Question

formula for critical speed ​

Answer 1

Now that you have calculated the static deflection (sag) of the shaft as measured at the impeller, we will use this number to calculate the first critical speed of the pump. For all practical purposes you can calculate the first critical speed by using one of the following formulas:

Nc = Critical speed

Y = The deflection that we calculated.

To maintain internal clearances of the wear rings in a closed impeller pump and to prevent the impeller from hitting the volute or back plate in an open impeller pump, most pump companies would like to limit shaft deflection to between 0.005 and 0.006 inches (0,125 and 0,150 mm.). Putting these desirable numbers into the formula we get:

As you can see, these numbers are well in excess of the 1750 or 1450 rpm. that we normally use for centrifugal pump speed. They are, however, lower than the higher speed pumps that run at 3500 rpm. or 3000 rpm. This means that higher speed pumps and variable speed pumps will experience shaft deflection as they pass through, or run at these critical speeds.

Since operation off of the pump’s best efficiency point (BEP) is common for centrifugal pumps, you will be experiencing shaft loads well in excess of those noted in the above examples; meaning that your critical speed will actually be experienced at a much lower rpm. than noted.

The numbers we calculated reference a shaft running in air. In actual practice the impeller and a major portion of the rotating assembly is immersed in liquid that provides a hydrodynamic support to help stabilize the assembly. Pump people call this hydrodynamic stabilizing the “Lomakin Effect.”

Answer 2

Answer:

Rayleigh Ritz method critical speed calculator - formula & step by step calculation to find the critical speed Nc of a rotating shaft. Nc = [(30/π) x √(g/Δst)].