CONSEQUENTIAL DAMAGE AND INSPECTION TECHNIQUES

FROM LACK OF LUBRICATION, STEAM TURBINE CASE

Author: Mr. Teerapat Chaocharoen

Electricity Generating Authority of Thailand, Bangkruai, Nonthaburi 11130, Thailand.

02-436-8789 ext.6958, teerapat.ch@egat.co.th

Corresponding Author: Mr. Kobchai Wasuthalainan

Electricity Generating Authority of Thailand, Bangkruai, Nonthaburi 11130, Thailand.

02-436-8789 ext.6958, kobchai.w@egat.co.th

Corresponding Author: Mr. Varach Sangthongstid

Electricity Generating Authority of Thailand, Bangkruai, Nonthaburi 11130, Thailand.

02-436-8789 ext.6958, varach.s@egat.co.th

ABSTRACT

Lubricating Oil system in Steam turbine is one of the most important systems which use for decreasing

friction between rotors and bearings and also, for dissipating the heat occurred from the rotation of steam

turbine. The problem of insufficiency of lubricating oil usually occurs in the power plant where there lacks of

appropriate operation, inspection and maintenance processes. This kind of problem will cause the severe

damage to the major equipments such as turbine rotor crack, rotor bend etc. To solve these damages,

maintenance engineer needs to use many complicated methods which take a longtime to investigate and

complete. This paper describes the root cause of lack of lubricating oil, the damage which usually found on the

rotor and blades, processes of investigation and analysis, renovation and reparation methods base on

Electricity Generating Authority of Thailand (EGAT) experiences to deal with the situation, and finally the

recommendation for avoiding from this problem.

Keywords: Lubricating System, Steam Turbine, Maintenance, Inspection, Damages

1. Introduction

Lubricating Oil System is a unit of lube oil substance which have many advantages in

the steam turbine. There are 3 subsystems of lube oil in the steam turbine which call High

Pressure Oil, use for controlling the system when the power plant was tripped, Jacking Oil

System, use for lifting the turbine shaft about 0.10 mm and reducing torque when you need to

rotate the steam turbine and finally Bearing Oil System, use for reducing friction between

rotating surfaces and dissipating the heat which generated from steam turbine rotation as

shown in figure 1. This system also have many equipment such as lube oil cooler, oil pumps,

and emergency oil pumps etc. as shown in figure 2. This paper describes about the

insufficient lubricating oil in function of Bearing Oil

Figure 1 : Lubricating Oil System

Figure 2 : Lubricating Oil Components

When the Bearing Oil is insufficient, it will be effected to the equipments. The lack of

steam turbine lubricating system only 2 or 3 seconds would cause the severe damage to the

major equipments as shown in figure 3 such as turbine rotor crack, rotor bend, heavy rubbing

etc. which may loss of time and money for renovation. Within 5 years, there are many cases

in Thailand which the lack of steam turbine lubricating system problem occurred. Base on

Electricity Generating Authority of Thailand experiences, EGAT also adapt the inspection

program, investigation and analysis process, for reducing the inspection period and making

the standard to deal with this situation.

Figure 3 : Overall damages of steam turbine caused by insufficient lubricating oil

2. Root Cause Analysis

The lack of steam turbine lubricating problem usually cause from the simple problem

which can be categorized into 3 main reasons. First is inappropriate design from

manufacturer. When power plant got accidents, emergency oil pump has unsuitable delay

time for starting up the equipment. It is just only 3 seconds but sometime may cause the

damages to steam turbine from insufficient lube oil. Second is inappropriate operation and

testing programs. When power plant is operated, the operator should perform the equipments

function test such as confirm the Unit Power System (UPS), and should monitor the

conditions of equipment such as temperature and vibration etc. These activities will confirm

the availability of equipment. When the power plant got the unexpected events, these

equipments must be started automatically. Finally is the inappropriate maintenance processes.

After the offline inspection period, the equipments should be able to work. Sometime, based

on the limited outage period, the power plant has been operated before the equipment testing

is completed. The equipments are unable to use and may cause of lack of lubrication

problem. In the other way, some equipments have not be planned to inspect such as lube oil

pipe. It may get accidents and effect to the system in future. Normally if these reasons have

been eliminated, the lack of steam turbine lubricating problem will not be occurred.

3. Investigation Process.

Steam Turbine Engineering Section, EGAT has adapt the inspection process and

adjustment criteria for dealing with this case as shown in figure 4. This process can be

classified into 2 parallel activities. First activity is finding the root causes of the failures and

solving the problems, they usually come from 3 reasons as mentioned above. The other

activities are finding the damages and way to retrofit them, which effected from the lack of

lubricating.

