sugar dust explosion protection techniques and technologysugar dust explosion protection techniques...

Julian Turner

Sugar Dust Explosion Protection Techniques And

Technology

5th April 2018

British Society of Sugar Technologists

European legislation

Explosion protection is controlled by the ATEX Directive.

Employers must protect their employees from the harmful

effects of an explosion when processing combustible

powders.

Suppliers have to demonstrate to an independent Notified

Body that their explosion suppression systems will deliver the

performance that are claimed. (Pred) This is achieved by

generating calibrated explosions in 10 Bar vessels and

measuring the results.

Explosion isolation must be proven too using real explosions.

Suppliers must demonstrate that their equipment is not an

ignition source in our customers’ processes.

Dust Explosions

• Any combustible material will burn with a speed that

increases with decreasing particle size

• For a dust explosion to propagate we require:

– Oxidising medium; usually oxygen

– Ignition source

– Adequate fuel concentration

ExplosionFast CombustionSlow Combustion

Explosion venting

▪ Standard

▪ Flameless

Explosion Suppression

▪ Unique dynamic detection (MEX)

▪ Range of suppressor sizes and features

Explosion isolation

▪ Active and passive

▪ Optical detection

Products / Systems

Confined Dust Explosions

• Maximum pressure (Pmax) dependent on fuel and initial pressure

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35

Time / s

Pre

ssure

/ b

ar(

g)

20 m3

6 m3

2 m3

Vessel Volume

STKVdt

dP 3/1

dP

dt

Example explosion values:

Kst values

Coal 75

Flour 100

Starch 150

Sugar 180

Pharmaceutical 240

Washing powder 280

Aluminium flake 555

Kg values

Methane 55

Propane 100

Hydrogen 550

Explosion Protection Options

Vessel Protection

Containment

•10 bar strong vessels

•Very expensive

•Not practical for large vessels


Vessel Protection

Containment Venting

• Very common

• Least expensive option

• Very common due to low cost and simplicity

• Weak panel on vessel – yields ~0.1bar

• Pressure relief only – does not extinguish flame

• Post-explosion fires likely

• Area of exclusion required - including flameless vents

• Flame ejection from vent – 8 x vessel volume

Explosion Venting

10

60m3 Vented Explosion


Vessel Protection

Containment

Pmax = Pred > 8bar

Suppression

Relieves pressure

Extinguished flame

Contains explosion

Venting

Relieves pressure

Inexpensive

Does not extinguish flame

Fireball 8x vessel volume

Explosion Suppression in detail

Ignition:

Time = 0 ms

Pressure = 0 bar(g)

Pressure wave ~300m/s

Flame front ~ 10m/s]

Detection:

Time = 70 ms

Pressure = 0.05 bar(g)

Suppressors Actuate:

Time = 80 ms


Suppression Complete:

Time = 100 ms


V0=10m3, Kst= 150 bar.m/s

Pressure Time Characteristics

Pa = Activation Pressure of Pressure Sensor

Ps –Plant Strength

Explosion Pressure [bar]

Maximum Explosion Pressure Pmax

Normal Explosion Development

Reduced Explosion Pressure Pred

Deployment of explosion suppressant

Pa

taTime [milliseconds]

1. Suppressant is a fine sodium bicarbonate powder

2. Suppressant has a large surface area per mass delivered.

3. Suppressant achieves extinguishing of the fireball by extracting heat . We aim to reduce the temperature of the fireball below the Auto Ignition Temperature of the product.

4. Suppressant isolates unburnt product from burning product.

How does suppressant work?

Radiation

Two unburnt coal particles

Coal particle

burns at 1,800

deg C

Unburnt particle

will self combust at

500 deg C (coal)Two burning particles – flame

propagation continues

Suppressant

engulfs

burning

particles

Suppressant forms a

barrier between

burning and unburnt

particles.

