Choosing the proper specific speed

Hi Muhammad,

The general guide is to approximate the four quadrant characteristics of the pump by selecting the Specific Speed closest to the value calculated using the equation in the following article: Estimating the Specific Speed of a Pump or Turbine

First ensure that the equation is properly used so that you know you are selecting the right Specific Speed.

If you are seeing large differences in results between two adjacent specific speeds, take a close look at the model results to get a firm understanding of the behavior of each case. Look for any instability or any conditions or specific sensitivity. If you see what appears to be irrational/unstable results, this could indicate other problems in the network that you may need to resolve before you can trust the results. Regarding sensitivity, some systems can be sensitive to small changes especially if there is significant vapor or air formation occurring. If you are seeing such things happening (animate profile paths) then the seemingly large differences in flow that you are seeing may still be acceptable. 

If your model appears to be stable and not sensitive in both cases (ex: the pump is settling down to a final steady state condition with no air or vapor present) then consider creating a custom four quadrant pump curve (new Specific Speed selection for your exact specific speed) based on the actual pump manufacturer data. This is a very advanced feature but you can find more information here: Defining custom four-quadrant curves (specific speed) for pumps and turbines in HAMMER

If a custom quadrant curve is also not an option, consider selecting the Specific Speed that results in the most conservative transient results.

Hi Muhammad,

The general guide is to approximate the four quadrant characteristics of the pump by selecting the Specific Speed closest to the value calculated using the equation in the following article: Estimating the Specific Speed of a Pump or Turbine

First ensure that the equation is properly used so that you know you are selecting the right Specific Speed.

If you are seeing large differences in results between two adjacent specific speeds, take a close look at the model results to get a firm understanding of the behavior of each case. Look for any instability or any conditions or specific sensitivity. If you see what appears to be irrational/unstable results, this could indicate other problems in the network that you may need to resolve before you can trust the results. Regarding sensitivity, some systems can be sensitive to small changes especially if there is significant vapor or air formation occurring. If you are seeing such things happening (animate profile paths) then the seemingly large differences in flow that you are seeing may still be acceptable. 

If your model appears to be stable and not sensitive in both cases (ex: the pump is settling down to a final steady state condition with no air or vapor present) then consider creating a custom four quadrant pump curve (new Specific Speed selection for your exact specific speed) based on the actual pump manufacturer data. This is a very advanced feature but you can find more information here: Defining custom four-quadrant curves (specific speed) for pumps and turbines in HAMMER

If a custom quadrant curve is also not an option, consider selecting the Specific Speed that results in the most conservative transient results.