Electrical Charges Produced by Flowing Gasoline S. S. MACKEOWN AND VICTOR WOUK California Institute of Technology, Pasadena, Calif. currents that may be measured. The reading of this meter is obtained by the milliammeter contained between the two anodes of the last two tubes. Before a reading is made, the meter is adjusted by varying the contact on the resistance between the anodes of the first two tubes so that the milliammeter reads zero. By selecting the proper input resistance, the sensitivity of the meter can be changed so that it will read from approximately lo-” to 10+ ampere. This range more than completely covers the magnitude of the currents which have been encountered. The meter has proved extremely satisfactory and can be used for days without recalibration. Figure 2 is a photograph of the instrument.
The flowing of gasoline in pipes generates static clectrical charges which are sometimes capable of producing a spark that may ignite a combustible mixture of gasoline vapor and air. Although suitable precautions have been taken in the oil industry to prevent the accumulation of these charges and eliminate the hazard due to an electric spark, very little is known regarding the magnitude of the charges. This work was undertaken to determine the magnitude of electrical charges which can be produced in the handling of gasoline. Measurements have been made on charges produced whes gasoline is pumped from a filling rack to a tank truck, when the truck unloads at a service station, and when an automobile receives gasoline at a service station. These measurements were made under normal operating conditions. Abnormally large amounts of moisture in the gasoline might alter the results.
T
Measurements
HE currents produced by flowing gasoline are so small that an ordinary meter will not measure them. TO make these measurements in the field, it was necessary to design an instrument which would be rugged and portable, operate under conditions of severe vibration. and vet measure very small .
A vacuum tube meier was built which has proved satisfactory t o cover the complete range of currents generated in these operations. The circuit for this meter is shown in Figure 1 and is practically the same as that described in detail by Goldberg’. The special feature is that, by the use of the push-pull arrangement of the tubes, the readings of the meter are made relatively insensitive t o changes in battery voltages. Across the i n m t of the first two vacuum tubes are resistances of 0.5, 2, 10, and 20 megohms, with a switch to select any of these resistances and thus control the range of 1
Goldberg, Harold, Elec. Eng., Jan., 1940
Approximately three hundred measurements were made of the currents produced when gasoline is loaded into a tank truck a t a filling rack. Three grades of gasoline, refined and sold by four different oil companies operating on the Pacific Coast, were used. The vacuum tube meter was connected between the tank truck and ground in place of the usual bonding wire as illustrated in Figure 3. Measurements were made in which the truck was insulated from the ground by placing wooden planks boiled in paraffin under the tires. It was soon found, however, that the insulation provided by the rubber tires of the truck was so high that no further insulation was needed. At no time was it necessary to use a resistance greater than 10 megohms in the input of the meter, and the measurements were made in the summer when the relative humidity was low. Under these conditions the tires of the truck always had a resistance which was very large compared t o 10 megohms. A wooden collar was built which could be quickly connected around the metal fill pipe used to
+
I
I-
-
- --2-- LL-=FIQURE1.
