Parameters Influencing Space Charge Density in Vessels by Spraying Water

Spraying water in vessels generates electrostatically charged clouds of droplets. If the resulting space charge density is sufficiently high, brush discharges to conductive earthed parts may occur. In vessels where potentially explosive atmospheres might occur, the prevention of hazardous space charge density is required. This work gives an overview of all the relevant parameters that influence the space charge density. For this purpose, own measurements are presented and compared with knowledge from the literature. The measurements have been carried out with earthed vessels and nozzles built of conductive material. To take the state of the art into account, pump pressures of up to 2500 bar have been used. The result of this work provides support for dimensioning vessels and water spraying techniques to minimize the risk of ignition hazards due to brush discharges of the electrostatically charged clouds of droplets.


Introduction
Charge separation takes place because of the Lenard effect.The Lenard effect is the detachment of very fine droplets from the outermost shell of a drop caused by impact on an obstacle or through hydrodynamic instability [1,2].The ions dissolved in water determine the charge separation when water is sprayed.In tap and deionized water, the resulting droplet cloud is negatively charged; in seawater, it is positively charged [3,4].The spray jet is charged oppositely to the droplet cloud.Deionized water has a comparably low electrical conductivity.In addition to the Lenard effect, charge separation occurs due to jet separation from the nozzle [4,5].
The level of space charge density in a vessel is the basis for evaluating the occurrence of brush discharges [4,6].Brush discharges will occur from the electrostatically charged droplet cloud to conductive earthed parts.The objective of this paper is to describe the following parameters: ▪ spraying technique: pump pressure, nozzle geometry (round, flat, rotating), nozzle diameter ▪ water: temperature, electrical conductivity ▪ vessels: diameter, volume, spraying angle, distance between nozzle and wall that lead to the maximization of space charge density when spraying water into a vessel.Through own experiments and knowledge from the literature, an overview of all the relevant parameters that influence the space charge density will be presented.

Experimental set-up
The conductive nozzles and vessels are earthed.The dimensions are given in table 1 and table 3.At the lowest point of the vessels, a drain is mounted.In the drain, the water temperature was measured with a calibrated type K thermocouple.The measurement uncertainty was ±2 K.The rotation axis of the vessels was horizontal.By performing "middle spraying", the jet follows along this axis.When spraying at an angle, the spray jet hits the vessel wall as directly as possible.Figure 1 shows exemplary the experimental set-up with the 1 m³ vessel.For the 6.2 m³ and the 16.8 m³ vessel the set-up is the same.In contrast to the other tests, the 44.0 m 3 vessel was set up vertically.In addition, an agitator was installed, reducing the effective diameter from 4.10 m to 2.50 m.The middle spraying was conducted from an eccentric filling hole in the top of the vessel.
The threshold for the force of the handheld spraying technique of 250 N was complied with [7].This threshold limits the flow rate, nozzle diameter and pump pressure.The electric conductivities of the water types are shown in table 2 [8].Electric field strengths () were measured with the air-flushed JCI 131 field meter of Chilworth Ltd. [9].It was mounted flush in the shell surface at half of the inner length of the vessel (figure 1, left).The measurement accuracy was ±1.5% of the upper range values of ±2 kV/m, ±20 kV/m and ±200 kV/m [9].The space potential Փ(0) in the centre was measured through a space potential probe (figure 1, right) [5].The isolated mounted electrode (tip: 10 mm length, 10 mm diameter) of the space potential probe in the vessel measured the potential of the droplets through contact.The electrode was coupled with a ±40 kV measuring potential attachment of the EMF 58 field meter of Eltex GmbH or the EFM 235 of Kleinwächter GmbH.The potential probe worked as an electrostatic voltmeter and measured the space potential in an unencumbered manner [5].The measurement accuracy was ±2.5% of the upper range values of ±1 kV, ±2 kV, ±4 kV, ±8 kV, ±10 kV and ±40 kV.Regardless of the vessel size, the electrode was always positioned in the centre of the vessel.For measurements within the cloud of droplets, the potential measurement will determine average values.When in direct contact with droplets, the electrode will assume the potential of those droplets.The potential gradient in the vessel from the centre to the earthed wall causes a currently unquantifiable reduction in the measured space potential.For measurements in the jet, this influence is insignificant.It is assumed that the amount of applied charges in touch with the jet dominates the influence of the electric field on the potential probe.
With the potential probe, the potential in the jet is measured with small deviations.With the field meter, the electric field strength of the homogenous cloud of droplets in the vessel is measured with small deviations.The potential can be calculated from this electric field strength [10].
The electric constant  0 was 8.854 [11].Droplets in air have a permittivity   of 1.01 [12].The following equations (1), ( 2) and (3) of the space charge density  apply to a cylindrical vessel volume with a homogeneous space charge density (table 4).The equations are not usable for middle spraying, because of opposite charges of the jet and droplet cloud.When spraying at an angle, the cloud of droplets within the volume of the vessel predominates and the equations are applicable [4].Table 4. Calculation principles for the space charge density in a cylindrical vessel [13,14].
Through measured space potential The measurement techniques were used in the same way for measuring the electric field strength and space potential on a spray jet without a vessel and impact.

