Exhaust Fan Structural Vibration Assessment and Strain Validation

Exhaust Fan Structural Vibration Assessment and Strain Validation

A modification intended to solve one engineering problem can sometimes create another.At a UK cement works, a main exhaust fan was identified as the source of a fluctuating tonal noise between 102 Hz and 128 Hz. The frequency range was associated with the fan blade pass frequency, leading to a proposed airflow modification designed to reduce noise generation at source. Before the modification was accepted for operation, the customer wanted confidence that the altered arrangement would not introduce unacceptable structural stresses or create a resonance problem as the fan moved through its variable operating speeds. AVT Reliability’s Asset Integrity team carried out an exhaust fan structural vibration assessment using Finite Element Analysis and strain gauge testing to validate the proposed installation under operating conditions.

Why structural validation matters when equipment is modified

Noise, vibration and structural behaviour are closely linked on large rotating equipment. A change to airflow, ducting, supports or attached components may solve the original issue while also changing mass, stiffness or excitation within the structure. On variable-speed equipment, this matters because operating speed can move through a range of forcing frequencies rather than remaining at one fixed point.

That creates two key questions:

• Will the modification introduce unacceptable stress into the structure?
• Could the altered system become vulnerable to resonance at any point in its operating range?

For this exhaust fan, those questions had to be answered before relying on the noise-control solution in service.

Using Finite Element Analysis before field validation

Finite Element Analysis was used to assess the proposed arrangement and identify areas where structural response and stress concentration required closer attention. FEA is particularly useful where engineers need to understand how a structure may respond to changes in loading, stiffness or geometry before or alongside physical testing.For this application, the analysis helped identify predicted stress hotspots, including areas around welded connections where local fatigue performance was important. Those locations then informed the field measurement plan. This is an important part of good structural investigation. Analytical modelling and physical measurement should support one another. The model helps determine where to measure, while measured data provides evidence of how the real structure behaves in operation.

Validating the model with strain gauge testing

Strain gauges were installed at the predicted hotspots around the exhaust fan modification. Strain measurement allows engineers to quantify the local deformation of a structure while the equipment is operating. From that measured response, the level of cyclic stress can be assessed and compared with appropriate fatigue criteria. This gave the engineering team a direct view of how the modified structure behaved under the tested operating conditions rather than relying on calculation alone. For equipment exposed to repeated dynamic loading, this type of validation is particularly valuable because fatigue risk depends on the repeated stress range, not simply whether a component can withstand a single static load.

Assessing fatigue against BS 7608

The recorded strain data was assessed against BS 7608:2014, the British Standard covering fatigue design and assessment of steel structures. The standard provides a framework for evaluating fatigue performance based on factors including stress range and the type of structural detail being assessed. This was relevant around welded connections, where fatigue performance can be influenced by local geometry and repeated cyclic loading.

Under the conditions tested, all measured strain results were below the Constant Amplitude Fatigue Limit used for the assessment. That gave the customer measured evidence that the proposed arrangement was operating within the assessed fatigue criteria during the validation period.

Why variable-speed fans need careful assessment

A variable-speed fan can pass through multiple excitation frequencies as its operating speed changes. If one of those forcing frequencies approaches a structural natural frequency, the response can increase significantly. This is the basis of resonance and is why an installation that behaves acceptably at one speed may respond very differently at another. For that reason, an exhaust fan structural vibration assessment should consider the operating range rather than relying on a single measurement taken at one speed.

Where required, techniques such as Finite Element Analysis, strain measurement, vibration testing and modal analysis can be combined to understand whether the structure is approaching a resonant condition and whether the resulting stress levels are acceptable.

Modification should be followed by verification

Engineering modifications are often introduced to improve reliability, performance, noise or maintainability. The strongest approach is to verify that the change has achieved its intended purpose without introducing a new mechanical or structural risk.

A practical validation process may include:

• Reviewing the proposed modification and operating conditions
• Modelling the expected structural response
•  Identifying likely stress or vibration hotspots
•  Measuring the real structure during operation
• Comparing measured results with recognised engineering criteria
• Defining any requirement for ongoing monitoring

This closes the gap between design intent and real operating behaviour.

Where permanent monitoring can add value

For some critical applications, a one-off validation provides sufficient confidence. In others, the operating environment, variability or consequence of failure may justify ongoing monitoring.

Permanent strain or vibration monitoring can provide additional visibility where engineers need to confirm that structural behaviour remains stable over time or across changing operating conditions. Machine Sentry Online can support this type of continuous monitoring where an application requires permanently installed sensors and remote access to condition data. The monitoring strategy should always be based on the engineering question being answered rather than adding instrumentation for its own sake.

Combining analysis with measured evidence

This cement-plant exhaust fan project demonstrates the value of combining analytical and physical techniques when validating a structural modification.

Finite Element Analysis helped identify the areas most likely to experience elevated stress. Strain gauge testing then measured the real response at those locations, while fatigue assessment provided the engineering framework for interpreting the results. Together, those methods allowed the customer to evaluate the proposed noise-control modification against the behaviour of the actual operating structure.

AVT Reliability provides Finite Element Analysis, Strain Gauge Testing and Stress and Fatigue Assessment as part of its wider Asset Integrity capability, helping industrial operators validate modifications, investigate structural vibration and make engineering decisions based on measured evidence.