Shaft Alignment of rotating equipment is a topic that should capture the attention of engineers and mechanics, especially when the operational vibration levels of the equipment make difficult the unit long-term functioning.
Here, we are presenting a case where Rotofix team was called by a client who experienced catastrophic failure (breakage) of an elastic coupling on Lube Oil pump (driven by an electric motor) that had just been put into operation after routine maintenance.
After initial discussions, the client provided a list of the work performed on the pump, which included reworking the pump’s baseplate (concrete repair), repairing the pump (including bearing replacement and inspection of the mechanical seal), and aligning the motor-pump shafts after the pump was installed on the newly repaired base.
Once the defective element was uncoupled (with the flexible elements being broken/damaged), a severe misalignment of the shafts (on the order of millimeters) was highlighted, preventing the reliable, long-term operation of the equipment.
Using the best practices for precise shaft alignment along with a high-performance EasyLaser XT440 shaft alignment device, Rotofix team identified the root-cause of the elastic coupling failure, namely the neglect to verify the stress induced by the suction and discharge pipes (pipe stress) of the pump (after the unit has been reinstated on the baseplate).
Excessive stress induced by the equipment’s pipes (commonly referred to as Pipe Stress) is one of the most likely factors negatively affecting shaft alignment and directly contributing to equipment failure.
The distortion of the equipment casing caused by high pipe stresses results in deformation of the bearing housings and, consequently, the rotor of the equipment. This deformation leads to reduced tolerances in the bearings and can quickly cause equipment damage, primarily to mechanical seals, couplings, and/or bearings.
A simple method to check for pipe stress is to directly measure the misalignment of the equipment nozzles relative to the pipe flanges in an uncoupled state (the acceptable value is generally provided by the equipment manufacturer, which also specifies the allowable suction/discharge nozzles force and moment values). Practically, a maximum of 0.50 mm can be used as a guideline—the idea is that the flange bolts can be easily installed without forcing the flanges onto the pipes.
Another method of checking for stresses in the pipes is to use dial gauges on the equipment coupling (one in the vertical plane and one in the horizontal plane) and sequentially tighten the suction and discharge flanges. Any reading on the gauges greater than 0.05 mm should be considered as pipe stress and proposed for correction.
For guidance in the area of precise shaft alignment, you can check the Library at www.rotofix.ro or give us a call.
Good luck!
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