Journal of Electrostatics, 16 (1985) 247--255 247 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

DI. G~nter L ~ t t g e n s

Co l l ec t i on o f Acc idents caused by S t a t i c E l e c t r i c i t y

I n t r o d u c t i o n

I t i s a remarkable f a c t t h a t " f r i c t i o n a l e l e c t r i c i t y " which has been known f o r many c e n t u r i e s and which in i t s days led t o the phenomenon o f e l e c t r i c i t y s t i l l appears as a mys te ry even today . That could be the r eason why in some cases - f o r lack o f a p l aus ib l e exp lana t i on o f acc iden ta l f i r e s o r e x p l o s i o n s - e l e c t r o s t a t i c charges a r e s t i l l o ccas i ona l l y held r e s p o n s i b l e f o r e v e n t s which t hey did no t c r e a t e . This work shows some r e a l cases in which s t a t i c e l e c t r i c i t y was sou rce o f i g n i t i o n in t he p lant . But above a l l t h i s paper o f case h i s t o r i e s aims a t a m o r e a c c u r a t e assessment o f e l e c t r o s t a t i c hazards so in t he f i r s t p lace an approved method wi l l be i n t r o - duced f o r the a n a l y s i s o f t he inc iden ts . Al l Case H i s t o r i e s (OH-01 t o CH-05) wi l l be ana lysed accord ing t o the fo l l ow ing scheme:

1 C h a r a c t e r i z a t i o n o f t he i nc iden t ( s h o r t desc r i p t i on )

2 I gn i t ab le atmosphere

3 Add i t iona l d e c l a r a t i o n s ( res is tance~ capac i tance etc.}

4 Ana lys is and i n t e r p r e t a t i o n : .1 charge g e n e r a t i o n .2 charge accumulat ion .3 gas d ischarge

5 L o c a l i t y o f i g n i t i o n

6 Eva lua t i on o f t he p r o b a b i l i t y

7 Proposed s a f e t y measures

When i n v e s t i g a t i n g an acc iden t one has t o r e a l i z e t h a t one can neve r be a b s o l u t e l y su re what was t he r e a l cou rse o f e v e n t s which were cause o f t he ca lami ty . N e v e r t h e l e s s in view o f the proposed s a f e t y measures a v e r y t ho rough i n v e s t i g a t i o n i s compulsory.

0304-3886/85/$03.30 © 1985 Elsevier Science Publishers B.V.

248

CH--O!: Pouring Powder into an Agitator Vessel

An agitator vessel A (volume: 6 cubicmeter) was partially filled with ethylacetate E. Via a grounded funnel F of stainless steel placed on the manhole an organic powder P was emptied out of a metal drum D without inserted plastic bag (volume: 60 liters) by an employee L standing on a wooden platform W. T~wards the end oF that pouring procedure an explosion occurred in the funnel area.

CH - 01 Pouring Powder into an Agitator Vessel

Ethylacetate in the vessel was at room temperature. According to the flashpoint of ethylacetate (-4 oC) a combustible vapor~air-mixture had been formed in the funnel area.

The worker was wearing ord inary shoes S with ve ry poor conduc t i v i t y ( res is tance t o ground ~ 1 teraohm). The wooden p la t fo rm i t s e l f showed a res i s tance t o ground o f 200 megaohms. The man's capac i ty t o his environment was found t o be 160 p icofarads. According t o the "time cons tan t equat ion" [ T = R * C ] a r e l axa t i on t ime T o f the worker is ca lcu la ted t o 160 seconds (the f l o o r r es i s t ance is negl ible in t h i s case).

4.1

4.2

4.3

The u t i l i zed organic powder was chargeable mater ia l ( r e s i s t i v i t y > 1 teraohm * cm) and hence i t i s obvious t h a t the charge genera t ion took place between the powder and the metal drum. Charge accumulation resu l t ed on the system "drum and worker" because the ground res i s tance was too high to drain away the charge as f a s t as i t was generated. Discharging sparks might have ar isen between the charged

metal drum and the grounded funnel when the distance was s h o r t enough f o r the spark gap. For eva luat ing the energy W o f t h a t spark the "energy equat ion" i s requ i red: [ W = 0.5 * C * U * U ]. As known from experience the p o t e n t i a l on the drum may eas i l y in - crease to a t l e a s t 3 o r even 5 k i l o vo l t s . Taking 3 k i l o v o l t s as a basis the energy s to red on the capaci ty o f the worker holding the drum might have been 0.72 mi l l i jou le. That i s some what more than the minimum ign i t ion energy o f e t h y l a c e t a t e which is determined t o 0.46 mJ and r e v e a l s the possible sparks being incendive f o r the e t h y l a c e t a t / a i r - mixture. I t has t o be taken in to account t h a t the once ign i ted vapor could i gn i te subsequent ly the d u s t / a i r - m i x t u r e o f the powder.

