Industrial Gas Turbines

2.6 The fuel

Also in our car, fuel is an issue. Supposed are not ’table discussions’ about wondrously addings. Here effects on unimagined maximum speeds are connected with some ingredients. No, a true theme is the environment protection. So lead additives were minimized or banished. Lesser sulfur contaminations are essential.

There were to some extent unexpected negative consequences for the durability of important motor components. Not till adaptions to the new conditions brought a satisfactory operation behavior. However those requirements in a twofold way had a positive consequence. On one hand by minimizing direct emissions of pollutants. To the other to eliminate harmful portions like NOx and CO from the exhaust gas by the help of catalysts without damageing their effective structure. Sulfur and lead literally give a hard time to the catalytic converter and cause its early breakdown. Naturally we will try at first to decrease the emission of harmful gases by optimizing the fuel charge and the combustion. An improvement of the motor efficiency leads to a lower fuel consumption and so to less exhaust gases. Expensive procedures like direct fuel injection and the realization of meager mixtures are used to lower the fuel consumption together with less emissions.

Quite similar is the approach at gas turbines to achieve the conservation claims with acceptable power, favorable fuel consumption and high operation life. The degree of efficiency will be further raised and new technologies lower developing of pollutants. Not seldom, at least during the introduction of low-emission configurations, problems appear which were almost not seen before. Therefore also at low-emission gas turbines improvements compared to former versions, must be carried out.

Characteristics of a fuel influence the operation costs to a high degree. Not necessarily the fuel consumption is of essential importance. Fuel, through it’s composition, purity and combustioncharacteristics determine to a great extent the life and operation performance of our gas turbine. Particularly the life time of the hot parts in the area of the combusion chamber is affected.

The life of the combustion chamber itself and the first turbine guide vanes (nozzles) must be seen in connection with the heat radiation of the combusion flame. It ‘s crucial responsible for the heating, that means the temperature niveau and the temperature cycles. With this also damages by oxidation and thermal fatigue, dependent on the radiation of the flame. Fuels which tend to soot formation and therefor intensive heat radiation (luminous flame) can multiple the repair costs of the hot section. Reasons are a shorter lift time and/or irreparable destructions after a normal lifetime. The soot formation is governed by the hydrogen content ( "Ill. 2.6-2"). The lower the hydrogen content, respectively, the higher the carbon content, the more soot is produced. This stands in connection with the fuel specific blend of different hydrocarbons.

There is an urgent recommendation that guarantees given on the life time of hot parts and demostration runs apply to the fuel, the operator uses (example 3.2.2-1).

Intends the operator, e.g., because availability or cost problems to change the fuel type (that is also true for the provider) he schould inform and ensure himself at the manufacturer (OEM) about concerns and potential consequences for the guarantee. The best is, when this happened alredy before the acquisition of the engine.

The minimization of the pollutent emission puts additional requirements on the constancy and cleanliness of the fuel. This includes first the ‘Dry Low-Nox’ (DLN) combustion.

The admixture of plenty air in the region of the injection nozzles favors instabilities of the combustion ( "Ill. 3.2.2-5"). This is extreme susceptible for seemingly small differences in the fuel composition. Apparently even deviations within the specification can become unacceptable noticeable. (see "Example 2.6-5" and Chapter 3.2.3).

In natural gas yielded finest distributed liquid aerosols and hard parts have, correspondent to the literature, shortened the time between overhaul of DLN Gas Tubines down to 2000 - 3000 hours of operation (Lit 3.1.2.3-1).

Drops the temperature of the fuel gas under the dew-point, a condensation formation can happen. With this exists the danger of overheat damage of the hot parts. In this case the control unit is not in the position to react at the sudden much higher fuel mass of the liquid instead of the gas.

Additionally, there is the danger of destructive flame reversal ( "Ill. 3.2.1-5.2") . In order to avoid such dangers, gas turbine operators recommend before the fuel system a fuel gas temperature that lies at least 10°C above the dew point. This temperature is guaranteed by some manufacturers with a permanent pre warming. In addition, one has to take note that gas cools off noticeably with a pressure drop. If the gas is reduced from, e.g., 56 bars to 25 bars, one has to reckon with a drop of combustion temperature around 17,5 ° C. A corresponding pre warming can, however, become costly because of the additional energy consumption factor.