Visual Inspection

Solve the root causes

Yes

No

Run Out Check NDT Test

- Rubbing- Parts Damaged

- Hot spotting points- Scratch

- Rotor Bow - Cracks- Microstructure

- Hardness changed

Lack of Steam Turbine Lubrication

Find the Root Causes

Damages Inspection & Repair

Inspect - Lube Oil Strainer

- Bearing Temperature- Place on Turning Gear

Prepare for Starting Up

No Indications

Inspect- Indicated Bearings

Have some indications

Acceptable

Unacceptable

Disassembly- All unacceptable

components

Fixed Damaged Parts

Acceptable

Back to Operation

Unacceptable

Inspect- All Parts

- Operations- Designs

- Equipments Failure

Figure 4 : Lack of lubricating inspection processes

EGAT categorizes the second activity to 3 level of inspection. The first level

inspection is called “condition inspection”. This inspection should be done by operator or

power plant’s owner which inspects the lube oil strainer, trend of bearing temperature and the

condition of steam turbine rotor when placed on turning gear. The judgment criteria for

restarting the machine are 3 indicators. First, the operator should not find anything in the

strainer basket especially the white metal scraps. Second, the bearing temperature trend

should not exceed the bearing limit temperature (trip setting point). Finally, the steam turbine

rotor must be placed on the turning gear automatically or the rotor can be manual turned

smoothly. If one indicator was found, the power plant should not be start and need to be

inspected by level 2 inspection processes.

Figure 5 : First level inspection indicators

The second inspection processes are called "severity inspection". In this step,

maintenance team should inspect the bearing conditions and estimate the turbine clearances.

All turbine, generator and reduction gears bearings should be inspected and found the babbitt

melting or burnt surfaces. The judgment criteria in this step are 3 indicators as follow. If one

of the equipment's fall of the criteria, this equipment need to be checked by the last inspection

processes.

1. Journal bearing babbitt conditions : If the bearing surface was melt over than 0.2

mm, the power plant should not be start and need to be inspected by the last inspection

processes. Unless the bearing should be replaced or dressed.

2. Thrust bearing babbitt conditions : The criteria is the same as journal bearing but

after the bearing replacement or dressing, the thrust bearing clearance and the axial clearance

between moving part and stationary part must be checked by moving the rotor in axial

direction. These clearance must be in the specification. Unless the power plant should not be

start and need to be inspected by the last inspection process.

3. The radial clearance inspection : When this power plant need to be started, the front

and end of equipment radial clearances need to be checked. This inspection is using for

estimating the rubbed or damaged surfaces which effect to the steam turbine or generator and

using for evaluating the performance loss from the excess clearances

Figure 6 : Second level inspection indicators (Unacceptable)

Figure 7 : Second level inspection indicators (Can be accepted)

The last inspection is called "damages inspection", in this step the power plant need to

disassemble for finding the overall damages which caused by this problem. The damaged

equipment should be removed and inspected. Only the steam turbine, the inspection processes

are done by non-destructive test. The damages will be found as follows.

1. Rotating Parts :- Steam turbine Rotor, Blades, Stub Shaft etc.

- Rotating seal & other parts rubbed damages : These damages occurred from the

dislocation of the rotor caused by the bearing thickness loss.

- The rotor hot spotted : These surface damages occurred because of the exceed heat

which generated from rubbing. The microstructure at hot spotted point must be degraded.

Figure 8 : Rubbed and hot spotted damages.

- The turbine blade permanent deformation or cracks : These damages occurred form

the exceed force which got from rotating and rubbing when the rotor is dislocation.

- The journal scratch and cracks : These damages occurred from rubbing surface

between journal and bearing.

Figure 9 : Blade permanent deformation and journal scratch damages

- The bended rotor : Since the heavy rubs occur at main rotor body, the rotor may

bended because of the excessive heat at rotor surface material.

Figure 10 : Bended rotor characteristic

- Foreign Object Damages : Since some parts got rubbed, the breakaway parts may

attack the turbine blades or rotor.

Figure 11 : Foreign Object Damages on rotating blade

2. Stationary Parts :- Stationary blade, Diaphragm or Blade Group, Oil deflector etc.

- Stationary seal & other parts rub damages : These damages occurred from the

dislocation of the rotor caused by the bearing thickness loss.

