Understanding Vibrations on Axial and Mixed-Flow Lineshaft Pumps

Vibration Wavelength Analyzer

Pumps can and do vibrate for a variety of reasons. First step in assessment is to decide if it is excessive or tolerable. The type of machine, installation environment, amount of use, etc. need to be considered, and to help in those decisions various specifications are available for guidance. For example, organizations such as ISO, API, HI (Hydraulics Institute) have significant information for assistance.  

Once decided that the vibration needs to be reduced, it needs to be analyzed to determine the source of the energy. There are a variety of possible sources for this, so one cannot assume that it is automatically the pump’s fault. The sources of vibration can be periodic or random. Periodic vibrations can be orders (multiples) of shaft speeds and harmonics of non-synchronous sources. Periodic vibrations have many sources; sometimes the source is the pump system itself, sometimes it is outside of the pump (i.e. significantly uneven 3-phase electrical power; another machine running nearby exciting a pump’s natural frequency, etc.). Quite often the sources are imbalance, misalignments, damaged parts (gear teeth, electric motor issues, etc), resonance, or blade pass matching. These can be confirmed or denied using a good vibration analyzer by a knowledgeable operator. The analyzer collects time-domain data then using an FFT circuit converts this time-waveform to a frequency-domain “spectrum” graph. Both the time waveform and spectrum information are used to get to the root of the source(s) by knowing the dominate frequencies of the vibration and comparing to known shaft speeds, their orders and other types of sources. If the scenario entails 1x shaft speed, imbalance may be the issue; 2x can be misalignments; 3x /4x /or 5x may be a blade pass problem if the pump is a 3 /4 /or 5 blade propeller/impeller machine. Blade pass isn’t necessarily the pump’s fault, it can be subsurface vortexes impacting the blades as the propeller rotates and might only be solved by changing the flow pattern coming to the pump. 

Random vibrations in lineshaft pumps are almost always fluid related. A good example of random vibrations related to fluid are cavitation, vortexes or general turbulence. These sources are usually caused by the geometry of the system feeding the pump and sometimes just the wrong pump in the application (typically cavitation) which can be expensive to fix. In a spectrum graph from an analyzer, this type of vibration is a broad raised floor covering a higher frequency range. 

When it comes to resonance, every structure has mass and stiffness and therefore there are infinite natural frequencies, however typically only the lower frequencies are of concern. When a periodic source’s frequency is near a natural frequency then resonance is possible. Depending on damping involved, the source doesn’t need to be very strong to generate large amplitudes. Solutions are either: change the mass, change the stiffness, change the source’s frequency, add damping, or detune using another mass/spring attachment.