increased life for gas turbine combustion systems burning … · increased life for gas turbine...

INCREASED LIFE FOR GAS TURBINE COMBUSTION SYSTEMS

BURNING RESIDUAL FUEL

R. W. MACAULAY Associate Member ASME

Aircraft Gas Turbine Development Department Genera/ Electric Company, Evendale, Ohio

and

C. M. GARDINER Member ASME

Gas Turbine Department Genera/ Electric Company

Schenectady, N. Y.

Contributed by the Gas Turbine Power Division for presentation at the ASME Gas Turbine Power Conference in Washington, D. C., April 16-17, 1956.

Written discussion on this paper will be accepted up to May 20, 1956.

(Copies will be available until February 1, 1957)

Printed by the General Electric Company

The Society shall not be responsible for statements or opinions advanced in papers or

in discussion at meetings of the Society or of its Divisions or Sections, or printed in

Copyright © 1956 by ASME

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FUEL NOZZLE

AIR INLET

-

:t1 I LINER ASSEMBLY

cow(' / �INER CAP SECTION

.et2 LINER SECTION

-

SHEATHING AIR

0 000 0 0

COMBUSTION AIR

cl, d,�d, d J,

fl 3 LINER SECTION

;> ;; > ,_;; 7 r;;..... _ ___ ._, ____ _.,.

LOUVER TAB

ATOMIZING AIR

HOOK 0 I 2 3 4 5 6 I, I, I, I, I, I 1 I

SCALE - INCHES

Fig. 1. Section view of locomotive gas turbine combustion chamber with hook liner and 18-tab cap

0 5 I ! I I I. I I I ! I

SCALE-INCHES

Fig. 2. Section view of locomotive gas turbine combustion chamber with louvered liner

DISCHARGE TO TURBINE


INCREASED LIFE FOR GAS TURBINE COMBUSTION SYSTEMS

BURNING RESIDUAL FUEL

by

R. W. Macaulay(a) and C. M. Gardiner(b}

ABSTRACT

ASME Paper 48-A-109* described combustion liners and fuel nozzles which were originally used in General Electric gas turbines. Operating experience has shown certain shortcomings of these, particularly in regard to liner life and frequency of changing required by the fuel nozzles. This paper describes a new type of liner and fuel nozzle which, on the basis of limited field experience, have shown considerable improvement in these respects. It also gives a brief review of test data and operating experience on combustion liners and fuel nozzles.

INTRODUCTION

The combustion system originally used on General Electric gas turbines employed a cap and four overlapping liner sections with annular gaps at each overlap to admit cooling air along the following section. It also used an air atomizing fuel nozzle of the type known as ''pintle vortex.'' Both the liner and nozzle are described in ASME Paper 48-A-109*. An early modification of this system, shown in cross section in Fig. 1 , used what was known as the 18-tab cap and hook liner. The liner sections were made of 1 /8-inch thick Type 31 0 or 309 material (25Cr-20Ni or 23Cr-13Ni). This cap and liner, with the pintle vortex nozzle, has been used as standard equipment on all General Electric locomotive gas turbines up to the present time. Operating experience, however, has shown that in this service, burning No. 6 fuel oil, and with frequent starts and stops and rapid changes of load, both the liner and fuel nozzle have certain shortcomings.

The liner life proved to be undesirably short, and the fuel nozzle was found to require frequent cleaning and reassembly. Damage to the cap and liner con-

* Combustion System for Burning Bunker C oil in a Gas Turbine, Bruce 0. Buckland and Donald C. Berkey

(a} Aircraft Gas Turbine Development Department General Electric Compan"y, Evendale, Ohio

(b} Gas Turbine Department General Electric Company, Schenectady, N. Y.

sisted of the burning away of louver tabs, or liner "hooks" in local areas, and warping, which then led to further overheating, closing off of cooling air slots, and rapid deterioration of the next section downstream. It was found that when properly made and assembled these liners had rather good life, but the conditions required to assure this were so difficult to maintain that actual experience was poor. The caps had short life in any case and often required replacement after 200 to 300 hours of operation. The No. 2 section lasted longer and the No. 3 section usually ran 1000 to 2500 hours.

Changeout time on the fuel nozzles was chiefly set by the life of 0-rings sealing the high-pressure oil from the lower-pressure atomizing air. These would become hard and crack, permitting leakage, which resulted in formation of carbon in the air passages, poor atomization, and damage to caps and liners. Under some conditions varnish or carbon would also form in fuel spin slots, but this seldom happened when the fuel system was adequately flushed with diesel oil during shutdown. It was found that if 0-rings were changed and nozzles cleaned and adjusted every 50 to 75 hours of operation excellent performance could be obtained, but such frequent changes, of course, are inconvenient.

