Computer simulation of passive device for elimination of static electricity.
Elimination of static electricity in loading and refilling operations of oil and oil products and gas into the metal storage tanks and tank trucks is a very complex problem. Commonly used solution for the elimination of static electricity by grounding of metal tanks is proved to be incorrect and very dangerous, since it provides condition for a cumulative discharge from the surface of oil or oil product and gas also, to wall of metal grounded tank.
Result of the described cumulative discharge is occurrence of a spark with ignition energy which is greater than minimum ignition energy (MIE) of combustible mixture in vapor space of a tank. In this way, an ignition of explosible mixture will occur and there will be an explosion and fire.
Thus, it was necessary to propose an approach for the controlled elimination of static electricity (controlled redistribution of generated charge), as suggested above-mentioned passive device for elimination of static electricity. So, the main role of the device is controlled transmission of generated electrostatic energy (generated by superimposing of volume and surface charge on the surface of the considered liquid or gas phase condition) from vapor space of tank to a grounding system.
In this paper, we conduct a computer simulation of the operation of proposed passive device for elimination of static electricity in software package MATLAB/Simulink v7.6 (R2010b).
[FIGURE 1 OMITTED]
2 OBJECTIVES OF COMPUTER SIMULATION
The goals of the simulation of the proposed passive device operation for elimination of static electricity are:
a) To obtain the reduction effect results of charge generated on surface of considered liquid or gas phase condition, by observing the relation of input/output values of charge (electrostatic voltage) and thus the input/output values of resulting electrostatic energy. The resulting ratio is the ratio of controlled charge reduction on the surface of the considered liquid or gas phase.
b) Using the indirect method described above, it is possible to obtain the comparative results of input/output values of resulting electrostatic energy. In this way, we can simulate and estimate the risk of a possible explosion from the direct grounded metal storage tank or tank truck. In the same way, we can simulate effect of controlled reduction of resulting electrostatic energy, in order to avoid unwanted consequences of fire or explosion.
c) In this manner, through common objectives of the simulation, we made and prove a controlled redistribution effect of resulting charge on the surface of the considered liquid or gas phase condition, using the proposed passive device for the elimination of static electricity.
3 DESCRIPTION OF THE DEVICE
Principled electrical scheme of the electric circuit of the proposed device is given in Fig.2.
[FIGURE 2 OMITTED]
For the selected values of parameters of elements in the electrical equipment scheme for example: [C.sub.1] = [C.sub.2] = [C.sub.3] = [C.sub.4] = 310pF, [R.sub.1] = [R.sub.2] = [R.sub.3] = 10M[ohm], L=310pH, and glow lamp switching voltage U=37V, we get diagrams for the current and the voltage on the input and the output of the device as is shown in Fig.6. The duration of the simulation is t=300ns.
Elements of the device shown in Fig.1 are determined to meet the requirement of a controlled elimination of static electricity. Principal electrical sheme of a device shown, can be divided into two section: power block of a condensed spark discharge, and a block of a integration RC circuit.
Power block of a condensed spark discharge consists of following elements: coil L, glow lamp [T.sub.1,] high voltage diodes [D.sub.1] and [D.sub.2], and high voltage elements, resistors ([R.sub.1] and [R.sub.2)] and capacitors ([C.sub.1], [C.sub.2] and [C.sub.3]).
Changes in incendivity of a spark as well as a smolder discharge are function of value of electric current flow I through flammable mixture in vapour space of the metal tank or tank truck.
Further, value of electrical current flow I can be changed--controlled by changing the inductive resistance L. Change in the value of inductive resistance L in the electrical circuit sheme of the device can be done by changing the frequency f and/or duration of spark or smolder discharge. The above is achieved by adjusting the values of elements of the integration RC circuit of the device.
In the unforseen event of the increased inflammatory of electrostatic discharge and creating of the spark discharge, such a created energy is controlled balanced at the power block of the device, on the element of the block, glow lamp [T.sub.1].
It is important to know that the above mentioned unforseen event of a spark discharge can occur in the vapour space of the tank, and in the direction oil surface--grounded metal wall of the tank truck or tank and vice versa.
Time priority state of direction of the sequence of events is maintained with direction of conduction of high voltage diodes [D.sub.1] and [D.sub.2], determined by the sheme of the observed device, in order to balance energy of a spark (and also smolder) discharge .
That is the reason why is the first block of electrical sheme of the observed device is called power block of condensed spark discharge .
The second block of electrical sheme of the observed block is called block of an integration RC circuit ([R.sub.3][C.sub.4]).
Value of resistance R3 in integration circuit is determined in order to satisfy conditions in the procedure of setup a proper operational mode, in the following order :
1) value of the time relaxation constant [tau][s] of the integration circuit must be at least an order of magnitude greater than the duration of pulse discharge current, [I.sub.pr];
2) value of integration resistance [R.sub.3], must be large enough that the signal observed on an oscilloscope or computer screen has a sufficient intensity of light;
3) value of integration resistance [R.sub.3] must be small enough to provide occurrence of two successive signals (pulses) on the oscilloscope or computer screen with a minimum time interval between the given signals;
4) value of integration resistance [R.sub.3], must also have a certain minimum value, in order to provide sufficient low level of interference on the oscilloscope or computer screen.