1
- --- - --- - - -- -L - - -1I
CIRCUITDIAGRAM OF PORTABLE BALANCEDMICROAMMETER 659
660
INDUSTRIAL AND ENGINEERING CHEMISTRY
fill the tank trucks. The wooden collar served to locate the fill pipe so that it could not touch the truck and ground it (Figure 3). Every 15 seconds readings were made of the current produced during the filling of the truck. The total amount of gasoline discharged into the truck was measured, and since the time was recorded, the rate of flow could be calculated. Measurements were made with an insulated fill pipe of neoprene, with metal fill pipes having a canvas extension to the bottom of the truck, and with metal fill pipes which extended only a short v a y into the truck. A typical run is shown in Figure 4 in which the current is plotted against the time of filling. This run was made with the metal fill pipe having a canvas extension. The curve shows that a t the start of filling there is a slightly larger current than during the rest of the time. After this initial rise the current reFIGURE 2. PHOTOGRAPH OF METER mains constant A . Zrio adjustment knob until the gasoB . Input terminals line level reaches C. Range selector t h e m e t a l fill pipe; then there is a small decrease in the measured current. The decrease in measured current during the filling of the truck, when the metal fill pipe is in contact with the gasoline in the truck, is probably due to the fact that some of the charge produced in the filling of the truck is conducted to ground through the fill pipe, and consequently does not register on the meter. There is probably no difference in the total current produced in filling a tank truck when an insulated pipe is substituted for a metal pipe of the same diameter. The current measured must be the current carried by the gasoline from the fill pipe into the truck. Splashing can produce only a separation of charges and no net charge inside the truck. Since the truck is a complete metal shield, it forms what is knowri as a Faraday cage. Theory shows that inside such a Faraday cage the net charge, including charges induced on the inner walls of the cage, must always be zero. The only charges that can be measured by connecting to the outside of the truck are, consequently, the charges carried into the tiuck from the outside. These charges are the only ones that can raise the potential of the truck and thus be considered a hazard. As expected, our measurements showed no difference in the charging rate which could be attributed to splashing of the gasoline while filling. Measurements were made on the charge produced with a pipe of the same diameter over a large range of filling rates, from 800 to 5 gallons per minute. These results show that in practically every case the amount of current produced is directly proportional to the rate of flow. Curve A , company A, Figure 5, shows all of the measurements made on the second grade of gasoline refined by one of the oil companies. This gasoline contains some tetraethyl-
Vol. 34, No. 6
lead. Each point represents the average current determined by an individual run such as that shoxn in Figure 4. These runs were made a t three different filling racks, 1%-here different rates of flow could be obtained, with 3-inch fill pipes. Some of the pipes were metal and some were neoprene. Practically all of these measurements were made with the pipe extending well down into the tank truck so that filling TWS primarily submerged. The points clearly define a straight line, which shows that the current produced is directly proportional to the rat'e of flow. These points are scattered somewhat around this line, probably due to the fact that disturbances often produced current's as large as those nieasured. Curve B shows the results obtained for the first grade (Ethyl) of gasoline by company A, and curve C, for the third grade vhich contains no ethyl. Why the gasoline containing no ethyl shows a current intermediate between that of the second and first grades is not known. Figure 5 also s h o w the curves for gasolines sold by company B. Curve E for Ethyl gasoline differs from the other curves in two respects: First, it shows a negative charge. Neasurements on all other gasolines shom~that the tank trnck acquired a positive charge. Furthermore, curve B does not pass through the origin. Measurements were made a t low rates of filling on this gasoline and gave inconsistent results, Sometimes when the filling rate was low, the truck would initially acquire a negative charge which would change t,o positive during the filling operation. Sonietimes the truck would initially acquire a positive charge and then during the filling operation this would reverse. No reason is known for these peculiar results, nor why this gasoline a t high filling rates should give a charge which differs in sign from other gasolines. For the gasoline sold by company C, no difference was found between the current produced by Ethyl and secondgrade gasolines (Figure 5 ) . In the case of gasoline sold by company D, the third grade produced more current than the others. These measurements on three different types of gasoline sold by four companies indicate that in practically every case the current is proportional t o the rate of filling, and that consistent results are obtained on any one type of F~~~~~3, cONNEC- gasoline. Measurements made on TION OF METERWHEN the same type of gasoline during an interval of t'wo months gave MEASURING sT-kTIc GENERATED consistent results. These results TRUCK-FILLING PROCESS show that for some unexplained A . Mioroammeter reason there is considerable difB. Wooden collar ference in the charge developed C. Fill pipe by gasolines refined by different Extension pipe companies. In all cases currents for splash filling 2, of fill pipe from to 10-8 ampere were for submerged filling measured with the normal rate used in filling tank trucks a t a filling rack. We did not have detailed information regarding the chemical and physical differences in the various types of gasolines used. Therefore we could not determine why different types of gasoline gave different static charges or why a reverse charge was produced with one type of gasoline. It is hoped that some laboratory with the necessary facilities will undertake this problem, The measurement of the charge produced by flowing gasoline under laboratory conditions is relatively easy. It is possible that the charge produced can be correlated with certain physical or chemical properties of the gasoline. The measurement of these charges may be an easy
June, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
method of determining certain properties of a gasoline which might otherwise be difficult to ascertain. The capacitance to ground of a truck holding 2000 gallons of gasoline was measured and found to be approximately 0.001 microfarad.
