Vacuum Equipment And Procedures - Westinghouse PS-1002 Instruction Book

Table 4 Vacuum Treatment and Liquid Filling
15 PSI or Full Vacuum Tanks
Absolute Pressure
Condition in Tank Torr
Before Filling 5 M aximum
During Filling 6 Maximum
After Filling 5 Maximum
Before Filling 34 7 Minimum
During Filling 347 Minimum
347 Minimum
After Filling
Note : 1 Torr = 1 mm Hg = .0 1 9 3 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 Transf ormers Shipped in L iquid
Apply the required vacuum level prior to liquid-filling , as
specified in Table 4. If additional li quid 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 i s being added . Break
vacuum with dry gas to a positive pressure of 2 to 3 psi
gauge .
Final Filling of Transformers Shipped in Dry Gas
b.
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.
Lowering Liquid Below Top of Core and Coils
c.
If the li quid 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 w henever work is interrupted.
Vacuum Holding
Time Hours
4
2
8 PSI Tanks
4
2
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 li quid , 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
li quid-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
l arger, particularly on l arger 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 tan k , above the liquid level. Do not use pres­
sure measurements at locations other than the tank it­
self. For measuring the pressure in the t ank , 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.
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