Synthesis Gas Compressor Failure:a Case Study

Unconventional repairs were carried out on one of the barrels ofsyngas compressor due to lack of spare diaphragm assembly.The rotor was a total wreck and diaphragm badly damaged, butthe thrust and journal bearings were found to be intact.

Prem Kumar and P.S. GrewalNational Fertilizers Ltd., Bathinda, India

Normally the indication of increasedaxial displacement if not cared for intime,may lead to the failure of the thrust bear-ings and thereafter rubbing of the comp-ressor internals.

What happened on our compressor on22nd May, 1986 was unimaginable. Our syn-thesis compressor train consists of threecentrifugal compressors directly coupled to16 MW, 11500 r.p.m. turbine. All the threestages discharge through inter-coolers/aftercoolers (See the flow diagram Fig. l ) »The 3rd barrel houses the final compressorstage & a recycle stage also. The suctionto the recycle stage is from the dischargeof final stage through an intercooler andfrom synthesis section. The two sectionsin the third-barrel are separated by aninternal diaphragm which is designed towithstand normal pressure differential.

The rotor in each of the barrels has abalancing drum to take care of the axialthrust* A loadcell and an axial displace-ment measurement probe is provided on everybarrel of the compressor. The load-cellreadings and the axial displacement aremonitored on the panel in Control Room.Each bearing of the compressor has probes

National Fertilizers LimitedBathinda (Panjab) INDIAPin code - 151 003

for on line monitoring of the vibrations &the display is on the control room panel.Right from the time of commissioning whilethe thrust in 1st & 3rd barrels was actingtowards active side,it is used tobe towardsnon-active side in 2nd barrel.

The steam to the turbine is suppliedat 100 ata & 480«C. The steam extractionis taken at a pressure of 40.5 ata and at atemperature of 375°C. The extraction linehas a hydraulically operated quick closingnon-return valve & an isolation valve»

SEQUENCE OP EVENTS;

The compressor which was running abso-lutely normal at 98$ load, tripped on sealoil tank level low. On checking, it wasfound that the back plate of the low Press-ure Seal Oil(L.P.S.O.)pump had cracked fromwelding and the oil leaked out through it»Attempt was made to start the second pump,but it tripped on low suction pressure -this was evidently because of the reasonthat the first pump*s suction isolationvalve had not been closed*

When the compressor was re-startedafter making up oil (which got drained), asthe machine reached the minimum governingspeed, abnormally high vibrations(4,6 mils)were observed against the normal value of2.2.mils on which it was running earlier.

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Moreover, it vas observed that axialdisplacement reading was -f20 mils againstearlier normal value of (-)3 mils. It wasfurther observed that the vibrations hadthe tendency to increase with increasedr.p.&i.

The machine was as such tripped & itsbearings were taken up for checking»Interestingly, both sides journal bearingsand the thrust bearing were found to bealright. The clearance in the thrustbearing was also within permissible limits.

Compressor was again rolled, but againit was having high vibrations. In factcompressor tripped on high vibrations whilecrossing the critical speed. Once again themachine was started, but it tripped at10,000 r.p.m. without any indication on thepanel. As no explanation was found forfailure of the L.P.S.O., it was consideredthat may be the machine surged, beforetripping and there could be obstruction inthe suction of the compressor. A punchedplate strainer in the second barrel suctionwas opened. In addition to some tube plugs& welding electrode stubs, some fine metalchips were found and that was a sureindicator about something seriously wrongin the compressor.

DAMAGE & REPAIRS;

The compressor was dismantled,following damage was observed s

The

(a) Grub screw of the locknut for thrustcollar was found broken ft the lock nuthad unscrewed slightly ft was thus notpressing against the thrust collar(See Figure - 3);

(b) Rotor shaft ft imp ell ars were foundbadly abraded ft beyond repair ( SeeFigure - 4 ft 5)}

(c) Impellar seal labyrinths were damagedft their debris was found fused on theImp ell ars ft diffusera;

(d) The diffusers were badly distorted ftthere was a gap of 1 to 3 mm along theparting plane (See Figure - 6). Thediffusers were twisted in the planeperpendicular to the shaft axis also(See Figure - 7 ft 8); ft

(e) There was rubbing onbalancing drum also.

