Experience with a Gas Turbine in anAmmonia Plant

The gas turbine drive for a process air compressor is as reliable as the steam turbine drive. Anon-line washing system, however, performs better than the off-line method, because it cleans the

compressor often.

Wolfgang RailBASF Aktiengesellschaft, Ludwigshafen/Rhein, Germany

BASF AG, operates two ammonia plants InLudwigshafen, West Germany. The newer one,designed by CF Braun, has a capacity of1360 mt/d of ammonia and was started up bythe end of 1982.

This plant has been equipped with a gasturbine to drive the process aircompressor.

DESCRIPTION OF THE GAS TURBINE USED

The model 5001 R of General Electricis used in our plant.

The machine consists of the followingmain parts (see figure 1):

1) An AIR COMPRESSOR because thecombustion takes place under pressure.

2) The COMBUSTION SYSTEM.

4) The AUXILIARY EQUIPMENT.

Air is filtered and enters thecompressor. The air compressor is a16 stage axial compressor. The air flow iscontrolled by the inlet guide vanes of the

BASF AktiengesellschaftLudwigshafen/Rh., Germany

compressor. The compressed air is thenpassed to the combustion system. Thissystem is composed of ten concentricallyarranged combustion chambers. Natural gasis used as fuel. The combustion processoccurs at essentially constant pressure.Via ten transition pieces the hot gasesare led to the turbine. In the turbinesection the enthalpy of the hot gases ispartially converted into work. The turbineis a two stage single shaft machine. In asingle shaft configuration of a gasturbine there is one continuous shaft andtherefore all stages operate at the samespeed. The hot gases enter the turbinewith an inlet temperature of about930 °C - 940 °C and enter into the exhaustwith an off-gas temperature of about545 °C. As the combustion takes place witha high excess of air the exhaust containsabout 15 % of oxygen. The control of themachine is done by the Mark IISpeedtronic. For the start-up of the gasturbine an auxiliary drive is necessary.For this purpose a Coppus steam turbinehas been provided. Table 1 and table 2give some design data. The normal shaftspeed is 4 860 rpm with an mechanicaloutput of 12 990 kW. At 110 % load theshaft speed is 4 954 rpm and the output is14 320 kW.

Figures 2 and 3 present a view of thegas turbine with the process aircompressor.

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CONDITIONS TO USE A 6AS TURBINE IN ANAPHONIA PLANT

What are the reasons to use a gasturbine in an ammonia plant? Use of a gasturbine basic!y depends on the steambalance of the plant. Many plants have nopossibility for using excess steam. Insuch plants a maximum of steam turbinedrives is provided and use of a gasturbine is not very reasonable. On thecontrary the newer ammonia plant at BASFin Ludwigshafen has been designed for amaximum steam export because there wasneed for 16 bar steam. A network withmany consumers is available.

From a thermodynamic point of viewinstalling a gas turbine is useful inprinciple as heat is used at hightemperature level in a gas turbine. Toprofit from this advantage entirely it isnecessary to make use of the gas turbineexhaust as well which has a hightemperature yet (545 °C in our case). Inan ammonia plant the primary reformersection is qualified for it. That meansinstalling a gas turbine as process aircompressor drive and using the exhaust inthe combustion chamber of the primaryreformer. Remember the gas turbine off-gashas a content of 15 % vol. of 02 and canbe used as high heated combustion air.

Figure 4 shows how the gas turbine isconnected with the primary reformer. Themain part of the gas turbine exhaust isdirectly led to the burners of theradiation zone, the rest goes to theauxiliary burners in the convection bank.This bypass-flow is pressure controlled.

The flue gas of the primary reformerenters the atmosphere at about 160 °C. Sothe enthalpy of the gas turbine exhaust isused down to this temperature level.

Before starting-up the gas turbinefresh air passes via the gas turbine stackto the primary reformer burners. In caseof shut-down of the gas turbine the valvesof the stack open and combustion air isled via the stack to the primary reformertoo. Under these conditions theprimary-reformer can be run until 50 % ofthe normal heat load without gas turbine.

Figure 5 shows the primary reformerwith distribution tubes for the gasturbine exhaust to the burners and the gasturbine stack.

OPERATING EXPERIENCE

The operation of a gas turbine islargely influenced by the followingfactors, corresponding to the experienceof General Electric:

0 Type of fuel0 starting frequency° load cycle0 environment0 maintenance

The most favourable fuel is naturalgas which contains no corrosive substancesand gives the least combustion problems.The starting frequency affects the lifetime of parts and this is longer for unitsin continuous duty service. The load is oflittle importance if load is seldomsubstantially changed. The environment maybe of influence if abrasive or corrosivesubstances are in the inlet air. In anycase solid deposits on the axialcompressor diminish the performance of themachine. So the inlet filtration equipmentis important. A careful preventivemaintenance is obviously important forlong running periods too.

