ODS, Video and Modal: Great Techniques That Work Well Together
By Maki Onari
Every time I hear the words: “troubleshooting”, “vibration problems”, “repetitive failure”, or “catastrophic failures” on rotating machinery, I cannot avoid listening to the conversation and it makes me want to participate in the discussion. I think that it is in my DNA and it is inevitable. By nature, it makes me think about the following questions: How can I help the End User to identify the root cause and solve the problem? What kind of instrumentation should they use to quickly gather the information that the equipment is trying to tell them?
At this point in my career, there is no doubt that the first techniques that come to my mind and should be conducted, regardless the nature of the problem, are Operating Deflection Shape (ODS), its young cousin Motion Magnified Video (MMV), and Experimental Modal Analysis (EMA) testing, or commonly known as Modal Impact testing. These troubleshooting techniques go hand-in-hand and none of them should be ignored during the troubleshooting process for turbomachinery (like the key pieces of a puzzle, in this case they represent valuable information for the Root Cause Analysis (RCA)).
As I mentioned in other Blogs (see The Value of Operating Deflection Shape blog post here), ODS testing has been and will continue to be an essential troubleshooting tool that has helped MSI’s engineers and technicians to identify the root cause of the high vibration of any rotating machinery. The ODS will help visualize, in a 3D computer model or using Motion Magnification from high-speed video, the overall motion of the machine including the surrounding stationary components (i.e. piping, foundation, other rotating equipment, etc.) at a given frequency (i.e. 1x rpm, 2x rpm, VPF, etc.). It can also help visualize the motion of the rotor by adding radial and axial proximity probe vibration data.
The question typically is: Why is the amplitude of vibration so elevated to cause a premature failure? Often, based on MSI’s experience, if the amplitude is close to or over 0.5 in/s RMS, it is likely that a structural resonance is involved in the problem. In this case the natural excitation force is being amplified by a natural frequency by a factor of 2 to 25 times. This is where the modal impact testing plays a significant role in the troubleshooting process (see The Value of Experimental Modal Analysis here). This technique helps identify the structural natural frequencies and mode shapes of rotating machinery, as well as the rotor assembly (shaft lateral modes). If the primary vibration of the ODS animation or video matches the frequency and mode shape of one of the offending structural natural frequencies from the EMA test, there is no doubt that you are facing a structural resonance issue.
The following example demonstrates a typical vibration troubleshooting issue on a reactor feedwater pump installed in a nuclear facility, and how using one (modal) without the other (ODS) can leave problems left unsolved. In this particular case, all of the efforts that the End User and other consultants put in over the years had been exhausted. Because of previous modal testing, the plant was aware of an axial rocking mode of the pump near the upper end of the running speed range. This mode had been successfully shifted by welding stiffening plates on the pump pedestal on the sister pump, greatly reducing the overall vibration at the outboard bearing in the vertical direction. However, this modification did not affect the overall stiffness of the problematic pump. MSI’s troubleshooting methods clearly identified the offending axial rocking mode near 1x rpm. ODS testing identified soft-foot at the outboard right foot as shown in the ODS animation (Figure 1), which contributed to the overall issue.
Figure 1: ODS Animation at 1x rpm - Pump Soft-Foot
This soft-foot issue was also observed using VibVue® Motion Magnified Video (MMV) testing (Figure 2).
Figure 2: Soft-Foot Detected Using VibVue® Motion Magnified Video Testing
Further investigation revealed that another soft-foot condition was noted at each corner of the baseplate to the main concrete foundation, except the north-west corner. This would explain the reason why the stiffening of the pedestal on this particular pump was not responding favorably like the sister feedwater pump (Figure 3). When a problem solution eludes experienced plant personnel, we often find out that there are two separate problems unexpectedly occurring at the same time. ODS + Modal + VibVue® work well together to find that second problem!
Figure 3: Soft-Foot between the Baseplate and the Concrete Foundation