the face of

No spare diaphragm bundle wasavailable from any of the plants. Sparerotor was, however, got from a sister-unit.The only way to bring back the machine online was to repair the diaphragm.

fo correct the distortion of the dia-phragms, they are pressed in a 50T press ftheated by oxy-acetylene flame. Thiscorrected the distortion to within 1 mm.The parting plane faces of the diaphragmswere built up by welding with low heatelectrodes ft thereafter machined ft lapped.The labyrinth grooves were built up ftmachined. Light machining cut was given onthe face of the diaphragms. The compressorwas assembled ft the total downtime was tendays*

PROBABLE CAUSES»

(a) Since the commissioning of thismachine, the axial thrust has beenacting towards discharge side. Itappears that due to the thrust actingtowards discharge side, the thrustcollar was exerting force on the nutwhich got loosened ft the thrust collarshifted towards suction side. Thisalso explains the reason for no damageto the thrust bearings while the wholeof rotor got rubbed.

(b) It can further be safely presumedthat during the earlier overhaul ofthe machine in October, 1985, thepositioning of the rotor was not doneas per specifications & it was runningnearer to the discharge side than tosuction side. As soon as the machinetripped, because of L.F.S.0. failure,the barrel contracted faster thanrotor, this combined with the shiftingof thrust collar caused the rubbing.

(c) After the failure, during investi-gations,it was reported by the operat-ing personnel that high vibrations hadappeared on the compressor 1 0 - 1 2minutes after the machine had trippedand it was found to be running at 1500rpm. The quick closing-cum-non-returnvalve on the extraction line has ahistory of passing steam. It ispossible that after the tripping, themachine remained spinning at criticalspeed for sometime ft the vibrationswent high ft remained un-notieed. Beingthe shift change-over-time, the iso-lation valve on the extraction line

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could not be closed immediately. Thushigh vibrations at critical speed forlonger period could have caused theloosening of the lock nut for thethrust collar« Due to this» the rotorwould have shifted towards the dis-charge side without causing any damageto the thrust bearing pads*

(d) The failure of back plate of theL.F.SeO, vas attributed to the weldingfailure of the plate. This pump hadbeen recently installed.

REMEDIAL MEASURES?

(a) The diameter of the balancing drum wasreduced by 10 mm & a new modified bal-ancing drum labyrinths were provided.In fact,these labyrinths were suppliedby manufacturers in the earlier yearsafter commissioning when it was obser-

(b)

ved that the thrust is acting towardsdischarge side. It was not installedearlier as no problem faced in runningexcept for the resultant thrust actingtowards discharge side. This hadresulted in the residual thrust actingtowards suction side. The advantageof this lies in the fact that when thethrust acts towards suction side, thethrust collar is abbuting against astep on the shaft rather than towardsa nut on other side (See Figure -9).

A new modified design of quick-closingnon-return valve is being provided onthe extraction line. Moreover, theoperation group has been asked toensure that the main isolation valveon the extraction line is immediatelyclosed after tripping to prevent backflow of steam & running of compressorat critical speed for long period.

P. Kumar

NOR. FLOW=r7otjJiy.MAX.

P.S. Grewal

NET: 145-t/hv.MAX : 181 't/f.*-.

E*T. Pf. HIGH TRIP:4T5?tiEXT (V- Low TjKUP-'57-5«ij.

NITROGEN WASHrw

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Figurel. Synthesis compressor.

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SPEED

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POWERM-W

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11, 100

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11550

14.53

not

MAX

12,127

15-94

COMPRESSOR

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2»dBCL-4o7e*,

3rdßCL-4o7

RECYCLE

AHTISURaEVALVE

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FIC-60IZ

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COMRESSIOÜRATIO

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Figure 2. Capacity of the system.

•»•«*•'' A,

Figur® 3. Grub screw of the locknut broken andunscrewed.

Figure 4. Rotor shaft and impeller abraded.

110

Figure 5. Rotor shaft and impeller: beyond repair.

Figure 7. Diffusera twisted.

Figure 8. Diffusers twisted.

AXIAL. DISP. PROBE. D'SC THRUST CoLMR.

kt.iMPE.iLAR,

NUT/ STEP ON SHAFT

Figure 6. Gap in diffusers. Figure 9. Second barrel rotor with fixed thrust collar.

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