All remaining

particles

cooled to <500

deg C

The explosion is

extinguished

Validation by Experimental Data

100 150 200 250 300 350 400 450 500

0.5 m³ Vessel

50 mbar

100 mbar

150 mbar

100 150 200 250 300 350 400 450 500

Vessel Volume 0.5m3

100 150 200 250 300 350 400

100 150 200 250 300 350 400 450 500 100 150 200 250 300 350 400 450 500

Vessel Volume 25m3

Vessel Volume 250m3Vessel Volume 10m3

100 150 200 250 300 350 400 450 500

0.5 m³ Vessel

50 mbar

100 mbar

150 mbar

100 150 200 250 300 350 400 450 500

0.5 m³ Vessel

50 mbar

100 mbar

150 mbar

100 150 200 250 300 350 400 450 500

0.5 m³ Vessel

50 mbar

100 mbar

150 mbar

Kmax – bar.m/s

Kmax – bar.m/sKmax – bar.m/s

Kmax – bar.m/s

Tim

e (d

t)Ti

me

(dt)

Tim

e (d

t)Ti

me

(dt)

17

Suppression system design is critical to prevent

failure.

Principle of Explosion Detection

t

p t0

t0

p = pt0

p agw

t 0t

p

exp

losio

n p

ressu

re p

[b

ar]

time t [ms]

a

- t

- t

P static alarm pressure

p dynamic alarm pressureP =p pressure at the time of activation

P pressure at the time before activation

t dynamic alarm timet time of activation

t time before activation

agw

t0 a

t0-dt

0

t0-dt

Explo

sio

n P

ressure

[ba

r]

Time [milliseconds]

dt

dP

Introduction to dynamic explosion pressure

detection

Principles of dynamic explosion detection

• Pressure is monitored 1,000 times per second.

• The dynamic detector is programmed to respond

to a pressure rise of a known kst value in a vessel

of a given volume and aspect ratio (surface area).

• Including aspect ratio accounts for flame stretch

in an elongated vessel. This effects explosion

suppression and active explosion isolation

performance.

• dP is a value set by the designed. dT is calculated

by research based unique algorithm.

• Where process pressures rise and fall the

dynamic detector will assume a new reference

point.

• Typical set point is 50mBar in 80mSec.

Dynamic detection – the window of detection

dt

dP

Alarm

Ignore

Fault

Fault : Alarm : Ignore

Kreal

dt

dP

Alarm

Kmax

Slow explosion

“dPslow” - criteria

Pred Kreal< Pred Kmax

Ignore

Process pressure

fluctuation

Fault

e.g. Electrical noise

View in

presentation

mode

Dynamic detection – the window of detectionE

xp

losio

n P

ressu

re [b

ar]

Time [milliseconds]

dt

dP

dt

dP

Fault : Alarm : Ignore

dt

dP

Dynamic detection – the window of detectionE

xp

losio

n P

ressu

re [b

ar]

Time [milliseconds]

View in

presentation

mode

Dynamic Explosion DetectionP

roce

ss P

ressu

re [m

ba

r] 50mbar Static Pressure Detection Limit

Pneumatically conveyed batch processing

Dynamic pressure detector will not respond to this

overpressure because dt has not been met or exceeded.

Event Captured Pressure Time DataStrong explosion in bucket elevator HRD discharge only

EMC interface due to poor installationExplosion in large spray drier

All vessel protection options require explosion isolation to

minimise the risk of explosion propagation

Flame Transfer between connected plant leads to enhanced

explosion severity – Flame Jet Ignition

Explosion Isolation

Explosion Isolation much more complicated than Suppression!!

Introduction to Explosion Isolation

How to Isolate? Passive or active?

• Passive Isolation

– Explosion pressure actuates mechanical device;

• Active Isolation

– Mechanical – fast acting valves

– Chemical – Suppressant barriers

29

air flow ~16 m/s

DN500 duct - ~30 m long

vent panel

26 m3 vessel9.6 m3 vessel

DN500 air inlet

vent panel

DN500 to cyclone

V1V2

Active Explosion Isolation Tests at FSAFSA: Forschungsgesellschaft für angewandte Systemsicherheit und Arbeitsmedizin

FSA are a notified body for ATEX approvals

air flow ~16 m/s

DN500 duct - ~30 m long

vent panel

26 m3 vessel9.6 m3 vessel

DN500 air inlet

vent panel

DN500 to cyclone

V1V2

Explosion Isolation System

• System Design Premise – isolation device location

• Probability and consequence of flame propagation

dependent on primary vessel explosion protection used

– Vented and contained vessels ~95% chance of

flame propagation into a duct

– Suppressed vessels ~45% chance of flame

propagation into a duct

Based on actual research.

Why Isolate?

