Failure of Cooling Water CirculatingPump and Remedial Actions

An incident occurred relating to the failure of a cooling water circulating pump f or one of the twoammonia plants with the capacity of1,350 MT/d in KRIBHCO. Since March 1986, its startup in the

plant had consistently performed well at the capacity of over 110% until the incident of February1992.

P.P. Singh and D.K. PaulKrishak Bharati Cooperative Ltd., P.O. Kribhco Nagar, Pin-394 515 Dist: Surat, India

Description of Pumps

T here are eight cooling water recirculatingpumps for KRIBHCO's ammonia plants, out ofwhich six turbine-driven ones have been in

continuous service since 1986 without any problem.The design specification of the pumps are provided inTable 1.

Failure of Pump and Gearbox

One of the turbine-driven pumps (No. 2101-JB-2)intended for circulating cooling water to theAmmonia plant II broke down on March 20, 1992.The pump was in continuous operation until February20, 1992 when it was stopped for overhauling of theturbine by vendor specialist, M/S. TriveniEngineering, under a preventive maintenance programwhich included inspection of the turbine rotor, gearbox, bearings, main and auxiliary oil pumps, and tripdevices.

The overhauling of turbine was carried out from

February 20 to March 13, 1992. On March 13, thepump was started back and put into service. No abnor-mality was observed until the 18th when an abnormalnoise started coming from the gearbox, and the pumprotor bearing temperature was quite high at 77°C asopposed to the normal temperature of 55°C. The plantoperator promptly reported the matter to MechanicalMaintenance, which advised continued running of thepump following a checkup of the pump while running.

Two days later, the pump was stopped at 9:15 a.m. toattend to the tachometer which was not showing cor-rect rpm. The pump was restarted the same day at 4:30p.m. and was put on full load at 5:10 p.m. at 5,700rpm under the abnormal sound condition which wasnot considered serious. At 6:00 p.m., however, a lowcooling water pressure alarm appeared in theAmmonia Plant-II Control Room which promptlyinformed the cooling tower area field operator. Almostsimultaneously, a low C.W. pressure alarm appeared inthe cooling water tower field panel. The operator andshift engineer who immediately rushed to the pumpfound that the gearbox and the pump bearing housings

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Table 1. Design Specification of Pumps

Pump Manufacturer: Mather & Platt, India

Discharge Capacity (Turbine Driven): 5,849 M3/h

Discharge Capacity(Motor Driven): 7,507 M3/h

Discharge Head: 35 MWC

Efficiency: 90%

Pump Speed: 620 rpm

Pump Suction: Flooded

NPSH required: 6.3 MWC

KW Absorbed: 620

Turbine Rating, KW: 1,200

Liquid: Water

Temperature: 45°C

Specific Gravity: 1.0

Manufacturer of the Drive Turbine: M/S. Triveni Eng.Works, India

were on fire. It was also observed that the tachometerindicated a very high speed of 7,400 rpm, well beyondthe trip speed of 6,600 rpm. This indicated that the tur-bine overspeed trip device was not functioning proper-ly. The pump was manually tripped and a stand- bypump was placed on-line within 5 min (Figure 1). Theauxiliary oil system of the damaged pump which wason fire was stopped, and the fire was extinguished.However, flames peeped out intermittently from thebearing housings for some time after stoppage of thepump. The fire was completely extinguished within 5-

Figure 1. Cooling water flowchart: March 20,1992.Cooling water flow to ammonia plant not affected.

7 min by applying DCP extinguisher. The standbyC.W. pump was promptly lined up. The ammoniaplant was not affected and continued to run normally.

Details of Damage

The pump and gearbox were dismantled on March22 and several parts were found damaged.

Cooling water pump

The pump shaft sheared off from the edge of cou-pling side bearing housing (Figure 2); the inboardbearing housing was badly damaged (Figures 3a and3b).

Gearbox

The gear and pinion teeth were severely rubbed andthinned down (Figure 4); the gearbox top cover, bot-tom half, and bearing shells were badly damaged(Figures 5 a and 5b); the membranes of coupling

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Figure 2. Pump shaft broken at drive endbearing.between the gearbox and the pump were also damaged(Figure 6).

