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Electrostatic charge generation during tank washing. Spark mechanisms in tanks filled with charged mist
Journal o f Electrostatics, 1 (1975)295--309 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
ELECTROSTATIC C H A R G E G E N E R A T I O N D U R I N G TANK WASHING. SPARK MECHANISMS IN TANKS FILLED WITH C H A R G E D MIST J.M. VAN DE WEERD
KONINKLIJKE/SHELL--LABORATORIUM, Ne th erla nds)
(Shell Research B. V.), Amsterdam (The
(Received August 5, 1974; in revised form March 6, 1975)
Summary Under conditions of tank washing several possible sparking mechanisms were investigated involving conducting objects not bonded to the tank. Some of the tests were done in a full-scale 12,000 m 3 shore tank containing an electrostatically charged mist generated by a tank washing machine. Sparking mechanisms were identified associated with the formation of unbonded conductors in the form of water masses ("water slugs") by the water jets used for tank cleaning. Ignition experiments were performed using masses of water as unbonded conductors. They produced sparks of sufficient intensity to ignite h y d r o c a r b o n l i r mixtures under conditions relevant to the practical situation in tankers.
1. Classification of spark mechanisms Looking for spark mechanisms in tanks filled with charged mist is equivalent to looking for ways in which unbonded conductors present in such a tank can acquire a voltage substantially different from that of the tank structure when the gap between object and tank structure is narrow. We can distinguish a number of mechanisms: (a) A c c u m u l a t i o n o f charges This mechanism, which is the most obvious, is probably not very important for our case. Part of the charges present in the mist might be picked up by the insulated object. The contribution of the picked-up charges AQ to the voltage difference between object and tank wall,/~ V, is given by A V = AQ/C, in which C is the electrical capacity of the object in the position near the tank wall. Practical studies showed that the actual charge accumulation due to pickup is of minor importance as compared to that given by other mechanisms. This is due to the low mobility of the charge carriers in a tank-cleaning mist. For the case of a highly charged steam mist, however, indications have been obtained that pick-up could lead to dangerous sparks. *Paper presented in the Special S y m p o s i u m on Tanker Explosions at the 2nd International Conference on Static Electricity, Frankfurt, April 6, 1973.
296 (b ) Objects containing an induced net charge An object connected to the tank structure, and thus still at " e a r t h " potential, in a position in which it is very much exposed to the electric field (protruding) will contain a net (induced) surface charge of a sign opposite to that of the charges in the mist. Upon being severed from the tank structure, the object--now unbonded--is charged. When it touches the tank structure again in a place less exposed to the electric fields from the mist (e.g. a tank wall instead of a protrusion where it originated) a voltage difference will arise. Again, we can write A V = AQ/C, in which AQ is now defined as the difference between the charge the object actually contains and the charge it should contain at that place when it was at " e a r t h " potential (induced charges in the latter position). (c) Objects acquiring a net induced charge Let us consider the opposite of mechanism (b). The object starts its life as an unbonded conductor in a position where the electric field from the mist is low. It therefore hardly contains any net charge. Assume that this uncharged conducting body approaches a structure at earth potential that protrudes into the charged mist. In the electric field around such a protrusion the object is polarized and, upon close approach, one sign of the polarization charge on the object is neutralized in a spark discharge. Just prior to the spark there will be a voltage difference A V between object and protrusion, again given by ~ V = AQ/C, where Q is the difference between the actual net charge on the object before the spark goes and the charge present on the object after the spark has occurred by which the object has acquired earth potential. (d) "Free'" charges on the object due to corona or "water spray" mechanisms In this c o n t e x t " f r e e " charges are defined as charges in excess of those present upon earth contact due to the "ending of field lines". Depending on the shape of the object, it is conceivable that the electric fields at the surface of the unbonded object are locally sufficiently strong to give corona or "water spray" discharge. This might give " f r e e " charges and thus lead to sparking when the object approaches " e a r t h e d " tank structure. 2. Full-scale experiments Tests were performed in a full-scale set-up described earlier [1] - - a 12,000 m 3 tank equipped with facilities to produce charged mists. 2.1 Accum ula tion o f charges Charge pick-up was studied by dropping metal cylindrical objects from the top to the bottom of the test tank, onto an insulated plate connected to an oscilloscope. Figure 1 shows the set-up employed and Fig. 2 some of the results. Only in the case of a highly charged steam mist did we detect charge pick-up of sufficient importance to be considered a danger. In several instances, charges were measured of opposite sign to the charged mist. This suggests that in such cases not charge pick-up but, for example, corona effects are
297
:1
...............
L
Fig. 1. Experimental arrangement of insulated table on the b o t t o m of the 12 0 0 0 m 3 K S L A shore tank for charge detection on falling objects. OBJECT LENGTH r~IAMETER (ram) (m) • 5 1 • 8 1 • 13 1 • 18 1 • 13 1.5 60 1
MIST SOURCE WATER JET
ST'~'AMIN; STEAMING
CHARGE ON OBJECTS (nC) 20G 180 160 140
#
120
lO0 8O 6O 40 20
• I~
0 I I I I I 0 10 20 30 40 50 SPACE POTENTIALIN THE MIDDLE OF THE TANK (kV)
Fig. 2. Charge on objects dropped from the top of the tank.
