1996: failure of boiler feedwater pump turbine following

Failure of Boiler Feedwater PumpTurbine following Site Power Failure

Loss of electrical supply in the No. 4 ammonia plant led to a catastrophic failure of one steam drivenboiler feedwater pump. The event, the damage, and the cause are described.

D.P. Wallace, P. J. Nightingale, and A.P. WalkerICI Chemicals and Polymers Ltd., Middlesbrough, Cleveland, England

Introduction

No.4 Ammonia Plant operated by ICI atBillingham in the UK was designed for aflowsheet rate of 1,100 te/day and was com-

missioned in 1977. The plant operates on a natural gasfeedstock and is currently achieving an output inexcess of 1,400 te/day.

Summary of Incident

On Thursday, May 19, 1994 at 11:47 a.m., the planttripped due to an electrical supply failure.

The B and C steam-turbine-driven boiler feedwaterpumps were left in commission to maintain the levelin the steam drum which continued to supply steam toprotect the primary reformer catalyst.

At approximately 12:20 p.m., the decision was madeto shut down both boiler feedwater pumps. Between12:30 p.m. and 12:40 p.m., the turbine steam supplyand exhaust were isolated, casing drains were opened,and both turbines were observed to be depressured.

At approximately 12:48 p.m., a bang was heardfrom the boiler feedwater pump area, accompanied bya fire in the area of C pump train. Pieces of metal wereobserved to be thrown from C pump train between theturbine and the gearbox.

The fire, caused by released oil igniting, was extin-guished by plant personnel. The C pump train turbineand gearbox were observed to be severely damaged.In addition to the effect on No. 4 ammonia plant, thecause and effect of the interruption in power supply onthe rest of the Billingham Site are described, with par-ticular reference to safety issues.

No. 4 Ammonia Plant

The relevant sections of No. 4 ammonia plant asso-ciated with the incident are shown in Figure 1. Thisfigure shows the deaerator, the boiler feedwaterpumps, heat exchangers (C307, C303, and C506), andthe 1,500-psig (10-MPa) steam drum.

AMMONIA TECHNICAL MANUAL 23 1997

Boiler feedwater pumps

The No. 4 ammonia plant has three boiler feedwaterpumps, A, B and C, each capable of delivering 60% ofthe rate required by the plant. The A pump, normally astandby as it can be quickly started, is driven by anelectric motor, while B and C pumps are turbine-dri-ven. The turbine driving each pump is fed by steam ata pressure of 44 barg (640 psig) and a temperature of445°C (833°F).

Exhaust steam leaves each turbine at 4.5 barg (65psig) and 154°C (309°F). The normal operating tur-bine speed is 11,000 rpm and the normal pump speedis 2,980 rpm (see Figure 2).

Each pump takes water from the deaerator at 2.6barg (38 psig) and 120°C (248°F) and delivers it at apressure of 140 barg (2,030 psig).

Lubricating and control oil for each turbine/gear-box/pump is provided by a shaft-driven oil pumpwhich is located inside the gearbox housing and astandby electric-driven oil pump.

Sequence of Events

11:47 a.m. Total electrical power failure occurs.Plant personnel start making the plant safe. The B andC boiler feedwater pumps were left running at reducedrates to remove residual heat from the plant, maintaina level in the steam drum which continued to providesteam to the primary reformer to purge all hydrocar-bons from the catalyst.

12:20 p.m. Decision taken to shut the pumps down.12:30 p.m. The steam to the primary reformer cata-

lyst was isolated, and then the steam to both the B andC boiler feedwater pumps was isolated. This wasachieved by isolating the 44 barg (640 psig) steamsupply to each turbine, the 4.5 barg (65 psig) steamexhaust and opening the casing drains. An operatorrecalled later that the C pump speed indicated 1,500rpm but as the turbine had been isolated and the drainopened, the electronic speed indicator was assumed tobe faulty.