No Explosion Isolation: Pressure vs. time

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

Time / s

Pre

ssure

/ b

ar(

g)

V1

V2

Flame Entry into Duct Flame Exit from Duct

Explosion Isolation: Pressure

vs. time

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

Time / s

Pre

ssure

/ b

ar(

g)

V1

V2

Flame Entry into Duct

45

Explosion Isolation: Pressure vs. time

46

During this test programme, IEP Technologies performed 81 full-

scale interconnected vessel explosion tests.

In every case, flame entered the duct and propagated several

metres

77 out of 81 tests showed flame propagation over 21m

Thus for these geometrical configurations, dust concentration,

turbulence levels and ignition locations, flame transfer between

vessels is very likely.

In most cases, severe flame acceleration was observed leading to

enhanced explosion severity in the connected vessel.

Flame Propagation Probability

Schematic of an Explosion Isolation System

Detector, ta

Duct transit, tdDuct entry, te

Controlling Equation

ta + tb < te + td

d

tb

Active Explosion Isolation

0

50

100

150

200

0 50 100 150 200

experimental tff / ms

model t ff

/ m

s

perfect fit

1 m3 data

4.25 m3 data

9.4 m3 data

Maize Starch Explosions

?

Importance of Ignition Location

CFD simulation - 180ms from ignition as a function of ignition location

Importance of Ignition Location

Isolation Calculation Tool - SmartIS

SmartIS™ assumes worse case ignition locationPressure only assumes ignition close to duct mouth – long isolation

distances required

Pressure and Optical detection significantly reduces isolation distance

since ignition close to the duct mouth is detected very early by the optical

device

Isolation Calculation Tool - SmartIS

Passive Explosion Isolation Valves

Typical Passive Isolation valve▪ Uses the pressure of the explosion to

close the flap.

▪ Typical duct diameters of DN 160mm up

to DN 1000mm

▪ Typical values of Pred = 1.0 bar g. Must

therefore be used in combination with

explosion venting or explosion

suppression.

▪ Suitable only for low dust concentrations

▪ Can only be installed in the horizontal.

▪ Must be installed in line with the

manufacturer’s guidelines.

Thank You

Back-up Slides

Isolation

Full-Scale Testing

DN300 – 30m long

Vent Area=0.5m2

Vent Area=0.26m2

Fan

V1=9.6m3

V2=4.4m3

Air velocity ~16m/s

Air in

Air out

DN300 – 30m long

Vent Area=0.5m2

Vent Area=0.26m2

Fan

V1=9.6m3

V2=4.4m3

Air velocity ~16m/s

Air in

Air out

DN300 – 30m long

Vent Area=0.5m2

Vent Area=0.26m2

V1=9.6m3

V2=4.4m3

Air velocity ~16m/s

Control

Panel

DN300 – 30m long

Vent Area=0.5m2

Vent Area=0.26m2

V1=9.6m3

V2=4.4m3

Air velocity ~16m/s

Control

Panel

Explosion Isolation Trials at FSA

No Isolation Isolation 1 x HRD

65


66


67


Optical Detectors

Optical detection dependent on observing radiation from combustion

zone – line of sight

Limited emission wavelength signatures – black body

Dust laden environments bring difficulties

Typically not used for detection within process vessels

Are useful for flame detection within connections – duct work.

Explosion isolation systems

1.3 Products / Systems

Understanding Application of Optical Detectors

Optical detection efficacy will be dependent on the radiation obscuration,

detector sensitivity and duct diameter.

Dust concentration: 100g/m3 of Maize gives ~50% obscuration for every ~10cm!

Dust colour: Black dust attenuates radiation ~2 times more than white dust

Dust median particle size: 35mm attenuates radiation ~3 times more than 70mm

Other factors – cleanliness of optics and detector sensitivity

ALL OF THE ABOVE DETERMINE DETECTION EFFICACY

1.3 Products / Systems

70

Advanced Calculation Tools: CFD

We have at our disposal advanced calculation tools and expert

knowledge of flame propagation

This allows us to generate more advanced in-house calculations tools

and more secure design guidance based on this data

71

During this test programme, IEP Technologies performed 81

full-scale interconnected vessel explosion tests.

In every case, flame entered the duct and propagated several

metres

77 out of 81 tests showed flame propagation over 21m

Thus for these geometrical configurations, dust concentration,

turbulence levels and ignition locations, flame transfer

between vessels is very likely.

In most cases, severe flame acceleration was observed leading

to enhanced explosion severity in the connected vessel.

Flame Propagation Probability

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