Chemical Engineering November 2010 - 24

Beyond dust
W
hile many explosions are dust related, process incidents can
also be caused by flame propagation and over pressurization
(or implosion) of tanks. In this area of protection, API
(American Petroleum Institute) 2000, recently co-branded as ISO
2830, provides information for processors on how to determine
the required venting capacity based on their specific application.
Prior to being co-branded as an ISO standard, the earlier
API2000 code was developed specifically for the oil and gas industry,
so it did not fully cover other industries. However, since
the co-branding, some changes have been made that may have
an impact on the use of flame arrestors with vent valves and the
methods used to size valves in industries outside oil and gas, says
Mitch Anderson, director of operations with Groth Corp. (Stafford,
Tex.; www.groth.com).
" The previous version shied away from the use of flame arrestors
in oil and gas processing because of the risk of plugging, but
in the chemical industry where there's a cleaner process and the
material may be more flammable, the latest standard suggests the
addition of a flame arrestor where there's less chance of plugging, "
says Anderson.
Sizing issues may arise, as well. While vents that are already in
place according to previous versions of the code are grandfathered,
BS&B Systems
new methods of sizing may change venting requirements going forward
with new process lines or installations, explains Anderson.
And, because environmental safety is as of nearly equal importance
as personnel safety when it comes to tank venting, it is necessary
to keep emissions in mind when selecting valves based on
these new additions to the code.
For this reason, Groth offers products that protect the environment
as well as personnel and equipment. For example, the 1800A Series
full-lift-type valve is for applications requiring low fugitive emissions
and operation near tank MAWP (maximum allowable working pressure).
This relief valve attains full stable lift at 10% overpressure.
Also intended to protect from overpressure is the 3000-Series
Blanket Gas Regulators, which ensure that a constant gas pressure
is maintained in the vapor space of a storage tank. A blanket gas
regulator supplies an inert gas to prevent a vacuum from developing
when liquid is removed from a tank, to maintain the desired
blanket pressure when the temperature drops and to prevent outside
air from contaminating the tank or creating a flammable or
explosive environment. Because it prevents outside air and moisture
from entering the storage vessel and reduces the evaporation
of the stored product, the result is conserved product and greatly
reduced emissions, as well as protection from external fires. ❏
Rembe
Figure 2.
The Type IPD
Explosion Suppression
System
provides protection
where dust
suspended in
air represents
an explosion
hazard
formed decisions about which type of
equipment is right for their facilitiy
and their needs. "
And, there are several choices to be
made when it comes to dust explosion
prevention, starting with the choice
between active and passive systems.
Passive systems include explosion
vents that function completely based
upon the design and construction
of the item. There is no circuitry or
logic for making decisions. Included
in this group are explosion vents and
flameless explosion vents. Active systems
include explosion suppression
systems, which use sensing detectors
to identify that an explosion has
started. There's a logic decision in the
controller that activates the rapid release
of an agent into the offending
equipment to prevent a full explosion
from developing.
Venting versus suppression
There are pros and cons to each type
of system. " We see many advantages
of venting over suppression systems, "
says Gerd Mayer, president of Rembe,
Inc. (Charlotte, N.C.; www.rembe.us),
which offers both types of equipment.
" Suppression systems are subject to a
lot of faults and if you don't actually
have an explosion situation and the
suppression system is emptied, a facility
can be knocked out of production
for several days because everything,
not just the dust collector, will have
to be cleaned as the agent spreads
all over the processing line. If you're
in a food, chemical or pharmaceutical
plant, you don't want suppression
agents in your product. "
Also, there are high maintenance
costs associated with suppression systems,
says Mayer. NFPA 69 explains
that a suppression system should be
checked a minimum of four times a
year because the agent-containing
bottles can lose pressure, which would
result in faulty or no activation during
an event.
" In many cases vents are chosen
because they are an inexpensive
and readily available way to retrofit
an installation to meet standards, "
explains Christopher Driesel, engineering
manager with Oseco (Broken
Arrow, Okla.; www.oseco.com). " Active
systems are more expensive over the
longterm cost of ownership because of
the necessary inspections and maintenance
involved. "
24 ChemiCal engineering www.Che.Com november 2010
Figure 3. The Q-Rohr-3 guarantees
indoor venting without any flames or
dust propagation, allowing protection for
equipment that is hard to access or free
standing in the process area
However, it is important to remember
that venting systems are not " fit
and forget. " Brazier says that the latest
generation of NFPA standards is
very clear that even passive systems
do have a requirement for periodic inspection.
" This is something that the
end user community is very weak in
embracing, " he says.
Inspection here is necessary because
several factors could affect the operation.
For instance, process material
can build up inside the vent, causing it
to become heavy, which may result in
an inability to open fast enough, or at
all, during an event. Another example
is when a process changes, which can
have significant consequences to the
protection equipment if the material
being processed was changed to one
with higher combustion characteristics
and previously employed safety
measures might not be enough to con
http://www.groth.com http://www.oseco.com http://www.rembe.us http://www.Che.Com
Chemical Engineering November 2010

Table of Contents for the Digital Edition of Chemical Engineering November 2010
