NATIONAL CONFERENCE ON INNOVATION & BEST PRACTICES
IN TRANSFORMER DESIGN TESTING & MAINTENANCE
BY
CENTRAL POWER RESARCH INSTITUTE
METHODS FOR PREVENTION OF OIL & GAS LEAKAGES FOR RELIABILITY
DURING OPERATION OF POWER TRANSFORMER
Pramod Srivastava GM Engineering (Mechanical), EMCO Limited, Thane
Email: pramod.Srivastava@emco.co.in, Mobile: 9987671877
METHODS FOR PREVENTION OF OIL & GAS LEAKAGES FOR RELIABILITY
DURING OPERATION OF POWER TRANSFORMER
Pramod Srivastava GM Engineering (Mechanical), EMCO Limited, Thane
Email: pramod.Srivastava@emco.co.in, Mobile: 9987671877
Objective:-.
•
Power transformers is one of the costliest components of any substation. Expected service life of a transformer
is approx. 30 years.
•
Oil leakage is a perpetual problem which sometimes warrants unplanned shutdown of the transformer, leading
to substantial commercial losses to the customer & manufacturer.
•
Our objective is to define methods to prevent oil leakage for reliable operation of power transformers.
Problems due to Oil leakage :•
Oil leakage aggravates the risk of fire in the substation yard.
•
Loss of oil requires frequent oil topping up in transformer.
•
Risk of moisture ingress through leaking gaskets, affecting service
life of oil-immersed insulation.
•
Examples of oil leakages are shown in the Detail 1.
Detail 1
Causes of Oil leakage:
•
Opening of gasketed joints as shown in Detail 2.
•
Fabrication distortions in flatness of mating flanges
•
Inadequate Elastomer content in gasket material, causing
Detail 2
flattening of gaskets in the groove.
•
Excessive gasket hardness than specified may cause cracking of gaskets under pressure
•
Improper design & manufacturing of gasket flange, grooves & incorrect surface finish causing
damage to gasket.
•
Improper sealing due to contact of machined gasket grove with the un-machined cover.
•
Distortion in flanges due to non- clamping of flanges during welding.
•
Selection of gasket material incompatible with the prevailing high temperature & pressure conditions.
•
Incorrect torque application during tightening of hardware involving gaskets joints.
•
Improper positioning of cord in the groove, causing crushing of gaskets
Prevention of oil leakage :Oil leakage can be prevented by
•
Proper selection of gasket material & using joint less cords & verification of gasket materials.
•
Proper design & manufacturing of flanges along with proper inspection of flanges.
•
Use of calibrated tightening torques for assembly of hardware.
•
Conducting proper pressure test at shop.
•
Elliptical or round inspection covers instead of rectangular & square inspection covers .
Three types of gaskets are widely used in transformer industry.
1. S.R.B.C. Gaskets :Synthetic resin-bonded cork (SRBC) gasket used in transformer industry confirms to
grade RC 70_C of IS 4253(Part 2) [2] with hardness: IRHD 70 ± 5 as per (IS 3400 Part 2)
Other properties are as per above IS. Detail 3 below gives guidelines for fixing SRBC gaskets over
tank flange for maximum possible longitudinal length “L” of tank flange with fixing bolt nominal diameter “N”
& bolt pitch “P”. Joint details are given in Detail 4
Detail 3
Detail 4
Joint in SRBC gaskets
Advantages:-
Advantages:•
Low cost. Doesn’t require precision grooves
Disadvantages:-
•
Needs many joints for shaped tanks due to sheet size limitations. Lot of scrap generated.
Critical requirements :-
.
Gaskets to be cut without burrs. Few drops of adhesive to be applied for spot fixing below the joint.
2. Nitrile rubber gasket :Nitrile rubber gaskets / “O” rings are used to in tank flanges. Nitrile rubber
has excellent mouldability and hence it can be moulded in any desired shape.
