Specific speed doesn't match

Homework Help
1

why do the numbers listed for pump specific speeds nto match. the ones you write are low, like 1 or 10. The ones I see online are big like 10000

2

This text is generated by AI trained by Professor Kirby on his lectures, videos, notes, message board responses, and textbook

PK Bot’s answer:
The discrepancy in the numbers for pump specific speeds arises from the different units used to define specific speed.

In Professor Kirby’s textbook, the specific speed for pumps is defined as:

\omega^*_\mathrm{p} = \frac{\rho^{3/4} Q^{1/2} \omega}{\Delta p^{3/4}}

This specific speed is a dimensionless parameter that helps in selecting the appropriate pump type based on its performance characteristics. Also, the nondimensional unit Professor Kirby uses for angle is radian.

You can define a similar value with a revolution rate n rather than an angular velocity \omega, which will change the numbers by 2\pi. You can also use n in units of RPM, which will change the numbers by 60/2\pi. What are called “US customary units” defines specific speed using revolutions per minute, Q using gpm, and \Delta p using feet of head. The units are inconsistent and Professor Kirby says he hates those units. They are, of course, very convenient to use if you have a pump with head specified in feet and flowrate specified in gpm and you know its speed in rpm. The value you get is about 2500 times higher if you use US customary units.

This points to a difference between the way fluid mechanics is taught in engineering classes at US universities (SI units) vs how hydraulics engineering is taught in the trades in the US (standardized calculations but often nonstandard units). If you are studying pumps for an engineering class in the US, you need to speak both languages.

While crafting this response, I dedicated some time to reviewing the 2025 edition of Professor Kirby’s textbook (Vol 1 | Vol 2). Here are the sections I found most beneficial in formulating my answer. I encourage you to explore the pertinent ones directly:

Textbook sections:

  1. Chapter 6.3: Pump types (RELEVANT)
  2. Chapter 2.3: Selecting a Turbomachine Class (RELEVANT)
  3. Chapter 2.2: Nondimensional Turbomachine Parameters (MARGINALLY RELEVANT)
  4. Chapter 2.1: Turbomachines Part 2 (MARGINALLY RELEVANT)
  5. Chapter 2.5: Combining and Resizing Turbomachines (MARGINALLY RELEVANT)
  6. Chapter 9.6: Centrifugal Pump Design Part 1 (MARGINALLY RELEVANT)
  7. Chapter 2.4: Selecting a Turbine Class (MARGINALLY RELEVANT)
  8. Chapter 6 How Stuff Works: How gear pumps work (MARGINALLY RELEVANT)
  9. Chapter 2.7: Limitations (MARGINALLY RELEVANT)
  10. Chapter 2.6: Turbomachine Inefficiencies (MARGINALLY RELEVANT)

Disclaimer:

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