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Risks of ignition from ‘Type D’ ungrounded fibc
Journal of Electrostatics 68 (2010) 145–151
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Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Risks of ignition from ‘Type D’ ungrounded fibc John Chubb a, *, Paul Holdstock b a b
Infostatic Ltd, 2 Monica Drive, Pittville, Cheltenham, GL50 4NQ, UK Holdstock Technical Services, 3000 Manchester Business Park, Aviator Way, Manchester, M22 5TG, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 July 2009 Accepted 21 November 2009 Available online 4 December 2009
Avoidance of risks of ignition with unearthed Type D FIBC requires that electrostatic discharges cannot quickly access large quantities of charge via any conductive features of the material. The present studies show how the effective resistivity of conductive features within FIBC type fabrics can be measured, without electrical contact, from the variation of shielding performance with frequency. From such measurements an empirical acceptance criterion has been derived that matches well to the results of gas probe ignition tests. Studies are proposed to provide reliable criteria for assessing FIBC Type D bag materials and to help optimize FIBC fabric design. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Risk assessment Electrostatic ignition risks Material characteristics FIBC
1. Introduction This paper examines a method of testing that may enable the avoidance of risk of ignition from ungrounded Type D FIBC (Flexible Intermediate Bulk Containers) to be assessed without the need for time consuming and expensive gas probe ignition testing. Where FIBC are used with materials that may be flammable and/ or in atmospheres that may be flammable, precautions are necessary to avoid ignition risks from electrostatic discharges. The loading of flowable solids into FIBC is likely to create significant quantities of electrostatic charge. This would create high voltages on external surfaces if the bag material were just a simple dielectric fabric. One method of preventing the occurrence of electrostatic discharges at approach of any external earthed item is by the inclusion of high conductivity threads within the FIBC fabric with these threads reliably bonded to earth at the FIBC mounting arrangements. These are known as Type C FIBC. In this construction the occurrence of incendive discharges is avoided by suitably close spacing of the threads to encourage dissipation of charge by conduction to earth and limiting the area of charge available to discharges. An International Standard test method [1] is available for assessing such Type C FIBC, and other International Standards [2] can be used to assess the fabrics that are intended for use in Type C FIBC. The problem with Type C bags has been how to achieve reliable earth bonding of all conductive threads during long term * Corresponding author. E-mail addresses: [email protected] (J. Chubb), [email protected] (P. Holdstock). URL: http://www.infostatic.co.uk 0304-3886/$ – see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2009.11.012
industrial use. A Type D FIBC is in practical use and this offers the advantage of not requiring any earthing of the bag structure. The International Standard [1] also specifies test methods for Type D FIBC, but there is at present no standard test method to assess fabrics that are intended for use in Type D FIBC. For an electrostatic discharge to ignite a flammable gas or powder dispersed into the air there needs to be a certain concentration of energy in time and space. The requirements for ignition of various gas mixtures by spark type electrostatic discharges has been extensively studied [4]. For common hydrocarbon vapour air mixtures the minimum energy for ignition by a capacitive electrostatic spark discharge is around 0.2 mJ. This varies with ambient temperature, pressure, oxygen concentration, etc and with the discharge gap above a threshold minimum value [4]. For powders, minimum ignition energies are higher, but depend on the material and the fineness of the particles dispersed. Ignition by other forms of electrostatic discharge is less well defined. Brush discharges from an earthed projection approaching a highly charged insulating surface (solid or liquid) can certainly cause ignitions [5,6]. Ignition by propagating brush discharges, from an earthed projection approaching a charged insulating layer resting on an earthed surface, has been well characterized [6]. Corona discharges are generally felt to be incapable of causing ignitions – although some doubt remains with the most easily ignitable gases, such as hydrogen, etc. The types of discharge described above involve either the release of energy from electrostatic charge stored on highly conducting surfaces (spark type discharges) or a discharge to a highly charged insulator surface where the discharge reaches out over an immobile area of charge by propagation of the discharge
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through the air close to the charged surface. It was suggested in 1983 [7] that materials with resistivity within a certain range might inhibit the opportunity for occurrence of incendive discharges by limiting the rate of charge flow through the surface to stop full spark type discharges but allow sufficient charge movement to prevent the occurrence of air breakdown across the charged surface. For the rate of charge migration to be inadequate to support an incendive discharge the experimental work done at Culham [7] on electrostatic discharges between two resistive plane surfaces suggested that surface resistivities needed to be over 1–3 107U to avoid risks of ignition. Theoretical calculations suggested values over 107–108U. It seems reasonable to expect that somewhat higher values might apply for a localized earthed conducting probe approaching a single plane charged resistive surface. Recent studies with a single stage impulse generator circuit, with a source capacitance of 370 pF and a 1 kU load and with a 740 pF spark heating capacitor circuit, showed that for voltages up to 12 kV avoidance of continuity of discharge to the load required the spark heating series resistor to be over 5 MU. This is reasonably compatible with the Culham expectations and results. To prevent the occurrence of discharges propagating across the surface of a charged dielectric it seems plausible that the rate of charge migration across the surface should be sufficient to enable charge to concentrate in the high field region near the approaching earthed probe. When breakdown occurs the discharge will then only be able to propagate out to the boundary where the charge density is below some threshold level for propagation over the surface. If this model is plausible then the speed of approach of the probe relative to the level of ‘resistivity’ (charge decay time) of the surface will be a relevant assessment criterion. In charge decay time terms, a decay time under 0.1 s would seem to be appropriate in relation to likely rates of approach of earthed probes to an FIBC surface. On the basis of the above it seems plausible that avoidance of risks of ignition requires:
Gas probe testing has shown that some fabrics for FIBC provide the opportunity for occurrence of incendive discharges when an earthed probe approaches a highly charged fabric surface. Whether the occurrence relates to resistive features of the fabric accessible by a high voltage discharge or to propagation of a discharge across the dielectric surface cannot be assessed by traditional resistivity measurements. The structure of the fabric and/or the sheathing of the conductive component may inhibit appropriate electrical contact for reliable resistivity measurement. The technique developed for assessing the shielding performance of materials against electric field transients [9,10] has been shown able to measure the effective resistivity of materials without need for surface or invasive contact [11]. A resistive material (for example damp paper) shows a high attenuation at low frequencies and a fall off in attenuation with increasing frequency. The fall off frequency band relates directly to the resistivity. Shielding performance measured as a function of frequency over a range from 10 Hz to a few MHz provides the capability to measure resistivities in the range 0.1–1000 MU. A fully conducting element across the test area, even a single fine copper wire, can be detected and recognized by having a constant shielding performance with frequency over this range. This paper reports measurements of the variation of shielding performance with frequency for a number of FIBC materials and the interpretation of these measurements in terms of effective resistivity. Also included are measurements of charge decay time [12,13]. These results are compared with the results of earlier gas probe ignition tests to see if there is a convincing correlation – and hence a prospect of predicting the results of gas probe tests from shielding performance and charge decay measurements. 2. Method of testing 2.1. Shielding performance
- No component in the fabric material has a resistivity less than, say, 10 MU; - The surface of the fabric material has a charge decay time less than 0.1 s.
