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 Ī¼in for the baseline sample, 157.1 Ī¼in for the shot-peened sample, and 4.5 Ī¼in 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

Labels:

Comments

Post a Comment

Popular posts from this blog

5 Important Maintenance Metrics and How To Use Them

By  Bryan Christiansen,  Limble CMMS. Source : maintworld.com Effective maintenance of equipment is a critical factor in delivering quality operations that provide timely resources at a minimal cost. However, those in the maintenance field understand that equipment reliability does not come easy.  Organizations need to set quality benchmarks to measure the current effectiveness and predict future performance and use the data obtained to understand where to make improvements.   One way to do this is by using different maintenance metrics to understand the equipment performance. These metrics are very important as they can mean the difference between achieving the overall business goals and explaining how unexpected breakdowns caused yet another production delay.   Maintenance Metrics You Should Be Measuring What are the maintenance metrics? There are two categories of maintenance key performance indicators which include the leading and lagging indicators....
Post a Comment
[Read more...]

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...
Post a Comment
[Read more...]

Preventing Motor Bearing Damage from Shaft Current with the AEGIS® Ring

Electric motors are the workhorses of modern industry, powering everything from pumps and fans to conveyor belts and heavy machinery. But these vital components are not immune to wear and tear, and one common problem that can lead to premature motor failure is bearing damage caused by shaft current. Shaft current is a type of electrical discharge that occurs when there is a voltage potential between the motor shaft and the bearing, resulting in the flow of electrical current through the bearing. This current can cause a range of problems, including pitting and fluting of the bearing surfaces, which can lead to premature wear and failure. Shaft current Motor shaft current can have several harmful effects on bearings, including: 1.      Bearing pitting and fluting: Electrical discharges can cause tiny pits and flutes to form on the bearing surfaces, leading to premature wear and failure. 2.       Bearing noise and vibration: Shaft current can cause ...
Post a Comment
[Read more...]

WHAT IS THE ULTRASONIC IN-LINE INSPECTION (ILI) PIGGING?

In-line inspection (ILI) of pipelines has established itself as the most efficient tool for evaluating the condition of a pipeline and an indispensable part of pipeline integrity management. Historically, there have been two major technologies used in in-line inspection for corrosion, the magnetic flux leakage (MFL) and ultrasonics (UT), each having their distinct properties and fields of application.  Ultrasonic (UT) ILI has always provided unique quality of information about the pipelines, rendering highest accuracy and tightest measurement tolerances. In the 1990s ultrasonic tools for detection of cracks have become available. Ultrasonic measurement principle: ultrasonic transducer slides along the internal surface of the  pipe wall measuring distance to the wall and the wall thickness (top), yielding the stand-off and the wall  thickness (bottom two B-scans) Ultrasonic (UT) based In Line Inspection tools for all types of liquid filled pipelines. This includes Ultras...
Post a Comment
[Read more...]

Dry Gas Seal Failure Modes

BY BHUSHAN NIKAM. Invented in the mid-20th century and typically equipped in process gas centrifugal, dry gas screw compressors and expanders, dry gas seals (DGS) are the preferred gas lubricated dry seal solutions available on the market. They have become the standard for new machines. DGS are robust, simple, consume less power, and are more efficient in reducing leakage than their predecessor. Various configurations such as tandem with and without an intermediate labyrinth ( Figure 1 ), single ( Figure 2 ), and double ( Figure 3 ) are available & shall be selected based on process requirements. In this article, we discuss the various DGS failure modes and how they should be addressed:  PRESSURIZED HOLD/STANDBY Pressurized hold, also called settle-out condition, occurs when the compressor remains at a standstill, but the casing is pressurized. If an alternate process gas lacks sufficient pressure and flow, process gas enters the seal cavity through the process labyrinth ...
Post a Comment
[Read more...]