Also at this time, the deaerator relief valve wasobserved to be lifting. On close inspection, it wasnoted that the deaerator blowoff valve (PIC702B) wasfully open. This was the result of the loss of instru-

ment air (the reservoir on the plant contains enough airto last about 30 min). It was assumed that this was thecause of the deaerator relief valve lifting.

It is worth noting also that with no instrument air theboiler feedwater pump delivery control valves(FRC706 A-B-C) went fully open.

At this point, the operating team left the pump areato continue with other isolations.

12:48 p.m. A bang was heard from the area of theboiler feedwater pumps and a cloud of dust followedby a sheet of flame was observed originating from theC boiler feedwater pump train. The fire alarm was ini-tiated and two operators went to the area. The B pumpwas heard to be running (believed to be in reverse) sothe operators isolated the pump delivery (V7108) andthe pump was heard to run down and stop.

The fire on C boiler feedwater pump train was extin-guished. Seal water was seen to be leaking on the sup-ply line to the pump seal. The C pump was also isolat-ed by closing the pump delivery (V7111), however, itwas not noticed whether it was running at this stage.The C pump train was observed to be severely dam-aged in the turbine/high speed coupling gearbox pin-ion area, (see Photos 1 and 2).

3:00 p.m. Electrical power was restored to the plant.The steam dram pressure had now fallen to 40 barg(580 psig) and the 44 barg (640 psig) steam main wasat 20 barg (290 psig).

Damage to boiler feedwater turbines, gearboxesand pumps

B Boiler Feedwater Turbine, Gearbox and Pump.No external damage apart from a collapsed governorlinkage (believed to be coincidental). Inspectionshowed serious bearing damage, with repairs neces-sary to all bearings and shaft bearing surfaces on theturbine, gearbox and pump. The gearbox bull gearthrust bearing showed signs of reverse rotation.

C Boiler Feedwater Turbine, Gearbox and Pump.Visible external damage. The turbine shaft had frac-tured just inboard of the drive end bearing and out-board of the casing labyrinth. It was a classic "cup andcone" failure indicative of a single event and signifi-cant tensile (axial) load (see Photo 1).


NRV X V71Q3 NRV X V7106 NHV \ V7109

Figure 1. No. 4 ammonia plant boiler feedwater system.

Pump Highspeed Turbine Throttlecoupling cover exhaust valve chest

JJ.

Main oil pump(Geardnven)

Figure 2. Turbine-driven boiler feedwater pumparrangement.

Photo 1. lurbine drive end bearing housing andbroken shaft end.


Characteristic discoloration indicated that the 90mm (3 1/2 in.) diameter shaft had been severely over-heated (above 850°C/1,562°F) and must thereforehave been rotated at relatively high speeds withoutadequate lubrication.

The top cover of the turbine bearing/coupling hous-ing was fragmented and scattered around as was otherdebris (see Photo 3). The turbine/gearbox couplinghad disintegrated. One coupling flange was recoveredfrom 25 m (80 ft) away. The gearbox pinion was dis-placed axially away from the turbine by 50-60 mm (2in.) with sufficient force to fracture the gearbox pinionend cover (see Photo 4).

Boiler feedwater pump delivery nonreturn valves

Delivery nonreturn valves, V7106 & V7109, wereinspected after the incident. These are globe-typeNRVs with a piston guide above the valve plug (seeFigure 3).

The C valve was found not to be seated. The pistonand guide were found to be worn and operating veryunreliably when tested. The threaded valve seat wasfound to be loose, although still square and firmlyengaged in its thread. The valve was dry and clean.

The B valve was found to seat better, although wornin the guide area. The valve was full of water (nor-mal).

The nonreturn valve at the drum, V7112, was foundto be in good condition (see Figure 1).

Conclusion

It is clear that the C pump was running for sometime without lubrication before the failure. The powerfor this could only originate from either the turbinerunning normally powered by steam, or the pump run-ning backwards driven by boiler feedwater, or flashsteam.

