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CAUSES OF VIBRATION

Typical 1-times-rpm vibrations include imbalance, a soft foot, and a bent shaft. Mechanical looseness, misalignment, and resonance can generate vibrations at frequencies of two, three, or more multiples of rpm. Failing bearings typically produce non-integer multiples of pump speed. Hydraulic oscillations show frequencies that are a product of the number of impeller vanes multiplied by rpm.

Vibration frequencies are used to help locate the root cause of a pump's excessive vibration. Knowing the vibration frequency — the first step in the analysis process — can help narrow the possible causes of vibration.

Misalignment

Misalignment is the No. 1 contributor to vibration problems. This occurs when rotating centerlines of two shafts are offset and/or at an angle. Thus, proper coupling alignment is key to avoiding vibration problems. Shaft alignment should occur several times during a pump installation. Initial alignment should be completed prior to the piping of a pump. After piping is complete and the pump base is anchored (but not grouted), the shafts should be aligned again. The final shaft alignment always should be performed under actual operating temperatures. This allows for the contraction or expansion of all system components and ensures proper alignment under actual operating conditions. Alignment should be checked periodically and corrected if needed. Running a misaligned pump can cause extreme heat in couplings and bearings, bent shafts, and increased operating costs. To reduce angular misalignment, consider dial indicators or laser alignment equipment.

Coupling manufacturers publish alignment tolerances that should be followed if not addressed in a pump's installation manual. Different couplings will produce different vibrations, which are characteristics of design and construction materials.

Soft foot

Vibration also can be caused when the feet on a motor frame or pump casing are not on the same plane. Tightening the nut and bolt on a soft foot can distort the base, casing, and/or motor frame, leading to vibration. This is similar to pipe strain on suction and discharge flanges, which can induce a soft foot. Pump and motor shafts can be aligned perfectly and still exhibit excessive vibration because of the distorted frame caused by a soft foot.

To check for soft foot on a motor frame or pump casing, loosen the mounting bolt for each foot individually, checking the vibration level after each change. If a soft foot has occurred, the vibration level will decrease as a mounting bolt is loosened. Shims can be added under a soft foot to correct the condition. Alignment between shafts should be checked after shims are added.

Imbalance

Imbalance is caused when an object's center of mass is different from its center of rotation. As the leading cause of pump imbalance, impellers should meet G6.3 for allowable residual imbalance. For high-rpm pumps, rotating assemblies, including couplings, should be balanced as a unit to meet G6.3. It is reasonable to assume an impeller is balanced at startup, but after operation, an imbalance may be caused by wear. Impeller imbalance also is a frequent cause of mechanical-seal and bearing failure.

Another area of concern for imbalance is the shaft key used for a coupling and impeller. Using the wrong key could adversely affect rotating-assembly balance.

Mechanical looseness

Mechanical looseness should never be overlooked in the field. All mounting bolts, including base-plate, pump, motor, coupler, and guard bolts, can become loose over time. Pipe restraints also should be checked for looseness. Vibration can result from cracked foundations, insufficient contact between grout and pump bases, or cracked grout in base plates. When these problems occur, a pump, base, and foundation no longer are one solid unit. They become several individual units, each with its own natural frequency. It is important to use high-quality non-shrinking grout when installing a base designed for grouting.

Base installation

Poor base installation is another leading contributor to vibration. Foundation bolts of the proper size and number should be embedded in concrete prior to the installation of a pump base. The base should be supported and leveled by properly sized shims at a minimum of each anchor bolt. Nuts should be set to the correct torque based on their size. Any exceptions or modifications to the pump-base installation can lead to vibration issues.

Resonance

Every system has many resonant frequencies. If a system encounters vibrations at these frequencies, amplification will result in premature component failure and increased downtime. Resonance can be avoided by changing a system's frequency, which is determined by the mass, stiffness, and damping properties of all of the components involved, including the pump, base, motor, piping, coupling, guard, foundation, etc. If the resonance vibration is just below the system's natural frequency, the stiffness of the system should be increased so the vibration frequency shifts above the natural frequency. If the resonance vibration is just above the natural frequency, the system's mass should be increased, shifting the vibration frequency below the natural frequency.

Another solution to resonance vibration is to change the operating speed. When a centrifugal pump is operating on a variable-frequency drive, changing the speed in lieu of the resonant frequency can be the most economical option. Drives often are programmed to skip a frequency or two to avoid resonance vibration. This should not be a problem in an HVAC application. Another option is to determine which component in the system is being excited at resonance, then replace the component or modify its resonance frequency.

Cavitation

Cavitation also can cause pump vibration. It occurs when the system's available net-positive suction head (NPSHa) is less than the pump's requirement (NPSHr). Indications of a cavitating pump can include noise, fluctuating flow rates, a decrease in discharge pressure, and vibration. Cavitation-induced vibration is caused by imploding vapor bubbles that introduce shock waves in a pump, shortening the life cycle of all of the pump's mechanical components. The components that commonly fail prematurely are impellers, wear rings, and casings. If cavitation is present, NPSHa should be increased above NPSHr by making changes in the system design or operation to reduce or eliminate cavitation. Cavitation does not always produce pump vibration, and the induced vibration often is random and unmeasurable.

Motors

Finally, motors can be the root cause of vibration. Like centrifugal pumps, motors are built to vibration specifications. Guidelines are given by the National Electrical Manufacturers Association. Unfiltered velocity amplitudes should not exceed 0.12 in. per second zero to peak at bearings for 1,200-, 1,800-, and 3,600-rpm motors. These guidelines are for rigidly mounted uncoupled motors running at no load.

Electrically induced vibration results from unbalanced voltage, broken rotor bars, and air-gap distortion and can be checked by de-energizing a motor while in operation. If the vibration disappears after the motor is de-energized, the vibration is induced electrically. A broken rotor bar prevents current flow, resulting in a lack of a magnetic field. Because the magnetic field causes the rotor's rotation, its absence in one area results in an imbalance force. Air-gap distortion results when the space or gap between the rotor and stator is not perfectly even all of the way around. The strength of the magnetic field is proportional to the gap. Thus, the magnetic field is stronger where the gap is small and weaker where the gap is large. This variation in the magnetic field around the rotor also can produce an imbalance. By understanding vibration, its causes can be identified and corrected.

For past HPAC Engineering feature articles, visit www.hpac.com.

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A senior product specialist in the commercial HVAC pump group for ITT Residential & Commercial Water, Stan Kutin has experience with HVAC-system design, applications, and centrifugal pumps. He can be reached at stan.kutin@itt.com.

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