Unforunately, the author below has stated a number of inaccuracies in his comments that need to be corrected for the record. Rolls-Royce's David Taylor, Director of Technical Servcies, has provided corrections in the author's comments below.
Robert Wallace, Rolls-Royce Energyrobert.email@example.com
Comments on the Avon "200" upgrade package.
by j79guy on Tue Mar 03, 2009 9:00 pm
I was asked to comment on the Avon "200" upgrade package that Rolls has available for this product line. Here goes, I don't suppose I'll be getting a Christmas card.
Economic Alternatives to the Rolls Royce Avon "200" Modification Program
Rolls Royce has recently announced what they describe as a significant upgrade to the Industrial Avon gas turbine product line, promising; 1) Improved output. 2) Improved fuel efficiency. 3) Reduced exhaust emissions. 4) Improved overhaul life. 5) Reduced overhaul costs.
The following is an examination of these claims, based upon field and overhaul experience with the Industrial Avon product line, as a fully independent heavy maintenance and overhaul depot, without any affiliation to Rolls Royce, and an offer of economical alternatives:
An overview of the "200" modification revisions:
- DS cast Hp. Turbine Blades. (Hpt Blades.)
- Hpt Blade tip sealing, and Tip Shrouds.
- Hp. Turbine Disk and Blade root interface.
- Metallurgy, Ip and Lp Turbine Blades.
- Turbine Nozzle Guide Vanes. (Turbine Nozzles.)
- Combustion Liners and Fuel Injection Nozzles. (Swirler Burners.)
Let us now discuss each of the modification revisions as per above.
- DS cast Hp. Turbine Blades.
Improved, or really, increased output, for any gas turbine engine is strictly a function of increased airflow, (Mass flow.) or increased firing temperature. (Expansion ratio.)
As Rolls Royce is not proposing an increase in mass flow, then they are proposing to increase the output via increased firing temperature only.
[Taylor, David H] Actually the writer is wrong. The mass flow is increased as the rotor speed is increased. The average measured power increase is 11.5%, measured over 10 engines. Interested parties can contact Rolls-Royce for a copy of this data if required.
To do this, Rolls has changed the turbine blade materials to allow an increase in firing temperature. The root of this change is in the Hp Turbine Blades, which have been changed to single crystal cast. (As opposed to equiax cast in the "regular" Avon engines.) Rolls Royce commercial RB211 aero engine experience has demonstrated the ability of SC cast blades to better resist creep growth than equiax cast blades, when comparing, similar cooling regimens of like blades, and the only difference is the casting technique. Rolls, with the "200" modification has eliminated the cooling air used to keep Hpt blade temperatures under control, and has elected to use solid blades, similar to the Mk-1533 Hpt blades. Thus, when at full power, or increased power as proposed by Rolls, the Hpt blades will in fact be operating at significantly higher temperatures than in Mk-1535 engines.
[Taylor, David H] Wrong again. Due to the increased turbine efficiency the turbine temperatures actually decrease. The addition of improved materials allows the HP cooling to be deleted thus further improving the cycle efficiency.
It must be clearly understood, that in order to achieve this increased output, that the firing temperature of the engine must be raised, stressing not only the engine higher, but also significantly, the downstream equipment will be subjected to higher thermal and torsional stresses as well. As the Hpt blades are non-air cooled the operating temperature of the blades will be much higher than in the "regular" Avon engines, and the Hpt Stage-1 Disk will also be operating at a higher temperature as, the blades will be solely cooled through their roots into the disk.
[Taylor, David H] Wrong see above comment.
Thermodynamic laws dictate that any increase in firing temperature will indeed result in higher engine output and higher fuel efficiencies, so long as the customer's equipment can reliably take advantage of this increased output. What is not indicated in the Rolls Royce Avon 200 brochure, is that if increased outputs are used, that the overhaul life of the downstream equipment, particularly the power turbine assembly, may be reduced, and result in higher overhaul costs per fired hour. If the end customer is operating their equipment in an environment where local ambient temperatures are high, then increasing the output of the package may not be practical, due to higher thermal loads that will need to be radiated through the oil coolers.
