Recirculation
Recirculation
(OP)
Can anybody explain clearly the phenomenon of Suction recirculation and discharge recirculation in a centrifugal pump?
Also, the effects of these and how to overcome these phenomenon?
Regards,
Aditya
Also, the effects of these and how to overcome these phenomenon?
Regards,
Aditya





RE: Recirculation
Some times there will be an internal passage to allow high pressure fluid to be sent to the suction end and that high pressure fluid will be used to apply force on the shaft to counter balance the trust created from high pressure on one and and low on the other, each acting on about the same physical amount of area, causeing a force on the shaft.
Whew?
RE: Recirculation
Recirculation within the pump itself, is a different thing. There is some recirculating form the discharge of the impeller back to the eye of the impeller. Some flow slips between the impeller and the wear ring, (high pressure side to low pressure side) and re-enters the eye of the impeller. This is dependant on the clearance between the impeller and the wear ring. This clearance becomes larger, and recirculation increases, as the impeller and the wear ring wear down over time.
There is also recirculation within the impeller. As the discharge flow is lessened, flow within the impeller also lessens. When there is very little flow through the impeller, some of the fluid will just bounce around in certain pockets and eddies, and stay inside the impeller.
Both of these kinds of recirculation will cause cavitation like wear on the impeller. The worm hole effect you see, will be in different areas of the impeller, depending on the type of recirculation.
Sufficient flow is usually the best way to reduce recirculation. When pumping cool water, very little flow is required to prevent excessive recirculation. You must maintain enough flow so that the temperature of the water does not increase very much. The higher the temperature, the worse the cavitation like wear. Except for some really large pumps, recirculation at fairly low flow rates are usually not much of a problem.
Large pumps or small, if recirculation is a problem, impellers of higher tensile strength will resist the wear better.
RE: Recirculation
Shrouded Impellers just have a front (and rear on large pumps) wear ring of which the clearance governs the recirculation. Also, many will have balance holes to balance the hydraulic axial loads - great for clean non corrosive liqiids, bad for corrosives beacuse as surfaces corrode, clearances increase and so recirculation increases and efficiency decreases.
Semi Open Impellers have a back shroud only, and front face is machined. This Impeller also needs the casing to be machined, the essential dimension being the clearance between both components when assembled - the smaller the better to avoid recirculation and efficiency. The down side of this is the additional machining cost, but can handle solids better. Often installed with adjustment facility to continually tweek clearances and in turn efficiency.
Open Impellers - similar to semi open, best for handling solids, but needs tight clerances on both front and back faces of Impeller - even higher machining costs.
Ash Fenn
www.cdrpumps.co.uk
RE: Recirculation
RE: Recirculation
The way to avoid internal recirculation is to operate at BEP.
RE: Recirculation
RE: Recirculation
RE: Recirculation
"Pump Handbook" --Impeller design section
http:
-MJC
RE: Recirculation
Chapter 9 of Centrifugal Pump User's Handbook: Problems and Solutions by Sam Yedidiah (Chapman and Hall) deals with the subject of Recirculation.