The ign i t ion was located a t the funnels edge near the metal drum because the re combustible atmosphere and ign i t ion spark could have coincided.

The a b i l i t y o f a disharging spark t o produce an ign i t ion is governed l a rge l y by i t s energy which was in th i s case higher than the ign i t ion energy requ i red f o r e t hy l ace ta te .

Correspondingly the s a f e t y measures may be confined to the avoiding of e l e c t r o s t a t i c hazards which c l ea r l y come up as cause f o r th is incident. In th is case only grounding the metal drum is demanded e i t h e r v ia the employee (conductive shoes and standing on a grounded locat ion) o r by means o f a grounding wire t o the drum. The res i s tance to ground must be kept below 100 megaohms.

249

CH-02: Le t t ing o f f Ethylenoxide in to a p las t i c bucket

A tank vesse l f o r ethylenoxide ( l i qu i f ied gas) was near ly emptied and the remaining few l i t r e s were to be disposed of . Because the vesse l was under low ove rp ressu re deplet ing was done v ia the bottom va lve in to a p las t i c bucket (volume: I0 l i t r e s ) . To prevent too much vapor iz ing some water was given p rev ious ly in to the bucket to cover the ethylenoxide. According t o the workman's statement an ign i t ion occured immediately a f t e r opening the valve.

The temperature o f the vesse l was about 16 oC (boi l ing point o f ethyleno×ide: ~12 oC). A combustible mixture o f e thy leno×id- vapor in a i r had to be expected on the procedure o f expanding.

250

3 The workman had worn s a f e t y shoes wi th conduc t i ve s o l e s and was s tanding on a conduc t i ve f l o o r o f concre te . The r e s i s t a n c e t o ground was l ess than 20 megaohms r e v e a l i n g t h e r e had been no dangers by a charged person.

4.1 The assumpt ion o f an e l e c t r o s t a t i c i g n i t i o n became p laus ib l e when we heard t h a t in t h i s case a new p l a s t i c bucket had been taken from a p i le , where the buckets were s t o r e d one i n t o ano the r . For r e c o n s t r u c t i o n o f t h i s acc ident a n o t h e r p l a s t i c bucket was taken from the p i le . This bucket showed a s i g n i f i - c a n t l y high e l e c t r o s t a t i c charge which caused a f i e l d s t r e n g t h o f about 8 k i l ovo l t s / cm. Undoubtedly t h i s charge was genera ted by s e p a r a t i n g one bucket from the o t h e r and in f a c t the oppos i t e charge was e s t a b l i s h e d on the nex t bucket remaining in t he p i le .

4.2 After pouring 3 litres of water from another (old) plastic bucket into this charged bucket a voltage of 6 kilovolts to ground was measured with a static voltmeter via an insulated probe put into the water. The capacitance to the environment of the water in the held by hand bucket was about 30 picofarad. With the equation: W = 0.5 * C * U * U a spark energy of 0.5 millijoule is determined. The minimum ignition energy of ethylenoxide is 0.07 millijoule, and its resistivity is 10 megaohms*cm. From these data we have to assume the i g n i t i o n having occured as fo l lows :

4.3 The j e t o f e t hy l enox ide leav ing the v a l v e c o n s t i t u t e d a conduc t i ve path t o the grounded tank vesse l . As i t met the charged -and a l so c o n d u c t i v e - wa te r a d ischarg ing spark in which the above energy was r e l e a s e d occured between the two l iqu ids .

In the area of this spark a combustible mixture of ethylenoxide in air was created by the instantaneous vaporizing of the liquified gas.

