Engineering guide
Pump Skid Vibration: How Modal and Harmonic FEA Find the Fix
A skid that shakes is usually close to resonance. This guide explains how SLETEC finds the cause with FEA and designs stiffening that moves the problem out of the operating range.
By SLETEC Engineering · Updated 2026-11-06 · 4 min read
Why skids vibrate
Engines, pumps and motors excite the base frame at their running speed and its harmonics. If a natural frequency of the skid sits close to one of these excitation frequencies, small forces produce large motion. Typical causes are a frame that is too flexible, long unsupported beams under heavy equipment, missing cross-members, poor grouting or anchorage, and changes in equipment after the original design.
The three analyses
| Analysis | Question it answers |
|---|---|
| Static (equipment weight) | Are stresses and deflections acceptable under dead load and operating loads? |
| Modal | What are the natural frequencies and mode shapes, and are any close to the excitation frequencies? |
| Harmonic response | How much does the skid move at the equipment mounting faces under the actual engine and pump excitation across the frequency range? |
Mode shapes show where the frame is weak; harmonic results show how much it moves. A mode shape on its own is not operating vibration — it only shows the deformed pattern. The harmonic response with real excitation is what is compared with the acceptance criteria.
Fire water pump skid example (Ref. 81)
A diesel-engine-driven fire water pump skid showed vibration concerns. SLETEC carried out:
- A static FEA of the base frame under equipment weight to confirm displacement and stress.
- A modal analysis to identify natural frequencies and mode shapes relative to engine and pump speeds.
- A harmonic analysis under combined engine and pump excitation, with velocity response plotted at the engine and pump mounting faces.
- Stiffening options for the base frame, modelled and compared.
The outcome was a stiffening design with site-verification recommendations for vibration acceptance. See the project page.
Designing the stiffening
- Add cross-members or gussets where the mode shapes show the largest motion.
- Increase section depth under heavy equipment rather than simply adding mass.
- Check the new frequencies keep a margin from running speed and its harmonics, as required by the project criteria or OEM limits.
- Re-run the harmonic response with the stiffened model to confirm the reduction at the mounting faces.
- Issue fabrication details and a site-measurement plan to verify the result after installation.
Inputs that make the analysis accurate
| Input | Why it matters |
|---|---|
| Frame geometry and section sizes | Stiffness of the base frame controls the natural frequencies |
| Equipment masses and centres of gravity | Mass distribution shifts frequencies and mode shapes |
| Running speeds and number of cylinders or vanes | Define the excitation frequencies and harmonics to avoid |
| Support and anchorage details | Grouted, bolted or resiliently mounted supports change the boundary conditions |
| OEM vibration limits and project criteria | Acceptance criteria for velocity or displacement at the mounting faces |
| Site measurements, if available | Used to calibrate the model and confirm the diagnosis |
Common mistakes
- Treating a mode shape plot as the operating vibration. It is only a deformed pattern; the amplitude comes from the harmonic analysis.
- Adding mass instead of stiffness. Extra mass lowers frequencies and can move a mode onto the running speed.
- Modelling equipment as rigid blocks when the pump or engine casing is flexible enough to matter.
- Ignoring the foundation and grout; a perfect frame on a poor support still vibrates.
- Skipping site verification after stiffening — measurement closes the loop.
What to send us
Skid drawings or a 3D model, equipment weights and centres of gravity, running speeds, the OEM vibration limits, any site vibration measurements and photographs. Email enquiry@sletec.com or see our vibration analysis service.
Frequently asked questions
- What is the difference between modal and harmonic analysis?
- Modal analysis finds natural frequencies and mode shapes. Harmonic analysis calculates the actual response amplitude under a specified excitation across a frequency range.
- Can FEA fix skid vibration without replacing equipment?
- Often yes. FEA identifies the weak areas so stiffening can move natural frequencies away from the excitation frequencies; the result is then confirmed by site measurement.
- Do you need site vibration data before analysis?
- It helps but is not essential. Measured data is used to calibrate the model and to verify the stiffening after installation.
Start a project
Send your scope, drawings, codes and schedule to enquiry@sletec.com or WhatsApp +966 55 545 8298. An engineer usually replies within 24 hours.
Discuss your project with an engineer
Send drawings, datasheets or a short scope – an engineer replies with scope, schedule and price, usually within 24 hours.