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TRANSCRIPT
4/18/2016
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Failing protective measures: what happened?
Dr. Scott G. Davis
617-407-3300
GexCon US
Bethesda, Maryland
Generally accepted protective safety measures
• Containment
• Explosion venting including flameless venting
• Suppression
• Isolation techniques
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Standards protective systems in USA
NFPA 68
Vent sizing dust explosions
NFPA 68
Venting devices
NFPA 69
Explosion resistant products
NFPA 69
Explosion isolation
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Containment: explosion resistant equipment
• Designed to withstand maximum explosion pressure with or without allowance for deformation
• Possible design for lower overpressure when in combination with venting or suppression
• Shall allow for pressure piling in case of connected equipment
• Main reasons for failure: poor design and no isolation or design for pressure piling
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Failure of containment: pressure piling
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Pressure piling
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Explosion Venting
• a cover that opens at a given overpressure as a result of an explosion.
• leads to a “leakage” that stops the explosion pressure exceeding the maximum pressure that the process vessel can withstand.
Explosion vent panels
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Explosion venting
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Example of pressure development
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Explosion venting and what can cause it to fail
• Pros
• Passive system, low-cost, low maintenance
• Render the equipment effectively “explosion proof”
• Cons (things to look for in investigation)
• Should not be used in buildings
• If inside long vent ducts or quench pipes (reduce efficiency
• External blast and flames (can cause fires)
• Design
• Was adequate area utilized
• Passive - Maintenance forgotten
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Calculation of necessary vent area
• Calculated using various methods, e.g.:
• NFPA 68, Standard on Explosion Protection by Deflagration Venting, 2013 Edition (USA)
• EN14491 Dust Explosion Venting Protective Systems (March 2006)
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Necessary information for vent area calculation
• Explosion properties, Pmax and KSt-value (did the dust explosion properties change?)
• Geometry (volume, length, diameter, vent location, and presence of venting ducts if applicable) (were they all taken into account when designing the vent opening)
• Design/tolerance pressure of process vessel (Pred) (was there any calculation/assessment performed to determine the strength of the process vessel?)
• Opening pressure of the vent panels, (Pstat)
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Explosion venting – poor design
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Trends and relationships
• Large volume large vent area
• Small volume small vent area
• Weak process vessel large vent area
• Strong process vessel small vent area
• High (dP/dt)max or KSt-value large vent area
• Low (dP/dt)max or KSt-value small vent area
• High opening pressure (Pstat) large vent area
• Low opening pressure (Pstat) small vent area
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Cylindrical enclosures (silo)
• Venting device at end
High L/D ratio
• If venting device is located near the middle, the maximum flame length is reduced
Lower L/D ratio
• “Conclusion”: Position of vent panels (relative to ignition location) is IMPORTANT.
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Reduced venting efficiency
• Presence of flame arrester/dust retention
• Inertia of venting device
• Venting via ducts
• Blockage of vent area
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Influence of venting ducts
Reduces the efficiency of venting
0
0.5
1
1.5
2
2.5
0 1 2 3 4 5 6 7
pre
d(b
ar)
Vent duct length (m)
V=1m3; KSt = 200 bar-m/s, A = 0,196 m2
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Bag filters can block the vent area
Proper venting arrangements: vertical filters
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Improper venting arrangement: vertical filter
Reduce the effective vent area
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Examples of poor design: bag filter
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Bag filter after explosion
Big damage in spite of explosion venting
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Poor design: possible cause
Blockage of vent opening by filter bags
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External effects: flames
Flame length ejected from vent:
– K = 10 for metal dust; K = 8 for agricultural or chemical dusts
– when V = 50 m3, single vent, chemical dust LFH = 30 m
Flame deflector
𝐷 = 𝐾 ∙𝑉
𝑛
1 3
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External effects: pressure waves & recoil forces
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Other important design related aspects to consider:
“Vacuum breakers”
(when using self closing panels)
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Example – Implosion Damage Investigation
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Background
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Single burner, recirculating solids ring dryer
Not equipped with a vacuum breaker
4 deflagration incidents → each resulting from a unique chain of events
Similarities
observed overpressures
number of vent doors opened
Internal vacuum which collapsed sections of the dryer
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Model of the dryer
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Ring recirculation portion
Solids introduced
Recirculation fan
Equipment imploded during three of the four incidents
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1st Incident 2nd Incident 3rd Incident
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Considered dust clouds located throughout the dryer
Downstream of burnerIn exhaust headerIn drying column
In pre-separator In section of ring In a cyclone
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Pressure and flame front development with ignition in the exhaust header
Not enough. Needed cooling to obtain Pmin 20 sec after incident.
Flameless venting devices
Allows explosion venting inside buildings
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Potential causal factors with flameless venting devices into rooms
• Safety distance (combustion gases or hot)
• Increase of pressure inside building/room
• Reduced venting efficiency
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Venting into safe direction: hot combustion products are released
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Improper venting into building/ blocking of vent
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Possible reasons for deflagration vent failure
• Improper design
• Use of wrong explosion properties (e.g. no change of vent size after change of process)
• Strength protected vessel
• Geometrical aspects not taken into account• No account for external effects (pressure waves, flames,
safety zone)
• Blockage of vent opening (reduced efficiency)
• Other aspects
• Can lead to fires
• Passive system - maintenance is forgotten
Venting is not that easy – system must be evaluated
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Explosion suppression
• Fast-acting extinguishing systems that are activated at an early stage of an explosion thus quenching it.