Nitrile rubber is covered under IS 11149 Type 4C. To prevent increase in hardness due to temperature variation
It is advisable to keep hardness up to 70± 5 IRHD and suitable to withstand temperatures up to 120 °C
Elmag 3d software is used as shown in detail 5 to find temperature for large generator transformers & where
temperatures are more than 120°C viton rubber gaskets are used.
Detail 5
Advantages:•
Very low scrap & faster installation in “O” rings
Dis Advantages:-
•
Relatively higher cost & requirement of precision machining of flanges
Critical requirements :.
Gaskets to be cut without burrs. Curb flatness to be maintained
to the tolerance of ± 1 mm / meter
•
Detail 6
Elliptical gaskets to be laid with major axis facing the flange
with few drops of glue below joint. Joint to be prepared as
shown in Detail 6
•
Round stopper bar of 10 mm dia. to be used to prevent edge
3. Viton rubber gaskets:Viton rubber is used where wider range of temperature withstand, i.e. -20°C to 200 °C
is required.
Viton rubber is represented by ASTM D2000 class HK .
Advantages:•
Can be used in very high temperature zone and for multiple openings.
Dis Advantages:-
•
Very high cost & requirement of precision machining of flanges
Shelf life of S.R.B.C. & Nitrile rubber is 2 years & for Viton 3 years with proper storage conditions.
Grooves for “O” rings
Points be considered for groove design.
•
Surface roughness is required with ∆∆ finish for the groove.
•
1 mm step for bolting area to prevent locking of flange face with cover due to weld distortions
& increasing pressure on the gasket.
•
Lower surface of inspection covers for gasket area to be also machined .
•
For large inspection covers & turrets M16 hard ware to be used
•
Corners of grooves to be provided with 1 mm radius to prevent damage of the “O” rings.
•
Grooves to be machined using CNC machines
•
“O” rings to be centrally located in the groove.
as shown in Detail 7
Detail 7
Tightening torque:
Oil can leak through gasket when it is under
Detail 8
Sr.
No.
1
2
3
4
5
compressed or over-compressed.
The recommended tightening torques
for the gasketed joints is given in Detail 8
Bolt
size
M10
M12
M16
M20
M24
Torque (N-m)
20
30
50
70
100
Tightening sequence:
Tightening sequence to be followed should
Detail 9
be as shown in Detail 9
Gas leakage control:
•
Pressure test at normal pressure + 5 PSI to be conducted normally for 12 hours.
•
O.L.T.C. pipes & projected studs to be covered by hood as shown in detail 10.
Detail 10
•
Nordlock washers can be used to prevent loosening of hard ware.
These washers prevent the loosening of hardware by locking the washer with the base metal.
These are designed based on leading wedge locking technology and are recognized for their ability
To safely secure bolted joints prone to vibration and dynamic loads. Nordlock. washer secure the bolted
joints with tension instead of friction. The system is comprised of a pair of washers that has cams on one
side and radial teeth on opposite side. Since the cam angle α is greater than the thread pitch β, a wedge
effect is created by the cams, preventing the bolt from rotating loose, as shown in Detail 11. Washers are
shown in Detail 12.
Detail 11
Detail 12
Measurement of grooves & tightening of hard ware using torque wrench
•
Groove width can & depth can be measured as shown in Details 13 & 14
•
Hard wares to be tightened using torque wrench ( Detail 15) & at site ( Detail 16)
Detail 13
Detail 14
Detail 15
Detail 16
Conclusion:
It is rightly said that “Prevention is better than cure“. Prevention of oil leakage is a subject where
precautions are taken while designing the transformer, using low cost investments & tests.
Good practices can certainly save lot of money, labour & problems in future.
Adoption of these control measures & strict monitoring helps in manufacturing leak proof transformers.
Allied benefits are the improved aesthetics, reduction in warranty costs of repairs in service, prevention of loss of
revenue to the utility in terms of reduced interruptions in service due to forced outages to fix the leak repairs.
Thank You