Evil Math & NPSH margin

Ok maybe not so evil, but I was struggling for a catchy title....

In my previous post I talked about Hydraulic Institute (HI) 9.6.1 (2017) and how it can help guide to selecting the appropriate NPSH margin for your pump application.

However some applications are not covered by the standard (such as water injection). Additionally I know some people might want a more math based approach to margin determination. So fear not. If you are one of those people who enjoy some non-evil math, read on.

The formula I'm going to cover is a simplification of the one we use internally. I've tried to spare you the reader the need to wade through several different charts and extrapolate between them. If you are concerned that your application falls outside the limits of the formula, feel free to contact me and I can give you more specific guidance.

So onto the formula, which is applicable for pumps pumping water and operating in the POR (Preferred Operating Range of 70% to 120% of BEP) with a Nss of 11,000 US (231 metric) or less. It requires that you know 3 pieces of information:

1. The eye diameter of the impeller (D1 in inches, see the diagram below)
2. The rotational speed of the pump N in RPM
3. The NPSH3 (NPSHr) in ft. of the pump at the operating point of interest

Note that MNPSH is expressed as a non dimensional number.

The formula reflects the general sensitivity of pumps to the velocity at the impeller eye. It is well established that pumps with very high eye velocities ( > 80 ft/s, 24 m/s) require significant NPSH margins. ( I'd encourage you to try the different eye velocity values and see the effect on recommended margin).

Ok so now for a worked example to show you the formula in action:

The example is a pump @ 3560 RPM with an eye diameter (D1) of 7 inches (178mm) and a NPSHr of 26 ft. (7.9m)

u1 = π * 3560 * 7 / 720 = 109 ft./s

Putting this into the formula gives:

So in this application the required NPSH margin is 1.91 and the hence the recommended NPSHa is 1.91 * 26 = 49.7 ft.

I'd encourage you to take a look at any problem pumps you have in your installation and evaluate them where appropriate using this formula or HI 9.6.1. You may find it a useful tool in judging the quality of the installation and help guide as to whether insufficient NPSH margin is a contributor to the pump's reliability.

Any feedback is as always gratefully received.

Beatus Centrifuga