D11 Problems & Repair Solutions
- Jul 9, 2020 9:42 pm GMT
The common GE® D11 steam turbine, a single-casing HP/IP design used extensively in combined cycle applications, is demanding increased focus during normal outages, and ongoing attention for reliability, efficiency, and plant safety. Fundamental reasons are intensified by market-driven calls for continued cycling and low-load operation.
Units modified for higher efficiencies and compatibility with 7FA Gas Turbine exhaust temperatures are subject to even higher rotor to casing differential thermal expansions in the more tightly-packed seal areas of this single-casing design.
The key to continued efficient and safe operation of these machines is highly specialized NDE inspections, remaining life analysis and forward-looking repair option solutions.
HPIP Outer Casing/Shell Cracking
The HPIP Outer Casing has been shown to be susceptible to Low Cycle Fatigue (LCF) as well as Creep Rupture cracking, from different operating conditions. Both types of cracking are found adjacent to the N-2 male rabbet fit, which supports and positions the internal N-2 packing casing.
During a cold startup, the Hot IP inlet steam enters the IP steampath thru the two lower IP steam inlet bores. This steam rapidly heats the IP side of the HPIP Outer Casing N-2 male rabbet fit. The HP side of the N-2 male rabbet fit does not yet have any HP inlet steam so differential thermal expansion of the HPIP outer casing diameters occurs across this fit.
The differential thermal expansion puts the HPIP Outer casing into high compression adjacent to the fit. This compression creates LCF on the HP side of the N-2 fit. The LCF is cumulative with each cold startup. The accumulated LCF can then create radial and circumferential cracking on the HP side of the N-2 rabbet fit. The cracking is most severe in the lower half as this is closest to the IP steam inlets.
On the upstream (HP) side, Creep and/or LCF cracking has sometimes been known to sever the 4-inch-thick alloy steel HPIP Outer Casing wall in less than 5 years from time of crack initiation.
N-2 Inner Packing Casing
The N-2 Inner Packing Casing separates the HP inlet steam from the IP inlet steam near the midspan of the HPIP rotor and Outer Casing. It is located by the N-2 rabbet fit noted above.
During steady state full load operation at peak temperatures and pressures, there is a steady state steam pressure difference between the HP and IP sides of the N-2 packing casing. The net difference wants to force the N-2 packing casing towards the IP section. The female fit on the outer perimeter of the N-2 packing casing is resisting this unbalanced steam force.
This creates a steady state tension force at the HP side of the female rabbet fit. This steady state tension force occurs at the full operating temperature. At this high temperature, the N-2 Packing casing material is vulnerable to creep rupture damage. The creep damage is again cumulative and radial circumferential cracking has been sometimes known to occur at the female fit at about 100,000 operating hours. This damage has led to loss of axial positioning of the N-2 inner casing and internal damage to the rotor and diaphragms.
[N‐2 Packing Casing]
The key is to identify the cracking early, then pursue the optimum repair solution for safe and reliable long-term operation.
The hidden internal damage that exists is typically not evident until it surfaces as an emergent issue during an inspection outage.
Proper repair by Mechanical Dynamics & Analysis (MD&A) for outer shell IP inlet is on-site stress-relief weld repair and in shop repair and upgrade of N2 cracking casing. If needed, our experts also perform thermodynamic analysis, evaluating cycle efficiency to help isolate problems.
MD&A has more than 30 years of experience successfully applying its technology, knowledge and professionals on more than 500 casings worldwide on a variety of OEMs (Original Equipment Manufacturers.)
Horizontal Joint Leakage
The D11 lower HP/IP casing is long and slender, supported on the ends with the highest temperatures in the mid-span section. Over time the casing sags, with the lower half experiencing higher temperature transients during startups and high enough steady state midsection steam temperatures to permanently yield portions of the casing. The result can be steam leaks from inside to atmosphere and eventual steam cutting of the top to bottom flange, entering the insulation. The impacts are efficiency (heat loss), operability and safety.
The solution requires localized repairs and re-machining to correct both the steam cutting and the casing sag.
MD&A offers laser scanning equipment to determine the amount of distortion as compared to the original joint plane. Following weld buildup of the horizontal joint, a laser-guided mill blends the repaired section to the original plane. MD&A’s skilled technicians have been performing this on-site work successfully for more than two decades on a variety of OEMs and steam turbine models.
A related issue is mid-span outer shell creep and associated axial clearance shift from the N2 downstream as rotor length remains constant. Offset packing rings and modified diaphragm seal faces are two of several solutions offered by MD&A.
Bowed HIP and Clearance Control
The D11 rotor is long and slender with a significant distance between bearings. If the rotor rubs, it can easily become bowed, and there is no access point for a mid-span balance. Vibrations can become excessive during operation and roll up/down through critical speeds. Heat from metal-to-metal rubbing and water quenching during cooldown are the two main damage mechanisms. All lead to performance loss and high vibration.
During site inspection, MD&A technicians look at the specified clearances, the levels of distortion in the casing, rotor and diaphragms, and all other components. For the casing and diaphragms, MD&A alignment engineers can perform a realignment for improved efficiency.
Traditional bowed rotor repair is a shop process performed by MD&A’s Turbine-Generator Repair Facility in St. Louis, MO USA. If needed, straightening can be performed on-site with vertical or horizontal thermal straightening and stress relief. When required, high-speed balance can be performed in our state-of-the-art balance facility.
Straightening a Bowed HPIP Rotor
Diaphragm dishing/deflection is also a concern with these units due to high operating temperatures, insufficient main weld depths, reduced axial spacing, and problematic weld processes utilized in the original construction.
MD&A offers specialized diaphragm dishing repair services ranging from diaphragm inserts for upstream displacement, offset packing segments, restoration of diaphragm fabrication welds including increasing main weld depths, diaphragm stiffness improvements when possible and all required machining. MD&A also offers new custom
-engineered diaphragms with improved materials, deeper weld penetrations and increased stiffness.
MD&A is known for its catalog of field services, fast on-site and in-shop response, parts supply, and, above all, its approach to long-term better-than-original design solutions.
Although most D11 problems are in the HP/IP area, MD&A’s overall philosophy is to eliminate and resolve all reliability problems. The intent is to improve, not just recreate.
The best solution moves beyond simply restoring a unit to its original specifications. MD&A’s working philosophy is to consider all repair/replace options, look beyond the original, and move forward in consultation with the owner/operator with the optimum solution for real-world service going forward. Call MD&A at +1 (518) 399-3616 today.
Also read how, through its services experience and detailed engineering study of the D11 fleet and configuration, MD&A/MHPS have developed an HP/IP “Drop-In” retrofit solution based upon proven steam turbine technology.
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