Boiler Feed Pump

There are two approaches to balancing thrust on this sort of pump. The first is a back to back impeller design. In this design there are suction connections on each end of the rotor, and the discharge is in the middle. That way half the thrust load goes in one direction and half goes in the other direction so that the thrust load is balanced.

The other approach is to put suction on one end of the machine and flow through the pump to discharge on the other, but then to also insall a balance piston and balance line. The balance pistion will typically be on the suction end of the machine, and the balance line will hook to discharge pressure. The discharge pressure acting against the area of the balance drum is configured to count act the normal thrust load of the impellers. The size of the balance piston is designed to counter most of the thrust load and then the thrust bearing takes the rest. The problem is that if you loose the seal on the balance piston, the thrust load very quickly becomes so high that it overloads and damages the thrust bearing.

On top of what Steve misspelled (Oooops . . . I mean typed perfectly >>> he's a pal, not to worry), most boiler feedwater pumps are diffuser pumps with the balance piston pushing the element back to keep the largest thrust away from the thrust bearing, as Steve pointed out. However, if you do have a back to back volute pump, the clearances of the throttle bushing and center bushing, and to some lesser degree the impeller wear ring clearances, help greatly to balance the thrust. Especially on an uneven quantity of impellers. On a back to back volute pump, if your thrust bearing is hot or wearing, check the clearances of the throttle bushing (normally on the drive end side, but always on the crossover side) and also check the center bushing clearances. If flow or efficiency has changed, this might be a likely place to start.

Diffuser: I'm guessing you have a radial split diffuser pump. If the pressure in balance piston area is OK, and thrust still too high, then look for impeller wear ring clearances opening up and loosing their 'seal' and ability to breakdown pressure, therefore thrusting the element.

Cheers

I defer to the master of all things pumpness!!

(Grrrrrr the one bloody time I forget to run spell check, guess who shows up to pick a nit with me??? )

Another method used, similar to the balance piston, is a balance chamber at the rear of the impeller. The pressure is contained in the balance chamber by a axial or radial labyrinth type seal and this offsets the thrust of the impeller.

This type of seal can also be employed on some of the large turbo chargers.

Is the feed pump turbo or motor driven?