Disclaimer
Based on my experience there are very few experts that specialize in vibration analysis of piping and plumbing line applications. At Full Spectrum Diagnostics we get our share of piping problems, but we really can't consider ourselves experts. With piping problems we delve into a world of "rules of thumb," "trial and error," and "tribal knowledge." Now, with that said, let me set the table with some of the complexity associated with this type of analysis.
First of all, piping and plumbing lines will vibrate. They are, for the most part, flexible and under-supported lengths of tube of various diameters. Some lines extend a mere few inches, others can stretch hundreds of feet. It has been my experience that even "identical" pieces of equipment can have drastically different variations in the routing and clamping of their piping. Many piping systems will include routing bends in all three directions, will potentially change diameters and/ or thickness, and will include branching somewhere along their paths.
Next, all piping systems will have natural frequencies. Lots of them. All of the design and preliminary analysis in the world cannot eliminate all of these natural frequencies from the most dominant driving sources in the machines to which they are attached.
With all of this complexity, it is rare to come across a system with definitive limits on vibration acceleration, velocity, or displacement. The typical line is only under scrutiny following an unexpected failure. When a failure
occurs, the analyst will rarely have a set of baseline vibration readings to compare the repaired line with. The key question becomes, "How much vibration is too much vibration?"
Lacking a set of hard and fast rules for any given piping design, the vibration analyst is left with vibration measurements, and trial and error "Clamping & Damping" to ensure that the repaired system will be troublefree.
Preliminary Inspections
Before we start redesigning any plumbing line system it's prudent to inspect the system, and if possible compare it to any "sister" units. Some review of maintenance history is a good idea. Any mention of weld repairs, leaky flanges, or offset connections that needed coaxing to come into alignment during assembly (piping strain) can shed light on persistent problems.
A review of the piping clamps to note their locations, tightness, fretting, or damage can be a big help. Keep in mind that most problems develop for a reason. There is likely a root cause in the mix.
This article offers a case history in plumbing line vibration problems to establish a set of guidelines for analysis and maintenance that can assist the vibration analyst in resolving piping problems. The tools at our disposal are vibration analysis, natural frequency testing, and computer finite element modeling. The real key is the ability to "see" the vibrating shape via computer animation and simulation. This is a true case of "If you can't see it, you can't fix it. . . ."
CASE #1 Turbine Generator EH Control Lines
This case history involves critical plumbing lines on a steam turbine generator system in a Midwest power plant. The high pressure steam supply Intercept Valve system on many steam turbines is controlled via Electro-Hydraulic (EH) actuation systems. The plumbing lines contain 1800 psi hydraulic fluid. Any failure of these lines can have the potential to induce a fire and thus are considered very critical.
The line is actually a set of three with similar bends, joints and routing. Two sets of lines are found on each high pressure turbine. The configuration of the lines is similar but different on either side of the unit. An additional
note: the line configurations on the "identical" #1 and #2 sister units were NOT the same. The installations appeared to be constructed from a drawing with guideline dimensions and termination points. The actual routing was determined by the original pipefitters.
Figure 1 - EH Tubing Lines Unit #1 Steam Turbine
The Unit #1 south side middle tube failure was determined to be caused by High Cycle Fatigue (HCF) issues. The failure location was the termination point on the front of the unit (noted by red arrows in Figures 1 and 2). The line was replaced and put back into service. The on-site vibration analyst was instructed to take some vibration measurements and make an assessment. This was where the problems started. First of all, there was no baseline set of measurements for comparison. Secondly, the line was small (0.500 in) making his standard transducer a bit oversized for the application. The line was also made of stainless steel (non-magnetic).The best assessment formed was that the line was vibrating at very high levels. In fact ALL of the lines were vibrating at high levels.
Figure 2 - EH Tubing Animation Model
It was then that Full Spectrum Diagnostics was asked to take a look at the problem. An Operating Deflection Shape (ODS) analysis was recommended due to the fact that the unit could only be tested in an operating condition. The proposed analysis included all three of the lines in the set with the HCF failure, but also included the lines on the back side of the turbine as a comparison point. Note again that the north and south side line configurations were not the same.
