Steam Turbine Bearings | Turbine Fundamentals and Basics | Basic Aspects of Steam Turbine Maintenance

Steam Turbine Bearings

BEARINGS

The first activity in the Turbine Capital Overhaul is the checking  of the Turbine bearings.

1.) Turbine Bearing Checks:-

All the turbine bearings are checked for following:-

A.) Visual : No deep Grooves, aberrations. Excessive porosity and localized dents are allowed and are the reasons for rejection of the bearings.

B.) Bridge Gap reading : It is the reference of the axis of the shaft with respect to the parting plane of the pedestal.

It is measured by a special Fixture which is unique . Location at which this Fixture is to be placed on the bottom pedestal is marked on the pedestal. Bridge gap reading is very important when a new bearing is installed.

C.) Bore checking : It is measured by inside micrometer in three locations, vertical and two location at horizontal in cross fashion.

D.) Top Oil Clearances: Top oil clearance is measured by means of outside micrometer by measuring the thickness of the lead wire pressed between the top of the journal  and top half of the bearing, when both the bearing half’s are bolted tightly.

E.) Side Oil clearances: Side oil clearances are measured by means of the Feeler Gauges at four corners of the bottom half of the bearing with journal in position.

F.) Bedding: It is observed by putting blue on the Journal and then rotating the journal on the bearing bed by full rotation . After this Rotor is lifted and impressions of the blue on the bearing Babbitt is checked which give the indication of bedding.

-Less bedding will lead to more oil leakages from the side of the bearing.

– Extra bedding will lead to more friction loss.

G.) Dye penetrant checking: It is carried out to check any surface cracks/abnormality in the bearing. It can also give indication to Un bonding in the visual joints of the bearing Babbitt with the Shell.

H.) Ultrasound checking: It is done to check the complete de-bonding if any in the whole bearing.

If any of the bearing fails in having Higher Clearances OR in Dye Penetrant testing OR in ultrasonic  testing then that bearing is discarded and to be replaced with the new/Spare one.

2.) Turbine Bearing Types:-

2.1) According to Oil inlet:-

• Cylindrical Bearing( Single wedge ring) This has single oil inlet
• Elliptical Bearing( Double wedge bearing) This has double oil inlet
• Segment Bearing( Multi wedge Bearing)

• Cylindrical bearings are normally used for system where no transients are envisaged particularly in turbines without controlling stage, whereby one side radial impulse due to steam forces is not there.
• Multi wedge bearings are used by installing bearings in segments . Each Segment will have its own wedge.
• Multi wedge bearings can take more load ,can dampen the sudden disturbance on shaft and there is no formation of Oil Whirl and low frequency vibration components.

2.2) According to Torous/Pedestal Contacts:-

Three types of contacts are there

1. Cylindrical-cylindrical
2. Spherical- spherical
3. Spherical -Cylindrical

Cylindrical-cylindrical contact can be obtained by the manual scrapping, but Spherical- spherical and Cylindrical- Spherical should be brought with the machining only. In Urgency spherical-spherical can be rectified at site provided correction is very small.

3.) Interference of the Bearing with The Pedestal:-

• Interference is kept to ensure proper loading on the Bearing Top to ensure its proper clamping which helps in the damping of the bearing Vibration.
• It is in the limit 0f 0.15 to 0.35 mm
• This check is carried out after the completion of the TOC checking of the bearing.

4.) Overshoot Oil Groove of the Bearing:-

Overshot Oil groove is very- very important and it needs to be checked whenever a bearing is rebabbited as sometimes due to mistake,  machining of this groove  gets neglected in the shop. This overshot groove helps in allowing the sufficient quantity of the lubricating oil into the bearing wedge area and avoids the overheating of the bearing.

OVERSHOT OIL GROOVE IS VERY-VERY IMPORTANT AND ALWAYS CHECK FOR IT

• While measuring S.O.C ensure that the feeler should go into the gap neither less and nor more than                  appx. 25 mm.
• Never Exchange the bearing shell of thrust bearing with the yoke of other bearing and vice-versa.             Always use their Original set in pair.

5.) Check List follow during Turbine Bearing Inspection:-

1.) Acceptance of Visual, DPT & Ultrasonic results should be strictly as per OEM/ASME standard.