Steam turbine

The main oil pump and bearings of exhaust and frontends were damaged. No damage, however, occurred tothe high-speed coupling between the gearbox and tur-bine, and no damage occurred to the turbine internals.The governor (Model PG-PL) drive worm wheel wasbadly damaged.

Analysis of the Failure

The sheared pump shaft and overspeed trip devicewere examined in detail.

Sheared shaft

Two cut portions of the subject shaft containing eachof the fractured surface were analyzed metallurgicallyto find the cause of failure. The following examina-tions were carried out.

Visual Observation. Several features were observedfrom the samples: (a) rubbing marks, more prominenton one piece than the other; (b) the plastic nature ofthe pieces where rubbing marks were seen; (c) irregu-lar tear off fracture portions, more prominently visiblein one piece than the other (Figure 7); (d) high-tem-perature oxidation characteristics; (e) diameter of thesheared-off pieces (125 to 140 mm) larger than thespecified value of 120 mm.

Chemical/Metallurgical Analysis. The material ofshaft was AISI4340 alloy steel. Spot spectrochemicalanalysis was carried out on both the machined surfaceand the fractured surface. The results are given and

AMMONIA TECHNICAL MANUAL 103 1996

Figure 3a. Inboard bearing housing damaged.

Figure 3b. Inboard bearing housing damaged.

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y, ,, y ; ". " 't ». ' 7'<* " ^^ 'sfÇ*/***

Figure 4. Gear wheel rubbed and thinned down.melting and evaporation of important alloying ele-

Table 2. Chemical Analysis (Wt.%)

Element

Carbon

Silicon

Manganese

Nickel

Chromium

Molybdenum

Sulfur

Phosphorus

Specification

0.38 to 0.430.20 to 0.350.60 to 0.801.65 to 2.000.70 to 0.900.20 to 0.300.040 Max.0.040 Max.

MachinedSurface

0.430.250.731.960.980.230.0070.021

FractureSurface

0.530.290.721.880.920.220.0090.030

compared with the specified values in Table 2.There is a noticeable reduction in nickel and chromi-

um and an increase in carbon and silicon on the frac-ture surfaces in comparison to the machined back sur-face. This indicates that there has been some localized

ments. The nature of the rubbing marks on the fracturesurface visible is an indication of the flow ofplastic/semimolten/molten material. This flow onlyhas been responsible for the larger diameter of thepieces. It is inferred from these observations that therehad been intense heat near the fracture surface.

Micro examination and Hardness Measurement.From the microstructure of fractured surface at amagnification of 200X, martensitic structure can beseen (Figure 8). This is indicative of quenching from avery high temperature. The microstructure of the por-tion at a location far away from the crack shows typi-cal quenched and tempered structure. This is represen-tative of alloy steel manufactured for pump shaft.

Hardness measurements taken on the fracture sur-face and on the backside machined surface give thefollowing results: machined surface =19,20,20 HRC;fracture surface = 47,44,45 HRC.

The above observations lead to the following infer-ences: (a) the shaft has been metallurgically sound; (b)the shaft has had abnormally high temperature particu-

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Figure Sa. Gearbox top cover badly damaged.

Figure 5b. Completely womoiit portion of pinion teeth gearbox bearing and other parts.

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Figure 6. Damaged coupling membrane between gearbox and pump.

Figure 7. Closer look at one of the fracture surfaces.

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Figure 8. Structure of the ligament B close to thefracture surface (200X).

larly at the place where fracture has taken place; (c)crack initiation and final fracture have occurred at aplace where a very high hardness phase has formed.

Overspeed Trip Device

During the reassembly of gearbox and turbine, theoverspeed trip device was found defective. The worn-out drive wheel on the Woodward governor was alsoreplaced. The governor was tested before assembly.The test run for turbine was done independently forthe overspeed trip.

Sequence of Events Leading To Failure

From the investigation results, it is seen that thepump shaft was of proper design and there was nomaterial abnormality. The failure most likely occurreddue to malfunction of the overspeed trip device andgovernor of the turbine. After overhauling of the tur-bine, the pump was restarted on March 13, 1992 butfive days later abnormal sound was noticed from thegearbox. Also, the tachometer was not indicating prop-er speed.

