Should I worry about pump Nss ? (Part 2)

So after a brief interlude, it is time to swing back to discussing Suction Specific Speed (Nss).

Before I discuss the effect of Nss on life cycle cost, I want to continue from part 1 of this topic and reiterate how far we've come in terms of hydraulic design and pump construction in the 32 years since Jerry Hallam's study of reliability was published in 1982. Since I'm currently rebuilding my hobby car's engine (a turbocharged 2002 Focus SVT, if you are curious to know), I thought I'd continue with the car analogy theme.

The graph below speaks to how two relevant parameters have changed:

So you can see that since 1982, vehicle fuel efficiency has improved significantly through many small incremental technological improvements. However the same engineers that happily drive around in cars capable of 50 MPG/ton, also make the (incorrect) assumption that pump maximum safe Nss is fixed (or even declining) despite 30 years of similar incremental improvements.

For example in 2012 we did a study on the effect of impeller leading edge profiling and found that just by applying an optimized parabolic profile we could improve the NPSHr of the pump by 15%. So using just this one change an impeller with Nss =11,170 was improved to Nss = 12,644 with no adverse effect on part load performance, vibration or suction recirculation.

We also know that through the deployment of more advanced hydraulic design tools (we utilize several, some commercial, some developed internally), we can now have much more control over the impeller vane shape to provide such things as:

• Low blade loadings in the inlet region up to the impeller throat area. These help prevent the formation of low pressure zones where cavitation will begin.

• S shaped developments of the impeller camber line in order to achieve the required impeller throat area while minimizing the eye diameter.

• Backward swept blades to reduce the volume of any cavitation that develops at the leading edge.

• Impeller leading edge carried well forward at the impeller hub in order to reduce the formation of cavitation at part load operation.

Now granted the above might sound like geek speak to anyone who doesn't find joy in hydraulic design, but bear with me, there is a payoff. What this means in practical terms is that if no Nss limits are specified by the customer, I am confident in designing OH style pumps with the Nss limits shown in the chart below. For obscure reasons that are not terribly important the blue line that indicates the limit is what we call the SGsT curve:

The chart is read by determining the specific speed (Ns) of your pump and finding it on the X axis then moving vertically until the blue line is intersected, at which point you read off the recommended Nss limit from the Y axis. So for example a pump with a 2000 specific speed, could achieve acceptable performance with a 12,000 Nss impeller.

Those readers with sharp eyes will have noticed that above 2500 specific speed, the maximum Nss drops below 11,000. Why would this be ?

Well the clue is in the shape of the pump impeller, which I have included on the chart.

At 500 specific speed the impeller is like a skinny disc with a tiny impeller eye. For these impellers suction recirculation is not a problem (in fact the problem is often too little recirculation leading to unstable HQ curves). Consequently we can confidently design to 15,000 Nss knowing the pump will be reliable.

Contrast this with a 3000 specific speed impeller where the impeller eye is not much smaller than the outside diameter of the impeller. An impeller like this requires careful design in order to limit the onset of suction recirculation and consequently we would prefer to limit the Nss to around 10,000 or less.

When viewed in this context having a single Nss value regardless of the pump specific speed makes very little sense. Regardless of how conservative you want your Nss to be, I'd strongly recommend changing from a single value to a more graduated approach.

That's all for part 2. In part 3 I'll talk about the potential effects of Nss limits on life cycle costs. Until then, any comments or questions are most welcome.

Beatus Centrifuga