For this point of view the conditionsto attain long running periods with a gasturbine are favorable in an ammonia plantand especially in our plant. We usenatural gas as fuel, the startingfrequency is low, the load of the plant ismostly 100 % and varies only between 70 %and 100 %. Our environmental conditionsare not severe and precautions have beentaken for careful inlet filtering of thecombustion air.

The following table 3 gives somefigures about the operation period of theplant and the gas turbine between 1983 -1989. In this period the plant had 21shut-downs causing the shut-down of thegas turbine. As to the reliability of themachine only 3 of the shut-downs werecaused by the gas turbine itself (3trips). Until January 1, 1990 the gasturbine had a total running time of

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57 938 hours. Over this period (1983-1989)we had 2 759 running hours between twoshut-downs on an average.

MAINTENANCE

Tables 4 -6 present the kind ofinspection and the maintenance intervalsproposed by 6E and really carried out inthe plant. The combustion check comprisesthe inspection of combustion linerassemblies, fuel nozzles, transitionpieces cross-fire tubes, flame detectorsand etc.

The hot gas path inspection includesthe scope of work for the combustioninspection and a visual inspection of theturbine nozzles and buckets. The top halfof the turbine shell must be removed for ahot gas inspection.

The scope of work of a majorinspection comprises the hot pathinspection and in addition the inspectionof the axial air compressor that meanslaying open the complete machine at thehorizontal joints. Detailed maintenanceand inspection guides are available in theGE service manuals.

As to the inspection intervalsrecommended in the manual shutting-downthe ammonia plant every 8 000 hours for agas turbine inspection was not acceptablefor us (table 5). So in accordance with GEand because of the favorable runningconditions explained above we made a gasturbine inspection only at the periods ofthe normal plant turnarounds. Table 4gives the figures. Such turnarounds haveuntil now taken place 3 times in 1984, in1987 and in 1989. The first inspection wascarried out at nearly 13 000 hours, thesecond at nearly 35 000 hours and thethird at nearly 56 000 hours (table 6).Besides the first inspection the intervalsfor the two other ones exceeded 20 000hours. In order to be able to attain thelong operation periods we immediatelycarried out the hot gas inspection insteadof the combustion inspection. The runningtime until the major inspection was about56 000 hours. We combined the majorinspection with a preventive change of therotor because a spare rotor was available.So the removed rotor could be carefully

checked without beeing in a hurry during aturnaround. The last column of the table 4presents the inspection duration. Themachine was about 4 weeks out of operationfor the rotor change only.

The former hot gas inspections had thefollowing results: No cracks were found onthe combustion parts and the nozzles andbuckets of first and second stage of theturbine. But as preventive maintenanceeither in 1984 or in 1987 fuel nozzlecaps, combustion liners, crossfire tubes,crossfire tube retainer were replaced bynew parts. In addition first and secondstage buckets were replaced in 1987.Moreover some minor repair work was doneat each turnaround. A very important pointat a turnaround was washing of the axialair compressor.

GAS TURBINE PERFORMANCE

The gas turbine performance maydecline as a result of deposits on thecompressor blades. Many problems are adirect result of a dirty or fouled axialcompressor. Fouled compressors result inreduced air flow, lower compressoreffiency and lower compressor pressureratio. Substantial types of fouling areinorganic materials and oil.

In our plant we do not use the solidcompound cleaning procedure with nutshells or rice. We fear abrasive attack onthe machine. Another disadvantage of thisprocedure is the necessity of reducing theload.

Liquid cleaning involves washing thecompressor with water and/or detergents.This kind of cleaning is done during ashut-down of our plant. Some precautionshave to be made and some internal lineshave to be blocked. Washing with water ordetergent should be done at crank speed orslower. The cleaning water temperatureshould be in the range of 60 - 70 °C.

Filtering of inlet air

Another way to maintain the efficiencyof the air compressor are good airfilters. Normally the machine is equippedwith a gravity filter. In addition weinstalled on our gas turbine fine filters

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so called pocket filters. The filterelements consist of organic fibres and canbe used up to 70 °C. The filter elementscan be replaced during normal operation ofthe plant. On an average we have toreplace the filter elements once per year.

As to deposits to the air compressorinlet blades the problem becomes moresevere if the deposits consist of amixture of inorganic material and oil. In1984 we found oil containing deposits onthe inlet blades of the gas turbine aircompressor.

The cause was the waste air of the gasturbine lube oil tank which was blown intothe machine hall and got from here vialeakages in the suction line directly tothe guide-vanes of the gas turbine aircompressor. As modification we vented thewaste air of the gas turbine lube oil tankout of the machine hall away from the airfilter. This modification gave us asubstantial improvement.

Although we used fine filters we had adecrease of performance of the machine. Onrecommendation of General Electric weinstalled an on-line water wash. For thiswater wash no change of the operationconditions was necessary. The water washis performed once per day during30 minutes with démineraiized water. Thewater is warmed to about 60 °C. The basicgoal of an on-line compressor washingsystem is to keep the compressor clean bywashing it lightly and often. Thisprovides an increase in performance overthe conventional off-line method where theperformance drops to the lowest acceptablevalue before shutting down to wash thecompressor.