The approach developed for measuring the electric field shielding performance of film and layer materials [9,10], and from this to assess the resistivity of the materials [10], has five basic features:
Where the charge decay time is very long it may be that incendive discharges can be avoided if the propagation of discharges across the surface is limited to an adequately small area by proximity to a pattern of conductive threads. This possibility would need to be investigated by separate and additional studies. However, it is suggested that this would not be as secure a protection as an appropriate maximum charge decay time. It needs to be remembered that the speed of approach of the earthed probe to the charged surface is important. In studies relating to explosions in large crude oil tankers [8] it was shown how the velocity of approach needed to be adequately fast to prevent discharges (such as corona and water spray discharges) that removed charge form the surface – but not so fast that an incipient flame kernel was quenched before propagating to the surrounding gas atmosphere [8]. This speed of approach point is covered in present FIBC test procedures [1]. The fabrics from which FIBC are made may include a pattern of conductive threads. For the Type C bags these are often metallic and these must be fully linked across the whole surface of the FIBC so they can be reliably linked to earth. In concept, the fabric for Type D bags include quasi-conductive threads whose conductive elements are, in some cases, contained within a polyester sheath. A major question is clearly whether fabrics including such threads, if not earthed, could give rise to ignition risks.
a) A plane area of the test material, isolated from earth, is tested by applying a transverse alternating electric field stress over a defined area by capacitive coupling from a pair of nearby electrodes. b) The electric field stress is applied as a balanced bipolar sine wave signal relative to earth and covering a wide range of frequencies. Symmetry about earth potential ensures no common mode signal is impressed on the sample as a whole. c) Measurements are made of the signals capacitively coupled to a pair of electrodes on the far side of the sample in positions matching those of the driving pair of electrodes. Observations are made of the difference signal between these two electrodes. d) Shielding is measured at a variety of frequencies as the ratio of signals observed with the material present compared to that without. e) Shielding performance is presented as the variation of the ratio of signals as a function of frequency. This comprises a combination of in phase and quadrature signal components. The basic physical arrangement of the approach is shown in Fig. 1. The electrodes are 15 mm wide and 100 mm long and mounted flush in earthed plates to be parallel and with their centre lines 50 mm apart. The 100 mm length was chosen for
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Fig. 1. Basic arrangement for measuring the variation of shielding with frequency.
studies on the characteristics of fabrics including conductive threads where the threads might be spaced as much as 25 mm apart so at least several threads were covered. The arrangement of the observation electrodes matches the electric field stressing electrodes. The inner surfaces of the electrodes and mounting plates are 10 mm apart. The sample is clamped flat against a 3 mm thick sheet of good quality dielectric (polycarbonate) on the observation side by a second sheet advanced from the driving electrode side. This arrangement ensures the sample is held flat and in a well defined position relative to the stressing and observation electrodes. The present method for testing shielding performance [10,11] operates over the frequency range 10 Hz–10 MHz. This is a simplified version of an earlier arrangement that was developed to cover the full range of frequencies relevant to risks of damage to microelectronic devices – 10 Hz to 1 GHz [9]. The simplification of using antiphase sine wave drive signals with phase sensitive detection of in-phase and quadrature-phase signals provided good signal to noise ratios over the chosen frequency range. Combination of in-phase and quadrature-phase signals as the square root of the sum of the squares takes account of the phase shift that occurs as attenuation rises and provides a proper measure of shielding attenuation. The present more limited frequency range had been shown [10,11] to provide quite adequate information on the shielding characteristics of fabrics including conductive threads.
2.3. EN 1149-3 method 2
2.2. Charge decay measurement
4. Results
Measurements of the corona charge decay time characteristics of each sample were made using a JCI 155v5 Charge Decay Test Unit to the test procedure that has been described [12–14]. Measurements were made in normal laboratory conditions at a temperature near 20 C and humidity between 50% and 55%RH.
4.1. Shielding performance measurements
Measurements of half decay time (t50) and shielding factor (s) of each sample were made according to EN 1149-3 Method 2 [3] under conditions of 23 2 C and 25 5%RH. 2.4. Ignition testing Ignition testing involves filling FIBC with highly charged polypropylene pellets and bringing a gas probe up to the sides of the FIBC to provoke discharges that may cause the gas to ignitie. Ignition testing of full size FIBC was done in accordance with the procedures specified in IEC 61340-4-4 [1]. 3. Test samples A total of 15 samples were tested of fabrics as used in commercial FIBC. The fabric samples were cut from FIBC that had been previously assessed for ignition risk by gas probe ignition tests in accordance with the procedures specified in IEC 61340-4-4 [1] – but the results were not known to the test operator at the time of the present shielding tests. The samples were provided by Paul Holdstock as 100 mm 100 mm squares and identified just by a number. The description of the fabric samples is given in Table 1. Manufacturers are identified only by a code letter so as to maintain commercial confidentiality.
In previous studies [11] it had been observed that the variation of attenuation with frequency related to the effective resistivity of the materials over the test area. Fresh measurements of this have
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Table 1 Description of FIBC fabric samples.
1000
Sample Description
Manufacturer code
1
A
2 3 4 5 6 7 8 9 10 11 12 13 14 15
Lightweight, coated fabric with static protective yarns in 20 mm stripe pattern Standard weight, coated fabric with static protective yarns in 20 mm stripe pattern Plain, lightweight fabric (insulating fabric used to make Type A FIBC) Plain, standard weight fabric (insulating fabric used to make Type A FIBC) Standard weight, coated fabric with static protective yarns in 20 mm stripe pattern As sample 2 but the base fabric and coating contains 2.4% w/w antistatic additive Standard weight, coated fabric with static protective yarns in 20 mm stripe pattern Same as sample 5 (samples cut from adjacent areas) As sample 2 but the coating contains 1.2% w/w antistatic additive Same as sample 5 (samples cut from adjacent areas) Standard weight, coated fabric with antistatic finish Same as sample 11 but rinsed under running water for 20 min Same as sample 11 but cut from an old (2003) FIBC Standard weight, coated fabric with 50 mm 50 mm grid of yarns containing an antistatic additive Same as sample 14 but cut from a different FIBC
100
10
A A
1
A 0.1
A A
0.01 0.1
B
1
A
Series1
A
100
Attenuation
Series3
Transmission
Fig. 3. Variation of 50% transmission and 50% attenuation with resistance between test foils.
A C C C D D
been made, in the same way as before, using 0.1 MU, 1 MU, 10 MU and 100 MU resistors between thin foil electrodes matching the areas between each of the energizing and detection electrodes and mounted in the normal test position. A sample measurement is shown in Fig. 2. The variations of frequency (kHz) for 50% attenuation and for 50% transmission fraction with the resistance value (MU) are shown in Fig. 3. Resistance values R (megohms) may be derived from the frequency f (kHz) for 50% attenuation and for 50% transmission as: From 50% attenuation fraction: R ¼ 10 (0.947log (f)) From 50% transmission fraction: R ¼ 10(1.135log (f)) To express this as an equivalent resistivity over the test area it is necessary to take into account the 2:1 ratio of the length of the electrodes to their spacing in the test set-up. This means that the resistivity value is twice the effective value of equivalent resistance between the electrodes.
