Chemical Engineering June 2010 - 40

Feature Report
Discharge
to safe area
RD-1
* If a lined pipe system is required,
use the type requiring the liner to be
fused, a coupling installed and one
that is suitable for multi-axis bending
Threaded joints should be limited
to instrument connections and then
only if the instrument is not available
with a flange or welded connection.
If a threaded connection is used,
it should be assembled without thread
compound then seal-welded. This may
require partial dismantling of the instrument
to protect it from the heat of
the welding process.
It is recommended that piping systems
be welded as much as possible and
flanged joints be minimized as much as
possible. That includes using welded
end valves and inline components
where possible. If flanged joints are
necessary for connecting to equipment
nozzles, flanged valves, inline components,
or needed for break-out joints, it
is suggested that a spiral-wound type
gasket with graphite filler be specified.
This material can withstand temperatures
upwards of 3,000ºF. There are
also gasket designs that are suitable
for when a fluoropolymer material is
needed for contact with the chemical,
while also holding up well in a fire.
These are gaskets similar in design to
that shown in Figure 3.
Valves. A fire-rated valve meeting the
requirements of API 607 (Fire Test
for Soft Seated Quarter Turn Valves)
is designed and tested to assure the
prevention of fluid leakage both internally
along the valve's flow path, and
externally through the stem packing,
bonnet seal, and body seal (where a
multi-piece body is specified). Testing
under API 607 subjects a valve to well
defined and controlled fire conditions.
It requires that after exposure to the
fire test the valve shall be in a condition
that will allow it to be rotated
from its closed position to its fully
open position using only the manual
operator fitted to the test valve.
Quarter turn describes a type of
valve that goes from fully closed to
fully open within the 90 deg rotation
of its operator. It includes such valve
types as ball, plug, and butterfly with
a valve seat material of fluoropolymer,
elastomer, or some other soft, non-metallic
material.
Standards such as FM-7440 and
VA-1
LT-1
SG-1
VA-3
XV-2
Line B
Line C
VA-2
Pump
XV XV-4
-3
VA-4
PG-1
VA-5
Flammable
liquid out
Operating unit
battery limits
VA-6
Flammable
liquid to
recovery
Slope
Line A
Flammable
liquid in
Line D
XV-1
Figure 6. A simplified P&ID used in the discussion about process systems
BS-6755-2, touched on earlier, apply
to virtually any valve type that complies
with their requirements. Under
the FM and BS standards, valve
types such as gates, globes, and piston
valves with metal seats can also
make excellent fire-rated valves when
using a body and bonnet gasket and
stem packing material similar in temperature
range to that of a graphite or
graphite composite.
Process systems. At the onset of a fire
within an operating unit, initially unaffected
process piping systems should
not be a contributor to sustaining and
spreading what is already a potentially
volatile situation. There are basic design
concepts that can be incorporated
into the physical aspects of a process
system that will, at the very least, provide
precious time for operators and
emergency responders to get the situation
under control. In referring to the
simplified piping and instrumentation
diagram (P&ID) in Figure 6, there are
seven main points to consider:
1. Flow supply (Line A), coming from
the fluid's source outside the operating
unit, needs to be remotely shut off
to the area that is experiencing a fire
2. The flow path at the systems use point
valves (VA-1) needs to remain open
3. The flow path at drain and vent valves
(VA-2) needs to remain sealed
4. The external path through stem
packing and body seals needs to remain
intact during a fire
5. The bottom outlet valve (XV-2) on a
vessel containing a flammable liquid
should have an integral fusible
link for automatic shut-off, with its
valve seat, stem packing and body
seals remaining intact during a fire
6. Pipeline A should be sloped to allow
all liquid to drain into the vessel
40 ChemiCal engineering www.Che.Com June 2010
7. The liquid in the vessel should be
pumped out to a safe location until
the fusible link activates, closing the
valve. There should be an interlock
notifying the control room and shutting
down the pump
Those seven points, with the help of
the P&ID in Figure 6, are explained
as follows:
Point 1. The supply source, or any
pipeline supplying the operating unit
with a flammable liquid, should have
an automated, fire-rated isolation
valve (XV-1) located outside the building
or operating unit area and linked
to the unit's alarm system with remote
on/off operation (from a safe location)
at a minimum.
Point 2. Any point-of-use valve (VA-1)
at a vessel should remain open during
a fire. The area or unit isolation
valve (XV-1) will stop further flow to
the system, but any retained or residual
fluid downstream of the automatic
shut-off valve needs to drain to
the vessel where the increasing overpressure,
due to heat from the fire,
will be relieved to a safe location, such
as a flare stack, through RD-1. If the
Valves, XV-1 and VA-1, are closed in a
fire situation the blocked-in fluid in a
heated pipeline will expand and potentially
rupture the pipeline; first at the
mechanical joints such as seals and
packing glands on valves and equipment,
as well as flange joints, and then
ultimately the pipe itself will rupture
(catastrophic failure). During a fire, expanding
liquids and gases should have
an unobstructed path through the piping
to a vessel that is safely vented.
Point 3. Valves at vents and drains
(VA-2 & VA-6) need to be fire-rated and
remain closed with seals and seat intact
for as long as possible during a fire.
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Chemical Engineering June 2010

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