Operating Deflection Shape (ODS) & Modal Analysis

ODS Services:

VAI has extensive experience performing ODS tests, including motors, pumps, gearboxes, turbines, fans, piping and conveyor heads. In addition, we believe we have performed the largest ODS test in the power Industry, that being on a Nuclear Power Plant Turbine Generator and two (2) floors of the turbine building.

About ODS Tests:

The end result of all ODS tests is computer generated movie (computer animation) showing how any component, structure or pipe moves at any specific frequency.

ODS tests are rarely performed, but when needed, can greatly help determine what the cause of unusual vibration problems are. But since they are very time consuming to perform, they should only performed after all other tests have proved futile.

ODS tests typically take up to a week (or more) to perform depending on overall complexity of the test. Generally, it takes two (2) days to create the computer model (example shown above), two (2) days to collect the data, and one (1) day to create the animations and analyze them.  More time should be allowed if a detailed formal report is required.

ODS Modeling:

Points


Lines


Surfaces

There are three (3) main steps in creating an ODS model, as shown above:

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

During the modeling process, colors are selected, points are numbered, and the viewing angle is optimized to provide the view of the animation.

Testing:

After the model is complete, test data is collected on all accessible points using a dual channel analyzer. One probe (or optical/laser tach) remains in place at all times, while the other probe is moved from point to point. During the data collection, care must be taken to ensure all data is collected on the right point, and in the right direction. After that, the software is used to ‘mesh’ the data with the computer model, and the resulting animations show how the test subject moves at any frequency.  Typically, only frequencies that have high amplitudes are evaluated.

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

Some ODS Jobs Performed by VAI:

Nuclear Power Plant Turbine Generator

Encompassing 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 turbine operating speed (30 Hz).

Referring to the figure below, the upper and lower elevations of the turbine building are in gray (approximately 5’ between lines), the generator foundation is yellow, the sole plate is green, the generator housing is magenta, and the shaft centerline (casing) is sky blue.

Although this may be the single largest ODS test performed in the power industry, it only required one (1) week to perform by two (2) Analysts. The ODS test results identified a soft-foot condition on the generator sole plate and a cracked generator support column (link to animations below).

Condensate Pump:

This ODS test was performed on a condensate pump and 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 at 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). When plant personnel simply placed these supports in position (they were not carrying any noteworthy load) the vibration levels decreased 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 decreased by 80%.

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 better.

Plant personnel then performed a frequency response function test that showed the gearbox had a resonance 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. The gearbox casing was ½” thick, but the mounting pedestal was only ¼” thick and had two large coupling access openings that weakened it even more.  Based on the results of the ODS, the gearbox needed a much more robust mounting pedestal design, and this is a likely design issue within the industry.

For ODS Support:

To get 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

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