[Taylor, David H] Wrong again. At base load power the power turbine entry temperature actually drops by 40 degree C, thus the power turbine life is not reduced.
As the Avon 200 Hpt blades are not coated with any thermal barrier type coatings, these blades will run at much higher temperatures than the "standard" Avon Hpt blades. Despite the claims of Rolls Royce, who has yet to accumulate any significant run hours with any Avon engine with the "200" modification it is highly likely that these blades will require a significant percentage of replacement at each overhaul period, making such overhauls extremely expensive. Single Crystal casting techniques have on average 60%-80% blade rejection rates during their manufacture, which is the primary reason they are so very expensive to purchase, or replace.
[Taylor, David H] Wrong again. See above comments on turbine temperature. The fleet leader has completed over 15,000 hours and there are now over 150,000 hours with no problem being experienced. Scrap rate for single crystal blades is similar to that for equiax blades.
Should the end use customer require additional power from their package, and the downstream equipment can reliably utilise this extra power potential, the application of ceramic thermal barrier coatings to existing equiax cast blades has been proven to be an extremely effective and economical solution to reduce the thermal loads on the Hp Turbine Stage-1 disk assembly. For each Mil of ceramic coatings, the base metal temperature of the blading is reduced by 40 degrees C.
A second, slightly more costly option (But a fraction of the total cost of the Avon 200 Hpt Blades.) is to use Directionally Solidified (DS) cast blading, available competitively priced to the original equiax cast blades. DS casting is nearly as effective as single crystal casting in terms of creep growth resistance, and they retrofit directly into the customer's engine, with no other costly modifications required. A set of DS cast, Hp Turbine blades, with ceramic thermal barrier coatings will likely last the life of the engine, never requiring replacement, and in the event that a DS cast blade does require replacement due to some other reason, they are available cost competitive to "standard" blades. To date, no Avon Hp Turbine DS Cast blade, when combined with a ceramic thermal barrier coating, has ever needed replacement due to thermal distress or creep growth beyond acceptable limits.
[Taylor, David H] The authors initial premise is totally incorrect. The A200 has a far more efficient turbine which runs the engine faster thus increasing mass flow. The increase in component efficiencies allows the actual firing temperature to decrease by 29 deg C. Thus the power turbine and gas generator turbines actually run cooler, thus increasing life. The higher temperature capability of modern SC materials allows us to eliminate HP turbine cooling. We have been running single crystal materials in the RB211 for several years and the overhaul costs are comparable to equiax materials.
It would appear that the writer has some affiliation with a supplier of reverse engineered turbine blades and his comments could be considered as marketing for those ideas. Rolls-Royce is not aware of any Avon operators running the blades described and the claims made are unlikely to be substantiated.
- Hpt Blade tip sealing, and Tip Shrouds.
Rolls, with the introduction of the "200" modification, has revised the Turbine blade tip sealing and shrouds. The theory is that via revised blade tip sealing, gas path losses can be reduced at the blade tips. To achieve this, Rolls has revised the blade tip "fences", and the mating, stationary tip shrouds.
Reduced turbine blade tip leakage is undeniably an objective in maximising hot gas path energy, and transforming it into useable work. On paper, the revisions made by Rolls in the "200" modification appear to be a step in the right direction, however one must take into consideration as a whole, the operating of industrial Avon units, and the effectiveness of such modifications in light of long term operations. Avon units, in their "standard" configuration have shrouded blade tips, which is the single most significant step towards improved turbine cycle efficiency, as compared to traditional General Electric type blades, which do not have such turbine blade tip shrouds. GE, in an effort to minimise hot gas path losses, elect to run their turbine tip clearances extremely tight, and use abradeable tip shrouds to allow the blades to actually touch the stationary seals. It is common practise to have to "break in" a freshly overhauled GE engine, as the turbine blades run on the tip shrouds quite hard, and upon first shutdown, may indeed "lock" the turbine rotor from turning for several hours.
