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

Why Pump Shafts Often Break at the Keyway Area

By NTS Pump shaft failure can lead to significant downtime and repair costs in industrial plants. One of the most common locations for pump shaft failure is at the keyway area. In this article, we will explore the reasons why pump shafts often break at the keyway and what can be done to prevent such failures. The keyway is a high-stress point (weakest point)  on the shaft, where a key is inserted to transmit torque between the shaft and the pump impeller or coupling. During operation, the keyway experiences cyclic loading that creates a bending moment in the shaft, which is concentrated in the keyway area. Over time, this cyclic loading can cause fatigue failure in the shaft material, leading to a fracture at the keyway. In addition to cyclic loading, other factors can contribute to shaft failure at the keyway. Improper keyway design or installation can lead to stress concentrations or inadequate clearance between the key and keyway . Misalignment or overloading can also cause ex...

Grounding brush discharge monitoring

In recognition of the possibility of static charge build up in condensing steam turbines, API 612 (2005) specifies that grounding brushes be installed. The electrical flow to ground through these brushes  be monitored and useful information can be extracted. This article carries excerpts from the paper, “Babbitted bearing health assessment” by John K Whalen of John Crane, Thomas D Hess of Chestnut Run, Jim Allen of Nova Chemicals and Jack Craighton of Schneider Electric. Grounding brushes take current from the rotor to ground so that a charge does not build up on the rotor to the point where it discharges to ground though the best path possible – which is usually the closest point between the rotor and stator which is usually (hopefully) the point of minimum film thickness in a bearing. Typically this point of minimum film thickness is found in the active thrust bearing (as will be shown later). Shaft grounding brushes serve two purposes. The brushes are able to transmit modest amo...

Failures in babbit bearings

  There are literally dozens of ways bearings can fail. Some of the more common include: • Babbitt fatigue • Babbitt wiping due to rotor to stator contact • Babbitt flow due to high operating temperatures • Foreign particle damage • Varnish build-up • Electrostatic discharge damage (frosting) • Electromagnetic discharge damage (Spark tracks) • Oil “burn” or additive plating due to high temperatures • Loss of bond between babbitt and base metal • Chemical attack • Pivot wear in tilting pad bearings • Unloaded pad flutter • Cavitation damage This is taken from a paper, Babbitted bearing health assessment" by John Whalen of John Crane, Thomas Hess of Rotating Machinery Group, Jim Allen of Nova Chemicals Corporation and Jack Craighton of Schneider Electric. Babbitt fatigue Babbitt fatigue is caused by dynamic loads on the babbitt surface. Typically in bearings of this type, the dynamic loads are caused by vibration and result in peak film pressure fluct...

Failure investigation, remedies, and mitigation of a centrifugal pump.

  BY LUIS INFANTE & RODOLFO ALVARADO. A high energy pump at a water injection station in El Furrial, Venezuela exhibited extremely high vibration levels prior to an overhaul. It then suffered a catastrophic failure during startup following overhaul. The hydrodynamic bundle, rotor, and drive end (DE) bearing suffered damage.   High energy pump for boiler feed water. Courtesy of Flowserve. This centrifugal pump is a 3,000 HP, double-case volute, boiler feed water pump type. It has nine stages, outputs 750 gpm of water with suction pressure 1800 psi and discharge pressure 5250 psi. Rated speed was increased from 6000 to 6600 RPM to enhance the hydraulic performance. However, the pump’s actual discharge pressure was about 4,500 psi, well below the target value of 5,000 psi. The coupling was reportedly poorly fitted. The increased RPM created rotordynamic concerns of getting closer to a critical speed, thus the operator wanted to know about the synchronous regime. The...

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...