responsible for the charge on the objects. The tests performed were not sufficiently general with respect to object size and shape that charge pick-up can be completely ruled out as a possible source of dangerous sparks, but they do suggest strongly that other mechanisms are more likely. 2.2 Objects containing an induced net charge Objects suspended from an earthed wire in the middle between the top and b o t t o m of the shore tank were dropped onto an insulated plate mounted on the tank bottom. Again, charges of variable sign and low magnitude were detected. This suggests that effective discharging of the object might occur
298 during the fall, presumably due to corona and probably, to a much lesser degree, due to compensating charge pick-up. It is conceivable that not in all circumstances is the shape of the objects such that corona discharge of the net induced charge occurs. Recent studies by Klaver [2] indicate a loss of charge due to corona varying between 0 and 80%. He gives practical examples where no loss of charge occurs at all. Spark energies of sparks occurring as a result of this mechanism can be estimated according to:
( Cfree Vax)
(1)
in which Vmax is the maximum potential present in the tank due to the space charge; Cfree is the free-space capacity of the falling object; k is a factor which is always smaller than 1 and which depends on the shapes of the object and the protrusion and on the position in the t a n k where the object starts and ends its unbonded life. If, for example, the object were a nozzle of a strongly protruding tank cleaning machine that would drop off and hit the tank b o t t o m , the value of k, under the assumption that no charge would be lost during the fall, would be, at most, about 0.5. If the object were a water slug generated by the breakup process of a cleaning jet, the value of k would be about 0.2--0.4, assuming that the break-up of the jet occurs in a position where the jet is strongly protruding into the charged mist. 2.3 Objects acquiring an induced net charge Before discussing !Lndetail the experimental set-ups employed, we should like to go into some of the experimental problems and interpretational pitfalls encountered. What we are interested in is the spark energy, i.e. the charge transfer during the spark and the voltage across the gap just prior to the spark. This is necessary because we want to evaluate the incendiary power of the sparks w i t h o u t doing full-scale ignition tests. Both voltage and charge transfer are difficult to measure in a practical experiment. If we want to measure the full charge transfer to the object after it hits the protrusion, we should ideally have a charge-measuring probe of very small dimensions available at the point of contact between object and protrusion. Only in that case will all the charge flowing in the spark pass the measuring circuit and thus be detected. However, a measuring point of such small dimensions presents practical problems in that the object should exactly hit the point in which the charge-measuring probe is located. To overcome this one might decide to use a charge measuring system connected to the protrusion as a whole. In that case, however, the only charges measured are the extra charges induced when the object approaches the protrusion. But the contribution to the charge transfer in the spark from a shift in induced charges between protrusion and object is n o t measured. Direct measurement of the spark voltage is only possible if the gap width between object and protrusion at the m o m e n t of the spark can be determined.
299
Indirectly the voltage difference across the gap could be determined from the true charge transfer and the capacitance in the situation just prior to the spark. But measuring this capacitance in a practical test again presents experimental problems. One of the procedures employed to determine the spark properties prevailing when insulated objects approach a protruding structure is shown schematically in Fig. 3. An object connected to an insulator is lowered into the tank by means of an earthed cable. An adjustable spark gap between cable and object is provided. By adjusting the spark gap to the maximum distance still giving sparking after a lowering operation the voltage of the object in the
/
Fig. 3. Schematic representation of e x p e r i m e n t with a c o n d u c t i v e object c o n n e c t e d to an insulator arrangement and l o w e r e d in a t a n k filled with charged mist (Fig. 3(a)), for inductive effects simulating the situation of Fig. 3(b), an u n b o n d e d object nearing a protrusion.
position near a protrusion can be determined, the protrusion in this case being the suspension cable. Of course, a protrusion of this kind is one of a very serious nature and not likely to be the most c o m m o n type present in tanks. Another drawback of this technique is that the voltage distribution around a permanently present protrusion is likely to be changed by the occurrence of corona discharges. In the case of our experiment, the "protrusion" is created by the lowering operation of cable and object and thus not sufficient time might be available for the corona to change the potential distribution in a representative way. Table I gives the results of a number of tests done with objects of different lengths and under different charged mist conditions in the tank. Figure 4 gives an energy estimate of the sparks observed according to U=
1
2
(1.5
Cfm e +
C ~2 T72 o! V o b s
1.5 Cfree
(2)
300
TABLE I
Observed m a x i m u m voltages of sparks o b t a i n e d in e x p e r i m e n t of Fig. 3
insulated obJect-near-protrusion
M a x i m u m spark voltages (kV) o b t a i n e d at a rod length (cm) of 25
50
75
100
120
2 2 3.5 6--7
3.5 5.5--7 10--11
2.8 4.5 7--11 13.5--18
5 8--10 15
3.2 6 8.5 16
Space potential in the middle of the tank (kV) 150
7
8--10 15 20--25 30--45
Well-rounded 13 m m diam. rods with lengths from 25 to 150 cm
LENGTH OF 13 mm DIAMETER RODS (cm) 150 11.6
100 75
k5.3 1.6~
50 25 0
0
4.~
~ 0.9 I 10
0.9
13.6-21.3
48-
10 - 24.6
37--66
"~8 ~2.7
--
20-24 ---t."'--
7.8-10.6
I I I I 2O 3O 40 50 SPACE POTENTIAL IN THE MIDDLE OF THE TANK (kV)
Fig. 4. Energy estimates of the sparks observed in the e x p e r i m e n t of Fig. 3, expressed in 10 -4 J, as a f u n c t i o n of mist c o n d i t i o n s and object length, using eqn. (2) (see text). The arrows indicate variations in mist c o n d i t i o n s during part of the experiment. The dashed and drawn lines indicate the 0.2 and 0.5 m J limits.
Equation (2) includes a correction to the observed voltage values, Vobs as given in Table 1 because of the extra capacitance Co = 10 pF present in the suspension arrangement. The capacitance of the object near the protrusion in this situation without the suspension arrangement, is estimated to be 1.5 Cfree, Cfree being the "free space capacity" of an ellipsoid with axes equal to the length and diameter of the cylinder. This estimate is justified later.
301
Full-scale experiments with water slugs as unbonded objects were carried o u t in a set-up as illustrated in Fig. 5. A bar of 5-m length and 5.7-cm diameter was suspended horizontally in the centre of the shore tank and represented an earthed protruding structure. Water slugs were dropped from the r o o f onto or close to the bar. The slugs were released from a cylindrical tube of 5-cm diameter and 2-m length. Their length could be varied by partly filling the slug-producing tube. The bar arrangement as a whole was earthed via a resistor, a capacitor and an oscilloscope, all placed in parallel. A charged mist was created by a high-capacity cleaning machine giving a space voltage of a b o u t 9 kV halfway between the top and b o t t o m of the tank. When a water
\'/ 1~,50 nFOSCILLOSCOPE Fig. 5. Test set-up in t h e 12 000 m 3 s h o r e t a n k for sparks f r o m w a t e r slugs d r o p p e d o n a h o r i z o n t a l bar.
slug closely approached the bar, single-pulse type spark signals were observed on the oscilloscope. Figure 6 gives the magnitude of some of the signals observed, expressed in charge accumulated in the measuring capacity of 50 nF directly after a pulse. As stated before, it is difficult to relate the observed signals to the actual charge transfer in the spark. The signals observed are caused by e x t r a charges induced in the protrusion system due to the approach of the slug. But the contribution to the charge transfer in the spark due to a shift in induced surface charge from the protrusion to the slug is not observed. The experiments were repeated at higher charge density levels and then showed an increase in signal strength. The signal strength was found to vary approximately linearly with charge density {max. space voltage). Tests with a protruding structure of a more bulky nature gave w~aker signals. In view of the interpretation problems discussed extensively before, it is not possible to say
302 CHARGE TRANSFER, nC 150
1OC
50
l
O C~5 025 Q5 1.O 20 FILLING HEIGHT OF WATER-SLUG PIPE, m
Fig. 6. Signals f r o m water slug sparks obtained in the set-up of Fig. 5 charged mist from a cleaning jet with a space potential in the middle of the tank of 9 kV.
whether this reduction in signal strength is due to a major reduction in the actual charge transfer in the spark or mainly reflects a difference in distribution of induced surface charges between protrusion and tank wall.