Results
Table 5 includes the results of the 1.0 m³ vessel and table 6, those of the 6.2 m², 16.8 m³ and 44.0 m³ vessels.Round jet nozzles were used in most cases.The using of rotating (Rot.) and flat nozzles are marked in table 5.When middle spraying (middle spr.) tap water and seawater in the 1.0 m³ vessel, the measured electric field strengths and space potentials are lower than when spraying at an angle [5].Only the middle spraying of deionized water is included in table 5.The temperature in the "Type of water" column indicates that the water was tempered.7 includes the results gained by spraying a horizontally oriented spray jet in the environment without a vessel or impact.For these experiments, only round jet nozzles were used.Distances of 0.5 m, 1.0 m, 2.0 m, 3.0 m and 4.0 m to the nozzle along the jet were examined.The pump pressure from 100 bar up to 500 bar was increased in steps of 100 bar.A space potential of -48.04 kV was the limit of the useable measurement range.The actual space potential may be higher.Reducing the nozzle diameter by spraying deionized water led to higher space potentials and exceeded the limit of the measuring device.The space potential attained by spraying seawater was lower compared to tap water.To avoid unnecessary exposure to seawater, the electric field strength was not measured.

Discussion
Pump pressure: The velocity of the jet increases and smaller droplets arise when the pump pressure is raised [4].The Lenard effect becomes more effective because of the impact and the hydrodynamic instability, and this leads to a higher space charge density.The increase in charge density is not proportional to the pump pressure.At higher pressures, the increase is reduced (see, for example, table 5, tap water).By spraying deionized water, the charge reflux to the earthed nozzle decreases because of the low electrical conductivity and the higher exit velocity at a higher pump pressure [4].This results in highly charged spray jets (see table 5 and table 6 "Middle spr." along with table 7).

Nozzle geometry:
The nozzle geometry was examined by spraying tap water at an angle in the 1.0 m³ vessel (table 5).With the round jet nozzle, the jet is positively charged and hits the vessel wall.The negatively charged droplet cloud is distributed in the volume of the vessel.With the flat jet and the rotating nozzle, the jet counteracts this spatial charge separation through its expansion, rotation and turbulence.The space charge density in the vessel is reduced.

Nozzle diameter:
When spraying tap and deionized water at an angle and at a constant pump pressure into the 1.0 m³ and 6.2 m³ vessels, a larger nozzle diameter causes an increase in the space charge density (table 5 and table 6).Something similar was found in [15].Process water was sprayed into a 24 m³ vessel at a pump pressure of up to 400 bar.The tank-washing head had two nozzles.With a nozzle diameter of 1.5 mm, the electric field strength was 10 kV/m and doubled to 20 kV/m when the nozzle diameter was increased to 2.5 mm.During middle spraying or without a vessel and impact, when using deionized water, a smaller nozzle diameter increased the space charge density (table 6 and table 7).In [16] the same correlation was observed confirming that a smaller nozzle diameter increases the charge separation at the nozzle and the charge separation caused by the hydrodynamic instability of the jet.

Water temperature:
The surface tension and dynamic viscosity of water decreases with increasing temperature [17].The resulting higher droplet collapse favours the process of charge separation (Lenard effect).In the experiments described in [18], drops of deionized water fell onto a plate.The rebounding water was collected and its potential measured.At 15 °C it was 0.23 kV and at 95 °C it was 0.42 kV.This results in a factor of 1.83, which corresponds to the temperature difference.In general, it can be estimated that the space charge density increases 1% per 1 K.This corresponds to the experiments in the 1.0 m³, 6.2 m³ and 44.0 m³ vessels when spraying was carried out at an angle and with tempered deionized and tap water (table 5 and table 6).

