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Understanding
Flangeguard
The use of insulated flanges is common
practice in the construction of gas distribution systems. Typically, such
flanges are used between the supply line and the customer's facility to
prevent the cathodic corrosion protection voltage on the supply pipe from
entering the customer's system where it would otherwise find its way to
ground. During periods of disturbed atmospheric conditions and nearby
lightning strikes, strong earth currents and, thereby, voltage gradients
are developed in the earth. Such gradients can easily result in a voltage
potential difference of many thousands of volts between the supply pipe
and the customer's grounded system. This difference of potential appears
across the insulated flange and often exceeds the breakdown voltage rating
of the insulating material resulting in an arc-over and destruction of the
gasket and insulating sleeves.
To prevent the build-up of destructive
charges, buried siamese diodes are commonly installed at insulated
flanges. These devices work well but are expensive and require a
considerable amount of labor to install. The desire to find a simpler,
more economical way of protecting their flanges prompted one of the major
utilities to commission Thompson Lightning Protection, Inc. to design an
alternative protection system. The result was the development of the
Flangeguard.
At the conclusion of the Flangeguard
development work, a quantity of the developmental models were installed by
two major utilities at their most troublesome locations for testing. At
the conclusion of this two and one half year test, which included three
summer thunderstorm seasons, it was reported that not a single insulated
flange had been damaged. Before committing themselves to a more extensive
use of the arrester, however, they deemed it prudent to have the device
tested by an accredited testing laboratory. This was done at their own
expense and based upon the favorable laboratory report and the results of
their own field testing, an order was placed for a large quantity of the
arresters.
In practice, the Flangeguard is placed
across the insulated flange with connections to the pipe on each side as
per the attached drawing. Under normal conditions the arrester is
completely non-conductive and preserves the electrical insulation of the
flange as required to isolate the low cathodic protection voltage. As the
static voltage across the flange increases under the influence of
disturbed atmospheric conditions, the electrical resistance of the
Flangeguard automatically lowers, allowing the potential across the flange
to equalize. Conduction through the arrester begins at just over 100 volts
and its resistance approaches zero at about 300 volts. In effect, a
Flangeguard protected flange appears as a perfect insulator at cathodic
protection voltages but for the fraction of a second required to equalize
high-voltage static charges, it becomes, effectively, a non-insulating
flange. The device may be used either indoors or out. The active element
of the Flangeguard is expected to accommodate in excess of 10,000
discharges, the equivalent of many years of service. |