2.4 Conclusions on the importance o f the spark mechanisms related to tank cleaning It can be concluded from the experiments that spark mechanisms as described in Sections l(b} and l(c) are the most important for the practical situation. Both mechanisms involve objects in relation to protrusions. Spark energies are dependent on the size and shape of the object and the protrusion and their positions in the tank. In both cases, the spark energy can be represented by an expression as given in eqn. (1):
(1) in which k is mainly a function of the protrusion geometry and the path taken by the insulated conductor. We can evaluate the break-up process of a cleaning jet which is an important practical source of insulated conductors. The factor k in eqn. (1) representative of such a jet break-up process can be estimated to be about 0.2--0.4, depending
303 o n t h e slug size. T h e s e values f o r k relate to t h e b r e a k - u p o f a t a n k cleaning j e t in a s t r o n g l y p r o t r u d i n g p o s i t i o n a n d t h e resulting c h a r g e d slug t o u c h i n g an e a r t h e d t a n k s t r u c t u r e in a l o w v o l t a g e g r a d i e n t area. A l t e r n a t i v e l y , it applies to t h e reverse, t h e slug g e n e r a t e d in a l o w voltage g r a d i e n t area a n d t o u c h i n g e a r t h at a s t r o n g l y p r o t r u d i n g s t r u c t u r e . T h e values f o r k c a n be f o u n d b y e v a l u a t i o n o f t h e results of e x p e r i m e n t s as, f o r e x a m p l e , given in Fig. 4. T h e y can also be derived f r o m a c o m p u t a t i o n o f idealized t a n k c o n f i g u r a t i o n s as t h o s e given in T a b l e II. TABLE II Computed electric field at inner cylinder of a finite coaxial cylindrical arrangement Radius of inner cylinder (m)
Electric field at the inner cylinder halfway between top and bottom (kV/m)
Electric field at the inner cylinder 5 m from top or bottom (kV/m)
Maximum voltage between inner and outer cylinder (kV)
0.001 0.005 0.010 0.025 0.05 0.10 0.50 1.00
10266 2509 1387 645 369 212 66.6 42.5
3412 2063 1143 533 307 178 56.8 37.0
74.4 72.4 70.8 68.8 66.8 64.0 54.5 47.4
Radius of outer cylinder 10 m; height 20 m; charge density 40 nC/m 3. Electric field and maximum voltage are given as a function of inner cylinder radius. T h e m a x i m u m p o t e n t i a l Vmax has b e e n s h o w n to be at least 30 k V f o r t a n k cleaning o p e r a t i o n s as p e r f o r m e d o n t h e Mactra and t h e Marpessa j u s t p r i o r t o t h e i r e x p l o s i o n s . T a n k cleaning was p e r f o r m e d b y high c a p a c i t y t a n k cleaning m a c h i n e s . T h e w a t e r jets o f such m a c h i n e s m a y split u p in slugs o f a length of, say, 50 c m and a d i a m e t e r o f 4 c m . A c c o r d i n g t o ref. [3] this is quite a n o r m a l o c c u r r e n c e . Given t h e suitable slug t r a j e c t o r y , eqn. (1) p r e d i c t s f o r this s i t u a t i o n a s p a r k e n e r g y o f a b o u t 1.5 m J taking k as 0.2. F o r this t y p e o f s p a r k f r o m w a t e r slugs t h e m i n i m u m ignition e n e r g y has b e e n s h o w n to be a b o u t 0.5 m J , as we will see l a t e r on. We c a n thus c o n c l u d e t h a t s p a r k m e c h a n i s m s involving split-up processes o f the w a t e r jets used f o r t a n k cleaning are o f m a j o r i m p o r t a n c e f o r s a f e t y c o n s i d e r a t i o n s . Sparks o f i n c e n d i a r y p o w e r f o r explosive t a n k gas m i x t u r e s w o u l d be g e n e r a t e d even in cases w h e r e t h e t r a j e c t o r y o f t h e w a t e r slug w o u l d be such t h a t a l o w e r f a c t o r k w o u l d result. 3. L a b o r a t o r y s t u d y o f t h e b e h a v i o u r o f w a t e r slugs as u n b o n d e d c o n d u c t o r s T h e b e h a v i o u r o f free-falling w a t e r slugs w h e n t h e y a p p r o a c h a p r o t r u s i o n
304
surrounded by an electric field was studied in a laboratory-scale set-up. Slugs of a b o u t 65-cm length and 1.5-cm diameter were dropped on or close to a cylindrical probe 1 cm in diameter at a high voltage (10--30 kV). The signals of the sparks observed were studied and found to be similar to those from metal objects of similar shape. Photographs of the sparks were t a k e n using an image intensifier. In addition, we used a fast flash of low light intensity, triggered by the electric signal of the spark, to obtain an image both of the slug boundary and of the spark. Figure 7 shows one of the pictures taken. From the pictures, it could be concluded that the water surface of the slug is not y e t influenced by the strong electric field in the gap between slug and protrusion just prior to the spark. Obviously, the field strength in the gap
0.3S 13ram
4 Vldiv 2msldiv Fig. 7. Spark generated by water slug approaching probe. Probe voltage 30 kV; spark length 13 ram. Charge transfer in spark as estimated from electric signal 160 nC. Slug length 65 cm.
