Electrostatic Hazard Risk Management in
Coating and Printing on Moving Webs

Mark Blitshteyn
Ion Industrial
1000 Old County Circle, Unit 116
Windsor Locks, CT 06096
markblit@ion.com

Abstract

Accumulation of static charges leading to an electrostatic discharge (ESD) in flammable or explosive atmospheres is known to cause fires. This paper will identify hazards attributed to electrostatic charges on moving webs in a flammable-vapor environment, such as in coating and printing processes. Among the reviewed ignition sources are discharges of isolated conductors, discharges of insulating webs, and human operators as ignition source. Grounding of conductors and operators, and air ionization for prevention of static buildup on insulators will be examined. Safety and performance of various static neutralizers will be reviewed, including a review of the benefits of computer technology to provide information on static conditions and neutralizing equipment status. It will be shown that static neutralizing equipment's diagnostic and performance data can provide valuable information for internal safety evaluations and post-fire investigations. The paper will outline the key elements of a thorough electrostatic hazard risk management program in coating and printing.

Introduction

Accumulation of static charges leading to an electrostatic discharge (ESD) in flammable or explosive atmospheres is known to cause fires. Fires attributed to electrostatic discharges in coating and printing are not rare. In the informal survey of 17 coaters conducted by the author, out of those who responded, there were a total of 25 static-related fires at 13 plants in the last five years, and 9 fires in the 12 months preceding the survey.

Most such fires are quickly extinguished and result only in minor economic consequences. But not always… In the early 1999, a flash fire occurred at a Rexam Release site, involving the coating head of a silicone coater. The fire resulted in a serious injury to a machine operator, who sustained second-and third degree burns over 70% of his body. Due to the extent of his injuries, it took several weeks for his recovery. In addition, the company suffered losses as the production was on halt for several weeks during the investigation. Post-accident analysis by the company, Fire Marshall, and the insurance company concluded that "static was most likely the cause of ignition" [1].

Serious accidents and/or injuries from fires happen very seldom, but the consequences could be major. That is why the decisions related to managing static hazard risks must be guided not by the low probability of serious accidents but rather by the factor of the possible grave economic consequences and human life endangerment.

Five Rules for Dealing with Static Electricity in Hazardous Areas
Flammable vapors, oxygen and an ignition source must combine to ignite a fire. The most practical way to prevent fires is to control the ignition sources. Among the ignition sources static discharges are most unpredictable. This leads to the most important rule for dealing with static electricity in hazardous areas.

Rule 1:

Static electricity is a possible hazard in all installations in classified (hazardous) areas [2].

In simplest terms, you must be aware of the static and you must take the appropriate steps to manage the risk caused by static.

Among the ignition sources are discharges of isolated conductors, including operators, and discharges of insulating webs. Discharge of isolated conductors is most dangerous leading to the second most important rule for dealing with static electricity in hazardous areas.

Rule 2:

All conducting parts should be properly grounded, including operators.

Discharge of insulators is a rare occurrence. However, in the case of a moving charged film, conditions for a discharge to a metal roller or other metal grounded components may exist for periods of time, thus increasing probability of ignition (Fig. 1).

Figure 1. Typical coating head

Metal components of the machine are not the only conductive objects to which a charged web can discharge. Operators are often overlooked as conductors. The charged film can produce a direct discharge to an operator or induce charge on an operator's body, leading to a latent discharge of the stored energy to a grounded frame or controls, as shown in Fig. 2. To avoid such occurrences the charges on the moving webs need to be controlled. This leads to the third important rule for dealing with static electricity in hazardous areas.

Rule 3:

Static charges on surfaces of materials inside the hazardous areas must not exceed the safe limit.

Figure 2. Operator discharge as a source of ignition.

Static charges on non-conducting materials, such as plastic film or coated paper, must be controlled using air ionization from special static neutralizing devices. Specific devices for static neutralization in hazardous locations include tinsel and other passive devices, radioactive ionizers, and electrical ionizing blowers and neutralizing bars rated for the appropriate classified (hazardous) area.

Conditions for neutralizing static charges carried by a moving material are graphically shown in Fig. 3. The rate of charge transfer by a material moving at a constant speed can be considered an electrical current.

Figure 3. Neutralization of charges on a moving web with an ionizer.

The ions generated by the ionizer will be electrostatically attracted to the charged material. The polarity of these ions will be opposite to that on the charged material. That ion current, , flows to the charged material and neutralizes it.

