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Static Electricity

INTRODUCTION


Static electricity occurs commonly in industry and in daily life. Many of the effects are harmless and either pass completely unnoticed or are simply a nuisance.  Where, however, the generation of static electricity is encountered in commercial and industrial premises in the presence of an explosive atmosphere, a serious potential fire and explosion hazard will arise. In addition, static electricity can introduce operational problems during manufacturing and handling processes, for example, by causing articles to adhere to each other, or by attracting dust.


The purpose of this Technical Bulletin is to emphasise the need for Consultants to pay attention to the hazards associated with static electricity whenever flammable or explosive atmospheres are present, and that due regard is taken of such hazards when conducting risk assessments and formulating risk control programmes.


The hazards of static electricity should be a key component of a DSEAR risk assessment and therefore reference to this document, where it exists, should be a fundamental starting point of site enquiries.

 

GENERATION OF STATIC ELECTRICITY


Some of the more common processes in which static electricity may be generated include the following:


  • A rapidly moving belt running over pulley;

  • Paper or cloth passing through a machine and over rollers or under a doctor blade;

  • Non-conducting liquids flowing through a pipe or hose, or falling through the air in drops or as a spray;

  • Agitation of non-conducting liquids in a tank or mixing vessel;

  • Movement of material in grain handling and storage plants;

  • Processing plants for feed, seed, spices, sugar, starch and similar materials;

  • Manufacture of metal powders;

  • Mixing, grinding, screening or blending operations involving solid, non-conductive materials;

  • Pulverised materials passing through chutes or pneumatic conveyors;

  • The movement of pure gases or a mixture of gases generates little, if any, static electricity. Gases can, however, carry electrostatic charges on suspended liquid or solid particulate matter – e.g. LPG.;

  • Movement of personnel.


It is important to note that some solvents, referred to as “non-polar solvents”, have low electrical conductivity properties and are particularly prone to the accumulation of static charges. These include toluene and xylene and, where encountered, special care needs to be taken. However, this does not imply that “polar solvents” such as acetone and methyl ethyl ketone, which tend to be more conductive, can be disregarded as far as static generation is concerned; care should be exercised with all flammable industrial solvents.

                                                          

RISK CONTROL


The prevention of the generation of static electricity is rarely attainable, but dangerous accumulations can be avoided by conducting the electric charges away as fast as they are produced. If static electricity is not neutralised or eliminated as rapidly as it is produced, the charge builds up and will eventually develop energy enough to jump as a spark to a nearby grounded or less highly charged object, potentially with disastrous consequences. 


The hazards of static electricity and the control measures which should be taken are contained within British Standards Published Document PD CLC/TR 60079-32-1:2015 - Explosive atmospheres. Electrostatic hazards, guidance.


The objective of most static control measures is to provide a means by which separated negative and positive charges may re-combine or flow harmlessly to earth before sparking potentials are reached. The following is a summary of the key measures commonly employed which may be used, either individually or as components of an electrostatic protection programme.


1.     Earthing and Bonding (also known as “grounding”)

Earthing and bonding is generally the single most effective means of static control. This ensures that all conducting items of the plant that may produce static electricity, including tanks, silos, pipework systems, conveyors, chutes, etc... have a continuous electrical path to earth and that all the plant is at the same equal potential. Flanged metal pipes will normally require the fitting of earthling clips to maintain electrical continuity. When considering earth bonding it is important that due regard is given to vacuum cleaning systems, both fixed and mobile, which can generate large amounts of electrostatic charge. 


In the case of mobile/portable items such as drums, intermediate bulk containers (IBC’s) and tankers, purpose designed temporary earthing and bonding devices in the form of leads and clamps should be provided for which strict written procedures are required. To ensure that effective earthing and bonding in these circumstances is achieved, it is vitally important that purpose designed clamps are employed, instead of lightweight “alligator” clips. Intrinsically Safe, self-testing earthing clamps may be specified which provide the operator with a visual indication that a satisfactory connection has been made which, if required, can be extended to provide interlock switching contacts to transfer pumps, valves, alarms or control systems.