Avon units, on test cell operation, have as part of their acceptance tests, a timed coast down from the cut of fuel to the engine, from idle speeds. If the engine fails to meet a minimum run down time, the engine cannot be released to the customer. This is exactly the opposite to General Electric philosophy, where "tighter, is better", and their engines are expected to stop very quickly on test cell shutdown.
In order to meet this minimum run down time, Rolls elects to assembly the Avon units with generous turbine blade tip clearance, which limits the ultimate effectiveness of such.
Therefore, no matter what revised system Rolls elects to incorporate into the Avon product, without changing the break-in procedure on the test cell, or altering the minimum run down time, there is a distinct limit to the effectiveness of
revised tip shrouds.
[Taylor, David H] Incorrect. The A200 clearances have been selected and tested to give the optimum sealing. The seal design is based upon that used successfully in the RB211 for millions of hours.
Instead of electing the pay the premium price of the "200" modification, which includes revised turbine blade tip shrouds and stationary seals, it is more economical to incorporate stationary tip shrouds that have an abradeable surface (Typically an unfilled honeycomb type sealing surface.), together with extended turbine blade tip "fences", which can be added to "standard" blades during overhaul. During test cell running, the unit is run in sequentially, to gently wear the turbine components in to each other, minimising hot gas path blade tip leakage. Such is significantly more effective than the "200" modification, and is completely refurbishable at major overhaul intervals, at considerably less expense than what Rolls presents to their customers.
[Taylor, David H] Tip rubs are a known cause of turbine blade vibration which can lead to failure, unless the blade is designed to rub. Pre Avon 200 blades are not designed for that criteria.
- Hp. Turbine Disk and Blade root interface.
With the Avon "200" modification program, Rolls has revised the Hpt blade root profile, and the mating Hpt disk assembly, which holds said blades.
In the past, Rolls has experienced failure of the Hpt disk and/or Hpt blades at the fir-tree root interface, and thus has elected to revise the fir tree root profile on the promise of better fatigue resistance.
On the surface, revision of the Hp turbine root profile appears to be a good modification, however one needs to examine the past modes that led to the failure of the Hp turbine components; It has been demonstrated that the failure of these components was in nearly every case, caused simply by uneven combustor (Flame tube) loading. This uneven loading in turn, was caused by uneven fuel flows into the combustors, or a disruption of the combustion process in each combustor. Revision of the Hp turbine components, although may lead to a reduced incidence of failure, however this is treating the symptom, not the cause. As with all Roll Royce turbine products that have separate combustor cans, (Can-annular combustion.) it is imperative that the fuel flows into each combustor must be balanced and even. By flow matching the fuel injection system components and maintaining this system integrity, the incidence of Hpt turbine component failure is eliminated. (Those failures associated with uneven combustor loading.)
By incorporating the "200" modification, the customer is cutting themselves off from the traditional supply of commonly available "standard" turbine components and committing to unique turbine components that have yet to prove their ultimate effectiveness and longevity. Pricing of these new type turbine components have already been demonstrated as being extremely high.
[Taylor, David H] The revised root fixings and turbine blade design have been carefully designed using the latest techniques to eliminate HCF failures. Thus the through life costing of the A200 is cheaper by eliminating these costly failures. Interesting comments on pricing. If the writer had contacted Rolls-Royce for comparative costs he would have found the A200 parts are very similar to pre A200 parts.
