Skip to main content

DEEP COMPRESSION: A Repair Solution For Steam Turbine Compressor Failures

By Kyle Brandenburg.

Steam turbines provide an efficient means of producing electricity. Improving the corrosion fatigue performance and damage tolerance of steam turbine blades can offer overhaul and maintenance cost savings improved reliability and reduced outages.

The application of surface residual compressive stress to components can enhance fatigue strength and reduce the effects of applied tensile stresses.

Shot peening has been used for decades to reduce overall operating tensile stresses in steam turbine components. However, corrosion pits, erosion, fretting, and other damage can penetrate shot peening’s shallow layer of residual compression, providing a starting point for stress corrosion cracking and fatigue failures. Instead of settling for shallow compression, the introduction of a deep layer of compressive residual stress can extend service life.

To test the benefits of deep compression, high-cycle fatigue tests were done on Type 410 stainless steel, a common alloy widely used in steam turbine applications, to compare the corrosion fatigue benefits of Low Plasticity Burnishing (LPB) to shot peening. LPB is also a mechanical surface treatment, but it imparts a controlled layer of compression that is deeper than most surface damage

LPB was developed in the 1990s to keep cold work low when introducing compressive residual stress. 20 years of studying residual stresses led to the conclusion that residual compression introduced using high levels of cold working could be wiped out from in-service thermal and mechanical loads.

The first step in the investigation was to obtain sample specimens of 410SS machine-finished by low-stress grinding (LSG) to serve as a baseline. Compressive stress was introduced either by LPB or shot peening. The LPB process is performed on conventional CNC machine tools and robots. Shot peening was done using a conventional air blast peening system.

Noted by maintenance4.com:

What Is Shot Peening? A surface enhancement method for improving the fatigue strength of metals near the surface of the component, shot peening is performed by impacting part surfaces with spherical shot particles to induce compressive residual stresses.

DAMAGE TOLERANCE RESULTS

Next to be tested was the fatigue strength of samples subjected to either mechanical damage or simulated corrosion and stress corrosion cracking. Mechanical damage simulates what can happen to the surface of the turbine blade from common fatigue damage mechanisms like foreign object damage, fretting, corrosion pitting, or erosion.

Steam turbine blade being Low Plasticity Burnishing (LPB) processed in a milling machine.

Mechanical damage was simulated through an electrical discharge machining (EDM) notch with a depth of 0.01 in. (0.25mm). For a portion of the LPB-treated samples, a deeper notch depth of 0.02 in. (0.51mm) also was investigated.

High cycle fatigue tests show that shot peening of the 410SS samples provided a modest improvement over the baseline condition with mechanical damage. Residual stress distributions revealed that the 0.01 in. (0.25 mm) notch completely penetrated the compressive layer introduced by shot peening minimizing any fatigue life benefit from shallow compression.

However, the deep compression provided by LPB doubled the fatigue strength and improved fatigue life near the endurance limit by a factor of over 100 compared to the shot peen condition.

Even with damage twice as deep, the LPB-processed samples outperformed those that had been shot-peened. When subjected to corrosion damage and salt exposure, simulated by testing samples in an active corrosion medium of a 3.5% weight NaCl solution after first exposing them to stress corrosion cracking (SCC) in the same medium, the LPB-processed samples again demonstrated double the fatigue strength.

Surface roughness measurements indicated roughness values of 19.5 μin for the baseline sample, 157.1 μin for the shot-peened sample, and 4.5 μin for the LPB-processed sample. The roughness value for the shot-peened sample is nominally 35X higher than that of the LPB-processed sample. Shot peening dimples produce a rough surface that can adversely impact fluid flow at the blade surface.

Residual stress data revealed that LPB produced higher magnitude compression at the surface and about three times the depth of compression compared to what was achieved with shot peening. Compressive stresses are shown as negative values, tensile as positive.

Polarization testing results revealed roughly 20X higher corrosion rate in the highly cold worked shot peened samples compared to the lower cold worked LPB samples.


 
High cycle fatigue results for samples with notches.

 

High cycle fatigue results for samples with SCC and active corrosion.

Some steam turbine manufacturers are now implementing LPB on their blades upfront to reduce overhaul and maintenance issues. Others use LPB as a faster and more economical repair technique. Depending on the steam turbine, manufacturers can apply LPB with the turbine blades installed on the rotor. LPB as a surface treatment does not add material or change the blade’s balance like traditional welding repair methods.

 

Kyle Brandenburg is a Research Engineer at Lambda Technologies Group and has been supporting the research and development efforts of the company for over 10 years. He can be reached at info@lambdatechs.com.


Source: Turbomachinerymag.com

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