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Vibration Analysts, Inc., "The Global Leaders in Vibration Analysis"

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Operating Deflection Shape (ODS) and Modal Analysis

ODS Services:

VAI is highly experienced in performing ODS tests. Our experience includes motors, pumps, gearboxes, turbines, fans, piping, conveyor heads and what may be the largest ODS test performed in the power Industry (a turbine generator and two floors of the turbine building).

Background:

ODS tests provide a 3-D computer animation that shows how a component, structure or piping moves at any given frequency. ODS tests help determine the cause of vibration problems, but since they are time consuming, they should only be done after all other tests have failed to identify a cause. ODS tests typically take a week to perform: two days to generate the computer model (shown above), two days to collect the data and one day to analyze it (more if a formal report is required).

ODS Modeling:

  Points Lines Surfaces

As shown in the above figures, there are three steps to creating an ODS model:

  1. "Points" are created by entering the "x", "y" and "z" coordinates in the software.
  2. "Lines" are created by connecting any two points
  3. "Surfaces" are created by connecting any three points

During the modeling process, the colors for the points, lines and surfaces are selected, the points are numbered (can be turned on or off) and the viewing angle is optimized to provide the best view.

Testing:

Once the model is complete, test data is collected at various points on the assembly using a dual channel analyzer. One probe (or an optical/laser tachometer) remains in place at all times while the other probe is moved from point to point. During the data collection process, care must be taken to ensure all data is collected on the right point and in the right direction. After all data has been collected, the software merges the data with the model and the resulting animation shows how the item moves.

Animations can show abnormal movement caused by looseness, weak and flexible structures, resonance, vane pass, excessive forces, operating loads and more. Animations will also show how components move with respect to each other, such as with misalignment. They also show how components move with respect to the structure to which they are attached (soft-foot, looseness, etc.).

 

ODS Jobs Performed By Vibration Analysts, Inc:

Turbine Generator

This test was performed on a 1250 megawatt turbine generator and two floors of a main turbine building at a nuclear power plant. This test was performed because the plant experienced a sudden and significant increase in vibration levels at running speed.

Referring to the model below, the upper and lower elevations of the turbine building are in gray, the generator foundation is yellow, the soleplate is green, the generator housing is magenta and the shaft centerline (casing) is blue.

Although this ODS test may be the single largest ODS test ever performed in the power generation industry, it only required one week to perform by two analysts. The total billable cost for a job of this size would only be $10,000 plus travel expenses. The ODS test results identified a soft-foot condition on the generator sole plate and a cracked generator support column (link to animations below).

  Animations: Turbine Generator Overview
    Turbine Generator at 1x

 

 

Condensate Pump:

This ODS test evaluated a condensate pump and its discharge piping (model below). The horizontal pump is shown in blue and has four vertical support legs. The discharge piping is gray, and the original vertical support (red arrow) was located beneath the horizontal run of pipe.

This ODS test was performed because the plant had a history of very high piping vibration levels (5.0 IPS-Pk at vane pass), piping cracks and was frequently challenged with water leaks.

Prior to the ODS test, a negative linear averaging (NLA) test was performed and the results showed the pipe was resonant just below vane pass (which is why a temporary stiffener placed under the first discharge pipe elbow had no effect).

The ODS test results showed the discharge pipe was rocking back and forth on top of the original vertical support. Since the piping was resonant at vane pass, we decided the best solution would be to change the system stiffness by placing two 4x4s directly under the vertical pipe run (green arrow above). As soon as plant personnel placed the supports in position, (not carrying any significant load) the vibration levels dropped by 10%. As the supports were gradually tapped into place, the levels continued to drop. After the supports had been tapped into place enough to slide a piece of paper between the piping and original support, the levels dropped by 80% (from 5.0 to 1.0 IPS).

  Animations: Condensate Pump Overview
    Condensate Pump at Vane Pass

 

Right-angle Gearbox:

This ODS test was performed on a diesel driven, right angle gearbox because the gearbox had unacceptably high 2x vibration amplitudes. The gearbox and the diesel are shown in blue. The item shown in gray is a concrete wall.

Plant personnel first attributed the high 2x to an alignment issue, but after taking significant steps to correct the alignment, the post alignment vibration levels were not any better.

Plant personnel then performed a frequency response function test that showed the gearbox had a resonant in both radial directions at 2x. As a result, the plant took action to design and attach massive radial stiffeners between the gearbox and concrete wall to detune the resonance. However, these attempts were also unsuccessful.

An ODS test was then performed to evaluate the gearbox and the results showed the gearbox mounting pedestal—not the gearbox—was the cause of the high vibration levels. While the gearbox casing was ½" thick, the mounting pedestal was only ¼" thick. It also had two large coupling access openings that weakened it even more.  Based on the results of the ODS, the component needed a more robust mounting pedestal design.

  Animations: Right-Angle Gearbox Overview
    Right-Angle Gearbox at 2x

 

 

For ODS Support:

If you need an accurate ODS test performed at a reasonable rate, Contact Us. We will be glad to beat any competitor's price by 10% and provide accurate and informative ODS animations to your troubleshooting team.

Screen print - ODS software
Screen print from ODS software showing typical work area and data