Unloading of Gasoline at Service Stations Over a hundred attempts were made to measure the charge produced when a truck unloads its gasoline to an underground tank at service stations. Much difficulty was encountered in making these measurements. The charges developed were relatively small compared to those found when the truck was being loaded. This required a more sensitive scale on the meter and made the measurements much more susceptible to outside disturbances. Furthermore, since these measurements were made on or near the highway, many more disturbances occurred. The passing of automobiles, trolley cars, and even people produced a charge which was picked up by the truck, recorded on the meter, and therefore disturbed the readings. More than half the measurements gave such erratic results $hat they could not be used. Even working a t night was not satisfactory, since neon lights gave jnterferences which affected the readings. I n general, the charge produced was proportional to the rate of flow, using the same hose and nozzle. This agrees with the results obtained on the filling of trucks a t filling racks.
V
TIME AFTER START OF PUMPING
-
MINS.
SET O F READINGS FIQURE 4. TYPICAL
Measurements were made in which the meter was connected t o the truck and an insulating coupling was used between the faucet on the truck and the hose. The hose was grounded to the filling pipe at the underground tank. Measurements were also made in which an insulating coupling was used between the nozzle of the hose and the underground tank (Figure 6). These two conditions gave very different results. When the meter is connected at the faucet and the hose is connected through its bond wire to the underground tank, the charge measured consists of the charge produced by the flowing of gasoline through the faucet and an unknown portion of the charge produced by the flowing of gasoline through the hose. A bond wire was embedded in the hose but separated from the gasoline by a layer of synthetic rubber approximately inch thick. Charges produced on the inside surface of this inner layer can leak to the ground either over the surface of the artificial rubber or by conduction through the rubber to the bond wire. Consequently the measurements made when the meter was connected to the faucet of the truck would include those charges produced on the Inside of the hose which were conducted by the interior surface of the hose to the truck and through the meter to the ground. It did not include any charges conducted through the inner coating t o the bond wire or conducted by the inner surface of the hose to the nozzle and to the ground through the underground tank. When the meter is connected between the nozzle of the hose and the fill pipe of the tank, all of the charges produced,
661
TABLEI. CURRENTSGENERATED WHEN UNLOADING TRUCKS AT SERVICE STATIONS Grade of Gasoline Company A 2nd 2nd Ethyl Ethyl Corn a n y B Etiyl 2nd 3rd
Diarn. of Nozzle, In.
21’Q 2 “” 2 la/,
’”* 1> / a
Av. Current Arnp./Gal./Mh.
Meter Current When Connected Loading at: Amp./Gal./kin.
0.5 X 10-10 1 . 7 X 10-10 0.9 X 1 0 - 1 0 2 1 X 10-’0
Faucet Nozzle Faucet Nozzle
X 10-10 0.5 X 10-10
Faucet Faucet Faucet
-7.25 3.2
X 10-10
1 . 5 X 10-10
...... ......
5 . 0 X 10-10 -41.7 X 10-10 2 . 5 X 10-*0 13.0 X 10-10
both a t the faucet of the truck and by the gasoline flowing through the hose, will be measured by the meter. Table I shows the average current produced by the gasoline refined by companies A and B. The last column gives the measurements made in filling the truck a t the filling rack. The truck acquired a negative charge in all of the measurements, except that for the Ethyl gasoline refined by company B, where it acquired a positive charge. These results are in complete agreement with the sign of the charge measured when the truck was filled a t the filling rack. The sign of the charge on the truck is opposite in the two cases; for when the truck is being filled by gasoline, it acquires the sign of the charge oarried by the flowing gasoline, and when the truck is being emptied, it acquires a charge opposite in sign to that carried by the flowing gasoline. Table I shows the difference in the measurements when the meter is connected a t the faucet of the truck or between the nozzle and the underground tank. The latter case, which measures the total charge produced when the gasoline is emptied, gives a result two or three times greater than that measured when the meter is connected between the faucet of the truck and the hose. Further, the total charge produced when gasoliqe is discharged from a truck into a tank a t the service station is much less than that produced in filling the truck a t the filling rack.
