The rotating shaft (or rotor) generates vibrations that are transmitted to the split case pump and then to surrounding equipment, piping and facilities. Vibration amplitude generally varies with rotor/shaft rotational speed. At the critical speed, the vibration amplitude becomes larger and the shaft vibrates in resonance. Unbalance and misalignment are important causes of pump vibration.
However, there are other sources and forms of vibration associated with pumps.
Vibration, especially due to imbalance and misalignment, has been a constant focus of concern for the operation, performance, reliability and safety of many pumps. The key is a systematic approach to vibration, balancing, alignment and monitoring (vibration monitoring). Most research on split case pump vibration, balance, alignment and vibration condition monitoring is theoretical.
Particular attention should be paid to practical aspects of job application as well as simplified methods and rules (for operators, plant engineers and specialists). This article discusses vibration in pumps and the intricacies and subtleties of the problems you may encounter.
Vibrations in the Pump
Split case pumps are widely used in modern factories and facilities. Over the years, there has been a trend towards faster, more powerful pumps with better performance and lower vibration levels. However, to achieve these challenging goals, it is necessary to better specify, operate and maintain pumps. This translates into better design, modeling, simulation, analysis, manufacturing and maintenance.
Excessive vibration could be a developing problem or a sign of impending failure. Vibration and the associated shock/noise are seen as a source of operational difficulties, reliability issues, breakdowns, discomfort and safety concerns.
The basic characteristics of rotor vibration are usually discussed based on traditional and simplified formulas. In this way, the vibration of the rotor can be divided into two parts in theory: free vibration and forced vibration.
Vibration has two main components, positive and negative. In a forward component, the rotor rotates along a helical path around the bearing axis in the direction of shaft rotation. Conversely, in negative vibration, the rotor center spirals around the bearing axis in the opposite direction to the shaft rotation. If the pump is built and operated well, free vibrations usually decay quickly, making forced vibrations a major problem.
There are different challenges and difficulties in vibration analysis, vibration monitoring and its understanding. In general, as the vibration frequency increases, it becomes increasingly difficult to calculate/analyze the correlation between the vibration and the experimental/actual readings due to the complex mode shapes.
Actual Pump and Resonance
For many kinds of pumps, such as those with variable speed capability, it is impractical to design and manufacture a pump with a reasonable margin in resonance between all possible periodic perturbations (excitations) and all possible natural modes of vibration.
Resonant conditions are often unavoidable, such as variable speed motor drives (VSD) or variable speed steam turbines, gas turbines and engines. In practice, the pump set should be dimensioned accordingly to account for resonance. Some resonance situations are not actually dangerous due to, for example, the high damping involved in the modes.
For other cases, appropriate mitigation methods should be developed. One method of mitigation is by reducing the excitation loads acting on the vibration modes. For example, excitation forces due to unbalance and component weight variations can be minimized through proper balancing. These excitation forces can typically be reduced by 70% to 80% from original/normal levels.
For a real excitation in a pump (real resonance), the direction of the excitation should match the natural mode shape so that the natural mode can be excited by this excitation load (or action). In most cases, if the excitation direction does not match the natural mode shape, there is a possibility of coexistence with resonance. For example, bending excitations generally cannot be excited at the natural frequency of torsion. In rare cases, coupled torsional transverse resonances may exist. The likelihood of such exceptional or rare circumstances should be assessed appropriately.
The worst case for resonance is the coincidence of the natural and excited mode shapes at the same frequency. Under certain conditions, some compliance is sufficient for the excitation to excite the mode shape.
Furthermore, complex coupling situations may exist where a specific excitation will excite unlikely modes through coupled vibrational mechanisms. By comparing the excitation modes and natural mode shapes, an impression can be formed whether excitation of a particular frequency or harmonic order is risky/dangerous to the pump. Practical experience, accurate testing, and running reference checks are ways to assess risk in theoretical resonance cases.
Misalignment is a major source of split case pump vibration. Limited alignment accuracy of shafts and couplings is often a key challenge. There are often small offsets of the rotor center line (radial offset) and connections with angular offsets, for example due to non-perpendicular mating flanges. So there will always be some vibration due to misalignment.
When the coupling halves are forcibly bolted together, the rotation of the shaft produces a pair of rotational forces due to radial offset and a pair of rotational bending moments due to misalignment. For misalignment, this rotational force will occur twice per shaft/rotor revolution and the characteristic vibration excitation velocity is twice the shaft velocity.
For many pumps, the operating speed range and/or its harmonics interfere with the critical speed (natural frequency). Therefore, the goal is to avoid dangerous resonances, problems and malfunctions. The associated risk assessment is based on appropriate simulations and operating experience.