How to Protect Pumps from Costly Rotor Vibrations

The reliability of high-speed and high-pressure pumps is crucial to several industries in the Middle East and North Africa. Oil and refinery services and desalination operations, for example, depend heavily on pumps to perform consistently in demanding conditions. MENA operators should turn to practical guidelines for studying pump rotordynamics, especially the conditions that lead to dynamic malfunction and instability. Rotor vibration primarily limits how many impellers can be installed in a pump casing. Unbalance and hydrodynamic forces are two leading causes of rotor vibration during normal operation. This can exert excessive force at the bearings and deflect the rotor, shrinking internal clearances. Rubbing can then occur. Pump codes and standards specify maximum permitted out-of-balance forces and acceptable responses to them. The lateral critical speeds of the rotor supported on the bearing should not fall within the defined margins of a pump’s operating speed range. No contact between the rotor and stationary elements should occur at any speed at or below the operating speed.

Identifying Dynamic Effects

An undamped rotordynamics analysis identifies the rotors’ undamped critical speeds, or natural frequencies, and determines their mode shapes. The mode shapes help select the right rotor positions for setting the unbalances in the damped unbalance response analysis. For critical pumps, a calculated rotordynamics response should usually be verified at the manufacturing facility. The damped unbalanced rotordynamics analysis requires details on several dynamic qualities, including rotor mass, stiffness, accumulated fit tolerances, fluid stiffness and damping. Establishing a basis for the bearing stiffness and damping values is critical, especially for pump bearings. The pump’s complete operating speed range is also important to consider—thanks to the popularity of electric motors with variable speed drives. The difference between maximum continuous and first critical speeds affects a pump’s dynamic behavior. The margin usually decreases if the length of the pump rotor is increased—often a function of the number of impellers. The larger the rotor’s length-to-diameter ratio, the more likely a resonance or instability will occur. Results of the unbalanced dynamic response analysis include frequency, phase and response amplitude data, also known as Bode plots. Calculated major-axis, peak-to-peak, unbalanced rotor response amplitudes (at any speed from zero to trip speed) should not exceed 80 percent of the minimum design diametric running clearances throughout the pump. The operating clearances could differ from the assembled clearances.

Bearing Stiffness & Damping

Modern pump rotordynamics analysis programs calculate unbalance excitation lateral vibrations of a general multi-mass rigid or flexible rotor with bearings. The nonlinear stiffness and damping characteristics of bearings are usually linearized for a vibration close to the equilibrium static point for the bearings, which are modeled according to carefully linearized spring and damping coefficients. The bearing analysis program is critical for a correct pump rotordynamics analysis and the pump’s long-term reliability. Fluid forces originate where radial clearances between rotor components and stationary parts are small, such as bearings and seals. The behavior of rolling-element bearings in small pumps could be highly nonlinear. Sometimes, these small pumps require a nonlinear rotordynamics study for a realistic prediction of the dynamic responses. A pump should have a bearing that suits its application. Tilting-pad journal bearings, which are often used for large pumps, eliminate exciting forces for high-speed pumps. These bearings also inhibit cross-coupling forces.

Reducing Vibrations

Rotor stability analyses predict whether the vibrations of a pump rotor under operating conditions increase or decrease after an initial disturbance. A pump that is operating safely decreases vibration. Sources of increased vibration usually include the pump bearings, seals, impellers, shrink fits and shaft material. For some pump rotors, the minimum pumping load condition could represent the worst-case scenario for stability. The pump rotor’s unbalance depends on the unbalance of each attached component, as well as the mechanical run-out of the couplings and impeller fits. The actual unbalance at operating speed or within the operating speed range is an unknown unbalance distribution along the rotor axis. This unknown unbalance is particularly important for high-speed pumps. A low-speed residual unbalance may not give the same amount of unbalance at the operating speed—particularly for high-speed pumps—because of the elastic deflection of the rotor. The conventional rotor response analysis shows only the sensitivity of the rotor to a certain, discrete unbalance. This could be seen as a worst-case scenario because the real unbalance is distributed along the rotor axis. Some pump vibrations have reportedly resulted from blade-impeller excitations. Excitation frequencies could include the following:

• The blade-impeller passing frequency and related harmonics (usually up to fifth harmonics)
• The liquid passage splitter frequencies resulting from local flow disturbances
• The pump’s speed or speed range and harmonics (often up to eighth harmonics)

If any interference between a natural and excitation frequency cannot be avoided in the pump design, the pump manufacturer should be asked to provide adequate evidence that the material’s stress levels during resonance will remain at acceptable levels under all conditions. Natural frequencies should be at least 10 percent above or below any possible excitation frequency within the specified operating speed range of a pump. For a smooth operation, the pump manufacturer should demonstrate the pump’s hydrodynamic performance. The manufacturer should also indicate the pump’s temperature performance and critical pump tolerances, such as the tolerance of blade-tip clearance. Transient conditions, such as startup, emergency trip and other conditions, are also important cases. More than 50 percent of all reported pump dynamic problems occur in transient situations. The sensitivity analyses are important for long-term reliability of a pump, particularly in degraded conditions.