New additional instruments open the possibility to remove aerosoles or to monitor the combustion chamber continuously for this kind of impurity.

DLN combustion chambers ( "Ill. 3.2.1-5.1" and 3.2.2-2) are especially sensitive in comparison to conventional combustors, to carry oils in the gas. Oils like heavy hydrocarbons, amine and glykol, (oil smoke), that are brought in from preceding compressors must be removed from the combustor like aerosoles. If this does not succeed, there is the danger of fuel nozzle coking and of early ignitions ( "Ill. 3.2.1-5.1") or flashbacks ( "Ill. 3.2.1-5.2"). In order to remove this fine dispersed oil fog, a binder is necessary. The fine dispersed oil mist can be removed by means of a binding agent.

Unsatisfactorry quality of gaseous or liquid fuel is one of the most frequent reasons for problems with industrial gas turbines (Lit. 2-13). Causes are aberrations, respectively variations of the fuel specifications and/or not qualified handling the fuel. Shortages like during energy crises aggravate a trend towards poorer fuel quality. There are attempts to counter this with better fuel cleaning systems. Those are applied specially for heavy fuels.

Because contaminations like sulfur damage the expensive hot parts (see Chapter 3.4.2) overhaul intervals and its costs are influenced by the choice of the fuel ( "Ill. 2.6-1"). This is not only true for the turbine blading. So it was determined that silver can trigger in many ways sulfidation. ( "Ill. 3.4-4"). Even on superalloy rotor discs, the exposure to silver provokes dangerous pittings. Therefore in many cases silver-coated bolts are today no more used in the hot section.This problem seems to be considered so serious that galled, respectively jammed bolts are exchanged after loosening. Thereby the danger of an unidentificated damage by overload during loosening does not exist.

But not only hot parts are affected by poor fuel quality. Also deposits (fouling, "Ill. 2.6-3") in the fuel system (control system, injectors, "Ill. 2.6-4") can lead to malfunction of the gas turbine.

Example 2.6-1: (Lit. 3.1.2.3-1): A flashback ( "Ill. 3.2.1-5.2") in a DLN (DLE) equipped industry gas turbine was probably to be traced back to amine in the combustion gas, although the serious unit for gas treatment did not give any such indication. As it showed later, big amounts of amine ion had penetrated the gas pipe ten years earlier. These deposited on the bed of a river through which the pipe line was laid. A rise in the gas flow tore away the amine which had set down inside the pipe.

The insertion of a filter could prevent further flashbacks.

Example 2.6-2: (Lit 3.1.2.3-1): After about 2500 operation hours, far ahead of a planned overhaul, „oil smoke“ from an upstream situated pipeline compressor, arrived in the turbine. This may have lead to the fouling in the compressor. One succeeded in cutting off the oil smoke with an appropriate 2,5 my filter, so that the overhaul intervals up to 40,000 hours were reached.

Example 2.6-3: (Lit.3.1.2.3-1) In two big gas turbines equipped with DLN combustion, operated with low dew point gas (around 0°C ), flashbacks appeared. It showed that the dew point had fallen below in the overground gas pipes in winter. The condensed fluid led to the flashbacks. The pre heating of the fuel gas proved remedial.

Example 2.6-4: The operator changed the fuel gas supplier on account of costs. Because of this, fluid residue managed to get into the fuel gas pipes of the operator and led to the shut down of two turbines. The turbines were provided with a filter, FCGS, that prevented bigger damages.

Example 2.6-5: Before the delivery, the gas turbine was build up at the manufacturer (OEM). With the there available natural gas an extensive testing was carried out. This convinced about a complient free operation. At once after the delivery in overseas severe combustion instabilities were observed. Apparently the problems were very serious (see "Ill. 3.2.2-5"). Remedies led to existence-threatening costs for the OEM. An investigation of the used fuel gas showed differences to the gas in the testing. With this finding the unacceptable combustion problems could be explained.

"Illustration 2.6-1": The maintenance effort of a gas turbine depends, in the first place, on the life and the overhaul intervals of the hot parts, (combustion chamber and turbine). In this connection, it is important to recognize the used type of fuel. Comparatively clean, natural gas performs the best. The thicker and inhomogeneous the fuel, the more the impurities increase in cheap fuels. Cost intensive damages take place more frequently and /or overhaul intervals are shortened. The lifetime of hot parts is especially shortened by impurities that induce hot gas corrosion damages, e.g., sulfidation ( "Ill. 3.4-2" and "Ill. 3.4-3").