This spark could s e t f r e e an energy o f more than ten t imes the minimum i g n i t i o n energy o f e thy lenox ide .

The s a f e t y measure suggested he re i s t o rep lace t he p l a s t i c buckets with metal ones. For t hese metal buckets t h e i r g roun - ding is compulsory, which may be accompl~shed by t he grounded workman (v ia conduc t i ve shoe so les and conduc t i ve ground). I f t he man is wearing g loves t h e s e must be conduct ive , t oo .

2

CH-03: I gn i t i on Sparks from Meta l F langes o f a Glass Appara tus

They a r e o f t e n d iscussed bu t n e v e r t h e l e s s v e r y r a r e inc iden ts : spa rks f rom metal f l anges o f a g lass appara tus . In a 23 y e a r pe r i od we have had on ly one i g n i t i o n and i t happened on a d i s t i l l a t i o n a p p a r a t u s f o r s e p a r a t i n g t o l uene and t e t r a c h l o r c a r b o n on a cold w in te r day wi th v e r y low r e l a t i v e humidi ty in t he bui ld ing.

There was a leak ing f lange connect ion in t he t o l u e n e backst ream a r e a where a small amount o f t o l u e n e was dr ipping down t o the g lass pipe showing a t e m p e r a t u r e o f abou t 60 oC. The e v a p o r a t i n g t o l u e n e formed a combust ib le atmosphere in the envi ronment.

3 The l iquid had a spec i f ic e l e c t r i c a l r es i s tance o f 400 gigaohms*cm thus giving d ispos i t i on t o e l e c t r o s t a t i c charging. On the 2- inch-d iameter g lass pipe system were seve ra l metal f langes ungrounded showing a capacitance t o ground o f about 20 p icofarads.

4.1 The charged l iqu id f lowing through the glass pipes t r a n s - f e r r e d i t s charge v ia e l e c t r o s t a t i c induct ion to the f langes. One o f these f langes was located in the v i c i n i t y o f a grounded s t e e l cons t ruc t i on o f the building.

4.2 Survei l lance o f t h i s f lange showed within about 40 seconds a r i s e o f i t s p o t e n t i a l from 0 to 8 k i l ovo l t s .

4.3 A f t e r th is a spark discharge occurred to the s t e e l suppor te r . This incident happened over and over again, changed only in i t s per iod by the f lowing speed o f the l iquid in the g lass pipe.

The ign i t ion occurred a t the determined spark locat ion.

The minimum ign i t ion energy o f to luene i s determined to be 0.24 mi l l i jou le . The energy W re leased by the spark can be ca lcu la ted by the equat ion W = 0.5 * C * U * U thus leading to 0.6 mi l l i jou le which i s more than twice the minimum ign i t ion energy. That r a i ses the quest ion f o r the apparent low p robab i l i t y o f t h i s incident (only one in count less s imi lar operat ions) . The reason seems to be t h a t only in ve ry few cases a combustible atmosphere had been formed in the v i c i n i t y o f a possib le spark gap.

The proposed measure is grounding the metal f langes.

post scriptum: A f t e r each metal f lange had been connected by wires t o an ear th ing cable the crew f e l t safe . But a few days l a t e r an open-eyed worker observed sparks again a t the same f lange and he cal led us. I did not be l ieve an ear thed f lange could produce sparks but I became an eye-w i tness o f t h a t "miracle". What had happened was t h a t the ear th ing cable to which the f langes were connected had no connexion i t s e l f t o ground. The responsib le e l ec t r i c i an t r u s t e d with thought lessness only in the ye l low/green cable marking and did not check the proper grounding.

251

CH-04: Igni t ion in a Tank Vessel f o r Diphenyl

Diphenyl was s t o red in a ho r i zon ta l metal tank (A) o f I00 cubicmeter volume t h a t had a thermal i nsu la t ion on i t s o u t e r wall. In t h i s tank were 2 heat ing pipes (H) p a r a l l e l t o the axis through which warm water was streaming with a temperature o f 90 oC. The s o l i d i f i c a t i o n point o f diphenyl i s 69 oC. On account o f changing the product ion process the tank was emptied completely and cleaned. When f i l l i n g again the tank v ia pipe (P) an explosion occurred in the tank when f i l l e d about hal f -way. Due t o th i s explosion the tank's dome was thrown away and the dome area was se t on f i r e .