The vibration signatures showed that the lines were vibrating at levels that would be considered critical if measured on your typical rotating piece of equipment. But remember our first rule: Piping and plumbing lines will vibrate! The vibration signatures included numerous peaks of relatively high amplitudes. Our second rule: All piping systems will have natural frequencies. Lots of them!
The real insight in this analysis comes not from the single measurement, but rather from a fine mesh of measurements along the length of the line system. Approximately 180 measurements were collected on each tube set (see
Figure 2). The measurement sets included a response in each of the x, y and z directions. A small "teardrop" accelerometer was used and was clamped in place with a small lightweight spring clamp. See the measurement data in Figure 3. Note the box at 3,600 RPM turbine speed.
A few of the initial analysis hurdles were resolved, including:
1) The ability to collect numerous measurements, instead of a single point vibration assessment.
2) A transducer that was sized for the job.
3) A method of attachment that was convenient and repeatable.
Figure 3 - South Side EH Tubing Spectrum Plots
Now, what do we do with all of this data and what does it mean?
This is where Operating Deflection Shape (ODS) comes into play. Normally, a set of Natural Frequency measurements would be helpful, but this unit was not scheduled for a maintenance shutdown for quite some time. The ODS analysis provides the next best thing. Viewing the operating shapes from each elevated peak in the spectrum provides a glimpse into the response of the tubes and an indication of its driving source.
Table 1 - Data Table for North and South side tube sets
As noted in Figure 3, each tube (Top, Middle, Bottom) had significant vibration amplitude levels. A preliminary comparison of the North and South side tube sets (See Table 1) revealed that vibration amplitude alone is not good a potential failure indicator. The dominant response on the tube that failed (South side Middle tube) was found at 3,600 CPM, the rotating speed of the turbine. This suggested that a potential resonance amplification may be present. The operating condition of the machine did not permit a direct natural frequency test to verify this suspicion.
The ODS Shapes of the modes were analyzed next. Remember, piping systems are designed to be flexible. What is of interest is the motions that indicate a classic bending mode shape, or shapes that create a reverse bending motion in the line. This reverse bending motion at a high enough amplitude is a potential HCF driver.
The mode at 3,600 CPM on the South side Middle tube showed an unusual whirling or orbital motion in the longer elbow portion of the line. The image in Figure 4 is constructed in a "persistence" format to show the path of the deflected shape.
Figure 4 - Persistence ODS Shape @ 3600 CPM
The animation suggests two symmetric natural frequency modes are present in the line near 3,600 CPM. These "doublet" type modes share a single natural frequency, but have mode shapes in perpendicular directions about the line of symmetry.
This orbiting motion was suspected to be the root cause of the Middle tube failure at the mount location. When the tube orbits, it produces a multi-axis reverse bending mode at the attachment point(s). Along with an elevated
amplitude response, the energy is not dissipated through the flexibility of the line, but rather concentrated at a point where the moment is restricted (in this case at a weld joint).
Even with this analysis, the direct evidence for this line being "resonant" is still elusive. To add credibility to our assumptions, a Finite Element Analysis (FEA) was performed on this line set to better understand the motions. The
FEA modes analysis results did show a "doublet" mode near the 3,600 RPM operating point. The two modes at 3,710 CPM (within 3% of operating speed) produced simultaneous perpendicular mode shapes, similar to the operating response.
To resolve the suspected problems near 3,600 RPM, it was recommended that the line be restricted (clamped) at the anti-node location in the center of the tube. The restriction did not need to be a "hard" connection, which might create other problems, but rather, a soft restricting mount constructed to slightly stiffen the tube and at the same time add significant damping to the line (see Figure 5). If the line resonance is moved away from the dominant 3,600 RPM driving frequency (via stiffening) the amplitude of the response should be reduced; and if a softer (foam or rubber) material is used within the tube clamp the damping effect should also reduce the amplitude of the response.
