2 replies to this topic

[sandeep chk]

Dear all,

I will be much obliged if somebody enligten me regarding the significance of specific speed and suction specific speed for a cenrifugal pump. What is the importance of these two parameters in pump design?

Regards,

[sandeep chk]

Dear all,

I will be much obliged if somebody enligten me regarding the significance of specific speed and suction specific speed for a centrifugal pump. What is the importance of these two parameters in pump design?

Regards,

Waiting for a reply
regards
Sandeep

[ayan_dg]

Dear Sandeep ,

Suction specific speed ( N_ss) of a pump is a dimensionless number expressed by the equation

N_ss=( N* Q ^0.5 ) / (NPSHA)^0.75

Where
N_ss : Suction Specific speed
Q: Capacity (Flow rate) in US GPM ( To be divided by two for double suction pump) at the Best Efficiency Point
N: Pump rotational speed in rpm
NPSHA: Net Positive suction Head available at the site expressed in ft of liquid column

Significance:
We know that with increse in flow rate Q the NPSH required of the pump increses. suction Specfic speed gives an idea whether a pump will cavitate or not hence it helps the selection process of pump. As a thumb rule the value of N_ss should be kept below 8500 to avoid cavitation.


Significance of specific speed:

N_s (specific speed)= (N * Q ^0.5 )/H ^0.75

Where
N= Pump rotational speed in rpm
Q= Flow rate in US GPM at best efficiency point
H= Total head at Best efficiency point in ft of liquid column

Specific speed help us to predict the type of impeller
For radial flow pump they have low values (800 to 1200 ) for mixed flow pumps they have intermediate values (1500 to 6000) for axial flow pumps they have high values (7000 to 15000). Curves are available which are plotted having pump specific speed in X axis & Efficiency in Y-axis for different flow rates , this helps us to determine the most efficient pump for a particular use ( large flow small head or large head small flow rate).
I hope this satisfy you.