Pressure-vacuum gauge
Transformers may be furnished with a pressure-vacuum
gauge as optional equipment to indicate whether the gas
space in the tank is under positive or negative pressure.
The pressure will vary depending on the transformer
temperature. If the transformer is de-energized or operating
under light load in low ambients, the pressure may be
negative.
If sufficient air has been absorbed by the liquid during
shipment or storage, the transformer may operate
indefinitely in the vacuum range, depending upon the
loading conditions. This, in itself, is not cause for concern,
provided the pressure vacuum gauge does not remain on
zero for any length of time - an indication of a leak. The
transformer can safely operate in pressures ranging from -2
to +6 psig.
The transformer may have pressure vacuum switches
with two SPDT contacts for remote alarm on positive and
negative pressure. For wiring and contact ratings, refer to
the schematic furnished with the transformer.
Pressure relief device
Transformers may have a cover-mounted pressure
relief device (PRD) in addition to the faceplate-mounted
pressure relief valve (PRV) standard on all three-phase pad-
mounted designs. The cover-mounted PRD consists of a
self-resetting, spring-loaded diaphragm and a mechanical
operation indicator. Should the tank pressure increase above
that for which the device is set, the gas pressure will lift
the diaphragm and let the gas escape quickly. Immediately
after the pressure returns to normal, the diaphragm will
reset and reseal the transformer. A mechanical indicator will
protrude vertically. This must be reset manually to indicate
subsequent operations.
The PRD may have SPDT alarm contacts for remote
signaling of device activation. For wiring and contact ratings,
refer to the schematic furnished with the transformer.
Figure 6. Pressure-vacuum gauge.
Figure 7. Pressure relief device.
CAUTION
If the indicator reads zero and does not change under
any load condition, the transformer should be checked
for a possible leak in the seal.
6 Three-phase pad-mounted compartmental type installation and maintenance instructions MN202001EN August 2015
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