"Illustration 2.6-2": (Lit 2-9): Soot of the combustion process is crucial for the thermal load of the combustion chamber by heat radiation. The glowing soot particles are the main radiation source. It is responsible for the temperature of the combustion chamber walls because the radiation penetrates the protective veil of cooling air. So the repair costs of the combustion chamber rise due to exceeding damage (e.g. crack formation, deformations; upper sketch, "Ill. 3.2.3-1").

The soot forming carbon respectively the hydrogen content is destined by the ratio of the chain and ring hydrocarbons. Of course combustion conditions play an importan role for the soot generation (Lit. 2-12).

The diagram shows the tendency of the dependence from operation life time of the combustion chamber from the composition of a (liquid) fuel.

About 100°K lead to thermal stresses which cause disproportionately changes of about a order of magnitude in the operating life (load cycles to crack formation).

Notice:
Also a seemingly small change of the fuel specification and/or the use of different fuels can shorten the time to overhaul markedly and boost the repair costs. Therefore a most critical check with suitable operation conform test runs is strongly suggested.

"Illustration 2.6-3": (Lit. 2-13): Several derivate-engines in the oil production at the Middle East suffered dangerous fouling of the fuel system. Especially valve seats and control sections were affected. This influenced the start behaviour. Additional the engines responded slow at load changes.

An investigation showed, that formation of the deposits depended on the combination of the fuel contamination hydrogen sulfide with water. In the tube line for the fuel, fine iron sulfide particles formed and were suspended in the fuel. Those particles could pass the fuel filter and precipitate during expansion behind narrow cross sections. They form a very hard and brittle coating on wetted surfaces, which peels off in big flakes.

A similar problem are deposits, shown in the picture. They form in the fuel from a hot oil well at above 100 °C. To master the problem it was enough to bisection the size of the filter pores to about 2 μ. Therefore a combination of several measures were necessary:

Improved centrifugation/purification to reduce the water content. Including detailed maintenance and inspection of the water separator and similar acting installations.
Avoidance deeper located line zones, in which water under the dew point can settle from the fuel gas.
Suitable fuel system (hydraulic actuated), especially to overcome high adjustment forces.

"Illustration 2.6-4": (Lit. 2-13): It came in some derivate engines of mobile power generators to a fast formation of deposits (fouling) in the fuel jets. This fouling required frequent change/cleaning of the fuel filters. Additonal failures of the combustion chamber occurred.

The fuel was delivered in road tankers directly from the trailer. Because of those many separate shipments it was not possible to take fuel samples from every load for examination. With random tests the fuel contamination, obvious outside the specification, was not found.

A monitoring (Chapter 5.1) of the exhaust temperature together with borescope inspections of the combustion chambers ( "Ill. 4.1-6") and frequent cleaning of the fuel jets/burners could bring the problem to an acceptable degree. A detailed investigation of the fuel showed that the reason for the problem was bad handling (poor ‘housekeeping’) of the fuel. The contaminations were dust, crude deposites and tar. They originated from poorly cleaned road tankers. This awareness triggerd the following successful remedies:

Improved handling of the fuel.
Installation of additinal filters at the fuel entrance into the gas turbine.
Extended setting time before the gas turbine.

"Illustration 2.6-5": (Lit. 2-14): A continuous control of the fuel quality has proved of value for gas turbines in practice. Thereby primarily the lower heating value (LHV) is a matter of interest. A decline of the LHV can cause additional fuel costs in the range of a one family home (upper diagram) in a gas turbine of medium size. In the shown case the drop of about 1 % LHV generated additional costs in the region of 300 000 $ per year.

For the LHV-Monitoring continously incoming data from important components like compressor and turbine are observed and electronically evaluated (performance monitoring, Chapter 5.1). With these data it is possible to distinguish (upper diagram) between effects by less fuel flow and LVH-change.

For this purpose proven methods are available. They can be used for gaseous and liquid fuels and every gas turbine type. This can also be applied for water or steam injection ( "Ill. 2.1-3.3" and "Ill. 2.1-3.4"). There exists such a possibility for gas turbines which drive pumps of gas pipe lines which use this gas as fuel.