Sample results of measurements of the variation of attenuation with frequency for the 15 samples studied are shown in Figs. 4–6. The signals observed are phase sensitive detected in phase with the source signals – and in most cases also in quadrature. Calculation of the true attenuation factor requires these two values to be combined as the square root of the sum of the squares. The low level of observed signals in a number of the present studies made it impractical to make much use of quadrature component measurements. The results have therefore been considered primarily in terms of the variation of the in-phase signal components. It was noted in the measurements with samples 1, 2, 5, 6 8, 9 and 10 that the attenuation varies from a fairly flat maximum at the lowest frequencies and fell to zero at the higher frequencies. This indicates that the threads visible across some of the samples were only having a modest shielding effect at best because of their relatively wide spacing. On the other hand samples 7, 13, 14 and 15 showed a wide range of attenuation, indicating that the shielding mechanism was a feature of the whole area of the sample and not spatially localized. It is noted that samples 3, 4 and 12 showed no attenuation. To calculate the effective resistivity of the fabric samples, from the variation of shielding performance with frequency, it seems plausible to use the frequency at which the attenuation or
1.0
in phase component
0.9
0.20
0.8
0.18
0.7
0.16
0.6
0.14
0.5
0.12
0.4
0.10
0.3
0.08 0.06
0.2
0.04
0.1 0.0 10
10
Resistance (M)
0.02 100 Transmission
1,000 10,000 Fequency (Hz) Quadrature
100,000
1,000,000
0.00 -0.02
Atte nuation
1
10
100
1,000
10,000
100,000
Frequency (Hz) Fig. 2. Variation with frequency of transmission fraction, quadrature fraction and attenuation for 100 M between test foils at 28 April 2009.
Fig. 4. Shielding performance of Sample 8 at 30 April 2009.
1,000,000
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149
Table 2 Results of shielding performance measurements.
1.0 0.9
Sample
50% Trans frequency (kHz)
Effective resistivity (Megohms)
Low frequency shielding
170 100
0.16 0.27
0.12 0.11
0.5
1 2 3
0.4
4
0.8 0.7 0.6
0.3 0.2 0.1 0.0 10
100
1,000
10,000
100,000
1,000,000
Fequency (Hz) Transmission
Quadrature
Attenuation
Fig. 5. Transmission and shielding versus frequency for Sample 11 at 18 May 2009.
transmission is halfway between the low and high frequency levels. On this basis the results from the various studies are presented in the following Table 2. Included in this Table are values for the effective shielding at low frequency. 4.2. Charge decay measurements The results of corona charge decay measurements are summarized in Table 3. All samples apart from 7, 11, 13, 14 and 15 showed long charge decay times. Where it was not sensible to measure the time to 1/e the local decay time constant values at 1000 s are reported. With sample 11, two measurements were made: the first over a thread, the second between threads. In both cases the decay curve flattened out – and with the second the local decay time constant at 1000 s was 250,000 s! For the long decay time samples the tribocharging created at even careful handling gave initial surface voltage of several hundred volts!. It is clear that charge decay on samples 1–6 and 8–12 are very long. Only samples 13, 14 and 15 show good short charge decay times. Sample 7 shows an initial fairly quick charge decay but this slows up significantly with the progress of decay. 5. Assessment of results Table 4 (appended) lists the results of the measurements made of resistivity and charge decay times for the 15 samples tested. 1.0
No attenuation 10 Hz–1 MHz No attenuation 10 Hz–1 MHz
5 6 7 8 9 10 11 12
55 90 <0.010 50 6 18 0.136
13 14 15
0.024 0.012 0.013
0.5 0.3 >3000 (>3G) 0.5 4.5 1.5 200
0.13 0.11 0.13 0.09 0.13 w1 No attenuation 10 Hz–1000 Hz
1150 (1.1G) 2270 (2.3G) 2165 (2.2G)
w1 w1 w1
Results of the application of the two criteria proposed in Section 1 are listed. These criteria for the avoidance of risk of ignition were that the resistivity should be greater than 10 MU (Criterion 1) and the charge decay time should be less than 0.1 s (Criterion 2). Also listed in Table 4 are the results of the gas probe ignition tests and the results of tests of these 15 samples according to EN1149 Method 2. This latter test method is used to assess apparel fabrics intended for personal protective clothing for use in hazardous flammable environments. It is clear from Table 4 that there is not a good match between predictions based on either criterion 1 or 2 and the results of gas probe ignition tests. This also applies for assessment by EN1149 Method 2 – but that is not directly relevant because that method of testing was not designed as an appropriate test method for Type D bag materials. If a criterion is considered that says nothing more than that ‘the resistivity shall be less than 2 MU’ then it appears that for the 15 samples tested there is a direct match between this assessment and the gas probe test results! This is shown as criterion 3 in Table 4. This is an empirical result that is true for all the samples as tested. This empirical criterion does not match the ideas outlined in Section 1. It is hence necessary to consider how the criteria that were proposed may be in error and what needs to be done to establish necessary and sufficient criteria that will apply to any plausible materials considered for FIBC Type D bag application. Table 3 Charge decay measurement results.
0.9 0.8
Sample
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 10
Comment
100
1,000
10,000
100,000
1,000,000
Fequency (Hz) Transmission
Quadrature
Attenuation
Fig. 6. Shielding versus frequency for Sample 13 at 22 May 2009.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Decay time to 1/e (s)
Decay time to 10%
Local decay time constant at 1000 s 36,000 27,000 47,000 33,000 15,000 8000
0.707, 0.806
37.6, 84.5 18,000 8200 23,000 250,000 (b) 50,000
183(a) 0.035, 0.031 0.265, 0.176 0.089, 150, 146
0.105, 0.103 3.2, 2.02 0.489, 0.847, 0.846
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Table 4 Comparison to results of gas probe tests – Prediction: P pass, F fail. Sample
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Resistivity (M)
Decay time (s) to 1/e or time constant @ 1000 s
0.16 0.27
36,000 27,000 47,000 33,000 15,000 8000 0.7–0.8 18,000 8200 23,000 250,000 50,000 0.031–0.035 0.176–0.265 0.089–0.150
0.5 0.3 >3000 (>3G) 0.5 4.5 1.5 200 1150 (1.1G) 2270 (2.3G) 2165 (2.2G)
Prediction criteria 1
2
3
F F
F F F F F F P ? F F F F P P P
P P F F P P F P F P F F F F F
F F P? F P F P P P P
Gas probe ignition Pass Pass Fail Fail Pass Pass Fail Pass Fail Pass Fail Fail Fail Fail Fail
Comments
Insulator – plain fabric Insulator – plain fabric
>50% of approaches result in ignition 1 approach out of 400 results in ignition w5 approaches out of 400 result in ignition >50% of approaches result in ignition w5 approaches out of 400 result in ignition w5 approaches out of 400 result in ignition w5 approaches out of 400 result in ignition
EN 1149-3 Method 2 s
t50 (s)
Pass/Faila
0.20 0.21 0.01 0.01 0.25 0.20 0.00 0.22 0.24 0.28 0.00 0.01 0.04 0.00 0.54
>60 >60 >60 >60 >60 >60 7.09 >60 5.16 >60 <0.01 >60 0.21 <0.01 <0.01
(Pass) Pass Fail Fail Pass (Pass) Fail Pass Pass Pass Pass Fail Pass Pass Pass
Criterion 1: Resistivity > 10 M. Criterion 2: Charge decay to 1/e < 0.1 s. Criterion 3: Resistivity < 2 M. a Requirements specified in EN 1149-5 are s > 0.20 or t50 > 4 s.
