Table
4
Vacuum Treatment and Liquid Filling
15 PSI or Full Vacuum Tanks
Absolute Pressure Vacuum Holding
Condition
in Tank Torr
Time Hours
Before Filling
During Filling
After Filling
Before Filling
During Filling
After Filling
5
Maximum
4
6
Maximum
5
Maximum
2
8 PSI Tanks
34 7
Minimum
4
347
Minimum
347
Minimum
2
Note :
1
Torr =
1
mm Hg =
.0 1 93
PSI
Caution: Where "MINIMUM" is specified lower
pressures may result in tank damage or
permanent deformation with serious dam
age to internal parts.
a. Final Filling of Transformers Shipped in Liquid
Apply the required vacuum level prior to liquid-filling, as
specified in Table
4.
If additional liquid is required to
bring the liquid level up to normal as shown by the liquid
level gauge, hold the vacuum level during liquid-filling as
specified in Table
4
while the liquid is being added . Break
vacuum with dry gas to a positive pressure of
2
to
3
psi
gauge.
b.
Final Filling of Transformers Shipped in Dry Gas
Apply the vacuum level prior to liquid-filling for the num
ber of hours prior to filling as specified
in
Table
4.
Then
admit liquid into the top of the tank, while holding the
vacuum level during liquid-filling as specified in Table
4 ,
until li quid reaches the normal level as specified in Table
2 ,
page
7 .
Break vacuum with dry air or dry nitrogen t o a positive
pressure of
2
to
3
psi gauge.
c.
Lowering Liquid Below Top of Core and Coils
If the liquid must be lowered so that any part of the core
and coils is exposed, use dry air to ventilate the space
above the remaining liquid and maintain
2
to
3
psi posi
tive gas pressure whenever work is interrupted.
29
When the core and coil assembly is out of liquid, the
conditions are similar to those during shipment in dry gas.
It is, therefore , important to make repairs quickly and get
the core and coils submerged, in order to avoid the long
vacuum hold times required for initial fillings.
Drain any li quid which was not drained before repairs
were made. Apply the required vacuum level prior to fill
ing and hold it for the specified time according to Table
4.
If the time the core and coil assembly is out of liquid , be
fore evacuation for refilling is started, does not exceed
16
hours, liquid flow into the tank may start as soon as
the required vacuum is obtained. Then admit liquid into
the top of the tank, while holding the vacuum level during
liquid-filling as specified in Table
4
until liquid reaches the
normal level as specified in Table
2,
page
7.
CAUTION
Transformers should not be left under vacuum except dur
ing the vacuum fdling operation. Leaks in the temporary
piping and connections lead to danger of drawing mois
ture into the tank if it is under vacuum during periods of
high humidity or during a rain. It is recommended that
the tank be under p ositive pressure during a rain to pre
vent drawing moisture into the tank.
8.5 Vacuum Equipment and Procedures
In order to attain the vacuum levels specified and to main
tain these levels during liquid-filling, a good vacuum pump
of adequate capacity is required. For smaller transformers,
a
1 00
cfm pump will be adequate, but for larger units, a
pump of
1 40
cfm capacity or greater is desirable. The
pumps should be capable of attaining a blank-off pres
sure of
.02
Torr or less.
Connections from the pump to the transformer tank
should be as short and as large in diameter as possible.
The line should be at least
1
inch in diameter, preferably
larger, particularly on larger transformers. There should
be no low spots in the vacuum line in w hich liquid can
collect.
The pressure in the tank during vacuum operations
should be measured through a connection of the upper
part of the tank, above the liquid level. Do not use pres
sure measurements at locations other than the tank it
self. For measuring the pressure in the tank , a properly
calibrated aneroid gauge or thermocouple gauge is recom
mended. Use mercury gauges only if a trap is provided
which will prevent mercury from entering the trans
former.