2.) Even If, any one half of old bearing fails then bearing is to be replaced with both the halves together ( Never replace with single half)

• If any old bearing fails then replace with New/ Rebbabited bearings (both the halves needs to be replaced)
• If New / Rebbabited bearing is used then ensure blue matching of pads with respect to their female seats.
• Blue matching of pads of old good or New/ rebaabbited bearing with pedestal should be more than > 90 % with all the sides touching all around .

3.) The difference in the values of SOC and TOC when measured from two sides (front and rear) of bearing should not be more than 0.05 mm.

4.) No of shims in each pad should not be more than 4 nos. If it is more , substitute with suitable equivalent sheet of stainless steel only.

3.) Bearing should not be touched with hand, S.O.C and T.O.C of bearing should be made on machine only. In no case Babbitt to be removed manually more than 0.1 mm ( across diameter). Thick removal of Babbitt material manually will not ensure proper converging wedge and can damage the bearing during running.

4.) While measuring S.O.C ensure that the feeler goes into the gap neither less and nor more than approx. 25 mm.

5.) Never Exchange the bearing shell of thrust bearing with the yoke of other bearing and vice versa. Always use their Original set in pair.

6.) Thumb Rule : In any bearing the side oil clearance on one side will be approx. equal to D/500 mm,               where D is the rotor Diameter in mm.

7.) It is to be ensured that while placing the Bearing no 2 ( Thrust cum Journal) the level Or slope of the bearing parting plane should be same as of the its journal.

8.) Bedding should be 30-35 degrees all around on both sides.

– Less bedding will lead to more oil leakages from the side of the bearing.

– Extra bedding will lead to more friction loss.

9.) Always carry out DPT test before UT Test

CHECKLIST FORMATE

ACTIVITY	√ if Done
Always carry out DPT test before UT Test
If new / Redbaited bearing is used then ensure bluematching of Flat surface  of pads with respect to their Flat  female seats in                      the bearing shell.
Blue matching of bearing pads with pedestal should be more than > 90 % with all the sides touching all around .
Bedding should be 30-35 degrees all around on both sides.
The difference in the values of SOC and TOC when measured from two sides (front and  rear) of bearing should not be more than 0.05 mm.
No of shims in each pad should not be more than 3 nos. If it is more , substitute with suitable equivalent sheet of stainless steel only.

6.) Precautions during Turbine Bearing Overhauling:-

Bearing Babbitt material should never be scrapped manually by more than 0.05mm .If it is scrapped for larger thickness then it can lead to irregular wedges making lubrication difficult which can raise the temperature of the bearing and also the damage of the Babbitt. Best way is to make the SOC and TOC of the bearings by machining  Babbitt on the Lathe/Vertical boring machine. By this the wedge will be perfectly regular and congruent, leading to smooth lubrication.

 

7.) Procedure of Turbine Bearing Inspections:-

1.) All bearing housing & bearing tops are removed

2.) Removal of all top internals (bearing, glands & oil catcher)

3.) Lifting of Bearings

4.) Placing of all bearing tops at turbine floor

5.) Putting in dummy bearing

6.) Cleaning of bearing 

7.) Blue Matching of the Turbine Bearing

8.) Turbine Components (Bearing Pedestal):-

Bearings are usually forced lubricated and have provision for admission of jacking oil.

• Journal bearing – 6 Nos
• Thrust bearing   – 1 No

Journal Bearing

The function of the journal bearing is to support the turbine rotor.

• The journal Bearing Consists of the upper & lower shells, bearing cap, Spherical block, Spherical support            and key.
• The Bearing Shell are provided with a babbit face.
• Bearing is pivot mounted on the spherical support to prevent the bending movement on the rotor.
• A cap which fits in to the corresponding groove in the bearing shell prevents vertical movement of the brg shell.
• The brg shells are fixed laterally by key.
• Each key is held in position in the brg pedestal by 2 lateral collar.
• The Temperature of the brg bodies is monitored by thermocouple.
• Upper and lower shell can be removed without the removal of Rotor.
• To do this shaft is lifted slightly by means of jacking device but within the clearance of shaft seal.
• The lower bearing shell can be turned upward to the top position and removed.

