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How to Enrich a Pump Vibration Monitoring Program Database

Sotirios Christofi | Industrial Machinery Consultant

A large amount of machinery data is gathered when establishing a vibration monitoring program. This article recommends additional data to be taken into account, namely:

  • Operating flow rate vs. best efficiency point (BEP) flow rate
  • Operating speed vs. critical speed(s)
  • Shaft stiffness vs. construction code recommendation

It explains why this additional data improves the condition diagnosis, and it suggests three steps in the vibration measurement evaluation process, namely:

  1. The assessment of the operating position
  2. The evaluation of the current integrity condition
  3. A forecast for the future integrity condition

A Typical Equipment Database Audit Form

Users start their validation master plan (VMP) by preparing a database of the machinery to be monitored. This usually contains the type of asset (pump type, fan, compressor type, etc.), the type of drive (electric motor, diesel motor, steam turbine, etc.), the drive speed (fixed or variable), the coupling type (rigid or flexible), the speed change (belt or gear box), the mounting type (rigid or flexible), the rated power, the rated drive speed, the rated pump speed, the gear box speed reduction/increase ratio and possibly more.

Some vibration experts recommend users go even further and tabulate all expected faults and the frequencies at which these faults usually appear. If the machine is running at 1,800 rotations per minute (rpm) (30 hertz [Hz]), then unbalance will cause a vibration component at 30 Hz, misalignment will cause a component at 30 Hz and 60 Hz, and so on.

All data above is necessary in order to evaluate the results of vibration measurements compared to the allowable limits specified by international codes and standards, i.e., International Organization for Standardization (ISO) 20816.

However, three factors can improve the effectiveness of the program:

  1. The pump operating flow rate should fall within the preferred flow rate limits set by American Petroleum Institute (API) 610, §6.1.16—i.e., between 70% and 120% of the BEP flow rate.
  2. The pump rotating speed should be far enough away from the critical speed of the pump rotor.
  3. The shaft should be stiff enough for the task.

Let’s look at them one by one.

| IMAGE 1: Allowable and preferred operating regions of flow on the pump performance curve (Image courtesy of the American Petroleum Institute)

Operating Flow Rate vs. BEP Flow Rate

According to API 610, §6.1.16, the pump should operate within the preferred operating region (POR) of 70% and 120% of BEP flow rate.

If it does not, the pump will have substantially higher vibrations, but the reason will not be any defect of the pump. It will be that the pump is assigned a task different from the one it was designed for. Therefore, the operating flow rate vs. BEP flow rate should also be included in the asset database. The BEP flow rate can be found on the pump data sheet and the performance curves provided by the pump manufacturer. The operating flow rate can be obtained from operations personnel.

API 610, §6.9.4.6, Table 8 (Vibration Limits for Overhung and Between-Bearings Pumps) and Table 9 (Vibration Limits for Vertically Suspended Pumps) specify that the “allowable increase in vibration at flows outside the preferred operating region but within the allowable operating region is 30%.”

The POR and the allowable operating region (AOR) are provided by the manufacturer on the pump data sheet.

If the pump operates with a flow rate outside the area of 70% to 120% of BEP flow rate, the vibration allowable limits are 30% higher. Failure to take this into account may lead to unnecessary pump disassembly and maintenance works. A practical policy could be to proceed to pump stoppage and maintenance only after a vibration issue is detected at an operating point near the BEP.

| IMAGE 2: Vibration amplification vs. pump running speed (Image courtesy of the Hydraulic Institute)

| IMAGE 3: Geometry of an overhung rotor (Image courtesy of the American Petroleum Institute)

Operating Speed vs. Critical Speed(s)

The pump should operate far enough from its resonance frequency. Otherwise, any defect causing vibration close to the resonance frequency (e.g., unbalance, misalignment or any of their harmonics) will be amplified by resonance, causing serious integrity issues.

Centrifugal pumps are generally considered safe from critical speed issues due to a combination of inherent design characteristics and fluid dynamic phenomena that stabilize the rotor during operation. That is the reason API 610 data sheets require the manufacturer to specify the “First Critical Speed Wet” for multistage pumps only.

Still, it is unknown whether a harmonic of a low-frequency fault (e.g., 4X harmonic of mechanical looseness) produces vibration close to the critical speed or one of its multiples.

Therefore, the natural frequency or the critical speed of the pump should also be included in the database. This data can be provided for by the pump manufacturer, but it will be of the pump train alone.

Further on, my advice is that the overall natural frequency of the combined structure, including the pump train plus the foundation, be measured on-site so the effect of the whole structure, not just the rotor, is included in the measurement.

The most common available technique is a bump test (also called an impact or hammer test). By striking the machine, you "excite" its natural frequencies, making it ring like a bell.

Regarding codes recommendations, the Hydraulic Institute (HI) specifies that “there generally needs to be a minimum of 10% separation margin between a forced vibration and natural frequency unless sufficient damping is present to limit the amplification.” It recommends American National Standards Institute (ANSI) HI 9.6.8 Rotodynamic Pumps – Guideline for Dynamics of Pumping Machinery for additional guidance and recommendations.

If the pump speed or one of its integer multiples is within the range of critical speed ±10% or the number that results from ANSI HI 9.6.8 guidelines, then the cause for high vibration is resonance. Resonance can be dealt with by changing operating speed, pump structure stiffness, pump structure mass or system damping (e.g., shock absorbers).

