Cast iron impeller

Q  -  What is the maximum allowable peripheral speed for cast iron impeller?

A  -  It depends on specific grade of cast iron used: Class 30, 40, or 50. The class
number indicates its minimum tensile strength, in pounds per square inch (PSI).
The maximum allowable peripheral speeds, in feet per second (FPS), are:

ASTM A-48            Tensile                      Speed
Class 30                30,000 PSI               [ * ] FPS
Class 40                40,000 PSI               [ * ] FPS
Class 50                50,000 PSI               [ * ] FPS

These values are based on [ * ] degrees Fahrenheit maximum temperature.
Some manufacturers avoid using Class 30 cast iron because of quality and
machining issues with commercial sand castings.

The calculation for peripheral speed (S), in feet per second, is:

S  =  [ D x N ]  /  229

By rearranging, the equation can be rewritten as:

D  =  [ S x 229 ]  / N

N  =  [ S x 229 ]  / D

where:

D - impeller diameter, in inches
N - rotational speed, in revolutions per minute (RPM)
S - peripheral speed, in feet per second (FPS)

Example:

What is the biggest diameter a cast iron impeller can be used safely at 3560 RPM,
at ambient temperature?

Answer:        D  =  [ * x 229 ]  /  3560  =  [ * ] inches

[ * ] Some information are excluded in this article.