Filling of Automobiles at Service Stations Several measurements were made of the currents generated when an automobile is filled a t a service station. The rate of pumping was 12 gallons per minute, a normal value. As was to be expected, the currents generated were smaller than those encountered in either of the previous sets of measurements. However, although the rates of pumping were from one tenth to one fifth as great as those previously encountered, the currents generated were only about one fourth t o one half as large. This is explained by the fact that the size of nozzle used when loading automobile tanks is only about inch in diameter, as compared with the 2- and 3-inch diameter nozzles used when loading and unloading the tank trucks. Measurements were made only on the second grade of gasoline produced by company A. At 12 gallons per minute, current generated was approximately 2.8 X 10-9 ampere. This corresponds to a rate of production of charge of 2.33 X 10-lo ampere per gallon per minute. A few measurements a t higher rates of pumping indicated that the current was directly proportional to the speed of pumping.
Resistance of Tires Measurements were made of the resistances of tires on automobiles chosen a t random; they were determined by applying a high voltage between the car and an insulated metal plate resting under the tire. The voltage was measured
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 34, No. 6
FIGURE 5 . MEASUREMENTS ON GASOLINES REFIXEDBY POCRCOMPANIES A.
B. C
0
100
200
300 SPEED
400
500
OF PUMPIN5
-
600 GPM.
700
800
with an electrostatic voltmeter and the current with a microammeter. The resistance was then determined by Ohm’s law. The car was rolled onto the metal plate that formed one electrode; the plate was insulated from the pavement with a layer of highly insulating material, so that the leakage over the pavement from other tires was’negligible. A Wimshurst Influence machine was used as a source of high voltage, and an electrostatic voltmeter reading 10,000 volts was utilized. The results showed a range of resistances for individual tires from 12,000 megohms (12 X l o s ohms) to 2000 megohms. These measurements were made under conditions of fairly low humidity. I n general, resistances measured mere less for older tires. When the tire was damp, the surface leakage was great enough to make the reeistance readable on an ordinary ohmmeter. Tire resistances as low as 10 megohms (10 X 108) have been encountered under such conditions.
Static Charges as a Source of Electric Sparks The current measurements show that it may be possible to generate a sufficiently high charge in filling a tank truck to produce an electric spark, if the truck is insulated from the ground. If such a spark occurs where there is a combustible mixture of gasoline vapor and air, a fire may result. There is no method of preventing the production of these charges.
Second-grade gasoline Ethyl gasoline Third-grade gasoline
Safety demands, therefore, that means be used to prevent these charges from accumulating and raising the truck t o a sufficient potential to produce a spark. Figure 7 shows the electrical equivalent of a tank truck which is either being filled or emptied of gasoline. The truck has a capacitance to ground, which is represented by condenser C. It will also have some resistance t o ground, R . If the truck is completely insulated or if the resistance to ground is extremely high, a spark may occur across any small gap which may exist between the truck and a grounded conductor. If we measure the current, I , produced by the filling of the truck, it is easy to calculate resistance R which is necessary t o prevent any spark from occurring across spark gap S. The value of this resistance which will prevent a spark is independent of the magnitude of condenser C‘. Since the condenser discharges once the spark gap breaks down, it determines how much energy will be dissipated in the spark. No spark can occur between two metal contacts, no matter how short the gap is, unless a potential of approximately 300 volts exists across the gap. This is called “minimum sparking potential”. If the gap is increased, the potential necessary t o cause a spark increases. Consequently, no spark can occur from a tank truck to any other object unless a potential of a t least 300 volts exists. It is possible to obtain an arc, as
INDUSTRIAL AND ENGINEERING CHEMISTRY
lune, 1942
663
Q can be determined by multiplying the
1
Underground Tonk.