Failure to take this into account may mislead the vibration measurement evaluation and lead to unnecessary work.

Shaft Stiffness

Despite being an important reliability factor of a pump, shaft stiffness is often overlooked. In API 610, §9.1.1.3, it is stated that “if specified, the shaft flexibility index (SFI) shall be calculated by the vendor in accordance with Equation (K.1) and reported.” My experience is that this is seldom done.

Why is SFI important? A higher index means:

  • The shaft is more prone to bending under radial hydraulic loads, especially when the pump operates away from its BEP.
  • The natural frequencies are lower, which could trigger resonance.
  • The shaft is less able to "absorb" small alignment errors.
  • In general, pumps with a higher index are less forgiving. While they may run smoothly at the BEP, they can experience a sharp spike in vibration if the flow rate changes or if the fluid density differs from the design.

API 610, Annex K.1 provides a guideline on shaft stiffness for overhung pump types OH2 and OH3, where this definition is given:

“For a shaft of two diameters, D1 under the seal sleeve and D2 between the bearings (see Figure K.1), the shaft stiffness is inversely proportional to what is generally termed the shaft flexibility index, SFI or ISF, defined as given in Equation (K.1): ISF = L13/D14 + L1 L22/D24.”

According to the guideline, a shaft is considered to be stiff when:

For a detailed review of ANSI B73.1 and ISO 5199 specifications, I recommend the article “ISO 5199 Standard Addresses Today's Reliability Requirements for Chemical Process Pumps” by Pierre H. Fabeck and R. Barry Erickson.

Failure to check whether shaft stiffness lies within API recommended limits may lead to a misdiagnosis. High vibrations at operating points at the extremes or outside those limits may be interpreted as a sign of wear when the pump is actually in good condition.

Evaluation of Vibration Measurements

Step 1: Evaluate the operating position.

Make sure the pump operates at the preferred operating area. Check suction and discharge manometers, as well as the flow meter. If there aren’t any, request the data from operations personnel and explain to them why they should have them installed, starting with the critical pumps. Check whether the discharge valve is throttled, an indication that the pump is operating at a low flow rate. Check whether or not a recycle valve is open, and make sure the pump operates within the AOR.

Put the gathered data together and locate the operating point on the pump performance curve. If the pump operates within the POR, proceed to vibration measurements. If the pump operates outside the POR but within the AOR, proceed to vibration measurements/measurements increasing allowable limits by 30%. If the pump operates outside AOR, this should be immediately reported to plant management. The pump will probably have to stop running.

Step 2: Assess the current integrity condition.

Assess whether the level of the vibration measured requires action. Check whether the overall vibrations are within acceptable limits. Use ISO 20816 limits as reference values.

  1. If vibration velocity < ISO limits —> No immediate action (intervention/planning) is necessary.
  2. If vibration velocity > ISO limits —> Proceed to velocity spectrum to locate high-amplitude, low-frequency faults.
    1. Vibration component at running speed 1X —> unbalance
    2. Vibration component at running speed 1X or 2X —> misalignment
    3. Vibration component at running speed 1X, 2X or other multiples —> misalignment

Step 3: Forecast the integrity condition.

Assess whether faults have already initiated, are in their early stages or do not produce any high-amplitude vibrations yet but indicate they are going to in the near future. Proceed to the acceleration spectrum to locate low-amplitude, high-frequency faults.

  1. Compare acceleration root mean square (arms) with acceptable values. Generally, a value under 1.2 gravity acceleration (g) indicates a well-operating pump. High arms is an indication of inadequate lubrication and early stages of bearing damage.
  2. Check crest factor (CF) (= acceleration peak [apeak]/arms) for advanced bearing failure. CF > 4 indicates bearing failure, which can lead to catastrophic pump failure.
  3. Check spectrum for components at their early stages. Compare the component frequencies with the expected frequencies of bearing failures, gear failures, etc. Check their proximity to the resonance frequencies. Identify “suspect” components for inspection.

Check the vibration time waveform. An experienced eye can readily identify if there are high-impact loads.

Intervention

Begin with repair work that can be done without disassembling the pump, such as restoring adequate lubrication; correcting shaft misalignment and mechanical looseness; and aligning and/or replacing gear and drive belts. Once this is done, repeat the measurements.

If there are still issues, proceed to pump disassembly, repair work and inspection/replacement of all suspect components. If intervention is not possible at the moment and there are indications of pending failures still not visible, increase measurement frequency and report accordingly.

References

  1. Vibration Monitoring & Analysis – Category I Workbook, Simon R W Mills
  2. Resonant Frequency & Critical Speed, Joe Evans, Pumps & Systems, Feb. 2009
  3. EASA Bump Test Resonance, easa.com/resources/trade_press/how-to-conduct-a-bump-test-for-resonance
  4. Pump Vibration & Troubleshooting Strategies, Part 1, Hydraulic Institute, Oct. 2023. pumps.org/2023/10/12/pump-vibration-troubleshooting-strategies-part-1
  5. Pump Vibration Analysis, by Brian P. Graney, Pumps & Systems, November 2011

After a career of 38 years in the petrochemical industry, Sotirios Christofi provides consulting and training services for technical personnel on the design, construction, inspection, selection, condition monitoring and maintenance of industrial machinery.

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