It may be inferred that abnormal sound from thegearbox was due to running of the unit at a higherspeed than normal. It is likely that due to operation athigher speed for about two days during which timethe turbine was stopped for checking and recalibration

of the tachometer, the drive wheel of the governor wasdamaged resulting in malfunction of the governor dur-ing operation.

When the pump was stopped on March 20, 1992,the tachometer was set right. However, the governorand the overspeed trip device could not be taken noteof for their malfunctioning in spite of due provisionfor the overspeed test under the preventive mainte-nance schedule. Therefore, when the pump wasrestarted after rectifying the tachometer the turbinemight have oversped. At higher speed, movement ofthe pump rotor caused gear teeth to rub resulting intheir thinning down and generation of high heat lead-ing to the fire. The pump shaft in the process got mis-aligned and overheated, and finally sheared off.

From the chemical and metallurgical tests carriedout on the broken shaft, it is inferred that there was nomaterial/metallurgical defect in the shaft responsiblefor the fracture. Hence, it is concluded that the shaftfracture is an aftereffect of mechanical failure.

Remedial Measures

The following measures have been incorporated tofurther improve the built-in safety of the pump:

(1) A single channel vibration monitor with an alarmstrip and a velocity sensor, made by IRD mechanaly-sis, was installed to measure and monitor the vibrationof the turbine.

(2) A thrust wear trip supplied by TriveniEngineering Works was installed in a turbine consist-ing of a nozzle, a nozzleblock, a thrust wear trip relay,a pressure gauge, and a pressure switch. This systemtrips the turbine at a high thrust. The thrust is sensed interms of oil backpressure as shown in Figures 9a and9b. The pressure switch is used as an alarm of back-pressure of oil before tripping the turbine, i.e., thealarm is set at 2.0 kg/cm* and trip relay actuators areset at 2.6 kg/cm* of oil backpressure.

(3) The constant level oiler tube was increased from1/4 in. (6.35 mm) to 3/8 in. (9.53 mm) size to avoidchances of clogging.

(4) The level of vigilance by field operator has beenfurther improved.

(5) After every major overhauling of the turbine, ithas become mandatory to run an overspeed trip test

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ranunoH PCT-UPBOUCHON GEAR

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REF. DESCRIPTION

TUBBIHE COVH. EH1> BRG.ÏUBBDIB EXHAUST EHP HBO.PINION SHAFT* BBCk VJS3SSX END.ronoN SHAFT BBC. ALTESHATOK BID.GEAE WHKB. SHUT BHQ. TUHBWB BID.O&IH 1BQBEL SHAfT BRQ. AI3KBN£lYffi £ND.ILiraOUIDR BBC. DBIVXH BID.U.TXBMUOR BBS. IK» DSOVHC DB).

VffiRATION MONTTER

0-28 MM/SÏC BUS (AUTO RISST)OR mm srnnre: IE.O nu/a» BUS

mp sKnma-. IB.O mi/aac aioBT PANEL PUSH BUTTON)

SKTTDIG)is ew or TBK SHUTDOWN

Figure 9a. Vibration alarm and trip system.

DISC

HIGH PRESSURE OILTO REUT CÏTJNDER

(To b« Sent afterTripping by PullingOut tin Pin)

Figure 9b. Thrust wear trip (oil circuit) (axial displacement).

AMMONIA TECHNICAL MANUAL 109 1996

before going into operation.(6) Before the assembly of the governor, testing and

calibration of the governor in a specially availableWoodword governor test stand has been carried out.

Conclusion

Generally, the cooling tower equipment does not getthe same attention as the main plant equipment. Majordamage to a cooling water circulating pump occurred

in spite of the fact that KRD3HCO has a very strongNDT-based, predictive, preventive maintenance sys-tem. The pump failed, because the maintenancerequirement of the governor and overspeed trip devicewas overlooked when the turbine was overhauled aftersix years of operation. The turbine was coupled withthe pump without the overspeed test and governoroverhaul, which is a normal practice. If the overspeedtrip device had been overhauled and the turbine over-speed trip tested, the incident could have beenavoided.

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