The following reasons are suggested for the failures of criteria 1 and 2:
- Measurement of charge transfer in discharges to look for differences between incendive and non-incendive discharges.
- The 5–10 MU limit proposed on the resistance to avoid opportunity to support a spark type discharge applied for charge held on a lumped value of capacitance. In the case of resistive threads in an FIBC material charge is only accessed by flow along an extended distribution of R and C. There is no lumped source of charge. Hence quite different resistivity limits can be expected to apply. - In practical gas probe ignition tests the fabric is directly backed by highly charged pellets on which charge is likely not to be very mobile. This could restrict the movement of charge across the outer fabric surface as a way suggested for inhibiting the chance of ignition for an isolated charged sheet of fabric.
The above studies need to be pursued with all measurements made at the same time, and preferably, in conjunction with gas probe ignition testing. It will also be necessary to have available surfaces and threads that provide characteristics likely to be on either side of expected boundary criteria. The above discussion suggests that avoidance of ignition risk may rely upon criteria of the form that the resistivity of threads shall be above some lower level, below a resistivity value around 2 MU and that this resistivity is provided in a localized pattern of threads. The method of shielding performance measurement described provides both assessment of resistivity and indication of thread spacing via the low frequency shielding factor.
In addition to these it may well be that as the earthed probe approaches the charged bag surface there is corona at any conductive threads that drains charge from an area and so reduces the opportunity for an incendive discharge. For charge to be drained adequately quickly the resistivity of the conductive threads must not be too high. Also, the presence of conductive threads may limit the propagation of the discharge in the air across a charged insulating surface and so limit the incendivity of the discharge. If the influence of charge on powder within the FIBC prevents effective limitation of ignition risks by a resistivity distributed across the whole surface of the bag then it may be that prevention must rely upon the proximity of a pattern of conductive threads to segment the area of charge available to contribute to discharges. It is plausible that this requires threads of adequately low resistivity and of appropriate spacing – but of not too low resistivity. To progress prospects of assessing Type D FIBC fabrics in advance of, or as an alternative to, gas probe ignition tests the following studies are needed: - Low light level photography to find out how discharges spread out across charged surfaces and how the spread is affected by the resistivity/charge decay characteristics of the material, by the presence or not of charged material backing the surface and by the presence, pattern and resistivity of conductive threads included in the structure of the fabric. - Measurements of any corona charge transfer preceding occurrence of a main discharge
6. Conclusions The present studies have shown that it is feasible to use the variation of shielding performance with frequency to measure, without electrical contact, the effective resistivity of conductive components included within FIBC type fabrics. From such resistivity measurements on 15 samples of FIBC fabrics it has been possible to provide an empirical criterion for the avoidance of ignition that matches to the results of gas probe ignition tests for these samples. This criterion is that ‘the resistivity shall be less than 2 MU’. The empirical criterion is very different from the two criteria proposed on the basis of the conditions expected to be required for the avoidance of spark type discharges. These criteria were that the resistivity should be more than 5 MU and that the charge decay time of the fabric should be less than 0.1 s. Studies are proposed to provide an improved understanding of the features required for avoidance of ignition. This is needed for confidence in criteria appropriate for assessing FIBC type D bag materials in advance of, or as an alternative to, gas probe ignition tests. It will also enable fabrics to be designed that are optimized to avoid risks of ignition. The present work has been concerned with Type D FIBC bag materials. It may be that it will also have some relevance for garments worn by people working in flammable atmospheres. For such people the primary requirement is that the body and the garment are reliably connected to earth. Situations may arise
J. Chubb, P. Holdstock / Journal of Electrostatics 68 (2010) 145–151
however where this may not be feasible and in such situations having garments made of fabrics with the same ability at Type D FIBC to avoid direct incendive discharges would provide a useful degree of secondary protection.
References [1] I.E.C. 61340-4-4, Electrostatics – Part 4-4: Standard Test Methods for Specific Applications – Electrostatic Classification of Flexible Intermediate Bulk Containers (FIBC). International Electrotechnical Commission, Geneva, 2005. [2] I.E.C. 61340-2-3, Electrostatics – Part 2-3: Methods of Test for Determining the Resistance and Resistivity of Solid Planar Materials Used to Avoid Electrostatic Charge Accumulation. International Electrotechnical Commission, Geneva, 2000. [3] E.N. 11493, Protective Clothing – Electrostatic Properties – Part 3: Test Methods for Measurement of Charge Decay. European Committee for Standardisation, Brussels, April 2004. [4] B. Lewis, G. von Elbe, Combustion, Flames and Explosion of Gases. Academic Press, New York, 1961. [5] J.M. van der Weerd, Shell Report AMSR 0016, 1972.
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[6] M. Glor, ‘‘Electrostatic ignition hazards associated with flammable substances in the form of gases, vapours, mists and dusts’’ Inst Phys Confr ‘Electrostatics 1999’ IoP Confr Series 163, p. 199. [7] G.J. Butterworth, E.S. Paul, J N Chubb, ‘‘A study of the incendivity of electrical discharges between planar resistive electrodes’’ Electrostatics 1983 IoP Confr Series 66, p. 185. [8] J.N. Chubb, Practical and computer assessments of ignition hazards during tank washing and during wave action in part ballasted OBO cargo tanks. J. Electrostatics 1 (1975) p61. [9] J. N. Chubb, C. Hancox, J. Harbour, ‘‘Measuring the shielding performance of materials and assemblies’’ IEE Colloquium on ’Low cost EMC testing’ Univ Birmingham Sept 21, 1993 (Digest 1993/169) [10] J. N. Chubb, ‘‘How effectively do materials shield against transient electric fields?’’ IEEE Ind Appl Magazine 8 (5) Sept/Oct 2002 p13 (Based on paper presented at the 2000 IEEE-IAS meeting in Rome 8-12 October, 2000). [11] J.N. Chubb, Non-contact measurement of electric field shielding performance and the resistivity of layer materials. J. Electrostatics 64 (10) (2006) 685–689. [12] J.N. Chubb, Instrumentation and standards for testing static control materials. IEEE Trans. Ind. Appl. 26 (6) (Nov/Dec 1990) 1182. [13] J.N. Chubb, A standard proposed for assessing the electrostatic suitability of materials. J. Electrostatics 65 (2007) 607–610. [14] J. N. Chubb, ‘‘Test method to assess the electrostatic suitability of materials for retained electrostatic charge’’ Document prepared in 2004 for discussion as prospective British Standard www.infostatic.co.uk/cache/JCITestMethod.pdf.