Electrical conductivity of water:
The electrical conductivity of water has an influence on the Lenard effect and thus on the polarity of the jet and droplet cloud [3]: ▪ In deionized water, the  − ions form the outermost molecular layer of the double layer.The  + or 3 + ions remain in the main drop after the  − ions have been detached.The spray jet is positively charged, while the remaining droplet cloud has a negative charge.By spraying deionized water at high pump pressures, the charge reflux to the earthed nozzle decreases because of the low electrical conductivity and the higher exit velocity.This charge separation process dominates the Lenard effect and the spray jet is negatively charged.▪ When spraying tap water, mainly the ions formed from sodium and chloride influence the charging.
The  + ions displace some of the  − ions of the outermost layer.When the outermost layer is detached, the droplet cloud has a reduced charge compared to deionized water.▪ The salinity of seawater is on average 3.5% by mass, of which the proportion of sodium and chloride is about 3.0% by mass [17].The Na + ions in seawater displace and exceed the number of  − ions in the outermost layer.The droplet cloud is then positively charged.
Compared to the measurements on the jet, without a vessel and impact, the vessel reduced the electric field strengths and space potentials when spraying deionized water.When spraying seawater and tap water, the dominant charge separation process is the impact with a surface, leading to higher electric field strengths and space potentials in the vessel (table 5, table 6 and table 7).The polarities of the jet and droplet cloud and the dominant charge separating processes depending on the water type are summarized in table 8.  8.The charging of the jet is dominated by the opposite charging of the droplet cloud at high pump pressures and with large inner lengths of the vessels from a volume of 6.2 m³.Furthermore, differences in the polarities of the electric field strength and the space potential can be seen.In contrast to space potential measurement technology, the field meter cannot measure with spatial resolution.The measured value of the charged droplets in the measuring volume is averaged.It is not possible to record the charging of the jet separately [4,5].

Vessel diameter and volume:
Simulations are known from [19] in which the space potentials in large vessels were calculated.A constant space charge density of 40 nC/m³ was assumed.The initial condition was a vessel with the dimensions of 30 m x 30 m x 8 m.The maximum space potential was 35 kV.A change was made to the initial condition of the vessel, and the reduction of the space potential was examined: 1.The insertion of an earthed conductive rod, which pointed vertically upwards in the centre of the base area of 30 m x 30 m and corresponded to the height of the vessel of 8 m, did not reduce the room potential of 35 kV. 2. Reducing the length from 30 m to 15 m had little impact.3. Halving the height from 8 m to 4 m reduced the space potential to 9 kV.
As a result of changes no. 2 and no. 3, the vessel volume was reduced from 7200 m³ (initial state) to 3600 m³.This shows that it is not the vessel volume but rather the smallest dimension limiting the space charge cloud that is decisive for the height of the space potential.
In the experiments performed as part of this work (table 5 and table 6), a constant space charge density as in the simulations is not present.By comparing the 1.0 m³ vessel with the larger ones, the electric field strength and the space potential are reduced when spraying deionized and tap water at an angle, with a constant configuration of the nozzle diameter and pump pressure.While the vessel volume increases, the number of charge carriers remains constant.This leads to a reduction of the droplet density and space charge density.
When spraying deionized water, the larger the vessel, the higher the space charge density of the jet and the droplet cloud formed by the hydrodynamic instability (table 5 and table 6).It is highest when there is no space limitation caused by a vessel (table 7).

Spraying angle:
To determine the influence of the spray angle, tap water was sprayed at up to 500 bar into the lower and, for comparison, into the upper part of the 1.0 m³ vessel [4,5].The electric field strengths and space potentials were measured.The measured values were reduced when spraying into the upper part.For example, the space potential was up to -1.48 kV when spraying into the lower part and only up to -0.99 kV when spraying into the upper part.The reduction of the space charge density results from the positively and/or neutrally charged drops that fell through the volume of the vessel after the impact of the jet in the upper part.These drops reduced the charge of the negatively charged droplet cloud.

Distance between nozzle and wall:
The distance between the nozzle and the wall is researched by spraying tap water at an angle in the 1.0 m³ vessel (table 5).If the distance between the nozzle and the vessel wall is 0.65 m, a space potential of -0.90 kV was obtained.For comparison, it is -1.17 kV at 0.42 m and -1.66 kV at 0.19 m.An increase in distance is related to a reduction in the velocity of the droplets at the impact of the spray jet [4,5].The charge separation and the resulting space charge density are reduced.
At pump pressures of more than 500 bar, distances of over 0.19 m were necessary to avoid the transection of the vessel wall.

Summary
A summary of the parameters studied and of what influence they have on the space charge density is shown in figure 2. A differentiation must be made between tap water and deionized water.As shown in figure 2, a high space charge density can be avoided through suitable parameter selection.This reduces the risk of ignition hazards due to brush discharges from the electrostatically charged clouds to conductive earthed parts.

Figure 1 .
Figure 1.Experimental set-up with mounted air-flushed JCI 131 field meter (left) or the mounted space potential probe (right).

Figure 2 .
Figure 2. Parameters that increase the space charge density.

Table 3 .
Dimensions of cylindrical vessels.

Table 7 .
Results of horizontal jet without a vessel and impact.

Table 8 .
Polarity of the droplets charged by charge separation during spraying.