must be capable of withstanding system pressure and
MATERIAL REPLACEMENT
temperature during performance of the maintenance
When a section of an existing piping system must
be renewed, select the materials according to the
or external bypass features and may be used as a barrier
following general considerations.
only in one direction if the bypass is open. A gagged
Choose replacement materials of the same material,
relief valve may unseat with sufficient reverse pressure,
size, and wall thickness as the original. This will ensure
so consider a gagged relief valve a pressure barrier only
that the original design of the system's capacity, thermal
for maintenance performance downstream of the relief
expansion, and contraction is not changed. It will also
valve. Solenoid-operated valves that have a fail to shut
prevent the setting up of a galvanic couple between
feature tend to seat with system pressure, and do not
dissimilar materials, with its attendant electrolytic
have internal bypasses that cannot be isolated. These
corrosion.
valves may be used as barriers for maintenance
performance downstream in the direction in which
Materials used in piping systems on ships built to
pressure tends to seat the valve. Do not use other
specifications should be built to military standards.
solenoid-operated valves as pressure barriers. Do not
Military standards require that materials used in piping
use lift, swing, or stop check valves seated by pressure
systems on naval ships have a higher resistance to
only, ungagged relief valves, four-way valves, or feed
corrosion and shock damage. They also must be of the
regulating valves as pressure barriers. Valves used for
lightest weight possible.
throttling are subject to erosion and may not be good
In the past, there have been numerous incidents
pressure barriers.
involving the failure of piping system components.
Several of these incidents have resulted in lost of
At least two pressure barriers are required between
equipment and even the death of shipmates. Most of
the maintenance area and any high-temperature (200F
these incidents have resulted from the careless
or more) fluid. Where possible, open a tell-tale drain
maintenance practices and incorrect materials used
valve between the two barriers to warn if one barrier is
while performing maintenance or repairs. Therefore,
leaking. Provide at least two pressure barriers, since
you should always consult the applicable ships and
high-temperature (above 200F), high-pressure liquid
system drawings when choosing replacement or repair
will flash, providing a continuing pressure source,
materials. If the applicable blueprint is not available,
causing discharge of large quantities of high-
consult other MIL-STDS, technical manuals, operator
temperature fluid and endangering personnel in the
manuals, or repair manuals for correct materials and
maintenance area if leakage should occur. If necessary,
repair procedures. If you deviate from original design
an exception to this requirement may be made if the
specifications, obtain prior approval from competent
source of high-temperature liquid is separated from the
authority before proceeding with the repair.
maintenance area by a single-pressure barrier and a
sufficiently large volume of low-temperature (below
200F) liquid. The volume of the low-temperature
SYSTEM AND EQUIPMENT
liquid must be sufficient to allow personnel performing
ISOLATION WHEN CONDUCTING
the maintenance to get clear and to secure the source of
MAINTENANCE AND TESTING
high-temperature liquid in time to prevent its discharge
into the maintenance area if the single-pressure barrier
Pressure barriers, which prevent the escape of a
should fail.
pressurized fluid from a system, are necessary when
conducting maintenance on piping systems to prevent
A potential danger in component repair or
endangering personnel and equipment. These dangers
replacement lies in inadvertent or accidental operation
include personnel injury due to high temperatures,
of the isolation valves. Tag out all valves and their
projectures from high-pressure systems, wetting of
controls to prevent inadvertent operation. If the control
electrical equipment, or unintentional venting or
valve of an actuator is electrically operated, have the
draining of adjacent systems.
fuses removed from the circuit or disconnect the
electrical connector so that the operator of the control
When isolating a system for component repair or
valve cannot be accidentally energized. If the control
replacement, the type of barrier used for isolation is an
important consideration. Examples of allowable
valve of an actuator is air operated, shut and tag out the
pressure barriers are a closed valve, blind (blank) union,
air supply isolation valve so that the control valve
blind (blank) flange, or spectacle flange, all of which
cannot be accidentally operated by air.
16-43