For further guidance, reference  should be made to the NewsonGale article entitled: Mitigate the Static Ignition Risks of Drum Filling.


As with all critical plant safety systems, earthing and bonding provisions should be the subject of routine inspection and planned preventative maintenance.


Earthing and Bonding...


A typical earth bonding arrangement for dispensing highly flammable liquids


A purpose designed, monitored earthing clamp


 

2.     Static Elimination

A range of static eliminators are manufactured all of which have the purpose of neutralising the charge on non-conducting materials such as paper, cloth and rubber, by the ionisation of the atmosphere near the charged surface. As a result, the charge is conducted to ground through the ionised air, or the static charge on the object attracts sufficient oppositely charged ions from the air, from which it becomes neutralised.


In its simplest form, the static eliminator consists of a passive, static comb comprising of a metal bar with sharp teeth or brushes which is connected to earth. As an example, static eliminators are commonly employed on high speed gravure printing presses where they are installed across and in very close proximity to the web path, immediately downstream of rollers, doctor knives and other potential sources of static generation. Radioactive and electrically energised static eliminators are also available. It is important to recognise that the latter, as with any other form of electrical equipment provided for the measurement or mitigation of the hazards of static, must be suitable for the Zone in which it is employed (Technical Bulletin 23 refers).


Static Elimination


A passive static eliminator


An EX rated energised static eliminator


An EX rated static meter


3.     Humidification

Artificial humidification is an effective means of preventing the accumulation of static charges on some non-conducting materials such as paper and fabrics. The humidity produces a minute film of moisture over the material which increases its conductivity, to the point where a suitable path to ground is established for any charge that develops. A relative humidity of between 60-70% will normally be required for this purpose. In practical terms, this method of static control has limited application.

 

4.    Dissipative Materials and Additives

Some non-conducting materials such as rubbers and plastics are available in grades which are electrostatic dissipative and, as such, reduce the risk of static accumulation. The use of dissipative rubber conveyor and transmission belts are typical examples.


Similarly, charge accumulation in flammable liquids may be able to be reduced by the use of commercially available static dissipative additives.


With powders there is no equivalent to the dissipative additives used for reducing the resistivity of liquids.


5.    Plant Design and Process Operations

In many cases it is possible to reduce the generation of static electricity by giving attention to the design of the plant and the processes conducted. Examples of such measures can include:


  • Location of the source of static generation in an explosive free/safe atmosphere;

  • Since static generation involving liquids increases with the rate of flow, piped velocities should be kept to a specified level having regard to the properties of the liquid and the processes involved;

  • Splash filling of a flammable liquid should be avoided by employing bottom entry or by using a fill pipe terminating close to the bottom of the tank;

  • Mechanical mixing or agitation of liquids to be kept to a minimum;

  • Limiting the running speeds of conveyor systems.

 

6.    Fixed Protection/Inerting Systems

Despite employing the appropriate controls, the residual risk from static electricity and, for that matter other hazards, may still be such that additional methods of protection are required. This might include the employment of gas inerting of the vapour space within flammable liquid tanks and production vessels, the installation of explosion suppression systems on powder silos and dust handling plant, or other measures as considered appropriate.


7.    Static Electricity on Persons

Persons insulated from earth can easily acquire and retain an electrostatic charge, normally caused by the fact that the floor covering, or the soles of their footwear are made of non-conductive materials. If an electrostatically charged person touches a conducting objective, a spark can occur capable of igniting gases, vapours and some sensitive dusts. To combat this risk, floors should be conductive or dissipative, and operatives provided with electrically conductive or dissipative footwear according to the level of risk.


In many factory environments bare concrete floors and steel grids are likely to be sufficiently conducting, although this should not be taken for granted. In order to remain effective, the floor should not be covered by rubber matting or other insulating materials and kept free of contaminants such as resins and other non-conductive substances.


In addition, anti-static clothing may be warranted in high risk situations. Clothing, irrespective of the type, should not be unfastened or removed in explosive atmospheres.