If the neutralizing current is equal to the rate of charges transferred by the web then the outcome is complete charge neutralization. If the neutralizing current is lower than the charge carried by the web, as it would happen with a weak ionizer, then the web would carry a residual charge after passing by the ionizer. Therefore, a choice of an ionizing device, i.e. neutralizer, is important

The other important factor is the location of the neutralizers. Two approaches can be considered for identifying locations for static neutralizers. The NFPA Recommended Practice on Static Electricity [3] calls for a conservative approach, installing a static neutralizer after each roller (Fig. 4).

Figure 4. NFPA recommended installation of static neutralizers in
coating or printing applications.

A more practical approach can be considered where a static neutralizer would be installed where it is really necessary by correctly identifying where charges accumulate and assessing the ignition hazard at each such location.

Proper measurements with a hand-held fieldmeters will help determine the locations of charge accumulation within a process line. However, it is important to take measurements regularly and record all readings. Charge levels often vary day-to-day due to a variety of changes within the process [4]. Material differences, machine speed, temperature, tension and humidity are the factors that can produce a variety static conditions (Table 1).

Table 1. Static conditions on a coating line during twelve consecutive days at a custom coating company.

Most of the commercially available neutralizers have not changed since they were first introduced over three decades ago. We will refer to those devices as conventional neutralizers. Notwithstanding the aging designs, static neutralizers in the hazardous areas are loss prevention products and must be judged as such.

The safety of static neutralizers is an important consideration in selecting an appropriate device. For instance, when a neutralizer is inadvertently switched off or stops functioning for some other reasons, it no longer reduces the charges on the moving web which becomes capable of creating hazardous conditions. Yet, in majority of cases, the user would not even know that the neutralizer stopped functioning and a dangerous situation has ensued. A more dangerous situation could develop if a non-functioning neutralizer itself becomes a source of ignition. A detailed study of the safety of electrical static neutralizers [5] demonstrated that when some eliminators were switched off it was observed that, at high level of charges on the web, discharge took place between the charged surface and the metal body of the eliminator. The properly designed static neutralizers must be safe when they are working, as well as when they're off. When they fail, they must fail safely.

The European Union ATEX Directive concerning equipment and protective systems intended for use in potentially explosive atmospheres specifies that the requisite level of protection must be ensured even in the event of frequently occurring disturbances or equipment faults which normally have to be taken into account [6].

When considering whether a piece of electrical equipment is suitable for use in a flammable atmosphere the user must take into account fault conditions which could make the equipment more dangerous. These considerations lead to the fourth rule for dealing with static electricity in hazardous areas.

Rule 4:

The measures and devices employed to control static electricity should be capable of ensuring a high level of protection and should not provide sources for new hazards when they are working, as well as when they're off.

Majority of fires occur when a situation develops that is unusual in some regards, i.e. a human error, a process problem requiring quick operator's intervention, etc. For instance, a web break may create an abnormal condition that would cause the web to wrap around a roller and then to stop. Yet, the electrostatic hazard is still present. After the break, the web wrapped around the roller has to be unwrapped and removed. Unwrapping the web off the roller may generate static charges sufficient for ignition. The procedures for dealing with irregular events must take into account the static risk. This leads to the fifth important rule for dealing with static electricity in hazardous areas.

Rule 5:

The measures and devices employed to control static electricity should take into account all real-life circumstances.

Electrostatic Discharge Loss Prevention Equipment and Procedures

Managing risks of electrostatic hazards must rank very high on the list of numerous demands facing converters. Companies which use flammable chemicals for printing and coating have to manage the risk of electrostatic hazards on three levels,
· real-time,
· preventive, and
· post-accident.

The previously mentioned conventional static neutralizers can be rated with reference to the three levels of electrostatic risk management as follows:

· No real-time electrostatic risk management.
Conventional neutralizers provide no real-time risk management whatsoever because no information is available about their condition, operating status, or performance parameters.

· Arbitrary preventive electrostatic risk management.
The frequency of preventive measures with conventional neutralizers is often based on the history and/or visual observation. Routine maintenance, such as cleaning electrodes of electrical neutralizeror checking tinsel stringing, as well as performance tests or diagnostics checks to verify that the neutralizers are functioning, are performed on a periodic basis or whenever process problems occur. The reliability of the equipment in-between these checks is always questionable in the absence of the real-time diagnostics.

· No post-accident electrostatic risk management.
Conventional neutralizers provide no information for data analysis. Therefore, post-accident analysis of data is virtually impossible. Examination of the neutralizing device might be the only means of establishing clues to cause and effect.

Conventional neutralizers are inadequate products for the application in hazardous areas. No wonder that in the same informal survey, when asked if they were satisfied with their existing static control equipment and measures, 20 coating professional from 17 companies responded negatively, and only six responded positively.

Advanced static neutralizers with diagnostics and performance monitoring capability.