305 increases so rapidly that there is n o t enough time for surface disruption before the m o m e n t that the field is sufficiently strong to give spark breakdown. The photographs thus corroborate the evidence produced by the electric signals, i.e. the water slugs behave similarly to solid conducting objects. In a recent publication, Barreto et al. [4] give details on the spark discharge process between a metal and a liquid (water) electrode. By using highresolution oscilloscope techniques they were able to show that, apart from phenomena associated with the deformability of the water surface in the electric field, the lower conductivity of the water electrode as compared to a metal one may also affect the discharge phenomena. Their data on explosion tests, although not comprehensive, indicate a somewhat higher minimum ignition energy for water to metal sparks as compared to metal sparks. This is in line with results of ignition tests we performed using actual water slugs. Ignition tests were performed in a well-controlled flammable atmosphere of 5% propane in air. Figure 8 shows the set-up. Slugs were produced in a 75-cm-long, 2-cm-diameter tube closed with a shutter. When the shutter was opened, the slugs travelled downwards over a distance of a b o u t 3 m towards an earthed probe of 2 ~ m diameter. An electric field around this probe was produced by means of a cylindrical, cage-type high-voltage electrode of 1-m height and 60-cm diameter. A charge-monitoring arrangement, consisting of capacitor, resistor and oscilloscope placed in parallel, was connected to the probe. The flammable atmosphere was produced by enclosing the complete set-up in a plastic tent of a b o u t 2 m 3 volume. This large volume of gas was necessary to provide the proper gas mixture over the entire trajectory of the water slug. Only in that case would it be possible to guarantee the appropriate gas composition in a surface layer around the water slug. Ignitions due to falling water slugs in this set-up were observed at a minimum voltage at the polarizing electrode of 15 kV. At this voltage, careful positioning of the probe with respect to the trajectory of the slug was essential; only in cases when the water slug just passed the probe were ignitions observed. Estimates of the spark energy still giving ignition were of the order of 0.5--0.8 mJ. So sparks from water slugs gave ignitions at energies n o t far above the minimum ignition energy level of 0.25 mJ for the propane--air mixture employed. 4. Model studies It was attempted to calculate the energies of sparks occurring when uncharged conducting bodies approach a structure that is protruding into a charged mist confined in a tank. In the electric field around such a protrusion, the object is polarized, and upon close approach one sign of the polarization charges on the object is neutralized in a spark discharge. The charge transfer in such a spark depends on the shapes of the object and the structure, and on the intensity and size of the charged mist surrounding the structure. The number of parameters involved is therefore so large that only
306
Fig. 8. An ignition due to a water slug. idealized situations can be tackled effectively by calculations. As a model, a coaxial cylindrical arrangement was chosen, in which the inner cylinder represented the protrusion, and the out er cylinder and the to p and b o t t o m represented the tank walls. The objects were taken to be
307 cylindrically shaped and were t h o u g h t to approach the inner cylinder at right angles, the point of contact being located halfway between the top and b o t t o m of the inner cylinder. The spark properties were calculated in steps. The first step was the c o m p u t a t i o n of the electric field at the inner cylinder in the absence of the object. The second step was the derivation of an empirical relation between the charge induced in the object, the electric field just calculated, and the length and diameter of the object. The third step was to find the capacity of the object in a position near the protrusion which enables the spark voltage and energy to be derived. Table II gives the results of the first step, the electric field at the inner cylinder as a function of the inner cylinder radius. The dimensions of the outer cylinder were a height of 20 m and a diameter of 20 m, which is representative of a centre tank in a large tanker. For the charge density a value of 40 nC/m 3 was taken, as observed during the cleaning of large tanks with severely contaminated washing water. The results of Table II were obtained by using a computer program described earlier [5]. An empirical relation was derived for the net charge induced in a cylindrical object when brought into contact with the inner cylinder. This relation was set up in such a way that the electric field near the inner cylinder in the undisturbed situation (before the presence of the object) was the only parameter describing tank and protrusion shapes and the charging conditions. The relation was established by means of small-scale model tests in homogeneous and radial electric fields. For objects entering previously homogeneous fields it reads: Qind = 4.8
Q)ELI"5D°'5
(3)
in which E is the undisturbed homogeneous field strength at the boundary, L the length of the object, and D the diameter of the object. This equation fits the experimental data fairly well for L/D between 2 and 7. For objects entering radial electric fields, we found the following best fit to experimental data: Qind = 3.8
~oELI"5D°'5
(4)
in which E is the undisturbed field strength at the inner cylinder of the coaxial cylindrical arrangement, L the length of the object (cylinder), and D the diameter of the object. Equation (4) applies at L/D ratios between, say, 3 and 20. It fitted our experimental data within 20% for L/Ri smaller than 2 (Ri being the inner cylinder radius). Before we could assess the spark energy it was necessary to know the capacity of a cylindrical body of given L and D at sparking distance from the inner cylinder. In our approach, we tried to relate this capacity to the freespace capacity of a cylindrical body 2 u ~0 (L: -- D 2 )o.5 Cfree =
l n ( L + (L 2 - - D 2 ) ° ' 5 ) D
(5)
308 We did some laboratory tests in which : , o measured the capacity of hemispherically-ending cylindrical objects at sparking distance from a fiat plate. This resulted in a capacity estimate of 1.5 Cfree for objects at sparking distance from the inner cylinder with the same restrictions as those for the application of eqn. (4). Figure 9 shows the application of the equations derived to obtain the object length necessary to produce sparks with an energy of 0.2 mJ for different object diameters as a function of inner cylinder radius. The expression for the energy U we used was: U = Q2/3Cfree
(6)
in which Q is the induced charge in the object according to eqn. (4). OBJECT LENGTH L (m) 06 0.5 0.4
03
0.2
0.1 0.1
I 0.2
I 0.3
[ i I 1 I I 1 0.4 0.5 0,6 0.7 0.8 09 1.0 RADIUS OF INNER CYLINDER (m)
Fig. 9. Object lengths L and diameters D necessary to give spark energies of 0.2 m J upon approach of the inner cylinder for the model conditions given in Table II.
We can conclude from this model work that sparks with incendiary power are produced even when fairly small objects approach large-diameter protrusions, under the model conditions employed, i.e. charge density and tank dimensions representative of certain practical conditions in tank washing.
References 1 J.M. van de Weerd, Proc. Third Conf. Static Electrification, London, 1971, Inst. Phys. Conf. Ser. No. 11, p. 169.
309
2 R.F. Klaver, Charge transfer between isolated conducting objects and ground in a tank filled with an electrically charged mist, Chevron Res. Co., May 28, 1974. Part 1 of 2, API Statics Res. Program 1973--1974. 3 W.M. Bustin, Water slug formation during tank washing, Exxon Res. and Engineering Co., October 15, 1974, Part 2 of 2, API Statics Res. Program 1973--1974. 4 E. Barreto, S.I. Reynolds and H. Jurenka, Ignitions of hydrocarbons and the thermalization of electrical discharges, J. Appl. Phys., 45 (1974) 3317. 5 W. Smit, Proc. Third Conf. Static Electrification, London, 1971, Inst. Phys. Conf. Set. No. 11, p. 178.
