Boiler Feed Pumps Experience Cavitation

These are the massive centrifugal pumps that deliver pressurized water to the boiler.   The water is transformed into steam to drive steam turbines and spin electrical generators, which ultimately results in electrical power.   The noise was so loud, it sounded like rocks were being ground up.   I asked a coworker what was going on, and he replied matter-of-factly, “The pumps are cavitating.”

              Boiler Feed Pumps Experience Cavitation

    So what exactly is cavitation?   We’ll find out next time when we explore the mechanics of this noisy phenomenon as it applies to boiler feed pumps and other centrifugal pumps.

Centrifugal Pumps

     Last week we focused on various types of positive displacement pumps.  Today we’ll take a look at centrifugal pumps.  See Figure 1.

Figure 1 – A Centrifugal Pump

     Just like the positive displacement pumps we talked about last week, centrifugal pumps have rotating parts as well, but that’s where their similarities end.  Unlike positive displacement pumps that take “bites” out of liquid before trapping it between moving parts, centrifugal pumps rely on kinetic energy to move liquid in a continuous stream.  Kinetic energy is the energy of motion, and in the case of the centrifugal pump kinetic energy is developed by rotating parts within the pump and transferred to the liquid contained within the pump.  In other words, the liquid is moved through the pump by means of centrifugal force.

     To illustrate this concept, we can tie a rope to the handle of a bucket that has a small hole punched in the bottom.  Now, you know what will happen if you fill the bucket with water…  There’s a hole in the bucket, Dear Liza, Dear Liza…  That’s right, the water will just dribble out of the hole, thanks to gravity.  But before we fix the hole as Liza suggests, let’s do an experiment.  Pick up the rope and spin the bucket around as fast as you can in a circle.  You’ll notice that this rapid spinning creates centrifugal force, resulting in a rather powerful stream of water shooting from the hole.  The faster you spin the bucket, the stronger the stream.

     When it comes to centrifugal pumps, the idea is basically the same.  The objective is to forcefully spin water around in a circle, thus ejecting it from the pump.  This is accomplished with a rotating part called an impeller.  See Figure 2.

Figure 2 – Cutaway View of a Centrifugal Pump    

     In our illustration the impeller is attached to a shaft that’s powered by some source of mechanical energy, such as an electric motor.  The water enters the pump at the center of the rotating impeller, referred to as the “eye.”  The water then slides over the face of the impeller, moving from the center to its edge due to the action of centrifugal force.  That force pushes it off the impeller and into the pump housing.  You’ll note that the housing has a special shape, called a “volute.”  This volute looks a lot like a spiraled snail shell.  The shape of the volute helps direct the water coming off the impeller into an opening in the side of the pump where it is discharged.  The faster the pump impeller rotates, the more kinetic energy the water picks up from the impeller.

Boiler Feed Water, A Special Kind of Condensate

      Last time we learned how the condenser within a power plant acts as a conservationist by transforming steam from the turbine exhaust back into water.   This previously purified water, or condensate, contains valuable residual heat energy from its earlier journey through the power plant, making it perfect for reuse within the boiler, resulting in both water and fuel savings for the plant.   Today we’ll take a look at a highly pressurized form of condensate known as boiler feed water and how it helps the power plant save money by recycling residual heat energy in the steam and water cycle.

      Let’s begin by integrating the condenser into the big picture, the complete water-to-steam power plant cycle, to see how it fits in.   The illustration shows that both the make-up pump and the condenser circulating water pump draw water from the same supply source, in this case a lake.   The circulating water pump continuously draws in water to keep the condenser tubes cool, while the make-up pump draws in water only when necessary, such as when initially filling the boiler or to make up for leaks during operation, leaks which typically occur due to worn operating parts.

      In a nutshell, the condenser recycles steam from the turbine exhaust for its reuse within the power plant.   The journey begins when condensate drains from the hot well located at the bottom of the condenser, then gets siphoned into the boiler feed pump.

      If you recall from a previous article, the boiler feed pump is a powerful pump that delivers water to the boiler at high pressures, typically more than 1,500 pounds per square inch in modern power plants.   After its pressure has been raised by the pump, the condensate is known as boiler feed water.

      The boiler feed water leaves the boiler feed pump and enters the boiler, where it will once again be transformed into steam, and the water-to-steam cycle starts all over again.   That is, boiler feed water is turned to steam, it’s superheated to drive the turbine, then condenses back into condensate, and finally it’s returned to the boiler again by the boiler feed pump.   Trace its journey along this closed loop by following the yellow arrows in the illustration.

      While you were following the arrows you may have noticed a new valve in the illustration.   It’s on the pipe leading from the water treatment plant to the boiler feed pump.   Next time we’ll see how this small but important item comes into play in the operation of our basic power plant steam and water cycle.