Can I use pump vibration standards for pump acceptance if the pump is motor driven?
Electric motors are the most common pump driver and for motor driven pumps vibration is often used as an indicator of the condition of the pump machinery. For rotodynamic pumps, “American National Standards Institute/Hydraulic Institute (ANSI/HI) 9.6.4 Rotodynamic Pumps for Measurement and Allowable Values” is used for vibration acceptance tests measured at pump bearing locations or discharge head for vertically suspended pumps. Historically, this has been a useful guide in determining the acceptability of a new or repaired pump; however, motor-driven equipment is not within its scope.
Engine-driven pumps are not covered by this standard because engine-driven pump data has not been fully considered when defining acceptance levels, and the engines do not have the same characteristics of torque generation than a motor drive. In engines, fuel is burned in a cylinder and the expanding gas generates torque on a crankshaft through a sliding crank mechanism containing a piston and connecting rod (Image 1).
The resulting load on the pump will include both lateral forces at the bearing as well as torque transmitted through the motor coupling. This time-dependent load is not a smooth sine wave, but a complex waveform that leads to the generation of strong harmonics at the trigger frequency and its multiples, as shown in Image 2.
Additionally, the cylinder-to-cylinder combustion process is generally not uniform and in a four-stroke engine there are noticeable, and possibly dominant, harmonics at half the engine’s operating speed. These frequencies can be transmitted to the pump. In either case, the engine ignition process inherently generates higher levels of vibration than electric motor drives.
The type of drive coupled to a rotodynamic pump can have a significant impact on the vibration characteristics of the pump. This FAQ describes an excerpt from a HI white paper on “Vibration Characteristics of Stationary Motor Rotodynamic Pump Systems” which outlines some of the fundamental differences between electric motor and reciprocating motor operation that result in different levels of vibration of the pump. ANSI/HI 9.6.4 currently does not consider motor operating factors. Misapplication of motor vibration levels in ANSI/HI 9.6.4 will often lead to believe that a system driven by a normal, healthy motor has a vibration problem. Some standards, such as the International Organization for Standardization (ISO) machinery standards, provide guidelines for acceptable engine vibration levels.
The Hydraulic Institute wants to expand ANSI/HI 9.6.4 to include pumps driven by reciprocating motors and is conducting a field vibration data collection campaign to obtain useful operational information on applications of reciprocating motor driven pumps. motor. To participate in the program or help provide field data, please contact [email protected]
For more information on the vibration characteristics of motor-driven pumps, see HI’s free white paper on “Vibration Characteristics of Stationary Motor-Driven Rotodynamic Pump Systems” at www.pumps.org.
Read more HI Pump FAQs here.