P

252

CH-04 Explosion in Tank Vessel

Spec. Remtance 1016, Ohm, cm

10 tS.

10".

1013.

1012.

1011.

lo~O

i J I

i

I I

70 ~ ~ 1~o 1;o 1~o Teml~dure ~C

CH -04 Relation between Temperature and spec. Reetstance of DlphenyL

253

Due to the cleaning process a i r had come into the tank. The temperature of the del ivered l iquid diphenyl was about 120 oC. I t s f lashpoint is a t 113 oC meaning tha t a combustible atmosphere could have ben formed.

The diagram shows tha t l iquid diphenyl has a r e s i s t i v i t y s l i gh t l y above 10 gigaohms*cm tha t is -by experience- the lower l imit fo r charge disposit ion. But sometimes - p a r t i c u l a r l y on higher flowing speed- e l e c t r o s t a t i c charging was establ ished on values between 1 and 10 gigaohms*cm. I t is a remarkable fact tha t the r e s i s t i v i t y r ises a t the so l id i f i ca t ion point, in th is case more than 4 orders of magnitude. So we have to regard the sol id diphenyl an e lec t r i ca l insu lator . Streaming into the cold tank vessel the l iquid diphenyl so l id i f ied on the inner wall and so developed a highly e f fec t i ve insulat ing layer on the conductive wall material. Before heat-pipes work e f f i c i en t l y they have to be covered with the l iquid thus meaning the tank vessel being f i l l ed half a t minimum.

4.1 By flowing through the pipe (P) l iquid diphenyl was e l e c t r o s t a t i c a l l y charged. In a conductive vessel th is charge would be drained away more or less quickly.

4.2 But in th is case the insulat ing layer of sol id diphenyl prevented th is discharging and so managed a charge accumulation in the assembled l iquid (D).

4.3 When the surface of the highly charged l iquid approximated the heating pipe the f ie ld s t rength could increase to breakdown values thus evoking brush discharges.

From these considerat ions i t seems to be obvious tha t brush discharges on the heating pipes igni ted the diphenylvapor/air-mixture.

In general when brush discharges are suspected as an igni t ion source the evaluat ion of the i r igni t ion probab i l i t y is very d i f f i cu l t . But an unconstrained fol low-up of the given fac ts and conclusions seems to indicate brush discharges to be the only possible igni t ion source in th is case.

Proposed sa fe ty measures fo r preventing an igni t ion of the described type should not be confined to eleminating the suspected ign i t ion source but prevent the combustible atmosphere. This can be done e.g. by iner t ing the tank's atmosphere (by ni trogen or carbondioxide).

254

3

4.1

4.2 4.3

5

CH-O5A: Pneumatic Powder Conveying

In a pneumatic conveying system f o r t r anspo r t i ng ac ry l i c powder from a s i lo to a r a i l r o a d bulk conta iner the l a s t few meters o f a 2 inch s t e e l pipe had been replaced by a po lyethy len pipe f o r eas ie r handling. Only a f t e r a few hours running the system -on a ra iny and snowy winter day- an explosion happened in the conta iner .

Igni table atmosphere was formed in the conta iner by a combustible mixture o f the ac ry l i c powder dispersed in a i r .

I t was found t h a t every metal p a r t o f the system (pipe, va lves, bulk conta iner etc.) had been grounded except a 6 inch long metal coupling between 2 p a r t s o f the po lyethy lene pipe. This coupling had a res i s tance to ground o f more than 10 teraohms and showed a capacitance to i t s environment o f about 12 p icofarads.

Powders wil l be e l e c t r o s t a t i c a l l y charged in pneumatic conveying systems, a f inding which is va l id in general. Charge accumulation was assumed on the insu la ted coupling in the po lyethy lene pipe, leading to a discharging spark.

There was no idea f o r the spark's loca t ion between the coupling and grounded mater ia l . But i t had to be rea l ized t h a t possib ly by the explosion shock some pa r t s were displaced.

For t h i s al leged spark an energy was ca lcu la ted t o 0.6 mi l l i jou le on an est imated p o t e n t i a l o f 10 k i l ovo l t s . Of course th i s energy appeared t o be too low f o r ign i t ing a d u s t / a i r - m i x t u r e but as no o the r p o s s i b i l i t y seemed t o be conceivable f o r th i s incident the insu la ted coupling was accepted an ign i t ion source.