Figure 5 shows the arrangement of thenozzles at the inlet guide vanes. Table 7presents the conditions. We installed theon-line water wash during our turnaroundin 1987. The operation experience showedexcellent results in maintainingperformance close to the original levels.So we acctually got no remarkable decreaseof the efficiency of the compressor untilour turnaround in September 1989. This wasa large step forward. Before that wealways had a substantial decline ofperformance in the period between two

turnarounds although we used fine filters.Some remarks: On-line water washing doesnot affect the flame detector system.There is no need to lock out the flamedetectors of the control system duringwashing and therefore full security isguaranteed. We always made sure that theair/water mixture had a temperature higherthan 8 °C in order to avoid freezing whenentering the machine. In winter we carriedout washing at the highest possibletemperature of the day and warmed up theair with preheaters while washing. So wewere able to wash at every day all overthe year.

This presentation tried to give theconditions for installing a gas turbine inan ammonia plant and the experience withsuch a machine. Our conclusion is that gasturbine drive for process air compressoris as reliable as steam turbine drive. Thechoice only depends on the specialconditions of the plant.

Wolfgang Rail

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EXHAUST

COMPRESSEDAIR

Figure 1. Gas turbine package flow diagram.

Figure 2. Gas turbine.

Figure 3. Gas turbine and process aircompressor.

PROCESSNATURAL GAS

FUEL

PROCESS f-AIR

FUEL

i TURBINE I

STAC

±

<X

f^XA/

Runj '

VENT r

BEW t

- IMIXED FEED

GAS TURBINEEXHAUST

]

I

^^

T!̂ =— s=C

X

V B>

I NATURAL CAS

IAIR PRIMARY REFORMER SECONDARY REFORMER

Figure 4. Arrangement of the gas turbine In theammonia plant.

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Figure 5. Primary reformer and gas turbine stack.

GAS TURBIHE DESIGN DATA

Gas turbine model series

Number of turbine stages

Compressor, axial flow

Combustion

Fuel

Control

MS 5001 R

2 (single shaft)

16 stages

10 multiple combustors

Natural gas

Hark II Speedtronic

Table 1 Gas turbine design data.

GAS TORBIHE PESIBM DATA 1COHTIOTEP)

Rating

Ambient temperature [*C]

Output [KM]

Shaft speed [RE«J

Approx. firing temp. [°C]

Exhaust temperature [ °C3

Heat rate [KJ/KWH]

normal

26

12 990

4 860

932

545

16 050

110 %

15

14 320

4 954

938

545

15 500

Table 2. Gas turbine design data (continued).

SHUT-DOWNS OF ftHHOHIflL PLAHT WITH SHUT-POWHS

OF GAS TURBIHE

YEAR

1983 - 1989

TOTALSHUT-DOWNS

21

TURNAROUNDS

3

REPAIR SHUT-DCMHScaused bythe gas turbine

0

THIPScaused bythe gas turbine

3

REPAIR SHUT-DOWNSin the plantnot caused bythe gas turbine

4

TRIPSin the plantnot caused bythe gas turbine

11

Table 3. Shut-downs of ammonia plant with shut-downs of gas turbine.

Figure 6. On-line water wash.

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Tumniiomros WITH IHSPECTIOHS

OF GRS TUBBIME

OHLIHE WATER «ASH OF GftS TURBINE COMPRESSOR

Inspection

1

2

3

Date

«ay 3 - May 301984

March 5 - April 81987

August 8 - Sept. 301989

[tunning timebetween 2inspections

[h]

12 694

22 149

20 882

Duration ofinspection

[h]

651

819

719

Table 4. Turnarounds with Inspections of gasturbine.

Number of nozzles

Nozzle inner diameter

Flow per nozzle

Total quanity of water

Wash cycle

Wash frequency

Water quality

7

1,5 ram

3 1/min

630 1

30 rain

1 per day

deminerallzed water

Table 7. On-line water wash of gas turbinecondenser.

FROM GE PROPOSED INSPECTIONS FOR

NORHAL COMDITIOMS AS FUNCTION OF RUNNING TIME

Inspection

Combustion

Both gas path

Combustion

Major inspection

Totalrunning

[h]

8 000

16 000

Hot gas

32 000

time

- 24 000

+ 8 000

- 48 000

Table 5. From GE proposed Inspections fornormal conditions as function ofrunning time.

INSPECTIONS CARRIED OUT IM THE PLM8T

Inspection

Hot gas

Hot gas

Major inspection(change of rotor)

RunningIntervals

th)

12 694

22 149

20 882

Totalrunning time

[h]

12 694

34 843

55 725

Table 6. Inspections carried out In the plant.

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