Designed to be both efficient at baseload conditions and robust enough to be used in various operating modes, configurations, and climates, the GE D11 steam turbine is a popular choice. While it is known to have some issues, the experts at Allied Power Group (APG) have custom retrofit solutions that can help extend the life of your D11 and improve its efficiency.
Let’s take a look at the history of the D11 steam turbine, its known issues, some potential remedies for those issues, and why you should make APG your power partner.
The History of Steam Turbines
Although it was little more than a toy, the first reaction steam turbine was the aeolipile invented by Hero of Alexandria in the first century CE. Steam traveled through a hollow rotating shaft to a hollow rotating sphere and emerged through two opposing curved tubes, like water coming out of a rotating lawn sprinkler.
Although James Watt developed the steam engine, and many attempts at steam turbines were attempted by adapting his 1784 patent, the first successful steam turbine was built by William Avery in 1837. It had two hollow arms (which had nozzles on their ends) attached at right angles to a hollow shaft. Steam escaped from the nozzles in a tangential direction, turning the wheel.
Various inventors at the end of the 19th century laid the groundwork for today’s modern turbines.
In particular, British engineer Charles Algernon Parsons came up with the idea of using a large number of stages in series to extract thermal energy in small steps. He also developed the reaction-stage principle stating that equal energy release and pressure drop occur in both the moving and stationary blade passages. Parsons then built the first practical large marine steam turbines.
From there, steam turbines have continued to develop into the models we’re familiar with today, including the GE D11.
History of the GE D11 Steam Turbine
In the late 1990s, GE recognized that it needed to be more proactive to meet the demands of the competitive steam turbine industry, so it analyzed the market activity and decided to focus on a standardization effort of steam turbines for 207FA and 209FA combined-cycle plants. The resulting D11 turbine design comprises a combined, opposed-flow, HP/IP section with single-shell construction, and a two-flow LP section.
Due in part to the success of the D11, GE has supplied more than 30% of the world’s installed steam turbine capacity and 50% of the nuclear steam turbines, which are collectively generating more than 1,200 GW of power.
Known Issues With the D11 Turbine
Despite the success of the D11 steam turbine, it does have some issues. Luckily, many of the issues are fixable or even preventable. Let’s take a look at some of the common issues along with their potential fixes.
N-2 Packing Head Cracking
Separating the IP inlet steam from the HP inlet steam near the midspan of the outer casing and the HPIP rotor, the N-2 inner packing casing is located near the N-2 rabbet fit.
There is a steady-state steam pressure difference between the IP and HP sides of the N-2 packing casing during steady-state full load operation and peak pressures and temperatures, making the N-2 packing casing material vulnerable to creep rupture damage.
The radial circumferential and cumulative cracking tends to happen at the female fit at around 100,000 operating hours and leads to internal damage to the diaphragms and rotor as well as the loss of axial positioning of the inner casing.
To ensure reliable and safe long-term operation, the key is to identify cracking early and pursue the optimum repair solution. APG can inspect your turbines and look for small cracks before they become big problems.
HPIP Outer Casing/Shell Cracking
Creep rupture and low cycle fatigue (LCF) cracking have been known to happen to the HPIP outer casing, adjacent to the N-2 male rabbet fit, which positions and supports the internal N-2 packing casing.
During cold startups, steam rapidly heats the IP side of the HPIP outer casing N-2 male rabbet fit before the HP side of the casing gets any HP inlet steam, resulting in differential thermal expansion of the HPIP outer casing diameters across the fit. The cumulative effect of this repeatedly happening over the course of many cycles can create cracking on the HP side of the fit, especially in the lower half since it’s closest to the IP steam inlets.
Creep or LCF cracking can sever the casing wall within 5 years of the crack’s initiation.
Bowed HIP and Clearance Control
There is a significant distance between bearings on the long, slender D11 rotor which can lead to it becoming bowed and causing excessive vibrations and performance loss. APG’s experienced technicians can check for distortion and clearances and perform a realignment, if necessary, to help improve efficiency.
Horizontal Joint Leakage
Since the lower HP/IP casing of the D11 is long and slender, only supported on the ends, and deals with the highest temperatures in the mid-span section, the casing eventually sags, resulting in steam leaks and impacts on operability, efficiency, and safety.
APG can do localized repairs and re-machining to correct casing sag and steam cutting.
Due to problematic weld processes, reduced axial spacing, insufficient main weld depths, and high operating temperatures, diaphragm dishing or deflection is also a concern with the D11. APG may be able to repair or replace your diaphragm with one that has increased stiffness, deeper weld penetrations, and improved materials.
Make APG Your Power Partner
Whatever problems you may be having with your GE D11 steam turbine, Allied Power Group (APG) can create custom solutions to repair or replace problematic parts or sections of your turbine so you can experience increased efficiency and get more years of service out of it.
We’ve been bringing customized solutions and enhanced capabilities to the power generation industry in Houston and beyond since 2005, and we want to be your perfect power partner. Contact us today to see how we can help your plant reach its full potential.