Ingangsdatum: 22-12-1994
2.1Principles 2.1.1 Depending on their service and
location, devices are required to protect against the propagation of:
- moving
flames; and/or
- stationary flames from pre-mixed gases;
after ignition of gases resulting from any cause.
2.1.2 When
flammable gases from outlets ignite, the following four situations may occur:
- At
low gas velocities, the flame may:
- flashback; or
- stabilize itself
as if the outlet were a burner.
- At high velocities, the flame may:
- burn at a distance above the
outlet; or
- be blown out.
2.1.3 In order to
prevent the passage of flame into a cargo tank, devices must be capable of performing
one or more of the following functions:
- permitting the gas to pass through
passages without flashback and without ignition of the gases on the protected side
when the device is subjected to heating for a specified period;
- maintaining
an efflux velocity in excess of the flame speed for the gas, irrespective of the
geometric configuration of the device and without the ignition of gases on the
protected side when the device is subjected to heating for a specified period; and
- preventing an influx of flame when conditions of vacuum occur within the
cargo tanks.
2.2Mechanical design standards 2.2.1 The
casing or housing of devices should meet similar standards of strength, heat resistance
and corrosion resistance as the pipe to which they are attached.
2.2.2 The design of devices should allow for ease of inspection
and removal of internal elements for replacement, cleaning or repair.
2.2.3 All flat joints of the housing should be machined
true and should provide for a joint having an adequate metal-to-metal contact.
2.2.4 Flame arrester elements should fit in the
housing in such a way that flame cannot pass between the element and the housing.
2.2.5 Resilient seals may be installed only if
their design is such that if the seals are partially or completely damaged or burned,
the device is still capable of effectively preventing the passage of flame.
2.2.6 Devices should allow for efficient
drainage of moisture without impairing their efficiency to prevent the passage of
flame.
2.2.7 The casing and element and
gasket materials should be capable of withstanding the highest pressure and temperature
to which the device may be exposed under both normal and specified fire test
conditions.
2.2.8 End-of-line devices should
be so constructed as to direct the efflux vertically upwards.
2.2.9 Fastenings essential to the operation of the device, i.e.
screws, etc., should be protected against loosening.
2.2.10 Means should be provided to check that any valve lifts easily without
remaining in the open position.
2.2.11 Devices
in which the flame arresting effect is achieved by the valve function and which are not
equipped with the flame arrester elements (e.g. high velocity valves) must have a width
of the contact area of the valve seat of at least 5 §fi .
2.2.12 Devices should be resistant to corrosion in accordance
with 3.5.1.
2.2.13 Elements gaskets and seals
should be of material resistant to both seawater and the cargoes carried.
2.2.14 The casing or housing should be capable
of passing a hydrostatic pressure test, as required in 3.5.2.
2.2.15 In-line devices should be able to withstand, without
damage or permanent deformation, the internal pressure resulting from detonation when
tested in accordance with section 3.4.
2.2.16
A flame arrester element should be designed to ensure quality control of manufacture to
meet the characteristics of the prototype tested, in accordance with these Standards.
2.3Performance Standards 2.3.1 Devices
should be tested in accordance with 3.5 and thereafter shown to meet the test
requirements of 3.2 to 3.4, as appropriate.
2.3.2 Performance characteristics, such as the flow rates under both positive
and negative pressure, operating sensitivity, flow resistance and velocity should be
demonstrated by appropriate tests.
2.3.3
Devices should be designed and constructed to minimize the effect of fouling under
normal operating conditions. Instructions on how to determine when cleaning is required
and the method of cleaning should be provided for each device in the manufacturers'
instruction manual.
2.3.4 Devices should be
capable of operating in freezing conditions (such as may cause blockage by freezing
cargo vapours or by icing in bad weather) and if any device is provided with heating
arrangements so that its surface temperature exceeds 85 degrees C, then it should be
tested at the highest operating temperature.
2.3.5 Devices based upon maintaining a minimum velocity should be capable of
opening in such a way that a velocity of 30 §‰ is immediately initiated, maintaining an
efflux velocity of at least 30 §‰ at all flow rates and, when the gas flow is
interrupted, be capable of closing in such a way that this minimum velocity is
maintained until the valve is fully closed.
2.3.6 In the case of high velocity vents, the possibility of inadvertent
detrimental hammering* leading to damage and/or failure should be considered, with a
view to eliminating it.
2.4Flame screens 2.4.1 Flame screens
should be:
- designed in such a manner that they cannot be inserted improperly in
the opening;
- securely fitted in openings so that flames cannot circumvent
the screen;
- able to meet the requirements of these standards. For flame
screens fitted at vacuum inlets through which vapours cannot be vented the test
specified in 3.2.3 need not be complied with; and
- be protected against
mechanical damage.
2.5Sizing, location and installation of devices 2.5.1 For determining the size of devices to avoid inadmissible pressure or
vacuum in cargo tanks during loading or discharging, calculations of pressure losses
should be carried out. The following parameters should be taken into account:
- loading/discharge rates;
- gas evolution;
- pressure loss
across devices, taking into account the resistance coefficient;
- pressure
loss in the vent piping system;
- pressure at which the vent opens if a high
velocity valve is used;
- density of the saturated vapour/air mixture; and
- to compensate for possible fouling of a flame arrester, 70% of its rated
performance is to be used in the pressure drop calculation of the installation.
2.5.2 Devices should be located at the outlets to atmosphere unless
tested and approved for in-line installation. Devices for in-line installation may not
be fitted at the outlets to atmosphere unless they have also been tested and approved
for that position.
2.5.3 End of line devices
which are intended for exclusive use at openings of inerted cargo tanks need not be
tested against endurance burning as specified in 3.2.3.
2.5.4 Where end-of-line devices are fitted with cowls, weather
hoods and deflectors, etc., these attachments should be fitted for the tests described
in 3.2.
2.5.5 Where detonation flame arresters
are installed, as in-line devices venting to atmosphere, they should be located at a
sufficient distance from the open end of the pipeline so as to preclude the possibility
of a stationary flame resting on the arrester.
2.5.6 When venting to atmosphere is not performed through an end-of-line
device according to 2.5.4, or a detonation flame arrester according to 2.5.5, the
in-line device has to be specifically tested with the inclusion of all pipes, tees,
bends, cowls, weather hoods, etc., which may be fitted between the device and
atmosphere. The testing should consist of the flashback test of 3.2.2 and, if for the
given installation it is possible for a stationary flame to rest on the device, the
testing should also include the endurance burning test of 3.2.3.
2.5.7 Means should be provided to enable personnel to reach
devices situated more than 2 m above deck to facilitate maintenance, repair and
inspection.
* Hammering is rapid full stroke opening/closing, not intended by the manufacturer
during normal operations.