Skip to main content

Turbomachinery failures

BY AMIN ALMASI.

There are many reasons for turbomachinery problems and failures. Resonance, for example, is often overlooked. 

Rotating parts and components such as impellers and blade rows could be in resonance with any excitations generated by turbomachinery. Resonances for the first and second natural frequencies can be dangerous. Generally, there could be numerous cases of resonance. The second natural frequency of a rotating component, in one example, proved to be almost exactly an integer multiple of the first natural frequency. This led to excitation and operational problems. Fluid-induced vibration, oscillatory changes of fluid pressure, and turbulent flow (vortex formation) might also cause high vibration or even failure. 

Fatigue, too, is often a root cause in failures of rotating parts. Individual stress amplitudes should be analyzed to ensure associated components will not fail due to different forms of fatigue such as high-cycle fatigue (HCF) and low-cycle fatigue (LCF). 

For shaft failures, the reasons behind failures can be broken down into: 

1. Mechanical: such as overhung/bending/ torsional/axial load. 

2. Dynamic: vibration, cyclic, shock. 

3. Residual: manufacturing/repair processes. 

4.Thermal: temperature gradients, rotor bowing. 

5. Environmental: corrosion, moisture, erosion, wear, cavitation. 

Before the root cause of a shaft failure can be determined, it is necessary to understand shaft loadings and stresses. The ability to characterize the microstructure and surface topology of a failed shaft is critical. Visual inspection, optical scanning, electron microscopes, and metallurgical analysis can be used, for example. 

(source: r-e-v.co.uk)

Many failures can be diagnosed using a fundamental knowledge of shaft failure causes and visual inspections. This can later be confirmed through a metallurgical laboratory or other methods.

Based on case studies from several plants, the main reasons for shaft failure are: corrosion (35%), fatigue (32%), brittle fracture (16%), overload (11%), and creep/wear/erosion/abrasion (6%). Some studies found fatigue responsible for more than 50% of failures. Therefore, pay attention to surface discontinuities such as keyways, steps, shoulders, collars, threads, holes, snap ring grooves, and shaft damage or flaws. 

Keyway regions are often problematic. Keyways are commonly used to secure rotating components, rotor cores, and couplings to the shaft. The take-off end (or drive/driven end) is where the highest shaft loading occurs. Fatigue cracks usually start in the fillets or roots. A keyway that ends with sharp step(s) has higher stress concentration than one using a sled-runner type. In the case of heavy shaft loading, cracks frequently emanate from sharp steps. Avoid connections using keys if possible. If it can’t be avoided, obtain a sufficient edge radius. 

Fatigue-related failures usually follow the weakest-link theory: Fatigue leads to an initial crack on the surface; cracks propagate until the shaft cross-section is too weak to carry the load; and finally, a sudden fracture occurs. 

Remember that residual stresses or initial defects/deflections could be independent of external loadings. There are manufacturing or repair processes that can affect residual stresses, initial deflections, and defects. These include: drawing, bending, straightening, machining, grinding, surface rolling, shot blasting, and polishing. They can produce residual stresses and defects by plastic deformation. And thermal processes such as hot rolling, welding, torch cutting, and heat treating can lead to problems. 

Finally, shaft fretting can cause serious damage. Typical locations are points on the shaft where a press or slip fit exists. The presence of rust between mating surfaces helps confirm fretting took place due to movement between mating parts. Once fretting occurs, the shaft can become sensitive to fatigue cracking. Shaft vibration can worsen this situation.