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Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Risks of ignition from ‘Type D’ ungrounded fibc John Chubb a, *, Paul Holdstock b a b
Infostatic Ltd, 2 Monica Drive, Pittville, Cheltenham, GL50 4NQ, UK Holdstock Technical Services, 3000 Manchester Business Park, Aviator Way, Manchester, M22 5TG, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 July 2009 Accepted 21 November 2009 Available online 4 December 2009
Avoidance of risks of ignition with unearthed Type D FIBC requires that electrostatic discharges cannot quickly access large quantities of charge via any conductive features of the material. The present studies show how the effective resistivity of conductive features within FIBC type fabrics can be measured, without electrical contact, from the variation of shielding performance with frequency. From such measurements an empirical acceptance criterion has been derived that matches well to the results of gas probe ignition tests. Studies are proposed to provide reliable criteria for assessing FIBC Type D bag materials and to help optimize FIBC fabric design. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Risk assessment Electrostatic ignition risks Material characteristics FIBC
1. Introduction This paper examines a method of testing that may enable the avoidance of risk of ignition from ungrounded Type D FIBC (Flexible Intermediate Bulk Containers) to be assessed without the need for time consuming and expensive gas probe ignition testing. Where FIBC are used with materials that may be flammable and/ or in atmospheres that may be flammable, precautions are necessary to avoid ignition risks from electrostatic discharges. The loading of flowable solids into FIBC is likely to create significant quantities of electrostatic charge. This would create high voltages on external surfaces if the bag material were just a simple dielectric fabric. One method of preventing the occurrence of electrostatic discharges at approach of any external earthed item is by the inclusion of high conductivity threads within the FIBC fabric with these threads reliably bonded to earth at the FIBC mounting arrangements. These are known as Type C FIBC. In this construction the occurrence of incendive discharges is avoided by suitably close spacing of the threads to encourage dissipation of charge by conduction to earth and limiting the area of charge available to discharges. An International Standard test method [1] is available for assessing such Type C FIBC, and other International Standards [2] can be used to assess the fabrics that are intended for use in Type C FIBC. The problem with Type C bags has been how to achieve reliable earth bonding of all conductive threads during long term * Corresponding author. E-mail addresses: [email protected] (J. Chubb), [email protected] (P. Holdstock). URL: http://www.infostatic.co.uk 0304-3886/$ – see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2009.11.012
industrial use. A Type D FIBC is in practical use and this offers the advantage of not requiring any earthing of the bag structure. The International Standard [1] also specifies test methods for Type D FIBC, but there is at present no standard test method to assess fabrics that are intended for use in Type D FIBC. For an electrostatic discharge to ignite a flammable gas or powder dispersed into the air there needs to be a certain concentration of energy in time and space. The requirements for ignition of various gas mixtures by spark type electrostatic discharges has been extensively studied [4]. For common hydrocarbon vapour air mixtures the minimum energy for ignition by a capacitive electrostatic spark discharge is around 0.2 mJ. This varies with ambient temperature, pressure, oxygen concentration, etc and with the discharge gap above a threshold minimum value [4]. For powders, minimum ignition energies are higher, but depend on the material and the fineness of the particles dispersed. Ignition by other forms of electrostatic discharge is less well defined. Brush discharges from an earthed projection approaching a highly charged insulating surface (solid or liquid) can certainly cause ignitions [5,6]. Ignition by propagating brush discharges, from an earthed projection approaching a charged insulating layer resting on an earthed surface, has been well characterized [6]. Corona discharges are generally felt to be incapable of causing ignitions – although some doubt remains with the most easily ignitable gases, such as hydrogen, etc. The types of discharge described above involve either the release of energy from electrostatic charge stored on highly conducting surfaces (spark type discharges) or a discharge to a highly charged insulator surface where the discharge reaches out over an immobile area of charge by propagation of the discharge
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through the air close to the charged surface. It was suggested in 1983 [7] that materials with resistivity within a certain range might inhibit the opportunity for occurrence of incendive discharges by limiting the rate of charge flow through the surface to stop full spark type discharges but allow sufficient charge movement to prevent the occurrence of air breakdown across the charged surface. For the rate of charge migration to be inadequate to support an incendive discharge the experimental work done at Culham [7] on electrostatic discharges between two resistive plane surfaces suggested that surface resistivities needed to be over 1–3 107U to avoid risks of ignition. Theoretical calculations suggested values over 107–108U. It seems reasonable to expect that somewhat higher values might apply for a localized earthed conducting probe approaching a single plane charged resistive surface. Recent studies with a single stage impulse generator circuit, with a source capacitance of 370 pF and a 1 kU load and with a 740 pF spark heating capacitor circuit, showed that for voltages up to 12 kV avoidance of continuity of discharge to the load required the spark heating series resistor to be over 5 MU. This is reasonably compatible with the Culham expectations and results. To prevent the occurrence of discharges propagating across the surface of a charged dielectric it seems plausible that the rate of charge migration across the surface should be sufficient to enable charge to concentrate in the high field region near the approaching earthed probe. When breakdown occurs the discharge will then only be able to propagate out to the boundary where the charge density is below some threshold level for propagation over the surface. If this model is plausible then the speed of approach of the probe relative to the level of ‘resistivity’ (charge decay time) of the surface will be a relevant assessment criterion. In charge decay time terms, a decay time under 0.1 s would seem to be appropriate in relation to likely rates of approach of earthed probes to an FIBC surface. On the basis of the above it seems plausible that avoidance of risks of ignition requires:
Gas probe testing has shown that some fabrics for FIBC provide the opportunity for occurrence of incendive discharges when an earthed probe approaches a highly charged fabric surface. Whether the occurrence relates to resistive features of the fabric accessible by a high voltage discharge or to propagation of a discharge across the dielectric surface cannot be assessed by traditional resistivity measurements. The structure of the fabric and/or the sheathing of the conductive component may inhibit appropriate electrical contact for reliable resistivity measurement. The technique developed for assessing the shielding performance of materials against electric field transients [9,10] has been shown able to measure the effective resistivity of materials without need for surface or invasive contact [11]. A resistive material (for example damp paper) shows a high attenuation at low frequencies and a fall off in attenuation with increasing frequency. The fall off frequency band relates directly to the resistivity. Shielding performance measured as a function of frequency over a range from 10 Hz to a few MHz provides the capability to measure resistivities in the range 0.1–1000 MU. A fully conducting element across the test area, even a single fine copper wire, can be detected and recognized by having a constant shielding performance with frequency over this range. This paper reports measurements of the variation of shielding performance with frequency for a number of FIBC materials and the interpretation of these measurements in terms of effective resistivity. Also included are measurements of charge decay time [12,13]. These results are compared with the results of earlier gas probe ignition tests to see if there is a convincing correlation – and hence a prospect of predicting the results of gas probe tests from shielding performance and charge decay measurements. 2. Method of testing 2.1. Shielding performance
- No component in the fabric material has a resistivity less than, say, 10 MU; - The surface of the fabric material has a charge decay time less than 0.1 s.
