The ignition power of brush discharges — experimental work on the critical charge density

Journal of Electrostatics, 10 (1981) 161--168

161

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

THE IGNITION POWER OF BRUSH DISCHARGES

-

EXPERIMENTAL WORK ON TIIE CRITICAL CHARGE

DENSITY.

K.G. LOVSTRAND Institute of High Voltage Research, Uppsala, Sweden

ABSTRACT The properties of brush discharges were studied and ignition experiments were made for determination of the ignition power of brush discharges. At the ignition experiments a n-pentane - air mixture was used. Large PVC plates were charged in a corona discharge and discharged with spherica7 electrodes of varying diameter. The ignition probability was determined for negatively charged plates. No ignition was obtained with a positive charge on the plates. The experimental results are compared with similar experiments made by other authors. A method for estimation of the critical charge density for ignitions is suggested.

INTRODUCTION The ignition risks in connection with electrostatic brush discharges from charged insulator surfaces have been analysed by several authors. Heidelberg showed that a grounded metal sphere with discharges

(ref.

I-3)

15 Iron diameter can produce igniting brush

from a charged insulator surface in an explosive hexane - air atmosphere.

Gibson and Lloyd (ref. 4) found in similar experiments that a brush discharge transferring ]30 nC of charge can ignite inflammable vapours with a minimum ignition energy of 0.25 mJ with an ignition probability of about 50%. The ignition properties of brush discharges other authors

from insulator surfaces have also been investigated by several

(ref. 5-$)- Studies have also been made of the ignition properties

of brush discharges

from electrodes near the surface of charged insulating liquids

ref. 9-I~). This has provided further data of the ignition power of brush discharges. The aim of the present study was to establish the minimum charge density on large insulator surfaces which can produce igniting brush discharges from grounded spherical metal electrodes approaching the surface. Preliminary results from the investigation indicated a dependence of the size of the charged surface and of the electrode radius on the critical charge density (ref.

6).

0 3 0 4 - 3 8 8 6 / 8 1 / 0 0 0 0 - - 0 0 0 0 / $ 0 2 . 5 0 © 1981Else~erScientifie Publishing Company

162 RESULTS Experimental methods. The experiments were performed with 30x30 c ~ a n d

68x70 e~, 5 ~m~ thick PVC p l a t e s

A plate was suspended in a trolley hanging in a rail from the ceiling.

It was

evenly charged by corona at the 'charging' end of the rail and then transported through a field mill arrangement where the plate charge density was scanned. The charged plate was then positioned at the

'discharging'

end of the rail more than

50 cm away from any other conductive objects. At the discharging position a spherical electrode mounted at one end of a thin plastic tube was moved towards the centre of the plate at a constant velocity of 30 ~n/s. The electrode was connected via a thin shielded cable to a large capacitor in parallel with a static volt meter. The charge transferred in a discharge was thus registered. A position indicator connected to a recorder registered the electrode - plate gap at the discharge.

recorder

position

insulator plate

indicator

f

1

"0

discharging position

static _~leetrode voltmeter charge 7capacitor~

recorder

field

hr

mill ,~-~

[]

ii b

....

coronai" [] H.V.

s upp ly

Fig.

I. Experimental

discharges

I meter~

ge measurement

position

I

apparatus for investigation of the ignition power of brush

from large insulator surfaces.

A mixture of n-pentane - air was used at the ignition experiments.

The gas

mixture was controlled with a gas chromatograph and was kept at the optimum for ignitions, 3.h%vo I. The explosive mixture was fed through the plastic tube and emanated through a large ntmLber of tiny holes in the surface of the discharge electrode. The gas mixt'J~e produced in front of the electrode was tested with capacitive sparks and had an ignition energy less than 0.3 mJ. The minimum ignition energy of n-pentane - air is 0.22 mJ (ref.

12). The tiny holes in the electrode

163 surfaces had negligable discharges.

influence on the length and charge transfer of the brush

This was verified by comparisons with discharges

from smooth homo-

geneous electrodes. The brush discharges produced were photographed with a Polaroid camera and high speed film. The ignition events were also recorded with a rotating camera with 35 mm high speed film. The film velocity in the camera was I m/s.

Properties

of brush discharges.

