Table of Contents
1.2 The acceptance run
An experienced car-owner does surely remember the following situation: You assume your car from the garage after a repair, confident that everything is fixed. Under deadline pressure you save the time for a test drive. Soon you find, the happiness was apparently premature. The old problem attracts your attention once more. Even more aggravating is, when you realize that the complained failure disappeared, but you register instead new alarming appearances. For instance the electronic clock does not work, the bord computer is faulty or the engine output is disappointing. The repair shop swears when you claim, everything was functioning during delivery. They think its’ a problem of your manner of driving. Once more you pledge: The next shop visite will only end with a test drive, together with the responsible person of the shop.
A significant pass of test after purchase or an overhaul at the bringing into service of a gas turbine is even much more important compared with a car, correspondent with the extreme cost-potential. So we have to be especially crucial.
The pass of test takes place prior to the actual operation and should show that the technical conditions, regarding the fulfillment of the demands related to the engines, prevail.Because this proof is of great importance for the operator it will be handled in this chapter.The following expositions serve to understand the rules relevant to the pass of test. They do not claim to be complete.
A pass of test follows typically extensive work from the manufacturer for which he is responsible. It serves as proof of the said characteristics and their technical integrity. It should, therefore, be significant for all relevant conditions during operation. The necessity for a pass of test is standard for new engines, for overhauls as well as for big repairs. The test can take place at the producer’s, at the shop or on site, as long as the operation specific particularities are fulfilled. They include the impact of the fuel, for example natural gas, on modern „low-NOx-machines“. Apparently small differences in the composition affect the combustion remarkable ( "Ill. 3.2.2-5"). This can have different, adverse impacts on the machine and its’ operation behavior.
The purpose of the pass of test for gas turbines is, as with all engines, to facilitate the timely recognition of eventual serious manufacture and assembly related mistakes. The pass of test would naturally follow a previously worked out plan and should definitely take place on site with the typical operation periphery (Chapter 3.7). Provisional arrangements, at inlet and outlet areas, are to be avoided. The operation parameter must cover the spectrum of the application, e.g., partly, extreme surrounding temperatures, respectively, air intake temperatures. All features, before, during and after this test, should be protocolled together with the manufacturer (OEM). This later lightens, if need be, the understanding of problems as well as their treatment. In any case, one should have the manufacturer confirm the safety of particularities that may arise.
Some recommendations in this regard:
Before the run
- Visual inspection and/or borescope of all accessible modules as evidence for mechanical damages like FOD and OOD (definition see Chapter 3.1.2.2), or possible damages through transport and assembly.
- Examination of leaking, auxiliary materials, like lubricating oil or sealing compounds, including those on the auxiliary units and the periphery.
- If necessary, a washing of the compressor ( "Ill. 4.2-1.1") should be carried out ( pay attention to production details!).
During start, run and shut down
- If the producer permits (when in doubt ask): run up the power with the starter at maximum speed without ignition, then measure the run down time for later comparison.
- It can be necessary to start (new start) a gas turbine as soon as possible after shut down. In this case you schould make certain that the rotor does not stuck. Otherwise there is the danger of untolerable rubbing (blades, vanes and seals) and/or the possibility of harmful vibrations by rotorbow ( "Ill. 2.2-1 and 2").
- Pay attention to leakages.( "Ill. 3.6.1-2" and "Ill. 4.1-10").
- In case these prevail, pay attention to exhaust flow and flying sparks.
- In case accelerometer and /or sound sensors are present , their signals should be continuously and legibly noted down.
- The same is valid for all further existing probes and sensors.( e.g. for temperature, pressure, flowmeters).
- Run down time measurement ( with manufacturer’s consent!), as mentioned above, to serve as a comparison of run down times.
"Illustration 1.1-1": (Lit. 1.1-2) The most important emissions are the contaminants in the exhaust gases that are formed when the fuel is burnt. This concerns, in the first place, gases like
- nitrogen oxide (NOx),
- carbon monoxide (CO),
- sulfur dioxide (SO2),
- unburned hydrocarbons ( HC = Cx H y ,), and
- soot as a solid.
The sulfur dioxide portion in natural gas , as opposed to other usual fluid energies, should be negligible. The pollutant emissions of good, modern, gas turbine plants are very slight and fulfill the requirements of the latest laws.