ELECTROSTATIC C H A R G E G E N E R A T I O N D U R I N G TANK WASHING. SPARK MECHANISMS IN TANKS FILLED WITH C H A R G E D MIST J.M. VAN DE WEERD
KONINKLIJKE/SHELL--LABORATORIUM, Ne th erla nds)
(Shell Research B. V.), Amsterdam (The
(Received August 5, 1974; in revised form March 6, 1975)
Summary Under conditions of tank washing several possible sparking mechanisms were investigated involving conducting objects not bonded to the tank. Some of the tests were done in a full-scale 12,000 m 3 shore tank containing an electrostatically charged mist generated by a tank washing machine. Sparking mechanisms were identified associated with the formation of unbonded conductors in the form of water masses ("water slugs") by the water jets used for tank cleaning. Ignition experiments were performed using masses of water as unbonded conductors. They produced sparks of sufficient intensity to ignite h y d r o c a r b o n l i r mixtures under conditions relevant to the practical situation in tankers.
1. Classification of spark mechanisms Looking for spark mechanisms in tanks filled with charged mist is equivalent to looking for ways in which unbonded conductors present in such a tank can acquire a voltage substantially different from that of the tank structure when the gap between object and tank structure is narrow. We can distinguish a number of mechanisms: (a) A c c u m u l a t i o n o f charges This mechanism, which is the most obvious, is probably not very important for our case. Part of the charges present in the mist might be picked up by the insulated object. The contribution of the picked-up charges AQ to the voltage difference between object and tank wall,/~ V, is given by A V = AQ/C, in which C is the electrical capacity of the object in the position near the tank wall. Practical studies showed that the actual charge accumulation due to pickup is of minor importance as compared to that given by other mechanisms. This is due to the low mobility of the charge carriers in a tank-cleaning mist. For the case of a highly charged steam mist, however, indications have been obtained that pick-up could lead to dangerous sparks. *Paper presented in the Special S y m p o s i u m on Tanker Explosions at the 2nd International Conference on Static Electricity, Frankfurt, April 6, 1973.
296 (b ) Objects containing an induced net charge An object connected to the tank structure, and thus still at " e a r t h " potential, in a position in which it is very much exposed to the electric field (protruding) will contain a net (induced) surface charge of a sign opposite to that of the charges in the mist. Upon being severed from the tank structure, the object--now unbonded--is charged. When it touches the tank structure again in a place less exposed to the electric fields from the mist (e.g. a tank wall instead of a protrusion where it originated) a voltage difference will arise. Again, we can write A V = AQ/C, in which AQ is now defined as the difference between the charge the object actually contains and the charge it should contain at that place when it was at " e a r t h " potential (induced charges in the latter position). (c) Objects acquiring a net induced charge Let us consider the opposite of mechanism (b). The object starts its life as an unbonded conductor in a position where the electric field from the mist is low. It therefore hardly contains any net charge. Assume that this uncharged conducting body approaches a structure at earth potential that protrudes into the charged mist. In the electric field around such a protrusion the object is polarized and, upon close approach, one sign of the polarization charge on the object is neutralized in a spark discharge. Just prior to the spark there will be a voltage difference A V between object and protrusion, again given by ~ V = AQ/C, where Q is the difference between the actual net charge on the object before the spark goes and the charge present on the object after the spark has occurred by which the object has acquired earth potential. (d) "Free'" charges on the object due to corona or "water spray" mechanisms In this c o n t e x t " f r e e " charges are defined as charges in excess of those present upon earth contact due to the "ending of field lines". Depending on the shape of the object, it is conceivable that the electric fields at the surface of the unbonded object are locally sufficiently strong to give corona or "water spray" discharge. This might give " f r e e " charges and thus lead to sparking when the object approaches " e a r t h e d " tank structure. 2. Full-scale experiments Tests were performed in a full-scale set-up described earlier [1] - - a 12,000 m 3 tank equipped with facilities to produce charged mists. 2.1 Accum ula tion o f charges Charge pick-up was studied by dropping metal cylindrical objects from the top to the bottom of the test tank, onto an insulated plate connected to an oscilloscope. Figure 1 shows the set-up employed and Fig. 2 some of the results. Only in the case of a highly charged steam mist did we detect charge pick-up of sufficient importance to be considered a danger. In several instances, charges were measured of opposite sign to the charged mist. This suggests that in such cases not charge pick-up but, for example, corona effects are
297
:1
...............
L
Fig. 1. Experimental arrangement of insulated table on the b o t t o m of the 12 0 0 0 m 3 K S L A shore tank for charge detection on falling objects. OBJECT LENGTH r~IAMETER (ram) (m) • 5 1 • 8 1 • 13 1 • 18 1 • 13 1.5 60 1
MIST SOURCE WATER JET
ST'~'AMIN; STEAMING
CHARGE ON OBJECTS (nC) 20G 180 160 140
#
120
lO0 8O 6O 40 20
• I~
0 I I I I I 0 10 20 30 40 50 SPACE POTENTIALIN THE MIDDLE OF THE TANK (kV)
Fig. 2. Charge on objects dropped from the top of the tank.
