When measuring vibration on vertical pumps, does the allowable vibration depend on the height of the motor or on the height above the baseplate where the readings are taken?
The allowable vibration levels published by ANSI/HI Standard 9.6.4, Centrifugal and Vertical Pumps for Vibration Measurements and Allowable Values, are for one location only-usually near the upper motor bearing of vertical pumps with close-coupled motors or at the junction of the pump discharge head and the bottom mounting face of the motor. To determine the correct location of the vibration probes, consult the standard.
The probability of vibration damaging any pump is dependent upon two factors: 1) The stress placed on any components within the pump or any component, such as a gearbox or motor, which is mounted on the top of the pump and 2) the likelihood of interference at close running clearances inside the pump or driver. The scale of all components, including their close-running clearances, generally increases with the size of the equipment. Therefore, in larger pumps and motors, higher vibration displacement levels would be expected before undue deterioration occurs. However, larger equipment typically runs at slower speeds.
A user can approximately and simultaneously account for the increasing size and reducing speed of the equipment by using specifications based on vibration velocity, rather than vibration displacement. Worldwide specifications, including ANSI/HI as well as ISO, use vibration velocity as the preferred method of vibration measurement in pumps and their driving equipment.
When vibration velocity is used as the measurement parameter, the size of the equipment-including height above the baseplate-is of less significance. However, we recognize that actual vibration levels on vertical pumps will increase as the distance from the baseplate increases. Even this statement depends upon the assumption that the excited vibration mode is the "first reed bending" frequency (the top and bottom of the motor bend in the same direction). The best approach is to use the general specifications only at the measurement points to which they apply, and to seek the advice of the manufacturer concerning the proper interpretation of vibration readings taken at any other locations.