The approach developed for measuring the electric field shielding performance of film and layer materials [9,10], and from this to assess the resistivity of the materials [10], has five basic features:
Where the charge decay time is very long it may be that incendive discharges can be avoided if the propagation of discharges across the surface is limited to an adequately small area by proximity to a pattern of conductive threads. This possibility would need to be investigated by separate and additional studies. However, it is suggested that this would not be as secure a protection as an appropriate maximum charge decay time. It needs to be remembered that the speed of approach of the earthed probe to the charged surface is important. In studies relating to explosions in large crude oil tankers [8] it was shown how the velocity of approach needed to be adequately fast to prevent discharges (such as corona and water spray discharges) that removed charge form the surface – but not so fast that an incipient flame kernel was quenched before propagating to the surrounding gas atmosphere [8]. This speed of approach point is covered in present FIBC test procedures [1]. The fabrics from which FIBC are made may include a pattern of conductive threads. For the Type C bags these are often metallic and these must be fully linked across the whole surface of the FIBC so they can be reliably linked to earth. In concept, the fabric for Type D bags include quasi-conductive threads whose conductive elements are, in some cases, contained within a polyester sheath. A major question is clearly whether fabrics including such threads, if not earthed, could give rise to ignition risks.
a) A plane area of the test material, isolated from earth, is tested by applying a transverse alternating electric field stress over a defined area by capacitive coupling from a pair of nearby electrodes. b) The electric field stress is applied as a balanced bipolar sine wave signal relative to earth and covering a wide range of frequencies. Symmetry about earth potential ensures no common mode signal is impressed on the sample as a whole. c) Measurements are made of the signals capacitively coupled to a pair of electrodes on the far side of the sample in positions matching those of the driving pair of electrodes. Observations are made of the difference signal between these two electrodes. d) Shielding is measured at a variety of frequencies as the ratio of signals observed with the material present compared to that without. e) Shielding performance is presented as the variation of the ratio of signals as a function of frequency. This comprises a combination of in phase and quadrature signal components. The basic physical arrangement of the approach is shown in Fig. 1. The electrodes are 15 mm wide and 100 mm long and mounted flush in earthed plates to be parallel and with their centre lines 50 mm apart. The 100 mm length was chosen for
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Fig. 1. Basic arrangement for measuring the variation of shielding with frequency.
studies on the characteristics of fabrics including conductive threads where the threads might be spaced as much as 25 mm apart so at least several threads were covered. The arrangement of the observation electrodes matches the electric field stressing electrodes. The inner surfaces of the electrodes and mounting plates are 10 mm apart. The sample is clamped flat against a 3 mm thick sheet of good quality dielectric (polycarbonate) on the observation side by a second sheet advanced from the driving electrode side. This arrangement ensures the sample is held flat and in a well defined position relative to the stressing and observation electrodes. The present method for testing shielding performance [10,11] operates over the frequency range 10 Hz–10 MHz. This is a simplified version of an earlier arrangement that was developed to cover the full range of frequencies relevant to risks of damage to microelectronic devices – 10 Hz to 1 GHz [9]. The simplification of using antiphase sine wave drive signals with phase sensitive detection of in-phase and quadrature-phase signals provided good signal to noise ratios over the chosen frequency range. Combination of in-phase and quadrature-phase signals as the square root of the sum of the squares takes account of the phase shift that occurs as attenuation rises and provides a proper measure of shielding attenuation. The present more limited frequency range had been shown [10,11] to provide quite adequate information on the shielding characteristics of fabrics including conductive threads.
2.3. EN 1149-3 method 2
2.2. Charge decay measurement
4. Results
Measurements of the corona charge decay time characteristics of each sample were made using a JCI 155v5 Charge Decay Test Unit to the test procedure that has been described [12–14]. Measurements were made in normal laboratory conditions at a temperature near 20 C and humidity between 50% and 55%RH.
4.1. Shielding performance measurements
Measurements of half decay time (t50) and shielding factor (s) of each sample were made according to EN 1149-3 Method 2 [3] under conditions of 23 2 C and 25 5%RH. 2.4. Ignition testing Ignition testing involves filling FIBC with highly charged polypropylene pellets and bringing a gas probe up to the sides of the FIBC to provoke discharges that may cause the gas to ignitie. Ignition testing of full size FIBC was done in accordance with the procedures specified in IEC 61340-4-4 [1]. 3. Test samples A total of 15 samples were tested of fabrics as used in commercial FIBC. The fabric samples were cut from FIBC that had been previously assessed for ignition risk by gas probe ignition tests in accordance with the procedures specified in IEC 61340-4-4 [1] – but the results were not known to the test operator at the time of the present shielding tests. The samples were provided by Paul Holdstock as 100 mm 100 mm squares and identified just by a number. The description of the fabric samples is given in Table 1. Manufacturers are identified only by a code letter so as to maintain commercial confidentiality.
In previous studies [11] it had been observed that the variation of attenuation with frequency related to the effective resistivity of the materials over the test area. Fresh measurements of this have
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Table 1 Description of FIBC fabric samples.
1000
Sample Description
Manufacturer code
1
A
2 3 4 5 6 7 8 9 10 11 12 13 14 15
Lightweight, coated fabric with static protective yarns in 20 mm stripe pattern Standard weight, coated fabric with static protective yarns in 20 mm stripe pattern Plain, lightweight fabric (insulating fabric used to make Type A FIBC) Plain, standard weight fabric (insulating fabric used to make Type A FIBC) Standard weight, coated fabric with static protective yarns in 20 mm stripe pattern As sample 2 but the base fabric and coating contains 2.4% w/w antistatic additive Standard weight, coated fabric with static protective yarns in 20 mm stripe pattern Same as sample 5 (samples cut from adjacent areas) As sample 2 but the coating contains 1.2% w/w antistatic additive Same as sample 5 (samples cut from adjacent areas) Standard weight, coated fabric with antistatic finish Same as sample 11 but rinsed under running water for 20 min Same as sample 11 but cut from an old (2003) FIBC Standard weight, coated fabric with 50 mm 50 mm grid of yarns containing an antistatic additive Same as sample 14 but cut from a different FIBC
100
10
A A
1
A 0.1
A A
0.01 0.1
B
1
A
Series1
A
100
Attenuation
Series3
Transmission
Fig. 3. Variation of 50% transmission and 50% attenuation with resistance between test foils.
A C C C D D
been made, in the same way as before, using 0.1 MU, 1 MU, 10 MU and 100 MU resistors between thin foil electrodes matching the areas between each of the energizing and detection electrodes and mounted in the normal test position. A sample measurement is shown in Fig. 2. The variations of frequency (kHz) for 50% attenuation and for 50% transmission fraction with the resistance value (MU) are shown in Fig. 3. Resistance values R (megohms) may be derived from the frequency f (kHz) for 50% attenuation and for 50% transmission as: From 50% attenuation fraction: R ¼ 10 (0.947log (f)) From 50% transmission fraction: R ¼ 10(1.135log (f)) To express this as an equivalent resistivity over the test area it is necessary to take into account the 2:1 ratio of the length of the electrodes to their spacing in the test set-up. This means that the resistivity value is twice the effective value of equivalent resistance between the electrodes.