The turbine can be positively ruled out, because (1)it was comprehensively isolated, (2) the direction ofrotation was wrong, and (3) the shaft-driven oil pumpwould have continued to lubricate the machine and thefailure would not have occurred. Note the key pointhere that the shaft-driven oil pump does not deliver oilwhen driven backward.

Positive evidence for the above item 2, "direction ofrotation," includes damage to the pinion, turbine rotorand coupling, the condition of the pump nonreturnvalve when inspected (i.e., empty and dry) and the factthat the B pump was positively observed runningbackward.

It is not clear whether the "reverse rotation" alarmcame in. If it did, it was not noticed among the manyother alarms coming in at that time.

Further evidence may be addressed from the obser-vation that the speed indicator showed 1,500 rpm priorto the incident, and it is known that the speed indicatorwould give a reading whether the pump was runningforward or backward.

The configuration of the boiler feedwater pipeworkon the delivery side of the pumps suggests that duringthe incident the B pump was driven backward bywater, and the C pump was driven backward by steam.Hence, the power developed by the C pump would begreater and soon lead to seizure (Figure 4).

Although an initial look at Figure 1 would indicatethat to get a backflow of steam/water through thepumps would require two nonreturn valves to be pass-ing (this was in fact the conclusion of the original haz-ard and operability study), a closer study indicates thatthis installation depends critically on the single deliv-ery nonreturn valve on each pump. With approximate-ly 20 ton of water between the pumps and the boilerdrum, plus the energy input from three heat exchang-ers, C307 (exit LT shift), C303 (exit HT shift), andC506 (exit Synthesis converter) which would produceheat for some time after the shutdown, there is suffi-cient stored energy between the pumps and the non-return valve at the boiler drum (V7112) to drive thepump backward and cause the damage.

Confirmatory calculations

Calculations also show that the pump will operate asa turbine in reverse requiring 90 m3/h (330 gal/min) at60 barg (870 psig) inlet pressure to rotate the pump at1,500 rpm.

Calculations also show that to pass this rate of waterthe nonreturn valve only needs to be 2 mm (0.1 in.)


Photo 2. Gearbox pinion drive end. Photo 3. Debris collected from the area.

Figure 3. ASA 1500 piston-type check valve (8 In./203 mm).


off its seat.Between the pump delivery nonreturn valve and the

boiler dram, there is approximately 20 ton of water.This volume of water alone is sufficient to drive thepumps backward for 20 min.

Figure 6 shows the forces and torques applied whenthe whole BFW pumpset was running in reverse rota-tion. Figure 7 shows the forces and torques applied asthe pump seized due to overheating. When this hap-pened with the pump set reverse rotating, the piniongear screwed itself up the bull gear helix and pusheditself out of the gear casing (see Photo 4). Also, at thispoint in time the turbine shaft fractured.

Modifications to prevent recurrence

Although these pump nonreturn valves are regis-tered items and as such are inspected every four yearsat the major overhaul, a single nonreturn valve cannotbe relied on to prevent backfiow.

The circumstances leading up to this failure arethankfully rare. This is the first complete power failureever on the plant, and had it lasted for less than half-an-hour the events would not have happened. In theshort term, up to the next overhaul, a software solutionhas been applied. When pumps are shut down, thedelivery isolation valve on the pumps will also beclosed.

At the next overhaul a second nonreturn valve of adifferent type will be installed in the pump deliveryline (see Figure 5).

Billingham Site Power Supplies

Effect of interruption in power supply

There are two main supplies to the Billingham Site.A 132-kV supply via one substation called Syntheticsubstation and a 66-kV supply via another called NewRoad substation.

At 11:47 a.m. on Thursday, May 19, 1994, NewRoad substation was deenergized by the opening of allseven circuit breakers that can feed power into thesubstation. The circuit breakers were opened by thebus zone protection scheme which is designed to pro-tect the substation equipment from bus bar short cir-

cuits (faults).Extensive high-voltage testing had to be carried out

before any power could be restored, which progressedbetween 3:18 p.m. and 8:17 p.m.

