5.
3 POTENTIAL EFFECTS OF INSTALLATION
GEOMETRY
The geometry, size, and length of piping and piping systems
are important to consider when selecting a flame arrester. It is
possible that the level of turbulence generated by combinations
of these factors may render a flame arrester incapable of
quenching a flame front. Studies have noted that a correlation
of the performance of a flame arrester and piping size is not
always possible. It may be necessary to have tests performed
for the particular size of flame arrester proposed for use. This is
particularly relevant as piping diameter increases. For piping
systems, it is advisable to install only flame arresters that have
been designed and tested for detonations. In some situations,
this is required for regulatory compliance.
Placing two or more flame arresters in series is not advisable.
It has not been demonstrated that additional protection
would be provided. If the flame front propagates through the
fìrst flame arrester, it can be expected to propagate through the
second flame arrester.
Pipe diameter going into and out of a flame arrester should
be kept constant. Proprietary testing indicates that changes in
diameter can cause flame fronts to accelerate through an
arrester.
5.4 FLAME ARRESTERS NOT USING METAL
ELEMENTS
Some standards and testing laboratories have provided
for flame arresters that have a design that does not use metal
elements. Examples are hydraulic (water) type flame arresters
in CEN-EN 12874 and in the FM Approval Guide. The
performance of these flame arresters must be demonstrated
by testing.
In certain situations, the USCG regulations dow the
instdation of “water seals” and quick closing valves for
mechanical interruption of the flame path. These devices
must meet USCG certification requirements. Demonstration
of the suitability of these devices may require performance
testing of the design. By contrast, emergency shutdown
valves required by the USCG regulations for oil or hazardous
material transfer lines and cargo vapor shutoff valves are not
intended to act as flame arresters and are not required to function
as quickly.
Flame arresters using devices and techniques other than
metal elements are available and in use within the hydrocarbon
industry. Some of these flame arresters have been in service for
years without incident; however, without proof of performance
of the design by testing, it should not be assumed that the flame
arrester will be capable of performing properly. Examples of
flame arresters with designs that do not use metal elements are
discussed in the following sections.
5.4.1 Water Seals
Water (hydraulic) seals are often installed to prevent
reverse gas flow. Their design is capable of interrupting a
flame front. The gas mixture is bubbled through a reservoir
of water (or sometimes another liquid). Passage of the flame
front is prevented because each gas bubble is isolated by the
liquid water. There is no standard design for water seals.
Each installation presents a specific design problem involving
the rate of gas flow, the depth of the seal, and the size and
configuration of the vessel containing the water. If composition
of the process gas is such that a flame arrester using a
metal element could become frequently plugged, using a
water seal may be appropriate. If a water seal is used as a
flame arrester, some important design considerations are:
a. The water seal must be capable of withstanding the pressures
developed. Water seals are typicdy used immediately
adjacent to ignition sources such as flare stacks. In such a
case, the water seal would most likely have to withstand only
a deflagration. If a water seal is installed within a closed piping
system, it should be designed to withstand a detonation.
b. The water must remain in the seal for it to function as a
flame arrester. Automatic water level control and low level
alarms are desirable. It is doubtful that it is possible to design
for the water to be retained within the seal in the event of a
detonation and its accompanying pressure.
c. In cold climates, the water seal must be protected against
freezing which may require being heated or heat traced. In
some instances, anti-freeze has been added to water, or lower
freezing point materials (such as glycerine) have been used as
the fluid for hydraulic seals.
5.4.2 Packed Beds
Vessels with gravel, raschig rings, smd pebbles, a