Criterion for selecting the value of integration resistance [R.sub.3] is given by the following double inequality :
10 t /[C.sub.4][less than or equal to][R.sub.3][less than or equal to]10[t.sub.1]/[C.sub.4] (1)
for t - duration of the discharge current pulse [I.sub.pr] and [t.sub.1] - time interval between two pulses of discharge current [I.sub.pr].
Capacity of the capacitor [C.sub.4] of integration circuit of the device is determined based on the criterion of maximum generated electrostatic charge [Q.sub.m] = IRC, at the moment t of the occurence of the breakdown - discharge of static electricity in the vapour space, from the surface of liquid (oil) to the metal wall of the tank, now grounded by the here considered device (Fig.1).
Pulse discharge from the oil surface to the grounded metal wall of the tank can be controlled by adequate appreciation of the elements (R, C) of the integration circuit. This is the essence of the controlled elimination of static electricity.
The maximum amount of charge in the tank/tank truck has a value of [Q.sub.m] = IRC and the maximum energy due to the charge in the charged oil in the tank is given with Eqn. (2):
[W.sub.max] = [I.sup.2][R.sup.2]C / 2 = [Q.sub.m] / 2C = [Q.sub.m][U.sub.m] / 2 (2)
The theoretical maximum value of the charge released in the discharge will be achieved after an infinitely long period of time. Practically we can consider that the charge Q, on the tank/tank truck has a stationary value, if it reaches a value equal to 0.01 of the extreme or maximum charge, that is 0.01 [Q.sub.max].
[FIGURE 3 OMITTED]
From the foregoing it follows that the time t of reaching extreme/maximum value of charge is equal to:
t = RCln 1/0,99 (3)
and in this way is determined the time of reaching maximum potential difference between tank/tank truck and earth [U.sub.max], and also maximum value of energy [W.sub.max].
[FIGURE 4 OMITTED]
Establishing a state of breakdown in the vapour space of the tank/tank truck is also conditioned by the formation of the electrodes geometry, surface of the liquid (oil) in the tank/tank truck and the grounded metal wall of the tank/tank truck.
So, in the vapour space of the tank/tank truck, where explosible and flammable mixture is present, a state of breakdown voltage [U.sub.pr] is established, which leads to the discharge between the electrodes. The value of breakdown voltage is calculated from the condition of electrostatic safety, expressed in Eqn. (4):
[W.sub.iskre][less than or equal to] 0,4 [W.sub.min][J] (4)
[W.sub.iskre] = ([CU.sup.2.sub.pr]) / 2 (5)
which results in
[U.sub.pr][less than or equal to][(0,8[W.sub.min] / C).sup.0,5]. (6)
[FIGURE 5 OMITTED]
4 RESULTS OF THE SIMULATION
The principled electrical circuit diagram of the proposed passive device for elimination of static electricity is given in Fig.2. Taking the value for input electrostatic voltage of [U.sub.0] = 58 kV, which is considered as minimum surface voltage for incentive spark discharge to occur, the results of the simulation are:
[FIGURE 6 OMITTED]
In order to bring scientific expertise a valuable conclusion, we have created a MATLAB's m-file for the estimation of resulting electrostatic energy in the considered process of controlled redistribution of generated charge.
This means that the proposed passive device for elimination of static electricity carried out a controlled redistribution of resulting charge on the surface of the liquid or gas phase conditions and thus reduce the localized electrostatic energy in localized vapor space. Effect of a controlled redistribution of charge on the surface of considered liquid or gas phase condition, made using proposed passive device for elimination of static electricity can be expressed through coefficient of controlled impairment (ratio of energies on the input and the output of the device) of charges arising on the surface of considered liquid or gas phase, which in this case have averaged value of 240.
The result of the effects of controlled reductions resulting electrostatic energy, which amounts to 240, provides a completely safe operation of the technological operations of pouring oil and oil products and gas. This avoids the possible occurrence of latent danger caused by discharge of accumulated static electricity, manifested through the possible appearance of an explosion and fire.
[1.] Gacanovic. M, "Device for static electricity elimination during racking, transportation and loading/unloading of inflammable or explosive materials (type EL-1 N/S)". German Patent No. 699 14 225 (2003.); European Patent EP 1 169 88 (2003.); Canadian Patent No.2,401,517 (2007.); European-Asian Patent & Patent Russia No. EA 003411 (2004.).
[2.] API RP 2003, "Protection against Ignitions Arising out of Static, Lightning, and Stray Currents". API Standard 2003, Seventh Edition, Ballot Draft F2, -10-2006.
Nikola Dragovic, Mico Gacanovic
University of Banja Luka, Faculty of Electrical Engineering, Patre 5, 78000 Banja Luka, Bosnia and Herzegovina
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|Author:||Dragovic, Nikola; Gacanovic, Mico|
|Publication:||International Journal of Emerging Sciences|
|Date:||Dec 1, 2011|
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