Run-out condition

[sujins]

Please any pump expert explain the term of run-out condition? How it has been occured? and Does it have a criteria to select the max.flow rate, normal and min.flow rate for pump to operate?
Thanks in advance.

 

[quark]

When the system resistance to a pump is zero, you have run out condition. For example, when you don't pipe the pump discharge and run the pump, as the system resistance is zero, pump goes to the extreme right on its curve and discharges at the maximim rate(If the pump is non overloading type). This results in tripping of motor by overloading.

Safe minimum flow rate is decided by the manufacturer. Normal flowrate is what you require presently. Maximum flowrate is the extra capacity you should have incase of future expansion or for transient requirement.

 

[sujins]

Thanks for your answer.
Quark.

What i mean is that at the point where pump performance curve and system curve meet together, Is that point is the flow rate that will receive when we operate that pump? and does it have a criteria to choose where to determine the design flow rate? At that design flow the control valve will absorb the pressure drop, How can I know the suitable point to select for design case? I just wonder and please explain more!

 

[MikeHalloran]

Yes, the pump operating point is defined by the intersection of the system curve with the pump performance curve. You have some influence over the location of the intersection, e.g. moving the pump curve toward the origin by driving the pump at a lower rpm, or by adjusting the system resistance to scale that curve up or down.

The pump manufacturer provides a set of performance curves for a range of drive speeds and sometimes a range of impeller diameters. Superimposed on that you should find roughly elliptical curves representing the loci of constant efficiency operating points. It's customary, absent other constraints, to target your system's operaing point(s) somewhere within the highest efficiency region.



Mike Halloran
Pembroke Pines, FL, USA

 

[Bjegovic]

keep in mind that often the operating point (intersection of pump(s) and system curve) will change in time - for example, varying reservoir levels, consumers demand...also, pipe roughnes can increase gradually over time, affecting the system curve.

examine all possible operating points to evaluate your design!

 

[vanstoja]

To achieve best efficiency conditions, a centrifugal pump should normally be operated within a range of about plus or minus 15% of rated (design point) flow. Beyond that range, impeller inlet flow incidence angles generally become too great for smooth, undistorted flow through the impeller resulting in flow separation at the blade leading edges and suction or discharge flow recirculation which degrades hydraulic efficiency and possibly creates unstable flow conditions or damaging cavitation conditions. Manufacture's tests to determine flow recirculation onset may enable minimum continuous flow to be lowered to 60% rated or possibly even lower without encountering performance degrading flow passage recirculation conditions. Maximum allowable flow is generally determined by tests for cavitation flow breakdown conditions where the head drops precipitously toward zero as flow is increased beyond rated flow.