Overpressure protection is used to protect tubing, pipes,
desiccators, filters, reactors, etc. from being over pressurized.
Why use it?
Serious or fatal
accidents can occur
if overpressure
protection is not
used when it is
needed.
Texas A&M Jan 29, 2006
Ruptured liquid nitrogen tank.
(The pressure relief device had been removed)
http://ucih.ucdavis.edu/docs/chemistry_301a.pdf
Roughly 1200 psig passed
through a regulated valve
into downstream pvc piping.
Overpressure protection had
not been provided
downstream from the valve.
Overpressure protection is needed when a feeding system
generates/stores pressure higher than the maximum
working pressure rating of downstream pipes, tanks, and
other components. Feeding system examples include
compressors & compressed gas cylinders. The overpressure
protection protects the downstream components from being
over pressurized. If a regulator or pressure switch has
been installed then overpressure protection is needed.
Use overpressure protection to protect
objects (pipes, reactors, dryers, etc.) that
could develop pressures above their safe
maximum operating pressure (e.g. for
example runaway reactions).
Overpressure conditions might
arise due to:
1. Regulator failure (or other
device failures)
2. Runaway Reactions
3. Operator Error
4. Fire
5. Utility Failure (e.g. water,
steam, natural gas)
Exothermic reactions produce heat
and need to be cooled to be
controlled. If cooling fails then a
runaway reaction can occur.
Increased heat will increase the
speed of the reaction. Gases and
vapors can over pressurize their
containment leading to a catastrophic
failure if overpressure protection
isn’t provided.
Use engineering and administrative controls to avoid overpressure
accidents. This includes avoidance strategies as well as use of
pressure relief devices.
Avoidance Strategies
1. Design the system to
withstand the maximum
pressure that is present, or
that can develop.
2. Use of redundant cooling
systems.
3. Develop and use safe
operating procedures and
train operators properly.
Pressure Relief Devices
1. Buckling pin relief valve
2. Rupture pin relief valve
3. Spring loaded pressure
relief valve
4. Rupture disc
You must utilize a form of pressure relief unless you will
be utilizing the first avoidance strategy!
To prevent accidents the Chemical
Safety Board (CSB) recommends:
1. Identify & thoroughly evaluate
reactive hazards for your process;
2. Implement appropriate emergency
pressure relief systems and other
design safeguards;
3. Develop effective operating
procedures and training;
4. Carefully manage any changes to
existing processes; and
5. Plan for possible accidents
-1 fatality-
5 steps weren’t
completely
implemented.
Exothermic runaway
reaction and ensuing
vapor cloud ignition.
http://www.aristatek.com/Newsl
etter/JAN08/TechSpeak.pdf
If you find that pressure relief devices
won’t work for your application then use
series regulation. The risk of two
regulators failing is very low.
Drawback - it may be difficult to detect a
regulator that has failed because there is
another regulator masking the failure.
Contact Airgas, or another specialty gas
supplier for proper regulator selection.
Steps for selecting relief devices:
1.
2.
3.
Select the type of relief device you
would like to use based upon their
characteristics (see following slides);
Create a schematic of your setup,
identifying the maximum safe operating
pressure for each component of your
system; and
Contact a manufacturer and have them
help you select the proper pressure
relief device.
These valves can “simmer”
(release gas) at 95% capacity.
Spring Loaded Pressure Relief Valves
Require annual inspection
and bench top testing.
Open at a specified set pressure and
reseat after blow-down (2 to 20%
below set-pressure).
Outlet gases need to be channeled to
an appropriate location.
(diagram from Clarkson University)
A disc’s stress at
the burst point is much greater
than the yield stress.
http://oseco.com/na/home/
Rupture discs contain a one time
http://www.zookdisk.com/
use membrane that ruptures near a
specified set pressure.
Discs need to be replaced
once they are used.
Rupture discs fatigue over time and
need to be replaced according to
manufacturer recommendations.
Outlet gases need to be channeled
to an appropriate location.
(diagram from Clarkson University)
Buckling pins buckle at a precise
set pressure allowing the valve
to isolate upstream pressure.
Rupture pins buckle at a precise
set pressure allowing the valve
to relieve the pressure. The
vented gases will need to be
channeled to an appropriate
location.
Expensive, but there
are no size or
pressure limits for the
design of the valve.
Pins can be replaced
while the valves are in
service. Valves
close/open within
milliseconds.
Install, test, and maintain pressure relief devices
according to manufacturer recommendations.
Pressure relief devices can fail if not properly
installed and maintained.