Turbomachinery International

Comments

Popular posts from this blog

Maintenance 4.0 Implementation Handbook (pdf)

WHAT IS MAINTENANCE 4.0? Industry 4.0 is a name given to the current trend of automation and data exchange in industrial technologies. It includes the Industrial Internet of things (IIoT), wireless sensors, cloud computing, artificial intelligence (AI) and machine learning. Industry 4.0 is commonly referred to as the fourth industrial revolution. Maintenance 4.0 is a machine-assisted digital version of all the things we have been doing for the past forty years as humans to ensure our assets deliver value for our organization. Maintenance 4.0 includes a holistic view of sources of data, ways to connect, ways to collect, ways to analyze and recommended actions to take in order to ensure asset function (reliability) and value (asset management) are digitally assisted. For example, traditional Maintenance 1.0 includes sending highly-trained specialists to collect machinery vibration analysis readings on pumps, motors and gearboxes. Maintenance 4.0 includes a wireless vibration sensor conne

27 steps of the Gearbox Repair and rebuilding

 27 steps of the Gearbox Repair and rebuilding: Step 1 Cleaning exterior of Gearbox and identification. Step 2 Remove all bolts from the gearbox. Step 3 Disassembly for Gearbox preliminary evaluation of the condition and repair required Step 4 Mag inspect Gearbox. Step 5 check all Gears. Step 6 Customer communication of health of the Gearbox. Step 7 Parts to be repaired or, reverse engineered parts where needed required for Gearbox rebuild. Step 8 Failure analysis during complete disassembly and evaluation of the component wear and damage. Step 9 Cleaning all internal components and housing. Step 10 Check all bearings diameters in house. Step 11 Check all shafts Step 12 inspect all Gears. Step 13 Set up check line bore of the gearbox. Step 14 Repair and rebuild Gears back to O.E.M Step 15 Replacing all bearings seals and gaskets Step 16 Repair and rebuild all shafts again to O.E.M Step 17 Realigning all gears shafts and bearings back to O.E.M Step

Thermal growth: how to identify, quantify and deal with its effects on turbomachinery

Thermal growth, as used in the field of machinery alignment, is machine frame expansion resulting from heat generation. The generation of heat, of course, is caused by operational processes and forces. Materials subjected to temperature changes from heat generation will expand by precise amounts defined by their material properties. In turbomachinery, thermal growth results from the temperature differences occurring between the at-rest and running conditions. Generally speaking, the greater the temperature difference, the greater the thermal growth. The magnitude of the growth can be calculated from three variables: ∆ T (temperature difference) C   (coefficient of thermal expansion) L    (distance between shaft centerline and machine supports) When machinery begins to generate heat, the temperature difference between at-rest and running conditions will cause thermal expansion of the machine frame, thereby bringing about the movement of the shaft centerlines. This can produce changes in

John Crane's Type 28 Dry Gas Seals: How Does It Work?

How Does It Work? Highest Pressure Non-Contacting, Dry-Running Gas Seal Type 28 compressor dry-running gas seals have been the industry standard since the early 1980s for gas-handling turbomachinery. Supported by John Crane's patented design features, these seals are non-contacting in operation. During dynamic operation, the mating ring/seat and primary ring/face maintain a sealing gap of approximately 0.0002 in./5 microns, thereby eliminating wear. These seals eliminate seal oil contamination and reduce maintenance costs and downtime. John Crane's highly engineered Type 28 series gas seals incorporate patented spiral-groove technology, which provides the most efficient method for lifting and maintaining separation of seal faces during dynamic operation. Grooves on one side of the seal face direct gas inward toward a non-grooved portion of the face. The gas flowing across the face generates a pressure that maintains a minute gap between the faces, optimizing flui

Technical questions with answers on gas turbines

By NTS. What is a gas turbine? A gas turbine is an engine that converts the energy from a flow of gas into mechanical energy. How does a gas turbine work? Gas turbines work on the Brayton cycle, which involves compressing air, mixing it with fuel, and igniting the mixture to create a high-temperature, high-pressure gas. This gas expands through a turbine, which generates mechanical energy that can be used to power a variety of machines and equipment. What are the different types of gas turbines? There are three main types of gas turbines: aeroderivative , industrial, and heavy-duty. Aeroderivative gas turbines are used in aviation and small-scale power generation. Industrial gas turbines are used in power generation and other industrial applications. Heavy-duty gas turbines are typically used in large power plants. What are the main components of a gas turbine? The main components of a gas turbine include the compressor, combustion chamb