When a spherical electrode of brush discharges

approaches

a large charged insulator

surface a series

are produced several of which can be of equal strength. All

results presented below only apply to the first discharge when the electrode roached an evenly charged insulator The discharge

gap was strongly dependent on the plate charge density and for

15-20 mm electrodes The pictures

the gap was longer than LO0 mm at high plate charge densities.

of the discharges

branched but had a short bright

showed that the brush discharges were then very 'stem'. The length of the visible part of the

discharges was in the order of 10 cm. The luminous

channels

cross the gap but were confined to the metal electrode. with discharges the discharge

from heavily charged small insulator plates where the length of

generally

Fig. 2. Brush discharge to 68x70 c ~ P V C Gap length

did apparently not

This should be compared

can be in the order of 5 cm for electrodes with 15 mm diameter

These discharges

app-

surface.

also show considerable

from 25 mm electrode

plate with negative

155 mm. Visible discharge

charge. 70 mm.

(ref. 5)

branching but several discharge

Fig. 3. Brush discharge with surface discharge from 15 mm electrode to 75 2 cm PVC plate. Gap length 40 mm.

164 channels can bridge the discharge gap arid continu<, a,: :urfac~ discharg~:s (fig. i'

a~

~ ). Th,~

charge tra~l:3ferred in brush di,~ehurg ; i: :iel),'m,~ .t )ll th<
charged sl~rface sizes and the charge ~ensity
t~'ansfer a i~,rger u:n<}~ :]' <:~irg~

c o m p a r e d to larger charged area~ (fig. )i and 5). T)[s:charg~ fro~:~ ,.~mai! :!arged arenas cau~ thus~ be morL' ~ t e n s < . charges of equaN

A la~rg~

ar

a,

how~,wr,

~:~_tm

prod~.~<~ :~:v ~al !~ ~ -

strength.

200

nQc)100 0=2

0

2

4 6 qs(PC/m 2)

10

Fig. li. Charge transf<~rred, AQ, in a b r u s h discharg~ from

a

spherical

:i~ctrod( ~

as a function of the charge density, qs' on a n e g a t i v e l y charged ~Ox~O c ~ P V C

plat~

E l e c t r o d e radius, @ = 20 and 40 ~ml.

200' O=40mm Omrnffi

z~Q ( nC ) 100"

o Fig.

2

4

6

qS(PC/m 2)

5. Charge t r a n s f e r r e d ,

8

lO

AQ, in a b r u s h discharge from a spherical el~ctrode

as a function of the charge density, qs' on a n e g a t i v e l y charged 6~x70 cm~PVC p]at~ Electrode radius,

@ = 20 and 140 ~r,

165 l~nition experiments. Ignition experiments each electrode

were performed with electrodes

a large number of ignition

phere at varying charge densities the charge transferred

on the PVC plate.

The discharge

in each brush discharge were registered

the earlier measurements

of the properties

7 show the ignition probability a function

of 15-40 ~n diameter.

trials were made in the explosive

gap length and

and compared with

of the brush discharges.

of brush discharges

For atmos-

Figure 6 and

for varying electrode

size as

of the charge density on the surface.

100

"/5

Electrode -,25mm •30mm e4Omm

p ( ~ ) 50

25

6 qs ( p C / m 2 ) Fig. 6. Ignition probability, 30x30 c ~ P V C

8

10

p, for brush discharges

from a negatively

plate as a function of the surface charge density,

charged

qs"

100

75

Electrode • 35 mm e4

p (;) 50

25

qs ( p c / m 2 )

Fig. 7. Ignition probability, 68x70 c ~ P V C

p, for brush discharges

The surfaces were charged negatively discharges.

from a negatively

charged

plate as a function of the surface charge density, qs"

at these experiments,

producing positive brush

Several hundred ignition trials were made with positive

charge on the

166 surfaces. N o ignition was o b t a i n e d in any case. The ignition p r o b a b i l i t y for negative brush discharges is thus very low for the i n v e s t i g a t e d conditions. This can he e x p l a i n e d by the different character of the negative b r u s h discharge ~hich does not form luminous surface (ref.

channels but only very weak luminous cones at the electrode

5).

Ignitions were only obtained with electrodes of 35 nml or larger w h e n the 68x70 cm plate was charged. The 30x30 cm plate could produce ignitions w i t h the 25 m m electrode.

The charge t r a n s f e r r e d in the discharges

(fig. 4 and 5) indicated that

the discharges from the smaller plate can be more intense as a higher amount of charge is t r a n s f e r r e d by these discharges.