Sample results of measurements of the variation of attenuation with frequency for the 15 samples studied are shown in Figs. 4–6. The signals observed are phase sensitive detected in phase with the source signals – and in most cases also in quadrature. Calculation of the true attenuation factor requires these two values to be combined as the square root of the sum of the squares. The low level of observed signals in a number of the present studies made it impractical to make much use of quadrature component measurements. The results have therefore been considered primarily in terms of the variation of the in-phase signal components. It was noted in the measurements with samples 1, 2, 5, 6 8, 9 and 10 that the attenuation varies from a fairly flat maximum at the lowest frequencies and fell to zero at the higher frequencies. This indicates that the threads visible across some of the samples were only having a modest shielding effect at best because of their relatively wide spacing. On the other hand samples 7, 13, 14 and 15 showed a wide range of attenuation, indicating that the shielding mechanism was a feature of the whole area of the sample and not spatially localized. It is noted that samples 3, 4 and 12 showed no attenuation. To calculate the effective resistivity of the fabric samples, from the variation of shielding performance with frequency, it seems plausible to use the frequency at which the attenuation or
1.0
in phase component
0.9
0.20
0.8
0.18
0.7
0.16
0.6
0.14
0.5
0.12
0.4
0.10
0.3
0.08 0.06
0.2
0.04
0.1 0.0 10
10
Resistance (M)
0.02 100 Transmission
1,000 10,000 Fequency (Hz) Quadrature
100,000
1,000,000
0.00 -0.02
Atte nuation
1
10
100
1,000
10,000
100,000
Frequency (Hz) Fig. 2. Variation with frequency of transmission fraction, quadrature fraction and attenuation for 100 M between test foils at 28 April 2009.
Fig. 4. Shielding performance of Sample 8 at 30 April 2009.
1,000,000
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149
Table 2 Results of shielding performance measurements.
1.0 0.9
Sample
50% Trans frequency (kHz)
Effective resistivity (Megohms)
Low frequency shielding
170 100
0.16 0.27
0.12 0.11
0.5
1 2 3
0.4
4
0.8 0.7 0.6
0.3 0.2 0.1 0.0 10
100
1,000
10,000
100,000
1,000,000
Fequency (Hz) Transmission
Quadrature
Attenuation
Fig. 5. Transmission and shielding versus frequency for Sample 11 at 18 May 2009.
transmission is halfway between the low and high frequency levels. On this basis the results from the various studies are presented in the following Table 2. Included in this Table are values for the effective shielding at low frequency. 4.2. Charge decay measurements The results of corona charge decay measurements are summarized in Table 3. All samples apart from 7, 11, 13, 14 and 15 showed long charge decay times. Where it was not sensible to measure the time to 1/e the local decay time constant values at 1000 s are reported. With sample 11, two measurements were made: the first over a thread, the second between threads. In both cases the decay curve flattened out – and with the second the local decay time constant at 1000 s was 250,000 s! For the long decay time samples the tribocharging created at even careful handling gave initial surface voltage of several hundred volts!. It is clear that charge decay on samples 1–6 and 8–12 are very long. Only samples 13, 14 and 15 show good short charge decay times. Sample 7 shows an initial fairly quick charge decay but this slows up significantly with the progress of decay. 5. Assessment of results Table 4 (appended) lists the results of the measurements made of resistivity and charge decay times for the 15 samples tested. 1.0
No attenuation 10 Hz–1 MHz No attenuation 10 Hz–1 MHz
5 6 7 8 9 10 11 12
55 90 <0.010 50 6 18 0.136
13 14 15
0.024 0.012 0.013
0.5 0.3 >3000 (>3G) 0.5 4.5 1.5 200
0.13 0.11 0.13 0.09 0.13 w1 No attenuation 10 Hz–1000 Hz
1150 (1.1G) 2270 (2.3G) 2165 (2.2G)
w1 w1 w1
Results of the application of the two criteria proposed in Section 1 are listed. These criteria for the avoidance of risk of ignition were that the resistivity should be greater than 10 MU (Criterion 1) and the charge decay time should be less than 0.1 s (Criterion 2). Also listed in Table 4 are the results of the gas probe ignition tests and the results of tests of these 15 samples according to EN1149 Method 2. This latter test method is used to assess apparel fabrics intended for personal protective clothing for use in hazardous flammable environments. It is clear from Table 4 that there is not a good match between predictions based on either criterion 1 or 2 and the results of gas probe ignition tests. This also applies for assessment by EN1149 Method 2 – but that is not directly relevant because that method of testing was not designed as an appropriate test method for Type D bag materials. If a criterion is considered that says nothing more than that ‘the resistivity shall be less than 2 MU’ then it appears that for the 15 samples tested there is a direct match between this assessment and the gas probe test results! This is shown as criterion 3 in Table 4. This is an empirical result that is true for all the samples as tested. This empirical criterion does not match the ideas outlined in Section 1. It is hence necessary to consider how the criteria that were proposed may be in error and what needs to be done to establish necessary and sufficient criteria that will apply to any plausible materials considered for FIBC Type D bag application. Table 3 Charge decay measurement results.
0.9 0.8
Sample
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 10
Comment
100
1,000
10,000
100,000
1,000,000
Fequency (Hz) Transmission
Quadrature
Attenuation
Fig. 6. Shielding versus frequency for Sample 13 at 22 May 2009.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Decay time to 1/e (s)
Decay time to 10%
Local decay time constant at 1000 s 36,000 27,000 47,000 33,000 15,000 8000
0.707, 0.806
37.6, 84.5 18,000 8200 23,000 250,000 (b) 50,000
183(a) 0.035, 0.031 0.265, 0.176 0.089, 150, 146
0.105, 0.103 3.2, 2.02 0.489, 0.847, 0.846
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Table 4 Comparison to results of gas probe tests – Prediction: P pass, F fail. Sample
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Resistivity (M)
Decay time (s) to 1/e or time constant @ 1000 s
0.16 0.27
36,000 27,000 47,000 33,000 15,000 8000 0.7–0.8 18,000 8200 23,000 250,000 50,000 0.031–0.035 0.176–0.265 0.089–0.150
0.5 0.3 >3000 (>3G) 0.5 4.5 1.5 200 1150 (1.1G) 2270 (2.3G) 2165 (2.2G)
Prediction criteria 1
2
3
F F
F F F F F F P ? F F F F P P P
P P F F P P F P F P F F F F F
F F P? F P F P P P P
Gas probe ignition Pass Pass Fail Fail Pass Pass Fail Pass Fail Pass Fail Fail Fail Fail Fail
Comments
Insulator – plain fabric Insulator – plain fabric
>50% of approaches result in ignition 1 approach out of 400 results in ignition w5 approaches out of 400 result in ignition >50% of approaches result in ignition w5 approaches out of 400 result in ignition w5 approaches out of 400 result in ignition w5 approaches out of 400 result in ignition
EN 1149-3 Method 2 s
t50 (s)
Pass/Faila
0.20 0.21 0.01 0.01 0.25 0.20 0.00 0.22 0.24 0.28 0.00 0.01 0.04 0.00 0.54
>60 >60 >60 >60 >60 >60 7.09 >60 5.16 >60 <0.01 >60 0.21 <0.01 <0.01
(Pass) Pass Fail Fail Pass (Pass) Fail Pass Pass Pass Pass Fail Pass Pass Pass
Criterion 1: Resistivity > 10 M. Criterion 2: Charge decay to 1/e < 0.1 s. Criterion 3: Resistivity < 2 M. a Requirements specified in EN 1149-5 are s > 0.20 or t50 > 4 s.