These disadvantages were recognized at an early date, and a continuing program has been carried on to develop a combustion system which would burn bunker C as effectively as the original one, but have longer life and require less frequent nozzle changes. This paper reports the progress which has been made to date. Briefly this includes development of a thin-walled, punched-louver liner, similar to those used in many aircraft gas turbines, but suited to this application and development of a new fuel nozzle.

Originally it was felt that locomotive liners should be at least 1/8-inch thick in order to have good life and to resist damage during shipment, handling and installation in railroad shops. This eliminated the punched-louver construction, which is best adapted to materials 1/16-inch thick or less, and was one of the chief reasons for development and use of the hook design. Experience obtained since has shown that a 1/16-inch louvered type does have adequate

3


strength, and that it can also be designed to have excellent life, at least while burning an essentially ash-free residual oil. To date all use of these liners has been in turbines burning such a fuel, and it is possible that when burning a high-ash residual, even though it has been treated to prevent corrosion, some plugging of louver openings or other impairment of liner performance will result. This is not considered likely, but the possibility cannot be ruled out until actual operation with high-ash fuels has been obtained

DEVELOPMENT PROCEDURE

For optimum performance, cap, liner, and fuel nozzle must all be designed to work together. Experience has also shown that once a liner and nozzle have been obtained which work fairly well with one cap, still further progress can often be made by modifying the cap alone. In addition, testing done in a laboratory burner is not adequate to evaluate a system completely, and must be supplemented by relatively long trials in actual service. In this case, therefore, when a liner and nozzle had been obtained which worked well in the laboratory with one cap, other types of caps were tried until several were found that seemed to give good performance. A number of these combinations were then installed, with co-operation of the Union Pacific Railroad, in gas turbine locomotives operating on No. 6 fuel, and run until a reasonable estimate of their life and performance in actual service could be obtained.

THE LINER

In cooling the hook liner it is necessary to use a small number of relatively wide annular slots, in order to minimize the effects of manufacturing variations and thermal distortion on slot dimensions. As a result the sheathing air must travel a considerable distance between slots, and it may not always provide adequate cooling on the downstream edge of the section over which it sweeps. In the louvered liner this situation is improved by admitting air through a large number of closely spaced openings, formed by punching small portions of the liner surface outward. The use of many such openings permits a nearly continuous thin sheathing film, and by proper choice of louver size, spacing, and location, cooling can be increased in local areas which tend to run hot, and decreased in regions which run cool. Since the louvers are small, well cooled, and relative to their size quite rigid, they are less affected by thermal distortion than slots in the hook liner, and are able to perform effectively for longer periods of time.

A number of louver designs and spacings were tried. It was found that square louvers (in which the opening is a straight tangential slot) gave excellent cooling in a narrow converging streak behind each opening, but between openings, the cooling was poor. The type finally selected has a crescent-shaped opening which introduces the air in a spreading

4

Fig. 3. Louvered liner for locomotive gas turbine

combustion system

sheet, and gives good cooling over a much wider area. A liner using this opening has now been developed which, with a suitable cap, has several times the life of the hook liner and is also as good or better in most other characteristics. A cross section of this is given in Fig. 2 and a photograph indicating louver shape and arrangement is shown in Fig. 3.

THE CAP

On the basis of preliminary tests the three designs of caps shown in Fig. 4 , 5, and 6 appeared to be of particular interest. These are all installed in a portion of a spherical dome, the upstream end of which contains a perforated plate which meters the air supplied to the cap.


DIRECTION OF AIR FLOW

Fig. 4. Louver cone cap

Figure 4 illustrates the louvered, cone-type cap which consists of spherical dome, air metering plate, and a louvered, conical piece. In the face of the louvered cone around the fuel nozzle there is a series of 1 /4- to 3/8-inch diameter holes which act to minimize smoke. The louvers are arranged to discharge radially outward along the inner surface of the cone. The arrangement of the louvers and the area of the louvers, holes, and metering plate are all important in the control of flame stability, temperature profile, and other characteristics.

Figure 5 illustrates the swirl louver type cap in which the louvers discharge circumferentially along the inner surface of the cone. This cap has an overlap arrangement near the nozzle mounting hole to permit air to flow radially inward around the nozzle. This also affects performance characteristics. With this overlap design, louvers seem to be more effective when directed tangentially rather than radially.

Figure 6 illustrates the flat-cap type, which consists of a flat, conical dish. At the dish's outer periphery is a reverse flow deflector ring. Air metered by the cowl plate is partly deflected by this ring toward the nozzle and in sweeping over the

MOUNTING BRACKET

Fig. 5. Swirl louver cap

DIRECTION OF AIR FLOW

Fig. 6. Flat cap

conical plate helps to keep it cool. Additional air penetrates through holes in the center of the plate, and the remaining air by-passes the outer periphery of the ring.

