critical speed

Critical speed is the speed at which equipment has highest vibration amplitude.

Critical speeds are not different types.

A pump should not be operated at its critical speed.

Example: If you have a pump that starts from 0 RPM and 890 RPM is its maximum speed. If pump critical speed is 800 CPM then design changes are required.

This is some basics for your understanding.
critical speed is the speed when the natural frequency is excited by the mass residual unbalance force.
at this state the mass factor and the stiffness factor are against each other in direction which cause to lower the system resistance to vibration force.
any type of rotor will have one or more critical speed. so it will be like 1st critical speed 2nd critical speed and so on.

It is the Speed which is equals to its natural frequency. e.g-if a pump has 1st  natural frequency of 20 Hz (A natural frequency is a band of frequencies so it wont be a exact value ) then , then the 1200 cpm or anyway nearer 1200 cpm  will be its critical speed and we will recommend to operate at-least +/- 20 % from the critical speed.B

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