The gas turbine as turbo engine, with insignificant unbalances, emits hardly any vibrations and structure- borne sound at the foundation and ground level. Consequently, vibrations from gear boxes, additional appliances and driven accessories such as compressors or generators gain in importance. When necessary, an intermediate frame is to be recommended that carries the whole engine unit and is supported by elastic elements on the ground. Sound is normally mainly emitted from the gas turbine; although high frequency noise radiated from the gear box casing should not go unheeded. The relatively low, frequent, exhaust noise can be decreased to more permitted values, with the help of a following noise suppressor (Lit.1.1-8). The high frequency sound emitted from the compressor inlet can penetrate to the outside through the inlet duct. This sound can be well moderated through the use of the filter and noise suppressor. In order to control the radiated sound from the surface of all aggregates: gas turbine, gearbox, generator, auxiliaries, with one stroke, a single sound cowl has proved particularly cost effective.
The relatively low frequent exhaust noise can be minimized to a acceptable level with a downstream sound suppressor (Lit. 1-8).
Modern gas turbine units have such low emissions that they are chosen for decentralized power stations that can also lie in residential areas.
"Illustration 1.1-2": Important criteria for the acquisition of a gas turbine are the external influences to be expected. Primarily this is a question of the condition of the inlet air. This book deals in detail in the following positions with those aspects:
- Contaminations ( "Ill. 3.1.2.2-1").
- Flow disturbances ( "Ill. 3.7.1-5" and "Ill. 3.7.1-6").
"Illustration 1.1-3": The efficiency of a gas turbine is life dependent (Lit. 1.1-3, Chapter 5.1.1) and deteriorates, other than by a car engine, typically for new engines, especially clearly during running in. This sort of functioning is explicable because of the alteration of different components under operation influences. The initial, heavy loss is due to the running in and abrasion of air and gas seals (Chapter 3.1.2.4-4). Through the latter mechanism, a minimum of gas leakage is, indeed, produced for the condition of operation with the biggest, radial overlap ( ( "Ill. 3.1.1-3" and "Ill. 3.1.2.4-3"), but this must not necessarily be the actual operating point for continuous operation (cruise), so that here the degree of efficiency is noticeably reduced. The running in procedure does not repeat itself in the same measure with each new start/ shut down procedure, as the sealing clearances are already so heavily rubbed off that only a slight, moderate deterioration takes place during long operation time. It is important that the acceptance run is in accordance with a praxis relevant operating program. If not, this can lead to dangerous rubbing. If, during the pass off test, a performance curve is completed which does not cover the maximum overlap that emerges later, the performance of the engine at this point is not expressive.
A decline in efficiency, during longer periods of operation, may be attributed to long running effects. Typical is roughening by erosion and/ or the change of the profile by accumulations (fouling, "Ill. 4.2-1.1" and "Ill. 4.2-1.2"). Also labyrinths and blade tips are subject to long term aging and abrasion.
"Illustration 1.1-4": Areo-derivates at base load in industrial use are clearly differently loaded as compared to the flight engines from which they are derived (Lit.1-4). Engines for base load with typical long running times (10,000 hours) are usually operated at a lower gas temperature niveau (“derated“) compared to derivates. The load, however, with regard to peak load units, is very similar, at least in connection with the number of starts, (mostly a start with less than ten operating hours) . Consequently, the cyclic life of the hot parts by peak load (thermal fatigue, "Ill. 3.3-16") and possibly also other rotor components ( "Ill. 3.1.2.1-0"), is life determining. Contrary to this, e.g., the life of the blades of the high pressure turbine during long periods of operation , is dependent, primarily on hot gas corrosion, respectively, oxidation and creep load ( "Ill. 2.3-2"). The serious influence of the number of starts is represented in "Ill. 2.2-5". The influence of the base load niveau is shown in "Ill. 2.3-3".
"Illustration 1.1-5": Typical characteristics that influence the maintenance of a gas turbine (Lit.1.1-5 and 1.1-6). They are summarized under the term „Human Factors“:
„A“: Module design (also modular design, "Ill. 4.2-4"), implies the exchangeability of individual modules. One such is the high pressure compressor. It enables rapid availability in the case of repair
„B“: Axial split casings allow the engine to open without elaborate strip. In contrast, this is not so in casings that are built up as rings. With axial split casings, there is the possibility of exchanging individual stator vanes in a relatively simple manner, e.g., after an FOD.