From the explained presumption grounding the coupling alone seemed to s u f f i c e as a measure t o come.

CH-O5B: Pneumatic Powder Conveying (continued}

Having mended the pneumatic conveying pipe, grounded the quest ionabled metal coupling and changed the damaged r a i l car the system was s t a r t e d again. ASter running i t seve ra l minutes I got an urgent telephone ca l l informing me t h a t another explosion had j u s t occured in the same manner, a lso meaning t ha t another r a i l car was damaged. The crew was helpless and I pul led a long face on t h a t bad news. There was no p laus ib le ign i t ion source a t hand but I had to f ind one because I did not want to loose my sight. For t h a t ve ry reason I ordered to s t a r t the system f o r a rerunning, however, not blowing with a i r now but with n i t rogen.

Gauging the f i e l d s t r e n g t h on s e v e r a l l o c a t i o n s and keeping under s u r v e i l l a n c e gas d ischarges by r a d i o f r equency moni tor ing we hoped t o f i nd o u t t he a c t u a l i g n i t i o n source . But as a ma t t e r o f f a c t we did no t need any o f t h a t i n s t r u m e n t a t i o n because we could see f l a s h e s in the t r a n s l u s c e n t p o l y e t h y l e n e pipe where i t was connected t o the metal pipe o f t he r a i l car . These f l a s h e s had a leng th o f n e a r l y I meter and were running o b v i o u s l y on the inner wal l o f t he p o l y e t h y l e n pipe. They caused sudden breakdowns on t he f i e l d s t r e n g t h moni tored on the p ipe 's s u r f a c e and t h e y evoked even c rack l ings c l e a r t o be heard. According t o t he i n e r t i n g o f t he t r a n s p o r t gas t h e r e were no i g n i t i o n s o f course . What we were observ ing were the h ig ly incendive PROPAGATING BRUSH DISCHARGES, a t y p e o f gas d ischarge we were no t f a m i l i a r with a t t h a t t ime.

* see CH-O5A *

The p o l y e t h y l e n e pipe was covered wi th r a i n w a t e r and snow g iv ing i t a conduc t i ve o u t e r coa t i ng t h a t was i n e v i t a b l y grounded a t s e v e r a l l o c a t i o n s .

4.1 * see CH-O5A * 4.2 Charge accumulat ion was assumed by forming a double l a y e r

charge on t he p o l y e t h y l e n e pipe thus leading t o a v e r y high charge d e n s i t y on the pipe's inner wall.

4.3 Con t i nua l l y when the charge d e n s i t y on the inner wall was high enough a breakdown occu r red in the kind o f t he today ' s well known propagat ing brush d ischarge.

No doubt the d u s t / a i r mix ture in t he pipe was combust ib le,

A f t e r t h i s bad exper iences we knew t h a t t h i s t y p e o f d ischarge could occur and e a s i l y i g n i t e t he dust .

In pneumatic conveying sys tems f o r combust ib le powder no i n s u l a t i n g ma te r i a l a t a l l - n e i t h e r as pipe i t s e l f nor as metal p ipe 's l i n i n g - i s pe rm i t t ed in f u t u r e .

p o s t scr iptum: This i nc iden t happened about 14 y e a r s ago. From t h a t day I have been v e r y dear t o p ropagat ing brush d ischarges because when I had done my paperwork on t h i s a f f a i r t he p lan t manager sen t me the invo ice f o r r e p a i r i n g one damaged r a i l car ,

255

Conclusions

The 5 Case H i s t o r i e s i n t r oduced above should c l e a r l y d e m o n s t r a t e t h a t no t on ly a sys tema t i c but a l so a v e r y c r i t i c a l e v a l u a t i o n o f an inc iden t i s necessa ry when ass ign ing S t a t i c E l e c t r i c i t y t o being the i g n i t i o n source . The audience c e r t a i n l y w i l l agree t o the obv ious conc lus ion t h a t s a f e t y measures based on an i n v e s t i g a t i o n wi th an imper fec t exp lana t i on o f an acc iden t can b a r e l y enhance f u t u r e s a f e t y .