- The stationary blade permanent deformation or blade crack : These damages

occurred form the exceed force which got from rotating when the rotor dislocation.

Figure 12 : Stationary seal and blade damages

- Foreign Object Damages : Since some parts got rubbed, the breakaway parts may

attack the stationary blades or main equipments.

Figure 13 : Foreign Object Damages on stationary blade

When the condition inspection program is completed, EGAT can conclude the overall

damages of the main parts as shown in figure 14 and finding the way to recondition the

power plant.

Figure 14 : The conclusion of turbine damages which caused by lacking of lubricants

3. Repair Process.

After the damages were detected, repairs process must be done carefully base on the

damages classification. For EGAT experiences, EGAT perform the repair processes as

follow.

- Rotating & Stationary Seal Damages : The common process is seal replacement.

The replacement steps are removing the damaged seals, confirming the seal slots and re-

machining (if needed), installing the new one and finally adjusting the seal height for setting

the clearances.

Figure 15 : Seal replacement processes

- Rotor hot spotted : The countermeasures of this damages are removing the hot

spotted areas or stress relieving treatment. First, remove the hot spotted areas. These areas are

removed by machining. The machining will be limited to 0.1 mm, after this step we perform

the hardness inspection compare with the other surface with have not got any significant sign

of hot spotted. If the hardness is higher than 1.2 times of cleaned surfaces, the machining

should done and hardness test should be repeated until the hardness value is acceptable. The

other process is stress relieving, the hot spotted area will be recovered by using heat treatment

process to transform the bad microstructure to normal microstructure.

Figure 15 : Machining and polishing processes

- The turbine blade permanent deformation or crack : For the permanent deformation

problem, if the deformation is unaffected to the axial movement of steam turbine, EGAT

usually leaves the damages and monitors the damages. Unless these parts must be replaced.

For the crack problem, if the crack is not over than 5mm, EGAT usually grinds the crack

away and confirms by NDT. Unless the welding and heat treatment or replacing new parts

must be done. The welding process must be carefully controlled for avoiding other damages

such as material degradation, other crack growth.

- The journal scratch and crack : In the scratch problem, EGAT countermeasures by

calculating the surface stresses and pressure value. These values use for comparing with the

OEM criteria. If these value is over than the criteria, the surface machining should be

executed. The acceptance of journal surface roughness is grade number N7. For the crack

problem, EGAT perform the crack removing by machining. The process of crack removing is

same as hot spotted removal process as mentioned above. The critical criteria is minimum

size of journal or maximum thickness which can be removed. Normally the journal diameter

can be reduced about 10-20 mm. If the journal diameter loss more than 20 mm, welding

processes must be performed.

Figure 16 : Welding and post weld heat treatment processes

- The bended rotor : For this problem, if is the rotor run out is not over than 0.10 mm,

we perform only balancing. Unless the rotor straightening process should be performed. The

First is the stress relieving process in vertical direction, and perform the replica, run out

testing. If the microstructure and run out value is unacceptable, the hot spot heating process

will be performed. After the hot spot heating process, the stress relieving, microstructure

check and run out check should be done consequently. These process must do repeatedly

until the bend damages are eliminated.

Figure 17 : The stress relieving and hot spotted heating process

- Foreign Object Damages : In the case of these damages, EGAT usually performs the

cold bending by using the copper rod as much as possible and polishes the deformed surfaces

by using sand papers. For the stationary blade, EGAT will add the welding process to this

countermeasures if the bended surface or the damages still appear.

Figure 18 : Rotating blades cold bending processes

Figure 19 : Stationary blade welding processes

- Bearing damages : EGAT will solve this problem by reconditioning the babbitt and

machining the rough surface away until meet the criteria.

Figure 20 : Bearing reconditioning processes

After this damages and the root causes have been solved, this steam turbine is ready to

commission. When the steam turbine is started or in commissioning periods, operators should

strictly monitor and control the steam turbine condition such as pressure, temperature etc.

corresponding with vibration analyzer.

4. Conclusion

When the steam turbine has been faced with the lack of lubrication problem, there are

2 processes to deal with this problem. First is finding the root causes processes. Normally this

problem is generated from 3 under standards programs, poor design, poor operation and poor

maintenance. Second is finding the condition of damages and how to recondition the

damaged parts. In this process, EGAT has set the condition inspection program which has 3

step of investigation. First is condition inspection, second is severity inspection and last is

damages inspection. After all damages were detected, the special techniques will be applied

to solve these damages.