Balance Drum Working in Centrifugal Pump

I believe these are used in magnetically coupled seal less pumps for "automatic" adjusment of thrust loads. Check out http://www.dickow.de/e/content/MultistageCentrifug alPumpsTypeHZMR.asp for a description and picture. Also, search for info on the specific manufacturer of the pump you are interested in.

you can ask mr. ikram in utilities machinery

what is the purpuse of balance drum and whwre it is installed

The combination balance disc and drum arrangement used in some multi stage centrifugal pumps basically maintains uniform hydraulic axial loading through small changes in the axial clearance between a rotating and stationary disc. As the pressure rises on the balance chamber side it forces the stationary and rotating discs closer together, reducing the clearance between the disc faces and slowing the flow across the drum / bushing component of the arrangement, causing the pressure on the discharge side of the disc faces to build and push the rotor back towards the balance chamber until a state of axial balance is reached. The reason for using the drum / bushing component with the disc is to better control the pressure in the balance chamber. This a very basic explanation, more info on this subject can be found in a few of Igor Karassik's books. (Pump Handbook, Centrifugal pumps)