„C“,“D“,“F“: Individual, on site, exchangeable rotor blades, without having to dismantle the rotor
„E“: Individually exchangeable high pressure turbine guide vanes (‘nozzles’), or rather guide vane segments. The exchange follows on opening the casing, without further disassembling the engine.
„G“: Good , accessible combustion chamber that permits exchange, or rather the repair of individual sections to take place.
„H“: External, good, acccessibly positioned accessories, control unit, pumps etc., cables, probes and monitoring systems, with good control and exchangeability.
„I“: A sufficient number and a convenient arrangement of borescope openings for all important mainstream components ( "Ill. 4.1-5").
"Illustration 1.1-6": Parts that do not, or only with rarely available, expensive machines and appropriate expertise, admit of repair-weldability can raise the costs for the operator noticeably. An on site repair is mostly not possible. The trend is valid: the higher the heat resistance of the material, particularly in connection with it’s Al- and Tiportion (Lit. 1.1-7), the more problematic it’s weldability. Criterion is the so called heat cracking (heat tears). For this reason, high tensile, powder metallurgical materials are to be observed with special scepticism, relative to their welding reparability. Thus, a labyrinth, intermediate ring, produced from especially high resistance material, can make an exchange of this expensive part inevitable, since it lacks the possibility of welding the integrated labyrinth fins ( "Ill. 3.1.2.4-7.1"). Usually, high tensile materials in operation are also correspondingly heavily loaded. Even small, welding induced cracks can lead to crack propogation ( "Ill. 5.3-1") on such parts.
The worse the weldability, as a result of micro and macro crack formation, strength reduction, residual stress and deformation, the more complicated is the work in the repair shops, if at all it can be carried out there. Not only is the welding process itself important, so is the subsequent quality assurance, e.g. crack detection of inner micro cracks (hot cracks). The operating performance of weld repairs can be influenced through a reduction of the strength in the weld and heat affected zone (HAZ). A demonstration of the reparability of the parts can therefore be an acquisition criterion.
Instruction:
All observations and measurement results are to be documented and, as far as possible, engine specifically evaluated and assessed!
Literature of chapter 1
Lit. 1-1 H.J.Willcocks, P&W Aircraft, „Icing Conditions on Sea Level Gas Turbine Engine Test Stands“, AIAA-82-1237 (1982).
Lit. 1-2 A.Jäschke, KHD AG Köln, „Stand der Gasturbinentechnik“, VIK-Berichte Nr. 195. Januar 1986.
Lit. 1-3 P.E.Soinne,R.J.Stephens,R.R.plc Ansty,“Industrial RB211-15 Years of History and Development“, ASME Paper 90-GT-145 (1990).
Lit. 1-4 P.Lowden,J.Liburdi, Liburdi Engineering,“Observations on the Life and Overhaul Requirements of Aero Derivative Engines in Base Load Industrial Applications“, ASME Paper 87-GT-105 (1987).
Lit. 1-5 R.H.Knorr, GE Co. USA, „Gas Turbine Maintenance“ (1974)
Lit. 1-6 B.Porter, Shell UK, „Maintenance of Aero-Derived Industrial Gas Turbines“, ASME Paper 81-GT-192, (1981).
Lit. 1-7 J.W.Sawyer, „Sawyer´s Turbomachinery Maintenance Handbook I“, Turbomachinery International Publications USA, (1980).
Lit. 1-8 R.A.Morris,“Gas Turbine Exhaust Systems-Design Considerations“, ASME Paper 87-GT-238 (1987).
Lit. 1-9 J.F.Rudy, Beitrag zur Asian Aircraft Engineering and Maintenance, 1986. Conference Proceedings Page128.
Lit. 1-10 P.H. Wulff, „Optimierung der Unterhaltskosten von Gasturbinen“, aus „Gasturbinen in Praxis und Entwicklung“, VDI-Gesellschaft Energietechnik, VDI-Berichte 1721, ISBN 3-18-091721-0, Pasge 157 to 168.