Compared to the two previous power interruptionsin the past which had lasted for less than 42 min and 1s, this interruption of several hours did not result in atotal site shutdown, which, with the exception of theNo. 4 Ammonia and its consuming plants, made restartmuch easier, as all the site steam mains were kept atoperating pressure.

This improvement had been achieved by reconfigur-ing the site power supplies to ensure that in the eventof the loss of one of the main supply points, thechances of keeping some plants on-line, and hence siteservices such as steam, had been improved.

It is notable that with the exception of theturbine/pump failure on the No. 4 ammonia plant anda number of other minor issues listed later, no safetyincident occurred.

Reason for the interruption in power supply

As stated earlier, the bus zone protection operatedwhich isolated all supplies to the New Road substa-tion. The bus zone protection operates when both theframe leakage (FL) relay and the check relay (CR) areenergized (see Figure 8). The FL relay requires anearth fault on the switchgear frame. The check relayconfirms that a fault exists by detecting any earth faulton the system. Subsequent investigations revealedthree faults/errors which, when combined, resulted inthe maloperation of the bus zone protection.

These were the inadvertent earthing of the NewRoad substation switch gear frame, the unapprovedclosing of a transformer direct earth switch, and an 11-kV cable fault due to mechanical damage.

In Reverse Order: Examination of the faulted sec-tion of cable revealed that it had suffered mechanicaldamage. On Thursday, May 19th, a mechanical diggerhad been working in an area near the perimeter of theBillingham Site. The land was soft and it appears thatone of the stabilizing outriggers sank into the soil anddamaged the cable.

It is also worth noting that the cable was not in thelocation as shown on the drawing. It was much nearer


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Figure 4. Boiler feedwater delivery lines.

New secondnon return valve

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Figure 5. New boiler feedwater delivery nonreturn valve.


Photo 4. Gearbox pinion pushed out of thegearbox away from the turbine.

Pump

Pimion Gear

Figure 6. Reverse rotation with pump driving.


Pump

j Coupling Pinnion Gear

Figure 7. Reverse rotation with pump seizes.Turbine, gears and shafts "wind up" to absorb rotational energy.

Frame LeakageCurrent Transformer

Switchgear Frame (FLCT)

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11KV Earth FaultBillingham BeckPumphouseFeeder Due toDamage by Digger

Figure 8. Summary of electrical power failure.


the surface than it should have been, and it was notsanded and tiled as indicated on the installation draw-ing. It had been installed in the 1960s, and over 30years later this poor workmanship was the initiatingcause for a major site upset.

Secondly, the direct earth connection at the incom-ing transformer, which is normally only closed toallow maintenance on the earthing resistance, hadbeen left in the closed position. No record has beenfound of this switch being closed since it was lastrecorded open. The earthing resistance is provided tolimit the current flowing in the event of an earth fault.The closure of the direct earth results in higher thannormal earth fault currents, but not unsafe conditions.

Finally, further testing revealed that there was adirect connection to earth from the switch gear framein addition to the frame leakage current transformer(FLCT). This was eventually traced to the redundantcircuit that had supplied the main compressor motoron the No. 2 nitric acid plant. It was found that theinsulating spacer separating the earthed cable glandfrom the frame had not been reinserted after the cablehad been removed. This created a second earth con-nection to the frame.

Note: The cable had been removed as part of theproject to decommission the No. 2 nitric acid projectin 1989. So, like the cable location it was another mis-take that had been made in the past that was waitingto cause a problem. All that was needed now was afault anywhere on the system.

Figure 8 indicates how these faults combined to pro-duce the effect that caused the bus zone protection tooperate.

Other site safety issues

One plant, the "Nitram" fertilizer plant, lost almostall of its lighting. The emergency backup system on allother plants worked well. A backup system has nowbeen installed at "Nitram."