responsible for the charge on the objects. The tests performed were not sufficiently general with respect to object size and shape that charge pick-up can be completely ruled out as a possible source of dangerous sparks, but they do suggest strongly that other mechanisms are more likely. 2.2 Objects containing an induced net charge Objects suspended from an earthed wire in the middle between the top and b o t t o m of the shore tank were dropped onto an insulated plate mounted on the tank bottom. Again, charges of variable sign and low magnitude were detected. This suggests that effective discharging of the object might occur
298 during the fall, presumably due to corona and probably, to a much lesser degree, due to compensating charge pick-up. It is conceivable that not in all circumstances is the shape of the objects such that corona discharge of the net induced charge occurs. Recent studies by Klaver [2] indicate a loss of charge due to corona varying between 0 and 80%. He gives practical examples where no loss of charge occurs at all. Spark energies of sparks occurring as a result of this mechanism can be estimated according to:
( Cfree Vax)
(1)
in which Vmax is the maximum potential present in the tank due to the space charge; Cfree is the free-space capacity of the falling object; k is a factor which is always smaller than 1 and which depends on the shapes of the object and the protrusion and on the position in the t a n k where the object starts and ends its unbonded life. If, for example, the object were a nozzle of a strongly protruding tank cleaning machine that would drop off and hit the tank b o t t o m , the value of k, under the assumption that no charge would be lost during the fall, would be, at most, about 0.5. If the object were a water slug generated by the breakup process of a cleaning jet, the value of k would be about 0.2--0.4, assuming that the break-up of the jet occurs in a position where the jet is strongly protruding into the charged mist. 2.3 Objects acquiring an induced net charge Before discussing !Lndetail the experimental set-ups employed, we should like to go into some of the experimental problems and interpretational pitfalls encountered. What we are interested in is the spark energy, i.e. the charge transfer during the spark and the voltage across the gap just prior to the spark. This is necessary because we want to evaluate the incendiary power of the sparks w i t h o u t doing full-scale ignition tests. Both voltage and charge transfer are difficult to measure in a practical experiment. If we want to measure the full charge transfer to the object after it hits the protrusion, we should ideally have a charge-measuring probe of very small dimensions available at the point of contact between object and protrusion. Only in that case will all the charge flowing in the spark pass the measuring circuit and thus be detected. However, a measuring point of such small dimensions presents practical problems in that the object should exactly hit the point in which the charge-measuring probe is located. To overcome this one might decide to use a charge measuring system connected to the protrusion as a whole. In that case, however, the only charges measured are the extra charges induced when the object approaches the protrusion. But the contribution to the charge transfer in the spark from a shift in induced charges between protrusion and object is n o t measured. Direct measurement of the spark voltage is only possible if the gap width between object and protrusion at the m o m e n t of the spark can be determined.
299
Indirectly the voltage difference across the gap could be determined from the true charge transfer and the capacitance in the situation just prior to the spark. But measuring this capacitance in a practical test again presents experimental problems. One of the procedures employed to determine the spark properties prevailing when insulated objects approach a protruding structure is shown schematically in Fig. 3. An object connected to an insulator is lowered into the tank by means of an earthed cable. An adjustable spark gap between cable and object is provided. By adjusting the spark gap to the maximum distance still giving sparking after a lowering operation the voltage of the object in the
/
Fig. 3. Schematic representation of e x p e r i m e n t with a c o n d u c t i v e object c o n n e c t e d to an insulator arrangement and l o w e r e d in a t a n k filled with charged mist (Fig. 3(a)), for inductive effects simulating the situation of Fig. 3(b), an u n b o n d e d object nearing a protrusion.
position near a protrusion can be determined, the protrusion in this case being the suspension cable. Of course, a protrusion of this kind is one of a very serious nature and not likely to be the most c o m m o n type present in tanks. Another drawback of this technique is that the voltage distribution around a permanently present protrusion is likely to be changed by the occurrence of corona discharges. In the case of our experiment, the "protrusion" is created by the lowering operation of cable and object and thus not sufficient time might be available for the corona to change the potential distribution in a representative way. Table I gives the results of a number of tests done with objects of different lengths and under different charged mist conditions in the tank. Figure 4 gives an energy estimate of the sparks observed according to U=
1
2
(1.5
Cfm e +
C ~2 T72 o! V o b s
1.5 Cfree
(2)
300
TABLE I
Observed m a x i m u m voltages of sparks o b t a i n e d in e x p e r i m e n t of Fig. 3
insulated obJect-near-protrusion
M a x i m u m spark voltages (kV) o b t a i n e d at a rod length (cm) of 25
50
75
100
120
2 2 3.5 6--7
3.5 5.5--7 10--11
2.8 4.5 7--11 13.5--18
5 8--10 15
3.2 6 8.5 16
Space potential in the middle of the tank (kV) 150
7
8--10 15 20--25 30--45
Well-rounded 13 m m diam. rods with lengths from 25 to 150 cm
LENGTH OF 13 mm DIAMETER RODS (cm) 150 11.6
100 75
k5.3 1.6~
50 25 0
0
4.~
~ 0.9 I 10
0.9
13.6-21.3
48-
10 - 24.6
37--66
"~8 ~2.7
--
20-24 ---t."'--
7.8-10.6
I I I I 2O 3O 40 50 SPACE POTENTIAL IN THE MIDDLE OF THE TANK (kV)
Fig. 4. Energy estimates of the sparks observed in the e x p e r i m e n t of Fig. 3, expressed in 10 -4 J, as a f u n c t i o n of mist c o n d i t i o n s and object length, using eqn. (2) (see text). The arrows indicate variations in mist c o n d i t i o n s during part of the experiment. The dashed and drawn lines indicate the 0.2 and 0.5 m J limits.
Equation (2) includes a correction to the observed voltage values, Vobs as given in Table 1 because of the extra capacitance Co = 10 pF present in the suspension arrangement. The capacitance of the object near the protrusion in this situation without the suspension arrangement, is estimated to be 1.5 Cfree, Cfree being the "free space capacity" of an ellipsoid with axes equal to the length and diameter of the cylinder. This estimate is justified later.
301
Full-scale experiments with water slugs as unbonded objects were carried o u t in a set-up as illustrated in Fig. 5. A bar of 5-m length and 5.7-cm diameter was suspended horizontally in the centre of the shore tank and represented an earthed protruding structure. Water slugs were dropped from the r o o f onto or close to the bar. The slugs were released from a cylindrical tube of 5-cm diameter and 2-m length. Their length could be varied by partly filling the slug-producing tube. The bar arrangement as a whole was earthed via a resistor, a capacitor and an oscilloscope, all placed in parallel. A charged mist was created by a high-capacity cleaning machine giving a space voltage of a b o u t 9 kV halfway between the top and b o t t o m of the tank. When a water
\'/ 1~,50 nFOSCILLOSCOPE Fig. 5. Test set-up in t h e 12 000 m 3 s h o r e t a n k for sparks f r o m w a t e r slugs d r o p p e d o n a h o r i z o n t a l bar.
slug closely approached the bar, single-pulse type spark signals were observed on the oscilloscope. Figure 6 gives the magnitude of some of the signals observed, expressed in charge accumulated in the measuring capacity of 50 nF directly after a pulse. As stated before, it is difficult to relate the observed signals to the actual charge transfer in the spark. The signals observed are caused by e x t r a charges induced in the protrusion system due to the approach of the slug. But the contribution to the charge transfer in the spark due to a shift in induced surface charge from the protrusion to the slug is not observed. The experiments were repeated at higher charge density levels and then showed an increase in signal strength. The signal strength was found to vary approximately linearly with charge density {max. space voltage). Tests with a protruding structure of a more bulky nature gave w~aker signals. In view of the interpretation problems discussed extensively before, it is not possible to say
302 CHARGE TRANSFER, nC 150
1OC
50
l
O C~5 025 Q5 1.O 20 FILLING HEIGHT OF WATER-SLUG PIPE, m
Fig. 6. Signals f r o m water slug sparks obtained in the set-up of Fig. 5 charged mist from a cleaning jet with a space potential in the middle of the tank of 9 kV.
whether this reduction in signal strength is due to a major reduction in the actual charge transfer in the spark or mainly reflects a difference in distribution of induced surface charges between protrusion and tank wall.