THE FUEL NOZZLE

The pintle-vortex nozzle utilized a spin chamber into which the fuel was introduced through rather small passages, at approximately 300-psi pressure drop, to produce a thin uniform sheet which was then atomized by a concentric blast of air. Dr. F. J. Neugebauer, in the General Engineering Laboratory of the authors' company, suggested a type in which not only the atomizing but also the initial spreading of the oil would be done by air. From this suggestion was developed a new nozzle called the "air swirl" type, which is shown in Fig. 7 . In this nozzle, fuel is supplied through rather large passages into a central chamber where it is picked up by the spinning "primary swirl air" and fed uniformly across a narrow slot near the exit, through which a high energy blast of spinning "secondary swirl air" is admitted. This atomizes the fuel and forms a relatively narrow spray, having approximately a 60-degree included angle.

This nozzle contains one spark-plug type gasket which seals the air from the fuel. No other seals are required and even a small leak at this one is not serious, so long as the pressure of the atomizing air is kept higher than that of the fuel. Since the fuel passages are large, it is relatively easy to protect them from plugging by means of a guard screen which also has openings large enough to be quite free from stoppage troubles, and the nozzle can operate with a low fuel supply pressure. It has demonstrated excellent performance for periods up to 600 hours between changes, with as many as 50 shutdowns during that interval. At each shutdown and startup the fuel system and nozzle are flushed with diesel fuel, but the remainder of the running is on No. 6 oil.

One of the problems still apparent is a tendency to form carbon on the forward face and around the

5


SECTION "A-A"

\ \

PRIMARY SWIRL AIR DETAIL "II"

Fig. 7. Air swirl nozzle

lip of the discharge opening. This has not resulted in damage to liners but its presence must, at least part of the time, have a detrimental effect on the spray, and ways are being sought to keep it from forming. There are also some indications that the air swirl nozzle does not atomize as finely as the pintle-vortex, but the performance of combustion chambers in which it is used indicates that it is generally satisfactory in this respect.

The following is a list of conditions for which it was designed.

Normal Operation (Combustion chamber pressure 50 to 75 psig)

Maximum fuel flow . . . . . . . . . . . . . . 800 lb/hr Maximum fuel viscosity .... . . .. 20 centistokes Maximum fuel temperature . ... . .. . . .. 250 F Maximum atomizing air flow . . . . . . . 700 lb/hr Maximum atomizing air pressure ratio 2.0 Minimum atomizing air pressure ratio . . . . 1. 5

Firing (No. 2 fuel only. psig.)

Chamber pressure zero

Fuel flow ... Atomizing air pressure .

PERFORMANCE DATA AND

FIELD EXPERIENCE

. 5 gph 2 psig

The following is a list of characteristics which are investigated when evaluating a combustion system. The ideal system would rate high in all of these respects.

1. Pressure drop, measured as the difference between total pressure in the steam approaching the

6

head end of the combustion chamber, and total pressure at the turbine inlet. This should be less than 5 percent of the inlet pressure.

2. Turbine inlet temperature distribution, rated by a ''traverse number'' defined as:

(Peak Temp) - (Average Temp) Traverse No. = (Av Temp) - (Chamber Inlet Temp)

0.1 or below - Excellent 0.4 or over - very poor

The"peak and average temperatures are those at the turbine inlet, and the chamber inlet temperature is that of the air entering the combustion system.

3. Flame length, indicated in an inverse manner by the temperature rise at which tips of flame are first visible in a sight port at the liner exit. For a liner which is poor in flame length, tips of flame will appear at a lower rise than for one which is good. This rise should be at least 700 F.

4. Blowout rise, defined as the lowest temperature rise at which the chamber can run without the flame being blown out. For most applications this should be 250 F or less.

5. Carbon build-up, evaluated by visual examination of the liner after an hour of operation at conditions simulating the lowest idling condition it might encounter in service. Under this condition, operating temperatures are low and atomization may be poor. Carbon formations in the order of 1/16-inch thick after such a test are considered satisfactory, although a good liner may be entirely clean.

6. Liner temperature, evaluated by oxide color patterns on the outside of the liner after a one-hour run with inlet temperature of 500 F and discharge


temperature of 1500 F using bunker C fuel. The liner, for this test, is made of Type 310 material (25 Cr-20 Ni), freshly sandblasted with No. 60 carborundum grit, and its color pattern after the test can be related to temperature by comparison with furnace treated samples. The maximum temperature indicated by this tei;;t should not exceed 1400 F.

7. Ignition, evaluated by the fuel flow required to ignite immediately when fuel is turned on in a chamber with firing air flow already established and ignition already on. This flow should be less than 40 lb/hr.

8. Smoke, measured with a Von Brand smoke. recorder, the tape from which is evaluated by a photoelectric device. The smoke meter deposits a smudge on the tape which is darker as the smoke density is greater, and the reading device measures light transmitted through the tape. A reading of zero indicates very dense smoke and a reading of 100, none. Readings above 70 are considered good, and readings below 40 are poor.

