Unstable Pump Curves
Unstable Pump Curves
(OP)
Hello
I have a small question.
Sometimes in pumps (in point of fact in high specyfic speed pumps) the performance curve is unstable (by decrease capacity).
What is the reason of this effect.
Partial cutback of vanes at exit from the impeller in an inclined plane make that the curve get stalbe ...WHY?
I have a small question.
Sometimes in pumps (in point of fact in high specyfic speed pumps) the performance curve is unstable (by decrease capacity).
What is the reason of this effect.
Partial cutback of vanes at exit from the impeller in an inclined plane make that the curve get stalbe ...WHY?





RE: Unstable Pump Curves
Thank for answer
RE: Unstable Pump Curves
... by which I mean that a centrifugal pump is a kinetic device, so changing any part of its geometry is likely to change its behavior, and usually does.
Also because it's a kinetic device, it's not possible to test such a pump in isolation; it's always interacting with the system in which it happens to find itself, and the systems are not universally stable in their behaviors.
If memory serves, somewhere within the pile of books that I have read or bought about pumps, filing the vanes changes the shape of the capacity curve, which in turn moves the operating point within a given system. Sometimes the OP moves to a better place, sometimes not.
I don't think it's technically correct to say that a pump's curve can be unstable, but the pump _system_ can exhibit odd behaviors. Even more odd is the range of theories that local 'experts' will synthesize to explain said behaviors.
As for me, I'd clean out the, e.g., barnacles, and personally measure the operating point before filing anything.
Mike Halloran
Pembroke Pines, FL, USA
RE: Unstable Pump Curves
When you angle cut, you give the fluid space to align itself betore entering the diffuser/volute opening.
That's just a guess.
RE: Unstable Pump Curves
I remember Den Hartog's Mechanical Vibrations describes an unstable pump curve in similar terms to what Mike described.
I also seem to vaguely remember somewhere an explanation related to angle of attack reminiscent airplane stall. Has some resemblance to what Martin said.
I'll be interested to hear what other explanations appear. I'd like to know myself. The only plants at our plant that have this characteristic are the large slow speed axial flow circulating water pumps.... not sure if there is something about this style pump that creates that characteristic.
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RE: Unstable Pump Curves
Another probable explanation. Sam Yedidiah in his book Centrifugal Pump User's Handbook Problems And Solutions Chapman and Hall, refers to recirculation on performance at low NPSH saying,
RE: Unstable Pump Curves
RE: Unstable Pump Curves
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