This method was successful applied to monitor the gas quality in the line for natural gas and diesel fuel. It could be seen that the composition of the fuel, dependent on the source, can vary in a scatter of +-10%.

The fuel monitoring method has yet other highly interesting aspects:

Warning when emisson levels are exceeded by change of the LVH.
Avoidance of combustion instabilities ( "Ill. 3.2.2-5") which can cause in Dry-Low-NOx (DLN) gas turbines heavy damages.
This is also useful as far as the extinction (flame out) of the combustion chamber is concerned.

Literature of chapter 2

2-1 H.Löffel, D.Thinius, Energieconsulting Heidelberg, „Gasaturbineneinsatz im Rahmen der Kraft- Wärme-Kopplung“, VIK-Berichte Nr. 195. Januar 1986.

2-2 „Gasturbinen in der Industrie“,VIK-Berichte Nr. 195,Januar 1986, page 9-16

2-3 R.L.Casper, J.C.Rucigay, GE Co., „Design and Development of the General Electric LM5000 Industrial Package Power Plant“, ASME Paper 85-GT-26 (1985).

2-4 H. Huff, A. Rossmann, „Zur Kurzzeitermüdung von Turbinenrädern“, Allianz „Bruchuntersuchungen und Schadenklärung“, 1976, pages 98-103.

2-5 C. Marnet, B.Kassebohm, „Bau und Betriebserfahrungen mit der leichten und schweren Gasturbine“, VGB Kraftwerkstechnik 55, Heft 12 (1975).

2-6 R.Kennedy,GE Co.,Maintenance, Gas Turbine Reference Library (1968). page 7

2-7 R.H. Wulf, „CF6-6D Engine Performance Deterioration“, Paper NASA -CR - 15978G, 1978, page 1 - 103.

2-8 R.L. Martin, W.J. Olsson, „Operating Flight Loads and Their Effect on Engine Performance“, SAE-Paper 811071 des „Aerospace Congress& Exposition“, Anaheim, California, October 5-8, 1981.

2-9 C.A. Moses, P.A. Karpovich, „Fuel Effects on Flame Radiation and Hot-Section Durability“, Proccedings AGARD -CP-422 der Konferenz „Combustion and Fuels in Gas Turbine Engines“, page 15-1 to 15-15.

2-10 K.C.Ludema, „Failures of Sliding Bearings“, „Metals Handbook, Ninth Edition ,Volume 11 Failure Analysis and Prevention „, American Society for Metals (ASM ), ISBN 0-97170- 007-7, 1986, page 483-489).

2-11 F.Böckel, S.Verstege, „Lagerung-Grundlagen und konstruktive Gestaltung“, C.Lechner, J.Seume „Stationäre Gasturbinen“, Springer-Verlag, Berlin Heidelberg New York, ISBN 3-540-42831-3 2003, page 717-719.

2-12 J.Hellat, A.Eroglu, W.Krebs, „Technische Verbrennungssysteme“, C.Lechner, J.Seume „Stationäre Gasturbinen“, Springer-Verlag, Berlin Heidelberg New York, ISBN 3-540-42831-3 2003, page 447-538.

2-13 J.W.Sawyer, „Sawyer´s Turbomachinery Maintenance Handbook I“, Turbomachinery International Publications USA, (1980), page 7-25.

2-14 Fa. Gas Path Analysis Limited (GPAL) „ On-Line Fuel Quality Control for Gas Turbines“, www.gpal.co.uk, Mai 2008.

2-15 S.S.Florjancic, N.Lively, G.R.Thomas, „Mechanical Behaviour of an Industrial Gas Turbine under Fault Conditions, a Case History“, Proceedings of 7 th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery, ISROMAC Conference, Honolulu, 1997, page 1-10. (4655) 2-15 „T.Hansen, R.Smock, „Gas Turbines Aim at World Power Market“, periodical Power Engineering International, June 1996, page 1-9. (4650)

2-16 M.Maalouf, „Gas Turbine Vibration Monitoring - An Overview“, periodical ORBIT, Vol. 25, No. 1, 2005, page 49-62. (4654)

2-17 J.M.Robichaud, „Reference Standards for Vibration Monitoring Analysis“, www.bretech.com, page 1-10.(4653)

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2.6 The fuel

Literature of chapter 2