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Gas Turbine Disc Resurrection? PUBLIC ACCESS

[+] Author Notes
Lee S. Langston

Professor Emeritus, Mechanical Engineering, University of Connecticut

Mechanical Engineering 138(05), 56-57 (May 01, 2016) (2 pages) Paper No: ME-16-MAY5; doi: 10.1115/1.2016-May-5

This article discusses various aspects and need for gas turbine disc resurrection. Depending on the record keeping system used by the government, airlines, OEMs, and users, gas turbine discs are retired before they reach a critical state that might lead to their failure. Experts have reviewed current approaches to gas turbine life management. They point out that the high reliability and safety of modern gas turbines is largely due to a combination of improved materials, conservative design and maintenance philosophies, and improved life prediction capabilities. However, there are significant safety and economic concerns involved in the use of life predictions applied to extend disc life. Another resurrection path is the question of appropriating used discs to manage safe continued operation from unexpected field damage until new discs become available. Disc resurrection may be an attractive prospect, but lots of questions need to be answered before gas turbine users adopt the practice.

In axial flow gas turbines, discs in the compressor and turbine support and position rings of rotating blades and transmit energy to or from engine shafts. Their rotational speeds and power levels are high, so that each disc, composed of an inner bore, a web and an outer rim, are made of high-strength alloys, carefully manufactured to be as defect free as possible.

Typically records are kept on operation for both aviation and non-aviation engines. Depending on the record keeping system used by government, airlines, OEMs, and users, gas turbine discs are retired before they reach a critical state that might lead to their failure.

Gas turbine lore and legend has it that there are large warehouses storing many of these expensive used discs, particularly those from high usage applications, such as popular single aisle aircraft jet engines, many military jets, and high-sales electric power gas turbines. The thought is that many of these discs, presumably with significant life left, could be resurrected for future use. The means of resurrection might be some reliable reevaluation process (combining a new life law with testing, or a new yet-to-be discovered metallurgical procedure).

Let us look at just how feasible this concept might be. First, let us briefly consider what a disc failure can bring about. Then we can look at the disc life laws and the procedures used to retire them. We will end with an assessment of what is the possibility of their resurrection, i.e. the return of these discs to active service after their “certified” life has ended.

In an earlier column [1], I reported on the inflight turbine disc failure of a Rolls-Royce Trent engine on Qantas Flight QF32 on November 4, 2010. The super jumbo four engine Airbus A380 had just taken off from Singapore, bound for Sydney.

About 6 minutes after takeoff at 7,500 feet altitude over the Indonesian island of Batam, the Trent 900 intermediate pressure turbine disc on engine No. 2 failed, sending engine parts shrapnel through the engine nacelle and the left wing. Passengers saw several perforations take place on the upper surface of the wing above engine No. 2, resulting in one hole as large as 65 by 80 cm. Now powered by three of the four engines, the A380 circled to dump fuel (which was also leaking out of two wing tanks, above the failed engine). The Qantas plane then returned to Singapore, to land without thrust reversers, using emergency pressurized nitrogen to lower landing gear since the hydraulic system had been compromised by the uncontained engine failure. Controls to engine No. 1 had been damaged, so that the pilots were unable to shut it down after landing. Airport firefighters flooded engine No. 1 with foam to shut it down, further increasing the overall damage cost.

Fortunately, all Flight QF32 passengers and crew were safe and uninjured after the uncontained turbine disc failure. We can see that armed with enormous rotational kinetic energy, the disintegrated parts of a failed disc (see Fig. 1) and its blading become dangerous flying projectiles.

Figure 1. Recovered R-R Trent 900 intermediate pressure turbine disc segment from Qantas A380 Flight QF32. (Photo provided by Australian Air Transport Safety Bureau, courtesy of Aviation Week & Space Technology.)

Grahic Jump LocationFigure 1. Recovered R-R Trent 900 intermediate pressure turbine disc segment from Qantas A380 Flight QF32. (Photo provided by Australian Air Transport Safety Bureau, courtesy of Aviation Week & Space Technology.)

Vittal, Hajela, and Joski [2] review current approaches to gas turbine life management. They point out the high reliability and safety of modern gas turbines is largely due to a combination of improved materials, conservative design and maintenance philosophies, and improved life prediction capabilities.