The following reasons are suggested for the failures of criteria 1 and 2:
- Measurement of charge transfer in discharges to look for differences between incendive and non-incendive discharges.
- The 5–10 MU limit proposed on the resistance to avoid opportunity to support a spark type discharge applied for charge held on a lumped value of capacitance. In the case of resistive threads in an FIBC material charge is only accessed by flow along an extended distribution of R and C. There is no lumped source of charge. Hence quite different resistivity limits can be expected to apply. - In practical gas probe ignition tests the fabric is directly backed by highly charged pellets on which charge is likely not to be very mobile. This could restrict the movement of charge across the outer fabric surface as a way suggested for inhibiting the chance of ignition for an isolated charged sheet of fabric.
The above studies need to be pursued with all measurements made at the same time, and preferably, in conjunction with gas probe ignition testing. It will also be necessary to have available surfaces and threads that provide characteristics likely to be on either side of expected boundary criteria. The above discussion suggests that avoidance of ignition risk may rely upon criteria of the form that the resistivity of threads shall be above some lower level, below a resistivity value around 2 MU and that this resistivity is provided in a localized pattern of threads. The method of shielding performance measurement described provides both assessment of resistivity and indication of thread spacing via the low frequency shielding factor.
In addition to these it may well be that as the earthed probe approaches the charged bag surface there is corona at any conductive threads that drains charge from an area and so reduces the opportunity for an incendive discharge. For charge to be drained adequately quickly the resistivity of the conductive threads must not be too high. Also, the presence of conductive threads may limit the propagation of the discharge in the air across a charged insulating surface and so limit the incendivity of the discharge. If the influence of charge on powder within the FIBC prevents effective limitation of ignition risks by a resistivity distributed across the whole surface of the bag then it may be that prevention must rely upon the proximity of a pattern of conductive threads to segment the area of charge available to contribute to discharges. It is plausible that this requires threads of adequately low resistivity and of appropriate spacing – but of not too low resistivity. To progress prospects of assessing Type D FIBC fabrics in advance of, or as an alternative to, gas probe ignition tests the following studies are needed: - Low light level photography to find out how discharges spread out across charged surfaces and how the spread is affected by the resistivity/charge decay characteristics of the material, by the presence or not of charged material backing the surface and by the presence, pattern and resistivity of conductive threads included in the structure of the fabric. - Measurements of any corona charge transfer preceding occurrence of a main discharge
6. Conclusions The present studies have shown that it is feasible to use the variation of shielding performance with frequency to measure, without electrical contact, the effective resistivity of conductive components included within FIBC type fabrics. From such resistivity measurements on 15 samples of FIBC fabrics it has been possible to provide an empirical criterion for the avoidance of ignition that matches to the results of gas probe ignition tests for these samples. This criterion is that ‘the resistivity shall be less than 2 MU’. The empirical criterion is very different from the two criteria proposed on the basis of the conditions expected to be required for the avoidance of spark type discharges. These criteria were that the resistivity should be more than 5 MU and that the charge decay time of the fabric should be less than 0.1 s. Studies are proposed to provide an improved understanding of the features required for avoidance of ignition. This is needed for confidence in criteria appropriate for assessing FIBC type D bag materials in advance of, or as an alternative to, gas probe ignition tests. It will also enable fabrics to be designed that are optimized to avoid risks of ignition. The present work has been concerned with Type D FIBC bag materials. It may be that it will also have some relevance for garments worn by people working in flammable atmospheres. For such people the primary requirement is that the body and the garment are reliably connected to earth. Situations may arise
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however where this may not be feasible and in such situations having garments made of fabrics with the same ability at Type D FIBC to avoid direct incendive discharges would provide a useful degree of secondary protection.
References [1] I.E.C. 61340-4-4, Electrostatics – Part 4-4: Standard Test Methods for Specific Applications – Electrostatic Classification of Flexible Intermediate Bulk Containers (FIBC). International Electrotechnical Commission, Geneva, 2005. [2] I.E.C. 61340-2-3, Electrostatics – Part 2-3: Methods of Test for Determining the Resistance and Resistivity of Solid Planar Materials Used to Avoid Electrostatic Charge Accumulation. International Electrotechnical Commission, Geneva, 2000. [3] E.N. 11493, Protective Clothing – Electrostatic Properties – Part 3: Test Methods for Measurement of Charge Decay. European Committee for Standardisation, Brussels, April 2004. [4] B. Lewis, G. von Elbe, Combustion, Flames and Explosion of Gases. Academic Press, New York, 1961. [5] J.M. van der Weerd, Shell Report AMSR 0016, 1972.
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[6] M. Glor, ‘‘Electrostatic ignition hazards associated with flammable substances in the form of gases, vapours, mists and dusts’’ Inst Phys Confr ‘Electrostatics 1999’ IoP Confr Series 163, p. 199. [7] G.J. Butterworth, E.S. Paul, J N Chubb, ‘‘A study of the incendivity of electrical discharges between planar resistive electrodes’’ Electrostatics 1983 IoP Confr Series 66, p. 185. [8] J.N. Chubb, Practical and computer assessments of ignition hazards during tank washing and during wave action in part ballasted OBO cargo tanks. J. Electrostatics 1 (1975) p61. [9] J. N. Chubb, C. Hancox, J. Harbour, ‘‘Measuring the shielding performance of materials and assemblies’’ IEE Colloquium on ’Low cost EMC testing’ Univ Birmingham Sept 21, 1993 (Digest 1993/169) [10] J. N. Chubb, ‘‘How effectively do materials shield against transient electric fields?’’ IEEE Ind Appl Magazine 8 (5) Sept/Oct 2002 p13 (Based on paper presented at the 2000 IEEE-IAS meeting in Rome 8-12 October, 2000). [11] J.N. Chubb, Non-contact measurement of electric field shielding performance and the resistivity of layer materials. J. Electrostatics 64 (10) (2006) 685–689. [12] J.N. Chubb, Instrumentation and standards for testing static control materials. IEEE Trans. Ind. Appl. 26 (6) (Nov/Dec 1990) 1182. [13] J.N. Chubb, A standard proposed for assessing the electrostatic suitability of materials. J. Electrostatics 65 (2007) 607–610. [14] J. N. Chubb, ‘‘Test method to assess the electrostatic suitability of materials for retained electrostatic charge’’ Document prepared in 2004 for discussion as prospective British Standard www.infostatic.co.uk/cache/JCITestMethod.pdf.