charging rate by the time of charge. Capacitance G is approximately 0.001' microfarad (10-9 farad). If the truck is extremely well insulated so that the current leaking to the ground is small compared to the current generated by the flowing gasoline, then a relatively high potential may be produced on the truck and can cause a spark, which may ignite a combustible mixture of gasoline vapor and air. For a completely insulated truck the followine calculation can be made: Assuming current of 5 X ampere and a capacitance of the truck to ground of 1000 micromicrofarads, then after one minute the charge on the truck is:
I
(b )
(a) Meter at fttuaet, before hose (bl.
Meter .~
after hose. at entrance to underground tank
-
v
FIQURE6. TWOPOSSIBLELOCATIONS OF METERWHENMEASURINQ THE STATICGENERATED ON UNLOADING A TANKTRUCK A . Microammeter B. Insulating aoupling (Bakelite) C. Faucet D . Bonded ho8e
Q
=
5 X lod8 X 60 = 300 X 10-8 coulomb
The voltage on the truck will be: differentiated from a spark, with a very low voltage. For the starting of such an arc, however, a contact carrying considerable current must be broken. No static charges can produce a large enough current to,initiate an arc. If we know the current produced in the filling of a tank truck, it is easy to calculate what resistance is necessary to prevent a spark between the truck and any other grounded object. The potential of the truck is given by Ohm's law: IR = V where I designates the current, R the resistance, and V the potential. Referring to the circuit in Figure 7, I is the current produced by the filling of the tank truck, R is the value of the resistance between the truck and ground, and V is the potential which exists across spark gap S. As an example we might assume that a current of 5 X ampere is produced by a specific truck-filling operation, and that the minimum sparking potential is 300 volts. The resistance which will ensure no sparking during the filling operation can be calculated by substituting in the above formula. This calculation shows that any resistance less than 6 X 109 ohms (6000 megohms) will prevent a spark during the filling operation. This is an enormously high resistance and can probably obtain only under conditions of very low humidity and in the absence of any drag chain or bonding wire. Under these conditions there is no physical connection between the ground and the truck except the rubber tires. Even if we assume the highest resistance measured of 12,000 megohms per tire, the resistance to ground would be only 3000 megohms if four tires are used. If more than four tires are used, the resistance to ground will be correspondingly reduced. This study shows that under normal operating conditions the resistance to ground of the tires is sufficiently low to prevent a tank truck being filled with gasoline from acquiring a high enough potential to produce a spark. It is only under special conditions of very low humidity that a truck without a bond wire would present any hazard. If a truck is completely insulated so that none of the current can leak to ground, then the potential which the truck can acquire is given by the equation, V = Q/C
where Q = total static charge, coulombs C = capacitance, farads V = potential, volts
V
=
300&i0-8 = 3000 volts
I n the ordinary filling of a tank truck a t a filling rack, bonding wire of low resistance connects the truck to a grounded filling rack. In addition the metal fill pipe usually touches the metal of the truck. These precautions prevent the accumulation of any charge which may produce a spark. Even if these precautions are not taken, due to carelessness or accident, it will only be under conditions of very low humidity that the tank truck will be so well insulated by the rubber tires that the truck can acquire a sufficiently high potential to cause a spark.