The electrically operated entrance barrier to theRiverside Storage area could not be lifted to allowvehicular access.

A manually operated handle has now been installed.

Other site operational issues

Several plants had to be shut down as a result of los-ing half of the demineralized water export pumps.Also, the situation was reached whereby the site camevery close to running out of demineralized water (atmaximum rates the site only has two hours demineral-ized water capacity) due to the prolonged time that thepower was off.

Losing plants due to a loss of demineralized watersupply means they have to be shut down quickly andthis is always a risky operation.

The provision of "duplicate" electrical supplies tothe Site demineralized water plant is now being con-sidered.

Some plants were shut down due to loss of electric-driven cooling water pumps. For these plants there isno measurable safety risk, so whether backup suppliesare considered will depend only on the economic case.

DISCUSSION

Rudy Frey, M.W. Kellogg: I found this a very interest-ing analysis. I always wondered what would happen ifa check valve failed, and I think I now know. I alwaysthought the deaerator would be overpressured. I guessthat didn't happen. Your simplified diagram of the sys-tem didn't show a minimum flow recirculation, butyou did mention it towards the end of your talk. Didyou have an arc valve or a flow-control minimum cir-culation on these? I don't think it would have helped a

great deal, but it would have been of some interest.Wallace: Is that on the boiler feed pumps themselves?Frey: Yes.Wallace: Yes, there is a kickback circulation on thepump.Frey: Okay. So you just didn't indicate it. Check valvefailure is a common industry problem, and ASME hastwo committees studying this for the past eight years.


We have also been surprised in the recent past by thevolume of water between the pumps and the steamdram. In our case, it has led to problems with drumlevel control. I think this is something that we shouldwatch carefully in the design. My final comment is onthe failure position of boiler feed water control valves.You elected to fail these valves open, and I wonderedwhether that was your corporate practice. There are alot of arguments for the other way.Wallace: This depends on what you want to protectthe most. Is it the boiler?Frey: That's right. Thank you.Nightingale: Could I just add a brief comment aboutnonreturn valves? We had recognized a number ofyears ago the importance of nonreturn valves to thesafety of our plants. We identified about 30 criticalnonreturn valves, where their failure to operate effec-tively could lead to unacceptable situations and thesevalves are inspected and overhauled at every majorturnaround. All of these valves, including those on theboiler feedwater system, were inspected and over-hauled in January 1993. Apart from the fact that wehadn't recognized that we didn't, in practice, have twononreturn valves in series, what was a surprise to uswas how little they had to be opened to give us a prob-lem.Richard Wachter, Industrial Risk Insurance: Do youhave low lube oil pressure trips on your turbines?Wallace: Yes, we do have low lube oil pressure trips.

Wachter: Is there a reason they didn't function or pre-vent the turbines from operating?Wallace: The oil flow was kept by the gear box drivenoil pump, so they didn't come in when we had thepower failure.Wachter: Maybe, this was sensed on the gear box andnot on the turbine header.Wallace: It senses on the lube oil header.Wachter: You indicated that you don't have the flowwith the pumps running backwards.Wallace: With the pumps running backwards, therewas no oil flow and that's what caused the failure.They were running backwards with no lubrication atall.Wachter: I'm having trouble with that, but thank you.Nightingale: Perhaps I can help you. Most oil protec-tive systems isolate the power source. So, you eithertake the power off the electric motor or you isolate thesteam to the turbine. Because, at the design stage, itwas assumed that the nonreturn valves would preventreverse rotation, trip valves on the boiler feedwatersupply used to prevent reverse rotation of the pumpwere not considered necessary.Wachter: You indicated that the bearings show thatthe turbine had been running backwards for sometime. If it stays operating in one direction, and if youdon't reverse the direction, the shaft shouldn't snap.Wallace: The cause of the shaft snapping was the bot-tom feed water pump seizing.


1996: failure of boiler feedwater pump turbine following

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