Westinghouse PS-1002, Инструкция, Страница: 32 / 36

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It is important that the entire system be free from
leaks, otherwise it may be difficult or impossible to attain
the specified vacuum levels. Any leaks will permit moist
air or water to be drawn into the transformer. If a high
capacity pump is used, it is especially important to elimi·
nate leaks so that moisture drawn into the system is mini­
mized.
Openings for relief devices and other accessories must
be sealed in accordance with instructions furnished for
the particular accessory in order to prevent air from enter­
ing the transformer during vacuum operations. Valves i n
pipe connections between the main tank and liquid-filled
compartments should be open during vacuum, if neces­
sary, to limit differential pressures to values specified on
the transformer instruction plate.
If it is necessary to fill units in the field , apply the
vacuum which is permitted by the nameplate for the
length of time specified in Table 4.
The vacuum piping should be so related to the liquid
piping that liquid will not splash or spray into the vacuum
line. It is advisable to provide a liquid trap in the vacuum
line to protect the pump from the insulating liquid. The
efficiency of most vacuum pumps is dependent upon the
condition of the pump liquid. If the pump liquid becomes
cloudy from the moisture or thins out because of insulat­
ing oil contamination, it should be changed.
8 .6 Ventilation of Tank
When it is necessary to open a transformer, the following
procedure should be used.
Dry air should be used to ventilate the inside of the
tank when it is opened for internal fitting. When dry air
is used, the following restrictions should be observe d :
a. Temperature o f dry air entering the transformer shall
be at least as h igh as that of the transformer and at least
1 0°C higher than the dew point of the outside air.
b. Dry air shall be blown into the transformer so as to
create a flow of air out through the cover opening. Air
hoses may be taken into the transformer if they are clean
and made from an oil-proof material. In the case of a
transformer shipped without insulating fluid, blow dry air
into the tank through an opening near the bottom of the
main tank.
c. The dew point in the transforme r should never be
higher than 20° F.
d. In order to conserve dry air, place a light cover of ply­
wood, hardboard, or plastic over the manhole opening.
The work should be pe rformed in a sequence such that
when accessories such as bushings are being installed, only
one opening other than the manhole will be open at any
time.
Dry Air and Nitrogen. When nitrogen is called for the
nitrogen used should have a dewpoint not higher than
-50°C ( -58 °F), and total impurities not exceeding 0 . 1 %
b y volume. Nitrogen can be obtaine d i n high pressure steel
cylinders, or in some locations in insulating low pressure
containers in liquid form . In general, liquid nitrogen
which will boil in the container to yield gaseous nitrogen,
will have a lower dewpoint than gas in high pressure cylin­
ders.
Dry air should also have a dewpoint of -50°C (-58°F)
or lower. It is usually available
in
cylinders from the same
source which supply nitrogen. Air drying equipment is
also available which is c apable of producing dry air by
passing air through a desiccant bed to remove moisture .
When air or nitrogen are supplied from high pressure
cylinders the proper regulating valve must be used for in­
troducing the gas into the transformer tank. Cylinders
should not be completely emptied, but should be returned
to the supplier with at least 25 psi residual pressure.
Outside air may be used for ventilating the transformer
if dry air is not available . If outside air is to be used for
ventilation , open the transformer only if the outside
relative humidity is less than
65%
and if the temperature
of the transformer
i
s
at
least
I 0°C
h
ig
h
e
r
than the dew
point of the outside air. The maximum
total
time the
transformer should be open is 24 hours. If this time must
be excee ded, extend the length of the vacuum prior to
filling specified in Table 4 by one hour for each 4 hours
that the open time exceeded 24 hours. If work is inter­
rupted, the tank should be closed, evacuated, and refille d
with dry air or nitrogen.
8.7 Gaskets
All gaskets used on Substation Transformers are normally
made of a gasket material consisting of cork and synthetic
rubber; and for satisfactory results replacement gaskets
should be of the same material . Before replacing a gaske t,
carefully and thoroughly clean the steel surfaces between
which the gaskets are compressed to remove rust, oil ,
grease, paint and other foreign material . The cleaning may
be done by scraping or wire-brushing and then wiping the
gasket surface with de-natured alcohol. Use gasket cement