2.4 Conclusions on the importance o f the spark mechanisms related to tank cleaning It can be concluded from the experiments that spark mechanisms as described in Sections l(b} and l(c) are the most important for the practical situation. Both mechanisms involve objects in relation to protrusions. Spark energies are dependent on the size and shape of the object and the protrusion and their positions in the tank. In both cases, the spark energy can be represented by an expression as given in eqn. (1):
(1) in which k is mainly a function of the protrusion geometry and the path taken by the insulated conductor. We can evaluate the break-up process of a cleaning jet which is an important practical source of insulated conductors. The factor k in eqn. (1) representative of such a jet break-up process can be estimated to be about 0.2--0.4, depending
303 o n t h e slug size. T h e s e values f o r k relate to t h e b r e a k - u p o f a t a n k cleaning j e t in a s t r o n g l y p r o t r u d i n g p o s i t i o n a n d t h e resulting c h a r g e d slug t o u c h i n g an e a r t h e d t a n k s t r u c t u r e in a l o w v o l t a g e g r a d i e n t area. A l t e r n a t i v e l y , it applies to t h e reverse, t h e slug g e n e r a t e d in a l o w voltage g r a d i e n t area a n d t o u c h i n g e a r t h at a s t r o n g l y p r o t r u d i n g s t r u c t u r e . T h e values f o r k c a n be f o u n d b y e v a l u a t i o n o f t h e results of e x p e r i m e n t s as, f o r e x a m p l e , given in Fig. 4. T h e y can also be derived f r o m a c o m p u t a t i o n o f idealized t a n k c o n f i g u r a t i o n s as t h o s e given in T a b l e II. TABLE II Computed electric field at inner cylinder of a finite coaxial cylindrical arrangement Radius of inner cylinder (m)
Electric field at the inner cylinder halfway between top and bottom (kV/m)
Electric field at the inner cylinder 5 m from top or bottom (kV/m)
Maximum voltage between inner and outer cylinder (kV)
0.001 0.005 0.010 0.025 0.05 0.10 0.50 1.00
10266 2509 1387 645 369 212 66.6 42.5
3412 2063 1143 533 307 178 56.8 37.0
74.4 72.4 70.8 68.8 66.8 64.0 54.5 47.4
Radius of outer cylinder 10 m; height 20 m; charge density 40 nC/m 3. Electric field and maximum voltage are given as a function of inner cylinder radius. T h e m a x i m u m p o t e n t i a l Vmax has b e e n s h o w n to be at least 30 k V f o r t a n k cleaning o p e r a t i o n s as p e r f o r m e d o n t h e Mactra and t h e Marpessa j u s t p r i o r t o t h e i r e x p l o s i o n s . T a n k cleaning was p e r f o r m e d b y high c a p a c i t y t a n k cleaning m a c h i n e s . T h e w a t e r jets o f such m a c h i n e s m a y split u p in slugs o f a length of, say, 50 c m and a d i a m e t e r o f 4 c m . A c c o r d i n g t o ref. [3] this is quite a n o r m a l o c c u r r e n c e . Given t h e suitable slug t r a j e c t o r y , eqn. (1) p r e d i c t s f o r this s i t u a t i o n a s p a r k e n e r g y o f a b o u t 1.5 m J taking k as 0.2. F o r this t y p e o f s p a r k f r o m w a t e r slugs t h e m i n i m u m ignition e n e r g y has b e e n s h o w n to be a b o u t 0.5 m J , as we will see l a t e r on. We c a n thus c o n c l u d e t h a t s p a r k m e c h a n i s m s involving split-up processes o f the w a t e r jets used f o r t a n k cleaning are o f m a j o r i m p o r t a n c e f o r s a f e t y c o n s i d e r a t i o n s . Sparks o f i n c e n d i a r y p o w e r f o r explosive t a n k gas m i x t u r e s w o u l d be g e n e r a t e d even in cases w h e r e t h e t r a j e c t o r y o f t h e w a t e r slug w o u l d be such t h a t a l o w e r f a c t o r k w o u l d result. 3. L a b o r a t o r y s t u d y o f t h e b e h a v i o u r o f w a t e r slugs as u n b o n d e d c o n d u c t o r s T h e b e h a v i o u r o f free-falling w a t e r slugs w h e n t h e y a p p r o a c h a p r o t r u s i o n
304
surrounded by an electric field was studied in a laboratory-scale set-up. Slugs of a b o u t 65-cm length and 1.5-cm diameter were dropped on or close to a cylindrical probe 1 cm in diameter at a high voltage (10--30 kV). The signals of the sparks observed were studied and found to be similar to those from metal objects of similar shape. Photographs of the sparks were t a k e n using an image intensifier. In addition, we used a fast flash of low light intensity, triggered by the electric signal of the spark, to obtain an image both of the slug boundary and of the spark. Figure 7 shows one of the pictures taken. From the pictures, it could be concluded that the water surface of the slug is not y e t influenced by the strong electric field in the gap between slug and protrusion just prior to the spark. Obviously, the field strength in the gap
0.3S 13ram
4 Vldiv 2msldiv Fig. 7. Spark generated by water slug approaching probe. Probe voltage 30 kV; spark length 13 ram. Charge transfer in spark as estimated from electric signal 160 nC. Slug length 65 cm.