This fuel system flow matching can be done during engine overhaul at very low cost to the customer, and over the overhaul life of the engine, periodically checked during routine borescope inspection to keep the negative effects to a minimum. A spare set of flow matched fuel injection nozzles, (Burners.) can be rotated into the unit during routine inspection. The removed fuel nozzles are then sent to depot for cleaning, and flow balancing, which ensures the customer always has the lowest possible combustor loading, and subsequently reduced turbine system component stress. The original type Hp turbine components are retained, and reliably operate over the life of the unit, without incurring the expense of perhaps needlessly replacing such, with the goal of "fixing" a problem which may not exist.
[Taylor, David H] All burners are flow tested during overhaul and the flows must meet precise criteria. Burner flows cannot be checked by boroscope, only by checking on a calibrated flow bench.
- Metallurgy, Ip and Lp Turbine Blades
With the Avon "200" modification program, Rolls Royce has altered the base alloy of the Ip and Lp turbine blade materials. The claim is improved component longevity and overhaul costs.
Upgrading the base alloys of the Ip and Lp turbine blades, is potentially a good idea, as alloys have advanced considerably in the last 15 years, giving better thermal creep growth life, and resistance to hot gas path corrosion effects. However, the Rolls Royce Industrial Avon units typically do not have a problem with the Ip and Lp turbine blades. The coatings specified for "standard" Avon units is a very mature specification, dating back to the early 1960's. Currently there are a great many coatings available that offer vastly superior performance to the "standard" OEM coating, at economical pricing. It is most unfortunate that the Avon "200" specification Ip and Lp. Turbine blades cannot be slowly incorporated into the Avon product line during overhauls as required, rather than a single, expensive, mass change out.
[Taylor, David H] The coatings chosen are Sermaly J. This offer the best corrosion resistance/price/reparability of any coating. There are more expensive coatings like Platinum Aluminide but these are not required at the turbine temperatures experienced in the Avon turbine.
The application of modern corrosion and thermal resistant coatings to the "standard" Ip and Lp turbine blades, offers as much, or better of a component longevity advantage as the expensive "200" specification blades. Should the customer require increased engine output, then the application of ceramic thermal barrier coatings to these blades offers much more to the end customer in terms of reduced overhaul costs.
[Taylor, David H] The coatings used on the Avon 200 are the same as those on pre Avon 200 turbines. Ref the comment regarding TBC, see earlier comment about reverse engineering.
- Turbine Nozzle Guide Vanes.
Minor revisions to the turbine nozzles to allow a reduction in cooling air flow, and increase vane segment rigidity.
Any reduction in cooling air flow to the turbine nozzles will result in an increase in turbine cycle efficiency, as more of the hot gas path energy will be available for useful work. Again, parts commonality with hundreds of "standard" Avon units will be lost, and the user, if incorporating the "200" modification will be restricted to replacing these components in the future with like, low production and expensive parts.
[Taylor, David H] See above comments on parts pricing.
Once again, the application of highly effective thermal barrier coatings eliminates all the associated problems with operating Avon units at elevated firing temperatures. These coatings, when combined with intelligent minor revisions to the cooling regimens of the existing turbine nozzle guide vanes, has performed very well in the field, and keeps total overhaul costs low.
- Combustion Liners and Fuel Injection Nozzles. (Swirler Burners.)
With the failure of Rolls' DLE combustion system for the Industrial Avon, which had been under development for over 12 years, Rolls has settled for a modest modification of the combustion system by partially mixing fuel gas with a portion of the primary combustion air, via a "hole" in the fuel nozzles.
[Taylor, David H] Wrong. The writer has completely misunderstood the design of the swirler burner.
Close examination of the claimed benefits of the "200" Modification "swirler burners", in the product brochure shows that "no change in NOx emissions is expected", yet in the numbers posted a modest increase is indeed shown. This is simple combustion physics, as with a diffusion type fuel nozzle, which the "swirler burners" are, NOx is a function of firing temperature and residence time in the combustion zone. As these two parameters are not changed, NOx production volume cannot change. Therefore, any increase in firing temperature, will result in an increase in NOx production. Again, CO production is inversely proportional to NOx production in a diffusion nozzle type combustor. It is to be expected that CO production will drop, with an increase in firing temperature, and any Rolls Royce turbine model that fires hotter than the Industrial Avon, (RB-211, Spey.) shows a correspondingly lower CO production exhaust emissions signature.