The charge t r a n s f e r r e d in a discharge

from the 20 m~1 electrode in front of the 65x70 c1~plate was limited to less than 120 nC. The s~le electrode could transfer up to 200 nC in discharges from the 30x30 cm plate. A c c o r d i n g to the i n v e s t i g a t i o n s by Gibson and Lloyd (ref. 4) a charge transfer of 130 nC in a brush discharge indicates that the discharge m a y ignite vapours with an ignition energy of 0.25 mJ. The 20 ~m~ electrode p r o d u c e d discharges which t r a n s f e r r e d a higher charge but no ignitions were obtained with this electrode. The experiments p e r f o r m e d by Gibson o

and Lloyd i n c l u d e d charged plates with a maximtm~ area of 225 cm ~ . Brush discharges from such small plates are less b r a n c h e d ~ d

might have a somewhat higher ignition

p o w e r with the same amount of charge t r ~ i s f e r r e d although the electrode d i a m e t e r is equal. As a comparison the charge t r a n s f e r r e d in capacitive sparks m i n a t i o n of the m i n i m u m ignition energy can be calculated. For

for deter-

an ignition e n e r ~

of 0.25 mJ capacitors of 7-10 pF are u s e d Jn the circuit. The charge J~l the capacitor for a 0.25 mJ spark is then 60-70 nC. At all ignition events the e l e c t r o d e - p l a t e gap length was larger than 20 ~i. Heidelberg a hex~e Asano

(ref.

I) found that only brush discharges longer than 30 ~i ignited

- air m i x t u r e at experiments with small insulator surfaces.

(ref.

]]) who i n v e s t i g a t e d brush discharges

found that the surface p o t e n t i a l of the

F~'~mer ~ d

from charged liquid surfaces

liquid should exceed 58 kV for igniting

brush discharges. This also confirms the e m p i r i c a l findings

that a length of 20-30

imm is n e c e s s a r y for igniting b r u s h discharges. The development of ignitions A f t e r an ignition delay of about

could be studied at the sweep p h o t o g r a p h y recordings I ms a luminous flame front p r o p a g a t e s

from the

bright stem part of the discharge w i t h a flas~e front velocity of about 5 m/s. The ignition delay time corresponds well w i t h that r e p o r t e d by Barreto

(ref.

]5).

E s t i m a t i o n of the critical charge density for i~nition. Felici

(ref.

13) has d e t e r m i n e d the critical field strength, Ec, on the surface

of a spherical electrode

for a b r u s h discharge. An electrode of 20 ~ml diameter thus

requires 60 k V / c m and a 35 ~n electrode requires 50 k V / c m for a brush discharge. H e i d e l b e r g has derived a relation between the charge density, qs' on an insulator

167 surface w i t h a radius, R, the distance, d, to an e l e c t r o d e w i t h the radius, a, and the field strength, E, on the surface of the electrode

E = ~

qs

(3 - d/a +

(d-a)(d/a-3) + R2/a ((d-a) 2 + R2) I/2

)

(ref.

1):

(E = m a x i m u m field strenght)

(I)

If the c r i t i c a l field strength, Ec, is put into this relation we get a relation b e t w e e n the gap distance and the charge density for any p a i r of electrode and surface radii. Now assume that the m i n i m u m length of a b r u s h at the i g n i t i o n experiments,

discharge should be 20 ~mm as found

for an i g n i t i o n of explosive m i x t u r e s with ignition

energy in the o r d e r of 0.25 mJ. We can put this value into equation

(I) and we

thus get a relation b e t w e e n the critical charge density and the radius of a charged surface for any electrode diameter. H e i d e l b e r g h o w e v e r found that only electrodes w i t h a d i a m e t e r larger than 14 m m can p r o d u c e i g n i t i n g b r u s h discharges

in an ex-

p l o s i v e m i x t u r e of hexane - air (ref. 2). At the p r e s e n t experiments ignitions were o b t a i n e d w i t h the 25 m m electrode for the 30x30 c ~ p l a t e

and w i t h the 35 m m

electrode

for the 68x70 ci~plate.

electrode

diameters for ignitions are 20 m m and 30 mza respectively.