Bearing Pedestal

1.) HP Front Bearing Pedestal

Location:- The Front Bearing Pedestal is located at the turbine side end of the turbine generator unit.

Functions:- Its function is to support the turbine casing and bear the turbine rotor.

Components:-    (i) Journal bearing

(ii) Hydraulic turning gear

(iii) Main oil pump with hydraulic speed transducer

(iv) Electric speed transducer

(v) Over speed trip

(vi) Shaft vibration pick-up

(vii) Bearing pedestal vibration pick-up

• The bearing pedestal is aligned to the foundation by means of hexagon head screws that are screwed                in to it at several points.
• The space beneath the bearing pedestal is filled with non shrinking grout.
• The bearing pedestal is anchored at to the foundation by means of anchor bolts.
• The anchor bolt holes are filled with gravel, it gives a vibration damping effect.

2.) HP Rear Bearing Pedestal

Location:- The Bearing pedestal(2) is located between the HP and IP turbine.

Functions:- Its function is to support the turbine casing and bear the HP IP rotor.

Components:-    (i) Combine Journal and Thrust bearing

(ii) Shaft vibration pick-up

(iii) Bearing pedestal vibration pick-up

(iv) Thrust Bearing trip(electrical)

(v) Combined Journal and Thrust Bearing

• The magnitude and direction of axial thrust of the turbine depends on the load condition
• The Journal bearing is elliptical sleeve bearing.
• The bearing liners are provided with a machined babbit face.
• Located at each end of bearing shell, babbitted thrust bad forms 2 annular surfaces.
• These collars and thrust pads permit equal loading of thrust bearing.
• Thrust pads are of tilting type.
• Metal temperature of the journal bearing and thrust pads is monitored by the thermocouples.

3.) IP Rear Bearing Pedestal

Location:- The bearing pedestal is located between the HP and IP turbines.

Functions:- Its function is to support the turbine casing and bear the HP and IP turbine rotors.

Components:-           (i) Journal bearing

(ii) Shaft vibration pick-up

(iii) Bearing pedestal vibration pick-up

(iv) Hand barring arrangement

(v) Differential expansion measurement device

4.) LP Rear Bearing Pedestal   

Location:-The bearing pedestal is situated between the LP turbine and generator.

Functions:- Its function is to bear the LP rotor.

Components:-           (i) Journal bearing

(ii) Shaft vibration pick-up

(iii) Bearing pedestal vibration pick-up

9.) Important Data for Turbine Bearing:-

1.) Journal Bearing pads Thickness measurement:-

Front Bearing pads   :           28.25 , 28.25 , 28.25 , 28.24 , 28.25   mm

Rear Bearing pads    :           28.25 , 28.25 , 28.25 , 28.24 , 28.24    mm

 

2.) Turbine Front Bearing (FB) and Rear Bearing (RB) Top Oil Clearance (TOC) & Side Oil  Clearance (SOC) measurement:-

Turbine FB & RB Top oil clearance:- 

(a) Journal diameter                                                                  175 mm

(b) Bearing Shell ID (-) pad thickness                                         231.80 (-) 2 X 28.25

      Bearing bore                                                                      175.30                                                                           

(c ) Oil Clearance (b-a)                                                               0.30 mm

(d) Interface between bearing shell and bearing housing         0.03 mm

 

Turbine FB & RB side oil clearance:-                                    0.30 + 0.30 = 0.60 mm

 

Generator FB & RB side oil clearance:-                              FB                              RB

Journal diameter                                                                      299.65 mm                  299.65mm

Bearing Bore                                                                           300.06 mm                   300.10 mm

Clearance                                                                                0.41 mm                        0.45 mm

Thrust Pads Thickness:- 

Active side         28.56                                                                                                                Non active side  28.55

10.) Protocol for Turbine Bearing:-

Component name: JOURNAL DIA & BEARINGS OIL CLEARANCE
Radial clearance Designed value =……………….. MM					Side Oil Clearance Designed value =
Srl. No	Brg. No	Brg. Bore	Journal dia	Oil clearance	Left side	Right side	REMARK
1
2	Thrust Float
3	Total bump
4	Bump active to forward
5	Bump in active to backward