVs in high rise buildings and potentially save building owners over $250,000 replacing PRVs and associated testing through NFPA-25 over the life of the building. 

• Conserves water during testing NFPA-25 requires fire pump controllers to run for 10 minutes per week.This ends up using a lot of water in buildings with static pressures greater than 175 PSI and relies  on system relief valves to control pressure: (50–100% of pump rating) x (10 minutes) x (52 cycles/year) = water wasted. Conversely, under the same conditions, shifting does not waste water.