However, there are significant safety and economic concerns involved in the use of life predictions applied to extend disc life. For instance, disc cracking caused by the most common failure modes of low and high cycle fatigue, creep, and manufacturing defects is difficult to predict, so that statistical methods must also be relied upon.

One probabilistic life management algorithm [2] is the Life- To-First-Crack (LTFC) approach. LTFC is based on the premise that a safe service disc life can be gotten by testing a sample of engine discs in a spin pit.

It is assured that the discs are initially defect free. To get a life standard time, a disc is removed from spin pit operation, at a time just before the appearance of a fatigue-initiated “engineering crack” greater than 0.38mm in length, with a 95% confidence. This leads to a safety procedure whereby aircraft engine turbine discs are being retired at a time when one in 1000 discs has initiated a short fatigue crack of 0.38mm. This implies that over 99.9% of these expensive, high-strength alloy discs are retired before their useful life has been expended. The 1/1000 life limit is a “safe life” approach that is considered conservative [3] and even quite wasteful [2]. (It is a possible supply source for the large warehouses referred to earlier).

An alternate, newer life management algorithm is Retirement For Cause (RFC). RFC [2] allows an aircraft engine disc to be used for the full extent of its safe fatigue life, bypassing the conservatism of the LTFC algorithm. The new safe life is based on fracture mechanics analyses at critical disc locations, the engine service cycle and the inspection/overhaul cycle. A key element in RFC is the ability to predict crack initiation and growth in a probabilistic manner.

These very brief explanations of LTFC and RFC serve to give a flavor of two disc lifing models. These and newer life laws are used by the military, OEMs, government agencies, and gas turbine operators.

Suppose you are in charge of an MRO (maintenance, repair and overhaul) for an airline company. It has a supply of used turbine discs, stored after removal from service, based on the airline's lifing policy. You know that currently there are no metallurgical procedures to restore their life by removing any residual cracks. Should you drill holes in the used discs, assigning them to scrap, or consider their resurrection in the company's fleet?

If the company has a complete set of operating and service records on the discs and are comfortable with the OEM design criteria used to predict disc service life, you might choose to consider resurrection. Then, should you inspect all the discs for surface distress and cracks, and possibly test one to failure in a spin pit? What is the company's liability if an accident occurs, caused by a failure of an resurrected disc engine?

1. Please note that a disc failure is a disc failure. The incident I chose here was probably not due to a disc life issue.

These are some of the considerations to be made if used discs are to be returned to service after their certified life. Another resurrection path is the question of appropriating used discs to manage safe continued operation from unexpected field damage until new discs become available.

As the reader can see, disc resurrection may be an attractive prospect, but lots of questions need to be answered before gas turbine users, be they military, airlines, or non-aviation, adopt the practice.

Langston, L “Gas Turbine Progress through Trouble”, Global Gas Turbine News, February 2011, p. 51.
Vittal, S., Hajela, P., and Joski, A., “Review of Approaches to Gas Turbine Life Management”, AIAA 2004-4372, 10th AIAA/ISSMO Multidisciplinary Analysis and Optimization Conference, 30 August -1 September 2004, pp. 876-886.
Thompson, R.B., and Brasche, L.J.H., “Nondestructive Evaluation”, Chapter 13, Turbine Aerodynamics, Heat Transfer, Materials, and Mechanics, Shih, T.l-P. and Yang, V. editors, AIAA, 2014, pp. 556–558.
Copyright © 2016 by ASME
Topics: Disks , Gas turbines , Failure
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References

Langston, L “Gas Turbine Progress through Trouble”, Global Gas Turbine News, February 2011, p. 51.
Vittal, S., Hajela, P., and Joski, A., “Review of Approaches to Gas Turbine Life Management”, AIAA 2004-4372, 10th AIAA/ISSMO Multidisciplinary Analysis and Optimization Conference, 30 August -1 September 2004, pp. 876-886.
Thompson, R.B., and Brasche, L.J.H., “Nondestructive Evaluation”, Chapter 13, Turbine Aerodynamics, Heat Transfer, Materials, and Mechanics, Shih, T.l-P. and Yang, V. editors, AIAA, 2014, pp. 556–558.

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