Electric shocks due to static electricity rarely cause a person direct harm, but can give rise to accidents due to shock reaction such as falling from ladders, involuntary recoil and dropping or releasing loads, and should not be disregarded.


8.    Plastic Containers

The ever increasing use of plastics materials for tanks, containers and pipework systems can pose particular problems as far as static electricity is concerned and requires special consideration.


Plastic intermediate bulk containers (IBC’s) employed for the conveying of flammable liquids should be of the type manufactured with conducting surfaces which are surrounded by a conductive enclosure in the form of a metal grid or cage, whereby the risks of a hazardous static discharge are considerably reduced. In these circumstances, the conductive and dissipative items of the IBC should be bonded together and earthed.


A plastic IBC developed for use in  Zones 1 & 2 of multi-layer design with an external layer of an antistatic compound with uninterrupted connection to ground via discharge valve, cage and pallet. Such containers will normally be labelled.


Smaller plastic drums and other containers are more likely to be entirely non-conductive and, in these situations, a careful assessment of the potential static risk will be required, taking into account such features as the nature of the zone in which the operations are conducted, container capacities, flow velocities and the characteristics of the liquids/vapours involved, including the minimum ignition energy. Conductive plastic drums are available of a multi-layer design with an external layer of an antistatic compound similar to that employed in the IBC’s referred to above and according to a leading manufacturer should be identified by markings on the body of the drum. These are capable of being earthed in the same manner as metal drums.


PD CLC/TR 60079-32-1:2015 states that containers up to a nominal capacity of 5 litres made of non-conductive material may be employed in a Zone 1 area, subject to restrictions on flow velocity and other conditions.


Containers of larger capacity should only be used in Zone 1 if, following specialist guidance, it can be shown that the electrostatic ignition risk is acceptable, striking a balance between the risk posed by static and the benefits of using a non-conductive container.


Flexible intermediate bulk containers (FIBC’s – often referred as jumbo bags) have become commonplace in many manufacturing industries where large quantities of powdered, granular or palletised materials are handled. These bags which have traditionally been manufactured from woven polypropylene have the potential to generate high levels of static electricity when filling and discharging. In response, FIBC’s are now available as a so-called “Type D” antistatic container, in which the polypropylene has been chemically treated to modify its electrostatic properties, in addition to incorporating antistatic or partially conductive threads in the weave, thereby significantly reducing the electrostatic risk.


Smaller so called “antistatic” or “static dissipative” plastic sacks or liners are available; identification of these may not be easy and in some cases reference to the supplier’s product data will be required.


Dealing with the general use of non-conductive plastic liners, it should be recognised that they can on occasions create, by their own introduction, an electrostatic hazard, such as when used to line a metal container.


When considering the potential hazards posed by powders and bagging/handling operations, it is important not to lose sight of the atmosphere in which the activity is conducted. For example, an isolated single bagging or off-loading station might be such that a dust explosive atmosphere is not created, and the process can be disregarded. However, the same operation which might form part of an overall process operation in a flammable vapour atmosphere would be viewed entirely differently. In this context it should also be recognised that there is usually a higher risk of ignition when handling powders in the presence of flammable gases or vapours, owing to the fact that the minimum ignition energy (MIE) of most gases and vapours is much lower than those of dust clouds.

 

SURVEY EXPECTATIONS


It is recognised that static electricity is a complex subject on which this Technical Bulletin is intended to provide a general overview of the potential hazards involved and the various risk mitigation measures commonly employed.


Apart from Consultants with specialist chemical plant skills and the ability to fully appraise the electrostatic risk, it is the expectation that Consultants should be able to recognise industrial plant and processes involving explosive atmospheres where static electricity is likely to be generated, together with the degree of risk awareness displayed by the policyholder/building occupants and the protections that are in place from overall observation and enquiry. In the event of serious doubt, a risk improvement calling for an electrostatic safety audit by a specialist company is to be raised.

 

Firms providing such services include:


 

ADDITIONAL INFORMATION


Information relating to static electricity which is freely available on the web, includes the following:


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