Recirculation

[adityap707]

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

 

[dcasto]

I guess you are talking internal splippage of the fluids. These are from the mechanical fit of the impeller/difusser or case.

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?

 

[Valvecrazy]

Sometimes a recirculating line is used from the discharge side of the pump to the suction. Especially with hot water, a certain amount of flow, recirculating or going downstream, is needed to maintain the cooling for the pump and seal. Without some water being let out downstream, the recirculating line will continue recirculating the same water, until a critical temperature is reached and the pump will fail. Recirculating lines are usually used to maintain a certain flow for cooling, for a short amount of time, when the discharge from the pump is temporarily lessened or shut off.

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.

 

[pumpking]

Moving on from here a bit, you then look at the Impeller design, Shrouded/closed Impellers, Semi open Impellers and fully open Impellers.

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

http://www.cdrpumps.co.uk

 

[jet1749]

I think he refers to recirculation within and around the impeller vanes due to operating at low flow or high flow conditions. This is due to the impeller vane angles being designed around a flow rate envelope. Operating outside of these flowrates produces a stall condition similar to that experienced by aircraft wings when the angle of attack is too steep for airflow over the wing.

 

[hbw]

Internal recirculation occurs when operating at flow rates below the Best Efficiency Point. They are eddy currents and represent ineffiency.

The way to avoid internal recirculation is to operate at BEP.

 

[bk19702]

Suction specific speed will impact the propensity for re-circulation as well at lower flow rates. A higher Nss translates to a larger impeller eye; at lower flow rates the fluid flow into the vane inlet can become extremely turbulent, and re-circulation occurs.

 

[Artisi]

Suction and discharge recirculation are a fact of life if operating away from BEP and to answer the main question of "how to overcome these phenomena" - simple answer operate at BEP or very near to it, there is no other way to overcome internal recirculation.

 

[MJCronin]

giddyyap...

"Pump Handbook" --Impeller design section

http://www.amazon.com/Pump-Handbook-Igor-J-Karassik/dp/0070340323

-MJC

 

[25362]

Chapter 9 of Centrifugal Pump User's Handbook: Problems and Solutions by Sam Yedidiah (Chapman and Hall) deals with the subject of Recirculation.