Table 1 gives a summary of performance data, obtained in the combustion laboratory, on the hook liner with pintle-vortex nozzle and 18-tab cap, and on the crescent louvered liner with air swirl nozzle and each of the three caps discussed above From this it can be seen that all three louvered combina-

tions are better than the hook liner in traverse number and wall temperature, and two of them are better in blowout rise. The flat cap is especially good in traverse number and flame length. The louver cone and the swirl caps are both poorer in flame length, and all three combinations are poorer in smoke.

Starting in the summer of 1954, sets of these liners, plus several other combinations which have since been discarded, were installed in Union Pacific locomotives at Green River, Wyoming, for field evaluation.

Life data on combinations using the caps of Fig. 4, 5, and 6 are given in Table 2. From this it seems that, either the louver cone cap or the swirl cap, a life in excess of 2000 hours should be possible on nearly all liner parts. The swirl cap assemblies happened to be the first ones installed, and were subsequently selected as the ones most likely to prove satisfactory. For these reasons a larger number of them has been run than of the other types. In the meantime, the louver cone cap has also given a very good account of itself, and there is every indication that it will last as long as the other. Life of the flat cap was limited by overheating of the reverse flow ring and warping of the conical plate, and was so short relative to that of the others that further testing of it was discontinued.

TABLE I

Pressure Drop

Traverse No.

Flame Length Rise °F

c arbon Buildup

s moke No. (0 =Max)

B lowout Rise °F

Liner Temp °F Max

I gnition (PPH) Fuel

COMPARISON OF PERFORMANCE CHARACTERISTICS

OF VARIOUS COMBUSTION CHAMBERS AND CAPS

18-Tap Cap Louver Cone Cap Swirl Louver Cap Hook Liner Louvered Liner Louvered Liner

Flat Cap Louvered Liner

Pintle Nozzle Air Swirl Nozzle Air Swirl Nozzle Air Swirl Nozzle

4. 5 - 5% 4.5 - 5% 4. 5 - 5% 4. 5 - 5%

.25 .20 .18 . 12

800 780 6 50 930

Max 1/16 in. Max 1/16 in. Max 1/16 in. Max 1/16 in.

70 35 40 35

215 150 165 200

1400 1200 1200 1200

15 20 18 21

Desirable Limit

5

. 15

700

1/16 in.

60

250

1400

40

7


TABLE II

LIFE EXPERIENCE IN LOCOMOTIVE GAS TURBINE SERVICE

LOUVERED LINERS, AIR SWIRL NOZZLES, AND FIG. 4, 5. AND 6 CAPS

Data as of December 1 5, 1 955

Combination Fired Hours on Parts Removed Fired Hours on Parts Still in Use at Time Because of Burning or Damage of Report or at Conclusion of Test

Cap & Cap & No. 1 Section No. 2 Section No. 3 Section No. 1 Section No. 2 Section No. 3 Section

Air Swirl Nozzle, 2670 (1 ) 1730 (1 ) 1730 (1) 2050 (2) 2050 (2) 2050 (2) Louver Cone Cap & 2670 (1 ) 2670 (2) 3540 (5) 2970 (1 ) 2970 (1 ) Louvered Liner. 3540 (4) 31 90 (1) Fig. 4 and Fig. 3 3540 (2)

Air Swirl Nozzle, 1970 (1 ) 2320 (1 ) 1260 (1) 2450 (2) 371 0 (1 ) 1 520 (1 ) Swirl Louver Cap & 2260 (1 ) 2900 (1 ) 2600 (1) 4040 (3) 6310 (1 ) 2470 (3) Louvered Liner. 3050 (2) 3160 (1 ) 2900 (2) 5500 (1 ) 661 0 (3) 3910 (1 ) Fig. 5 and Fig. 3 4160 (1 )

5490 (1 )

Air Swirl Nozzle, 370 (1 ) 681 (1 ) 1 600 (3) 2030 (5) Flat Cap & Lou- 430 (6) vered Liner. 1 600 (5) Fig. 6 and Fig. 3

Note: Numbers in parentheses are the number of pieces which have run the length of time indicated.

All three combinations were noticeably poorer than the hook liner in regard to smoke. Whereas a locomotive equipped with six hook liners would have an exhaust barely visible under any conditions, the stack of one equipped with louvered liners would be clean at idle, but clearly visible at full load. The amount of smoke is estimated to lie between that of the average diesel and that of a steam locomotive.

CONCLUSION

Work is continuing in an attempt to improve these combustion systems still further, particularly in regard to smoke. In the meantime, based on the data and experience reported here, the crescent louvered liner with air swirl nozzle and swirl louver type of cap has been selected to replace the hook liner as regular equipment in oil-burning locomotive turbines.


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