• Requires fast and reliable detection system. Need to evaluate the design
• Often the only solution in many situations, e.g., Protection of coupled systems.
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Explosion suppression
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Pressure at
sensor
reaches
activation
levelIgnition
Opening of HRD Suppressant
reaches flame
Explosion
is
suppressed
time = 0 ms
pressure = 0
bar
time = 20 ms
pressure = 0.05
bar
time = 30 ms
pressure = 0.1
bar
time = 50 ms
pressure = 0.2
bar
time = 75 ms
pressure = 0.25
bar
Explosion suppression
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Venting vs. ….
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….successful suppression
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Successful suppression
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Explosion suppression in time
• Early detection of explosion
• Rapid identification of signal and action plan
• Rapid activation of suppressors
• Rapid distribution of suppressant
• Fast quenching of explosion flame
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Prevention of false activation:
dynamic pressure detection
t
p t 0
t0
p = pt 0
p agw
t 0t
p
Explo
sio
n p
ressure
p [
bar]
time t [ms]
a
- t
- t
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Failure due to explosion suppression: summary
• Improper design
• False activation
• Use of wrong explosion properties (e.g. no change of activation time after change of process)
• Strength protected vessel
• Geometrical aspects not taken into account
• Other aspects
• Active system - maintenance needs to be performed at least once every year; inspection every quarter
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• Problem: explosion will be able to propagate from one process unit to an interconnected one.
• “Pressure piling”, designing protective equipment for interconnected process units not possible with simple calculation methods (such as NFPA 68 for explosion venting)
• Result: uncontrollable explosion development and high explosion pressures may arise.
Explosion isolation
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Cannot be used for interconnected vessels
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Pressure piling in interconnected vented vessels
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Explosion isolation
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Can it really happen?
Passive isolation devices
• Self-closing explosion isolation valves
–Ventex valves
–Flap valves
• Rotary valves
• Explosion diverters
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Self-closing explosion valves
Ventex-ESI-E/-D/-C
Ventex valve
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Typical application: clean side of filter to protect fan
• Low dust concentrations only (< 50 g/m3); might fail at higher dust concentrations
• Should be placed horizontally, 5 m from where the explosion starts, shorter installation distances might make it fail
Ventex valve
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Self-closing explosion valves
• Is especially used in combination with dust collectors/filters
• Open during normal process conditions, closes in case of explosion due to flow reversal
Explosion isolation flap valve
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Explosion isolation flap valve
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Explosion isolation flap valve
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Possible reasons for isolationflap valve failure• Is the valve at the correct distance - dependent on closing
time valve (weight, dimensions); typically a few meters (is dust explosion property dependent)
• Can the valve withstand the expected explosion pressure
• Possible issues: re-opening during explosion venting event and dust settling at seal
• Regular inspection
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Rotary valve
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Rotary valve
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Possible reasons for rotary valve failure
• Poor design (dust explosion properties, maximum expected explosion pressure vs. strength of housing or blades)
• Erosion of blades: increase of gap width beyondcritical gap width
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Explosion diverter
Protective catching device
Explosion venting device
Flow reversal
In combination with
extinguishing barrier64
• Reversal of flow from inner pipe by 1800
• No full explosion isolation possible but avoids flame jet ignition and allows
for “normal” design of protective measures of coupled equipment
Possible reasons for explosiondiverters failing
• Explosion pressures generated exceed mechanicalstrength (poor design)
• High static opening pressure or specific weight ofexplosion venting device
• Poor design (shall be in accordance with EN 16020)
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Active explosion isolation systems
• Slam shut valves
• Extinguishing barriers
• Elements:
–Detection
–Control unit
– Isolation device
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Active isolation system using an extinguishing
barrier
Extinguishing
barrier
Infrared
detector
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Extinguishing barrier
• Installation distance dependent on KSt-value, process conditions, detection principle, geometry aspects, deployment time barrier
• Suppressant concentration in barrier shall be sufficiently high during explosion event
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Extinguishing barrier
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Extinguishing barrier
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Failure of extinguishing barrier
• Improper design
• False activation
• Use of wrong explosion properties (e.g. no change of offset distance after change of process)
• Geometrical aspects not taken into account
• Other aspects
• Active system - maintenance needs to be performed regularly
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Active isolation system using a slam
shut valve
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Slam shut valve
• Installation distance dependent on KSt-value, process conditions, detection principle, geometry aspects, closing time valve
• Valve shall be able to withstand maximum (reflected) pressure at valve location
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Maximum installation distance
• Strength of ducting, valve
• DDT = Deflagration Detonation Transition
– DDT for gas = 40D
– DDT for dust = 80D
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Explosion isolation: slam shut valve
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Explosion isolation: slam shut valve
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Failure of slam shut valve
• Improper design
• False activation
• Use of wrong explosion properties (e.g. no change of offset distance after change of process)
• Geometrical aspects not taken into account
• Other aspects
• Active system - maintenance needs to be performed regularly
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Failure of protective measures: summary
The main factors for protective measures failing include:
• Improper design
• No use of guidelines
• Use of wrong explosion properties (e.g. no change of vent size after change of process)
• Strength of protected equipment underestimated (incl. vacuum)
• Geometrical aspects not taken into account
• Blockage of vent openings (reduced efficiency)
• Other aspects
• Poor maintenance
• Safety zones
• Prevention of false activation of active systems
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