Currently, the world focus is on CO and CO2 emissions, however the production of NOx is not to be forgotten, and indeed is much more serious of a threat to the environment.
It is mandatory that the "200" modification combustion system be incorporated into the customer's engine at time of overhaul. You cannot choose to take only this or that part of the modification, rather, you must take the whole "kit" whether the operator really needs all of it or not. These one-off combustor components have no commonality with the rest of the world's fleet of Industrial Avon units, thus will be expensive when replacement or refurbishment is required at overhaul interval.
[Taylor, David H] Wrong. The swirler burner has been the standard for several years and was first introduced in the mid 1990's. Many operators have changed to the swirler burner and have benefited from the ensuing benefits.
The "Triple Dish" type combustors, when coupled to flow balanced fuel injection nozzles have demonstrated excellent overhaul life. In a pinch, even Mk-1535 operators can use the "old" style combustors and still operate reliably, so long as combustor loading is addressed. Once again, this is a case of keeping proven components in service, with the intelligent application of thermal barrier coatings, innovative refurbishment techniques, and keeping the 1f vibration signature of the engine as low as possible, via correct rotor balancing. These together result in a combustion system that runs reliably for the whole overhaul interval, and are readily refurbished at such overhaul.
Should reduced emissions be the goal, there are commercially available lean-premix combustors available for the Industrial Avon, that retrofit to the original engine cases, and fuel control systems. These lean-premix combustors offer significantly reduced NOx exhaust emissions , as well as reduced CO/CO2 emissions, and can be incorporated into any Industrial Avon unit without having to buy into a whole "upgrade package" that may be cost prohibitive.
It is commendable that Rolls Royce has elected to keep the Industrial Avon product line supported for an indefinite period of time. Most significantly, the downstream power turbine assemblies have been upgraded and improved to give dramatic increases in output and thermal efficiency, much more so than all of the model upgrades to the actual engines.
The Model changes over the years, from Mk-1533 to Mk-1535 have been significant, but most importantly, have been evolutionary, maintaining commonality with the lower output model designations. Today, you literally, can take an early 1960's commercial Mk-533 flight hot gas path engine component, and it will still physically fit into a Mk-1535 unit. This is not to say it will perform as well, but demonstrates the point of commonality. This has kept the Industrial Avon units cost competitive, and maintained a low total overhaul cost per fired hour.
What Rolls Royce now proposes with the "200" modification, is a step away from this commonality, sharply driving up total overhaul costs, and most significantly, will keep these units high in terms of total maintenance costs per fired hour.
Current Avon unit overhaul costs, when the "200" modification package is taken, are averaging approximately $3.0M USD.
[Taylor, David H] Wrong again. The writer clearly has not researched the costs of upgrading to Avon 200.
At less than 60% of this cost, any Industrial Avon unit operator should have the option of having their Avon engine assemblies overhauled, and demonstrated on the test cell, with whichever particular suite of hot section, or combustor section upgrades incorporated into their engine that best suits their operating conditions and duty cycles. Anything less should not be tolerated.
[Taylor, David H] Wow. 60% x $3 million USD is an expensive overhaul for an Avon engine. Rolls-Royce authorised overhaul bases can provide a much more competitive price and have OEM backing.
Many advances have been made over the last 15 years in terms of corrosion resistant coatings, maintenance techniques, thermal barrier coatings, and base component alloys, giving turbine engine operators a wide choice of options when it comes time to perform heavy maintenance to their units. A break from the OEM maintenance depots opens the doors to these options. Operators owe it to themselves to check out these maintenance options, as their competitors certainly are.
[Taylor, David H] Ask Rolls-Royce for an Avon 200 quote for comparison. Then choose.