Figure 5 - Potential Damping Clamp Design
The final tube clamp was designed and installed by the customer. The mount arrangement and clamp design is shown in Figures 6 and 7.
Figure 6 - Tube Clamp Installation
The results were dramatic; however, the tubes continued to vibrate at levels that appeared excessive. Again, all piping and plumbing lines will vibrate! Full Spectrum Diagnostics was asked to return to the plant and repeat the original ODS analysis to assess the modifications.
The results were indeed dramatic. Figure 8 shows a waterfall type plot of the piping line data from the original January 2009 measurements and the repeat August 2010 analysis. The overall look of the data is similar with
the notable exception to the 3,600 RPM frequency range. The new data shows that the problematic mode was all but eliminated.
Figure 8 - Before and After Waterfall Plot
Analysis of the other peaks in the data were found to be absent of "reverse-bending" type operating shapes that were suspected to be inducing the HCF problems.
A final note is in order. The hard plumbing lines still produce elevated vibration levels during operation. The goal in this type of analysis is to dissipate the energy of the line via its inherent flexibility rather than concentrate
the deflections and moments at a point where those moments are restricted, like a flange or mounting connection.
Some general guidelines can be drawn from this case history.
1) Inspections of the piping system should be performed to determine any deteioration of the existing clamps and mounts.
2) Review maintenance logs for previous failures, leaks, alignment problems, etc.
3) A simple "hand" inspection of vibrating lines can reveal node points (minimal vibration) areas of the line. Existing clamps at node points are not effective.
4) The analyst must have the ability to take the vibration measurements without "mass loading" the test article.
5) The method of transducer attachment must be convenient for taking numerous measurements and reliably repeatable.
6) Vibration measurements must be collected in multiple planes due to the three-dimensional routing configurations of manyline systems.
7) Vibration Amplitude is not the single indicator of plumbing line problems.
8) Amplified peaks near a known driving source should be investigated (rotating speed harmonics).
9) Define potential driving sources: Rotating Speed Harmonics, turbulence, cavitation, water hammer, etc.
10) Plumbing Line Natural Frequencies should be measured directly (impact testing), if possible.
11) Line Resonance should be avoided. Minimal margins should be +/- 10%.
12) ODS animations should not include orbiting or reverse-bending shapes that load fixed points on the line(s).
13) Clamps that add damping as well as stiffness are preferred.
14) Clamps should be used SPARINGLY. Over-clamping lines can eliminate flexibility needed to dissipate energy.
15) The best clamping practice is to collect vibration measurements while installing and adjusting the clamps.
16) Baseline Vibration Analysis surveys on Critical Piping is encouraged. A comparison point is always recommended.
17) Flexible (Braided) Lines can eliminate many hard line problems. Beware of jump rope modes in lengthy flex-line sections.
Finally, the tools required for the computer animation of vibrating systems is currently within the reach of most plant analysts. Even a simple ODS analysis can be performed with a single channel data collector and a sheet
of notebook paper. Whatever your method, analysis of plumbing line systems is made easier by making the analysis VISUAL. This format is valuable for the analyst in assessing the problem, and in his explaining the results to a supervisor with little or no vibration analysis background.
Dan Ambre, P.E. is a mechanical engineer and founder of Full Spectrum Diagnostics, PLLC, a full service predictive maintenance consulting company. Dan specializes in resonance detection, experimental modal analysis, and operating deflection shape machinery diagnostics.
Full Spectrum Diagnostics provides a series of vibration analysis level I, II, and III training courses and certification, as well as training in advanced diagnostic techniques. Dan is a certified software representative for Vibrant Technology, Inc., the creators of ME'scope VES software tools. He also provides ME'scope VES software training targeting the field vibration snalyst.
Animations referenced in this article are available via e-mail at modalguy@aol.com. Please visit our Web site at www.fullspec.net.