_. -E=-
FIGURE7. ELECTRICAL EQUIVALENT OF A TANK TRUCKBEINGLOADED WITH GASOLINE C
--
-
oondenser. R = resistanoe; S spark gap: ourrendgenerated by flowmg gasoline
I'
When a tank truck is unloading gasoline a t a service station, the current produced is very small because of the low rate of flow used. If we assume a rate of discharge of 80 gallons per minute and a charging rate of 2 X 10-lo ampere per gallon per minute, the current will be 160 X 10-10 ampere. The resistance between the truck and ground which will prevent the truck from acquiring the minimum sparking potential of 300 volts can be calculated by applying Ohms' law. This shows that, under the assumed conditions, a resistance of less than 18 X 109 ohms will prevent any spark. It is doubtful if even under conditions of extremely low humidity a resistance as high as 18,000 megohms can exist between a truck and ground. When a truck discharges gasoline into an underground tank at a service station, a hose is used which contains a metallic wire to make electrical connection between the truck and the underground tank. This wire offers such a low resistance that no appreciable voltage can exist on the truck. In the ordinary filling of a passenger car a t a service station, not enough charge is generated to create any hazard even if the car is perfectly insulated. If we assume a current of 3 X
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
664
10-9 ampere a t a filling rate of 12 gallons per minute and a capacitance to ground of 750 micromicrofarads, a filling of 15 gallons of gasoline will be required to produce a sufficient charge to raise the car to the minimum sparking potential of 300 volts, even if the car is completely insulated and no charge leaks to ground. The total charge produced by filling with 15 gallons of gasoline will be :
Q
15
= 3 X 10-0 X 60 X - = 225 X
12
lo-* coulomb
The voltage will be:
Vol. 34, No. 6
If an unlimited amount of gasoline is pumped into a car a t a filling station, the resistance of the tires will have to be
greater than 100,000 megohms to charge the car to the minimum sparking potential. Such a high resistance can probably never be obtained with four tires.
Acknowledgment We wish to express our appreciation and thanks to the American Petroleum Institute, Richfield Oil Corporation, Standard Oil Company of California, Shell Oil Company, Inc., and Union Oil Company of California. The expense of this work was covered by a grant from the American Petroleum Institute. The four oil companies cooperated in this work, and the measurements were made on their equipment and trucks.
Guarding against the Flammable Liquid Fire Hazard I
C. L. GRIFFIN Walter Kidde & Company, Inc., Bloomfield,
N.J.
T
HE flammable liquid fire hazard is chief among the threats which menace the process industries. The storage, processing, and testing of flammables present a serious problem in industrial fire protection. The way to extinguish a flammable liquid fire is to smother it, take away its oxygen. I n most cases, i t is vital to control the fire in a hurry. One of the answers to this question of fire protection is provided by carbon dioxide gas, stored in cylinders under pressure and released to create a n inert atmosphere in which fire cannot exist. The smothering action which kills flammable liquid fires can be produced most quickly by taking away the oxygen necessary to support combustion. Atmospheric air a t or near sea level contains roughly 21 per cent of oxygen. The action of the carbon dioxide extinguisher is to displace some of this oxygen with inert carbon dioxide gas until the content of the atmosphere surrounding the fire has dropped to 14 or 15 per cent oxygen. Under these conditions fire will usually be snuffed out. Some materials will burn in the presence of smaller quantities of oxygen than will ordinary substances. When we say that fire cannot exist in an atmosphere containing as little as 14 or 15 per cent oxygen, this rule holds for most flammable liquid fire hazards. Gasoline is taken as the standard in formulating this rule. There are certain notable exceptions TI hich 77 ill be discussed later. Khile portable carbon dioxide extinguishers fulfill a n important first-aid function in fire-fighting, this article is concerned entirely ITith the built-in carbon dioxide fire-extinguishing system which is fixed in position.
methods employs a detector for rate of temperature rise, which is part of a pneumatic detecting, actuating, and release system. The fusible link device is one of the simplest forms of thermostatic control for actuating fixed extinguishing systems, I t s principal use is for the automatic closing of fire doors but it is also employed for shutting dip tank covers, operating drain valves, etc. The fusible link is purely mechanical in
Detection of Fire The automatic detection of fire may be handled in several effective ways. Quartz bulbs or fusible link detectors may take care of this function. I n some instances thermostatic fixed-temperature devices will set the extinguishing system into operation. One of the fastest and most dependable
Portable carbon dioxide extinguishers are particularly suitable for protection of laboratories; here a 20-pound unit guards a range o n which batches of lacquers are being tested.