If the relations

Let us assume that the m i n i m u m values of the

found above are p l o t t e d in a d i a g r a m for an electrode diameter

of 10, 20 and 30 m m we can p l o t a new curve from R = 5 cm and a = 7-5 ~ml through R = 17 cm (equivalent

to

30x30 c~) and a = 20 m m to R = 39 cm (equivalent to

68x70 e~) and a = 30 mm. This curve can he used for e s t i m a t i o n of a m i n i m u m r e l a t i o n b e t w e e n the radius of a c h a r g e d surface and the charge density n e c e s s a r y to p r o d u c e i g n i t i n g b r u s h discharges

(fig. 8).

20 15 qs

(pC/m~

10

0

0

1"0

2"0

3"0

40

R (cm) Fig.

8. C r i t i c a l charge density, qs' as a function of the radius of the charged

surface, R, for 20 m m long b r u s h discharges from electrodes of 10, 20 and 30 Imm diameter ( (------).

). E s t i m a t e d m i n i m u m charge density for igniting b r u s h discharges

168 DISCUSSION The ignition

e×plosive high

c]~ctron

channel

experiments

gar mixture. density

for

or

The

the

the

bright

The

divergence

ignition

tile

power

stem

which of

the

con
by Barret<~

branched

I;t1,, e_uer&~'

h~w~ s h o w n t h a t

for

which

(n ,, =1C 17 CZu->~,, i ;

an i g n i t i o n .

a concentration

merits

show

C~lcuiation~

[rl

random

o]" t h i s

one

t~Te

aid

'h~/,'

charact.~r

can differ field

v~qti[red

char,act(r

of

the

in of

possibility

a smEtAl v o ! u r r ,

Especi:~,£1y

J : , s < k a i'

'i']:~ e x p , , r ]

[* v < r ; /

critic:~]

the: imerN, <['

a n d ]i)ri!<}]trl
surf~ce

,

:ou:it
cnartnd. can

< J :m

d t~t

<~t t h e

di',',<:har6<~',

ionized

length of

i gni t i o n

have i , N c ~ t

branches

discharge. in

lh)

brush

hSghly

o£ the

considerably

ii,,:a, t.o :m

can

(ref.

[mportax~ ~

u i s c h ~ r ; ~ < ; k.re o r b ) t "

irilport s~xt f a c t o r s . [Fh< e s t i m a t i o n

on a s s u ~ p t i o n s safe

values

~nd the

of

of

the

charg<

uncertainity

,'~pplied eg.

the

critical

from empirically

for

risk

of

charg~ ~ d e n s i t y

found

density.

the

The

assumDtions

evaluation',',

f'or i g n i t i n g

dat,~. ~and d o < ~ n o t

<~" f o r

random

behavi

n e c e ~ u . ~ r i b' ~'f th ~ b r u s h

r~quire

a g o o d m&~rgin

warning

applications.

when

i~

l>an~ i

constilut~ ([[z(:h~rg~s

t!l(

va!~::

~:r

REFERENCES

] E. H ~ i d e l n e r g , Static ~lccl,rifica%i<>n I ~(7, in.:t.fhy's.Conf. S r . No. i~ t P T4'/-~'~ 2 E. H e i d e l b e r g , Farbe und Lack 70(1964)>9[-99 3 N. H e i d e l b e r g , Advance~ in static e!ecbr[city, Eroc.1;:% Imt.(o~:F.Stnr~ic }d(ctr. Vienna, Hay 4-6 1970 pp3>1-5 } k H. C i b s o n a n d F . C . L l o y d , B r . g . A p ~ < . f h y s . ]6(1965)]619-<1 O. F r e d h o l m a n l k . S . L 6 v s t r e . n d , J.Sci. Ir:%rum. 5(1 )72)1058-62 £ K.G. LOvstrand ~ n d -'.. H 6 g b ~ . r g , [ z ' , ~ c . % ' d I n s . C o n f . S t a t i c !:iectr. ]!',no~l , A:ri I 2 9 - 2 2 1 9 7 7 , N o . -'3 Y if. B e r t a i n , Stable oL . ; < ~ t : { ::: ( ; ( i ' > " ) ) 3 6 1 - 7 1 I L!. --bi~t i < n , !ii'< [ (']'iski: Hr. 'L:I, ~JZlci, ;:~,nS%&J t i'[]r M:~ti:z' [ alpr[] ['/N,q, ], r L [ r: ] i i 1~ l~.J. F i [ c ] , Stu:~[<: c l e c t r i f i c u , t,i, I . E . R!yno/~is arlx K. ,[urc:lk~,~ , £ . A ~ i l . i ' h / s . hi(l<;) ~ }1['-;:7