Pump Specific speed and efficiency

[freefallingbody]

Hi,

I am using PSAT software from US DOE.

I want to know whether there is effciency penalty if specific speed is low as well as when it is high.

I had seen a graph showing specific speed on x-axis , pump efficiency on y-axis and a number of curves ( almost similar to motor eff. curves) for different flows. I think that was from Hydraulic institute. In that graph the efficiency increases with specific speed and flow. Tell me if this is true ( that efficiency will always be more for higher specific speed). In that case why a penalty for high specific speed?

Aslo please guide me to a relationship giving maximum achievable efficiency of a pump for a given head, flow and speed.

Thanks in advance

 

[MJCronin]

ffb,

I suggest that you re-post this question on the "pump engineering" forum on this website.....

MJC

 

[vanstoja]

The so-called "Worthington pump curves", published in several pump design texts such as Stepanoff (1957), show for different design flowrates (or ranges of flowrates), that there is an optimum specific speed giving maximum pump efficiency for the particular design flowrate. For pumps delivering 10,000 GPM and higher, the optimum pump specific speed is approximately 2600 (RPM,GPM,Feet)which is in the range of Francis vane impeller designs (ie, low mixed flow range). At that specific speed and flow range, the maximum pump efficiency is about 86%. Recognize that these curves are based on averages of many supposedly well-designed pumps so, at best, are only approximate values when applied to new pump designs. Hydraulic efficiency dropoff at lower and higher than optimum specific speeds is generally due to higher pump mechanical or hydraulic losses, respectively, for the particular design flowrate.

 

[vanstoja]

CORRECTION
The maximum hydraulic efficiency for 2600 specific speed at 10,000+ GPM is 90-92% vice 86%.