Weld-on tank cover removal
Transformer tanks are factory-sealed to prevent ingress
of ambient air and moisture. Do not open under any
precipitation conditions.
Explosive gases can develop inside a transformer tank
headspace during the life of the transformer. It is strongly
recommended that weld-on cover removal be done only
in a controlled repair facility by experienced maintenance
personnel. To prevent explosions, the following procedures
must be followed during weld-on cover removal:
1. Verify that tank is grounded. De-energize transformer
from a remote upstream source. Ground all bushings
and terminals before removing cover.
2. Relieve internal tank pressure by operating pressure
relief valve.
3. Remove manhole/handhole cover as outlined in the
Tank Cover Removal and Installation
section of this
manual.
4. Drain insulating liquid to the top of core clamp as
outlined in the
Insulating Liquid Maintenance
section
of this manual.
5. Allow transformer to sit with manhole/handhole
removed for a minimum of 24 hours to allow explosive
gases to escape.
6. Place a rubberized blanket on top of core clamp and
completely cover core/coil assembly to protect core/coil
assembly from weld spatter contamination during cover
cutting.
7. Remove pressure relief valve and attach a nitrogen
source in its place.
8. Fill headspace with nitrogen with manhole/ handhole
cover still removed. Nitrogen gas will help prevent a
volatile gas explosion while tank cover is being cut off.
9. While continuing to keep nitrogen flowing into the
headspace, use carbon air arc machinery or a hard-
wheel grinder to cut cover weld.
10. Thoroughly clean tank cover to help prevent
contamination of insulating liquid when removing cover.
Remove all dirt, grease, and moisture.
11. Remove tank cover from tank.
Weld-on tank cover installation
1. Grind smooth and then clean cover, removing all weld
spatter accumulated during cover removal.
2. It is recommended that the tank weld rope gasket
be replaced around the tank lip prior to re-installing
cover. The tank weld rope gasket helps prevent weld
spatter from entering the tank during cover weld. The
rubberized blanket installed during cover removal should
still be covering the core/coil assembly.
3. Re-position cover on tank.
4. Fill headspace with nitrogen through pressure relief
valve coupling, with manhole/handhole still removed.
5. Continue to keep nitrogen source flowing into
headspace, and weld cover fully around the tank lip.
6. Remove rubberized blanket that was installed during
cover removal.
7. Refill insulating liquid as outlined in
Insulating Liquid
Maintenance
section of this manual.
8. Re-install manhole/handhole cover as outlined in
Tank
Cover Removal and Installation
section of this man-
ual.
9. Pressurize headspace through pressure relief valve
coupling to insure that there are no leaks. The test
pressure should not exceed 7 psig. The established
pressure should be maintained for at least four hours to
ensure that all seals are proper.
10. Re-install pressure relief valve.
WARNING
Explosive gas. Can cause severe injury, death, or
damage to equipment. Before cutting open a weld-
on tank cover, fill tank headspace with nitrogen gas.
Keep nitrogen gas flowing while cutting. Keep tank
headspace filled with nitrogen until cutting is finished.
21
Three-phase pad-mounted compartmental type installation and maintenance instructions
MN202001EN August 2015