305 increases so rapidly that there is n o t enough time for surface disruption before the m o m e n t that the field is sufficiently strong to give spark breakdown. The photographs thus corroborate the evidence produced by the electric signals, i.e. the water slugs behave similarly to solid conducting objects. In a recent publication, Barreto et al. [4] give details on the spark discharge process between a metal and a liquid (water) electrode. By using highresolution oscilloscope techniques they were able to show that, apart from phenomena associated with the deformability of the water surface in the electric field, the lower conductivity of the water electrode as compared to a metal one may also affect the discharge phenomena. Their data on explosion tests, although not comprehensive, indicate a somewhat higher minimum ignition energy for water to metal sparks as compared to metal sparks. This is in line with results of ignition tests we performed using actual water slugs. Ignition tests were performed in a well-controlled flammable atmosphere of 5% propane in air. Figure 8 shows the set-up. Slugs were produced in a 75-cm-long, 2-cm-diameter tube closed with a shutter. When the shutter was opened, the slugs travelled downwards over a distance of a b o u t 3 m towards an earthed probe of 2 ~ m diameter. An electric field around this probe was produced by means of a cylindrical, cage-type high-voltage electrode of 1-m height and 60-cm diameter. A charge-monitoring arrangement, consisting of capacitor, resistor and oscilloscope placed in parallel, was connected to the probe. The flammable atmosphere was produced by enclosing the complete set-up in a plastic tent of a b o u t 2 m 3 volume. This large volume of gas was necessary to provide the proper gas mixture over the entire trajectory of the water slug. Only in that case would it be possible to guarantee the appropriate gas composition in a surface layer around the water slug. Ignitions due to falling water slugs in this set-up were observed at a minimum voltage at the polarizing electrode of 15 kV. At this voltage, careful positioning of the probe with respect to the trajectory of the slug was essential; only in cases when the water slug just passed the probe were ignitions observed. Estimates of the spark energy still giving ignition were of the order of 0.5--0.8 mJ. So sparks from water slugs gave ignitions at energies n o t far above the minimum ignition energy level of 0.25 mJ for the propane--air mixture employed. 4. Model studies It was attempted to calculate the energies of sparks occurring when uncharged conducting bodies approach a structure that is protruding into a charged mist confined in a tank. In the electric field around such a protrusion, the object is polarized, and upon close approach one sign of the polarization charges on the object is neutralized in a spark discharge. The charge transfer in such a spark depends on the shapes of the object and the structure, and on the intensity and size of the charged mist surrounding the structure. The number of parameters involved is therefore so large that only
306
Fig. 8. An ignition due to a water slug. idealized situations can be tackled effectively by calculations. As a model, a coaxial cylindrical arrangement was chosen, in which the inner cylinder represented the protrusion, and the out er cylinder and the to p and b o t t o m represented the tank walls. The objects were taken to be
307 cylindrically shaped and were t h o u g h t to approach the inner cylinder at right angles, the point of contact being located halfway between the top and b o t t o m of the inner cylinder. The spark properties were calculated in steps. The first step was the c o m p u t a t i o n of the electric field at the inner cylinder in the absence of the object. The second step was the derivation of an empirical relation between the charge induced in the object, the electric field just calculated, and the length and diameter of the object. The third step was to find the capacity of the object in a position near the protrusion which enables the spark voltage and energy to be derived. Table II gives the results of the first step, the electric field at the inner cylinder as a function of the inner cylinder radius. The dimensions of the outer cylinder were a height of 20 m and a diameter of 20 m, which is representative of a centre tank in a large tanker. For the charge density a value of 40 nC/m 3 was taken, as observed during the cleaning of large tanks with severely contaminated washing water. The results of Table II were obtained by using a computer program described earlier [5]. An empirical relation was derived for the net charge induced in a cylindrical object when brought into contact with the inner cylinder. This relation was set up in such a way that the electric field near the inner cylinder in the undisturbed situation (before the presence of the object) was the only parameter describing tank and protrusion shapes and the charging conditions. The relation was established by means of small-scale model tests in homogeneous and radial electric fields. For objects entering previously homogeneous fields it reads: Qind = 4.8
Q)ELI"5D°'5
(3)
in which E is the undisturbed homogeneous field strength at the boundary, L the length of the object, and D the diameter of the object. This equation fits the experimental data fairly well for L/D between 2 and 7. For objects entering radial electric fields, we found the following best fit to experimental data: Qind = 3.8
~oELI"5D°'5
(4)
in which E is the undisturbed field strength at the inner cylinder of the coaxial cylindrical arrangement, L the length of the object (cylinder), and D the diameter of the object. Equation (4) applies at L/D ratios between, say, 3 and 20. It fitted our experimental data within 20% for L/Ri smaller than 2 (Ri being the inner cylinder radius). Before we could assess the spark energy it was necessary to know the capacity of a cylindrical body of given L and D at sparking distance from the inner cylinder. In our approach, we tried to relate this capacity to the freespace capacity of a cylindrical body 2 u ~0 (L: -- D 2 )o.5 Cfree =
l n ( L + (L 2 - - D 2 ) ° ' 5 ) D
(5)
308 We did some laboratory tests in which : , o measured the capacity of hemispherically-ending cylindrical objects at sparking distance from a fiat plate. This resulted in a capacity estimate of 1.5 Cfree for objects at sparking distance from the inner cylinder with the same restrictions as those for the application of eqn. (4). Figure 9 shows the application of the equations derived to obtain the object length necessary to produce sparks with an energy of 0.2 mJ for different object diameters as a function of inner cylinder radius. The expression for the energy U we used was: U = Q2/3Cfree
(6)
in which Q is the induced charge in the object according to eqn. (4). OBJECT LENGTH L (m) 06 0.5 0.4
03
0.2
0.1 0.1
I 0.2
I 0.3
[ i I 1 I I 1 0.4 0.5 0,6 0.7 0.8 09 1.0 RADIUS OF INNER CYLINDER (m)
Fig. 9. Object lengths L and diameters D necessary to give spark energies of 0.2 m J upon approach of the inner cylinder for the model conditions given in Table II.
We can conclude from this model work that sparks with incendiary power are produced even when fairly small objects approach large-diameter protrusions, under the model conditions employed, i.e. charge density and tank dimensions representative of certain practical conditions in tank washing.
References 1 J.M. van de Weerd, Proc. Third Conf. Static Electrification, London, 1971, Inst. Phys. Conf. Ser. No. 11, p. 169.
309
2 R.F. Klaver, Charge transfer between isolated conducting objects and ground in a tank filled with an electrically charged mist, Chevron Res. Co., May 28, 1974. Part 1 of 2, API Statics Res. Program 1973--1974. 3 W.M. Bustin, Water slug formation during tank washing, Exxon Res. and Engineering Co., October 15, 1974, Part 2 of 2, API Statics Res. Program 1973--1974. 4 E. Barreto, S.I. Reynolds and H. Jurenka, Ignitions of hydrocarbons and the thermalization of electrical discharges, J. Appl. Phys., 45 (1974) 3317. 5 W. Smit, Proc. Third Conf. Static Electrification, London, 1971, Inst. Phys. Conf. Set. No. 11, p. 178.