CASE STUDIES - Eventsforce · 2019. 9. 1. · • Ignition during FIBC discharge into silo. • 2 x FIBCs already discharged. ... pneumatic conveying of powders”. ... trials it

The Global Experts in Explosion & Process Safety

CASE STUDIES

© 2013 DEKRA

Ian Pavey Principal Electrostatics Specialist

Chilworth Technology Ltd


Incident Investigations

• Flammable Atmosphere?

• Ignition Sources - Which are impossible? - Which are possible? - Which are likely? - Which could have ignited the

flammable atmosphere

• Conclude which are most likely / likely / possible

• Recommendations for future safe operation

© 2013 DEKRA

Static Electricity


Earthing of Conductors

• IEC/TS 60079-32-1 states in various places that all conductive

items should be properly earthed.

• Previous guidelines have said the same.

• The concept is simple and straightforward.

• The practice is almost always simple and straightforward.

• So how does it still get missed?

© 2013 DEKRA


Earthing of Conductors – Incident 1

• Ignition during FIBC discharge into silo.

• 2 x FIBCs already discharged.

• 3rd FIBC set to discharge and left.

• Explosion shortly afterwards.

• Severe damage to building.

• FIBC all but melted away.

• Minimal damage to silo.

• No personnel hurt.

© 2013 DEKRA



• All ignition sources except static eliminated.

• Powder MIE 15 – 20 mJ.

• Post incident investigation found: - Grille over hatch only fortuitously earthed.

- Some pipe sections in area not earthed.

© 2013 DEKRA



CONCLUSION – most likely scenario…

• FIBC sagged during discharge to partially block grille.

• Some powder dropped into workplace, some already there.

• Isolated grille charged by powder or FIBC.

• Spark discharge between grille and silo ignited dust cloud in

and around FIBC.

• Secondary explosion of powder in workplace.

• Other isolated pipe section could have led to a spark discharge.

© 2013 DEKRA



RECOMMENDATIONS

• Reduce powder in workplace (improve housekeeping).

• Positively earth all conductive items.

• Never rely on fortuitous earthing.

© 2013 DEKRA



• Transfer of fine powder from IBC to hopper.

• Platform-level IBC manually rodded.

• Internal ignition during transfer.

• Operator on platform next to IBC.

• Internal damage only.

• Operator unhurt.

© 2013 DEKRA



© 2013 DEKRA

• All ignition sources except static eliminated.

• Powder MIE up to 15 mJ.

• Post incident investigation found: - Spool adaptor isolated.

- Personnel issued with static dissipative footwear.

- Platform floor epoxy-coated.



CONCLUSION – most likely scenario…

• Powder falling through chute charged isolated spool piece.

• Spark discharge from spool ignited powder.

• Discharge from isolated operator via s/s rod to chute not entirely

ruled out.

• Operator protected by powder plug in IBC and loose fitting

cover on hopper.

© 2013 DEKRA



RECOMMENDATIONS

• Positively earth all conductive items.

• Where personnel must be earthed via footwear ensure floor

surface is sufficiently conducting and earthed.

© 2013 DEKRA


Flexible Hose Incident

© 2013 DEKRA

• Transfer of powder through wire supported

flexible hose.

• During transfer cracking and bang heard.

• Flexible hose blown off, smoke,

internal blackening and burnt powder.

• Nobody hurt and damage minimal.

• Down time for cleaning and review.



© 2013 DEKRA

• 2003 guidance (CLC/TR 50404) advised:

“Conductive pipes with non-conductive linings … should not be used for the

pneumatic conveying of powders”.

because of the propagating brush discharge hazard.



© 2013 DEKRA

• Through this and numerous similar incidents, and laboratory

trials it has become clear that the earthed spiral in an insulating

hose is enough to produce a propagating brush discharge.



© 2013 DEKRA

RECOMMENDATIONS

• The inside of wire spiral supported flexible hoses for powder

transport should always be static dissipative and earthed

(


The Operation

• Charge 10000 litres used toluene to vessel via dip pipe

• Once impellor covered start agitator

• Add water using hose via manway; typically 200 – 500 litres

• Add phosphoric acid / sodium hydroxide as required

• Stir for 10 minutes

• Settle

• Run off separate phases

Liquid Mixing Incident

© 2013 DEKRA


The Incident

• 4400 litres toluene already added

• Water addition started

• Phosphoric acid valve opened

• Flash fire in head space

• Remote witness reported “bang

and whoosh”

• Operator not seriously hurt


© 2013 DEKRA


The Investigation

• Pure toluene is highly insulating.

• Used toluene found to be insulating.

• Dispersed second phase significantly increases charge generation risk.

• Other ignition sources discounted.


© 2013 DEKRA


Recommendations

• Add a conductivity modifier to toluene.

• Check conductivity OK before adding aqueous phases.

• Ensure you do understand the hazards associated with the materials you are handling.

• Even if you have been operating for some time, make sure there is a proper hazard assessment in place.


© 2013 DEKRA


The Operation

• Powder charged to vacuum dryer

• Vacuum pulled with heating and

agitation

• Vacuum relieved, dryer inverted,

powder discharged

Vacuum Dryer Incident

© 2013 DEKRA


The Incidents

• Some rapid / explosive

decompositions as vacuum

relieved

• One resulted in loss of

containment

• Nobody hurt


The Material

• Highly insulating powder

• Very sensitive to ignition

• Actually, ignition starts as

decomposition for which air is

not necessary

The Investigation

• After extensive investigations,

settled on static

© 2013 DEKRA


Repulsive Force of Charged Particles

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Increase Potential Energy by Forcing

Charged Particles Together

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Cone Discharges


© 2013 DEKRA


Paschen’s Curve

0.1

1

10

100

1000

10000

0.001 0.01 0.1 1 10 100 1000

Product of Air pressure and Electrode Gap (bar.mm)

Bre

ak

do

wn

Vo

lta

ge

(k

V)


© 2013 DEKRA


Recommendations

• Limit vacuum to 20 mbara

• Ensure change control system is in place such that even the effects of “minor” changes are properly assessed


© 2013 DEKRA


Electrostatic Hazards

• Incidents due to static still regularly occur.

• IEC/TS 60079-32-1 is an excellent review and advice document.

• Anyone involved with handling flammable or unstable materials would be well advised to get a copy.

• If you struggle with IEC/TS 60079-32-1 or are unsure get help – it could save lives.

© 2013 DEKRA

Documents

CASE STUDIES - Eventsforce · 2019. 9. 1. · • Ignition during FIBC discharge into silo. • 2 x FIBCs already discharged. ... pneumatic conveying of powders”. ... trials it