Advanced static neutralizer technology has recently evolved and entered the marketplace [7]. These devices address the need for the electrostatic hazard risk management at all three levels, real-time, preventive, and post-accident.

These advanced static neutralizers are designed to monitor their own system diagnostic parameters to assure proper operation. These neutralizers are also capable of measuring the ion current that flows to the charged web to neutralize it. Neutralizing current is an important performance parameter that indicates the web charge that is being neutralized. The higher the web charge the higher the neutralizing current.

A typical advanced static neutralizer with computer interface consists of a high voltage power supply with a built-in monitoring circuit, a microprocessor-based controller, and a high-efficiency static neutralizing bar (Fig. 5). From a single PC terminal the software can simultaneously monitor a system with several neutralizing stations. The software displays performance and diagnostics for each individual neutralizer in the system, alerts operators and supervisors when it is time to clean the bar, or if there is a problem. The software can turn the system on and off automatically when the machine starts or stops, and allows users to store, retrieve and analyze production run records, maintenance logs, etc. [Ion Systems, 2000].

This type of systems offer many advantages in both hazardous and non-hazardous applications, and they can be rated with reference to the three levels of electrostatic risk management as follows:

· Complete real-time information on neutralizer status and performance.
The real-time diagnostic capabilities provide information directly related to the systems operational status. Indicators such as "system on/off", "malfunction", and "low ionizing efficiency" allow operators to determine the best course of action to maintain safe operation. Quick determinations can be made to repair or replace faulty system components.

· Comprehensive information for preventive risk management.
Performance parameters, such as total ion output and neutralizing current, are measured and continuously displayed. Operators are constantly aware of the systems capability to provide safe neutralization allowing them to proactively take preventive measures to assure low static charges without slowing down or stopping the process. For instance, cleaning neutralizing bars can be performed at convenient times during regularly scheduled shutdowns prior to the point where neutralization of the web is compromised.

· Extensive historic data for post-accident analysis.
When the information with a neutralizer's performance and status data is stored in an electronic database, this information provides a basis for post-accident analysis. Historical data can be utilized to determine if the neutralizer was operating properly at the time of the accident, if its maintenance was performed when required, if repetitive problems had occurred on the process line and if sudden changes in the process resulted in the changes of the electrostatic conditions on the web. The ability to retrieve and review historical data for post-accident investigations can assist in determining if electrostatic charges on the web and the performance of the neutralizing systems contributed to the accident. Analysis of stored data can also help to achieve constant improvements within total quality assurance programs.

Figure 5. Typical advanced static neutralizer with computer interface for hazardous areas.

Conclusions

Proper risk management of electrostatic hazards is the key to minimize the probability of the costly occurrences of flash-fires, explosions, and production losses in coating and printing operations. Static electricity is an ignition source, and must be understood to prevent accidents on the process line. Successful risk management of electrostatic hazards must be done on three levels, 1) real-time, 2) preventive, and 3) post-accident.

Advanced static neutralizers and conventional devices are available for converters to manage the risk of electrostatic hazards. However, each device is not equal in providing all three necessary levels of risk management. In order to compete successfully in today's market, converters must use the most appropriate equipment for each application. When specifying static neutralizers for use in hazardous areas, one must remember that it is an investment in loss prevention that must be judged and valued as such.

In summary, a comprehensive electrostatic hazard risk management program in coating and printing include the following elements:
1. Keep record of static conditions and events.
2. Establish and follow neutralizer maintenance schedule.
3. Remove misapplied ionizers.
4. Use the best static control equipment for each application.
5. Treat neutralizer installation as an engineering project.

Literature Cited

1. Sibrava, J., "Flash Fire Sparks Flammability Reduction Plan," Converting, Part 2, pp.10-13 (Dec. 2000).

2. Haaze, H., "Electrostatic Hazards," Verlag Chemie, New York (1973).

3. National Fire Prevention Association, "NFPA 77-2000 Recommended Practice on Static Electricity 2000 Edition", Quincy, MA, pp.32-33 (2000).

4. Niklarz R. and Blitshteyn M., "Static Neutralizing Problems are not without Solutions," Paper, Film and Foil Converter, pp. 58-61 (May 1999).

5. Tolson, P., "Assessing the safety of electrically powered static eliminators for use in flammable atmospheres," Journal of Electrostatics, 11, pp. 57-69 (1981).

6. "EC Directive 94/9/EC on the approximation of the laws of the Member States concerning equipment and protective systems intended for use in potentially explosive atmospheres" (1994).

7. Ion Systems, "Virtual AC™ Intelligent Static Neutralizer for Hazardous Locations," technical literature, Windsor Locks (2000).