STATIC ELECTRICITY
PART I

Static Electricity (static for short) has been blamed for many fires and explosions, sometimes correctly. Sometimes, however, the investigator has failed to find any other source of ignition. So he assumes that it must have been static even though he is unable to show precisely how a static charge could have been formed and discharged.

Whenever two dissimilar materials come in contact, a transfer of electrons from one material to the other takes place. As a result, one material acquires as excess of negative charges and the other material an equal excess of positive charges.

If the two materials are new separated, each will possess an excess of charge, and electrostatic charging has taken place. If both materials are conductors (e.g. 2 metals), the high conductivity allows an equalization of charges to take place along the contact surface during the separation, so that no measurable excess charge is present on complete separation. Therefore, measurable charges will only be created if one of the materials involved is a non-conductor. The human body may be regarded as a conductor in such processes. All solids having a surface resistance (measured according to DIN 53'482) higher than 1011 ohms are regarded as non-conductive.

Many static charges flow rapidly to earth as soon as they are formed. But if a charge is formed on a nonconductor or on a conductor, which is not grounded, they can remain for some time. If the level of the charge, the voltage, is high enough the static will discharge by means of a spark, which can ignite any flammable vapors, which may be present. Examples of nonconductors are plastics and non-conducting liquids such as most pure hydrocarbons. Most liquids containing oxygen atoms in the molecule are good conductors.


When a liquid with low conductivity (more precisely: with a conductivity below 10~8 ohm-1.m-1) flows through the pipe, only the diffuse accumulation of ions will move along with the liquid. As a consequence, the liquid accumulating in the receiving container will be charged. In the case of a conductive liquid, a balance of charges will be reached while the liquid is still within the pipe.

Whenever particles of dusts or fluids move in a gas phase, the generation of charges must be expected regardless of their conductivity.

A special type of charge separation takes place when a liquid (especially a polar one) is atomized (Lenard effect), e.g. in the case of water, negative charge will dominate among the smallest droplets, while the large droplets will mostly carry a positive charge.

Depending on the geometrical situation, on the materials involved and on certain other conditions, five different types of discharges are distinguished :

  • Spark discharge
  • Brush discharge
  • Corona discharge
  • Lichtenberg discharge
  • Lightning - like discharge

Characterizations of the different types of discharge are illustrated by sketches of the model experiments used to demonstrate them.

The Spark Discharge

> Discharge in a more or less homogenous fjeld.
> Discharge between two conductors.
> Luminous phenomenon bridges the entire gap between the conductors.
> Discharge occurs in a sudden burst.
> Typical radius of curvature of the conductors: >5 cm.

The Brush discharge :
> Discharge in the non-homogenous field.
> Discharge between a conductor and a charged conductive or non-conductive surface or a charged cloud.
> Luminous phenomenon is emitted from the conductor at the location of the highest field strength and ends in the gap.
> Several discharges in rapid succession give the overall impression of a "brush1.
> Typical radius of curvature: ca. 0.5 cm.

The Corona discharge :
> Discharge in the highly non-homogenous field.
> Discharge between a conductive point and a charged conductive or non-conductive surface or a charged cloud,
> Luminous phenomenon in the shape of a "crown" (corona) only immediately around the conductive point.
> Continuous discharge.
> Typical radius of curvature: <1 mm.

sharp point
The Lichtenberq discharge:
A further form of discharge became known as the Lichtenberg discharge. It can be observed along the surface of very highly charged non-conductive surfaces and is accompanied by luminous effects.

The lightning - like discharge:
It has never been proved beyond doubt that this type of a discharge can occur in industrial operations. It is thought that - similar to the phenomenon observed in nature - a lightning -like discharge could occur e.g. between a highly charged dust cloud in a large silo and the wall of the silo, or may be even between two dust clouds of different polarity created by sedimentation in a high silo. According to the present state of knowledge, this type of discharge cannot occur in vessels smaller than 60 (m)3, or in silos of any height when the diameter is below 3 meters.


THE HAZARD CREATED BY AN ELECTROSTATIC DISCHARGE
An electrostatic discharge is capable of causing ignition when the energy released is greater than the minimum ignition energy of a combustible mixture present at the time. For a gas/air or vapor/air mixture to be flammable, the ratio of the concentration of the flammable material to the concentration of air (oxygen) must lie between the lower and upper explosion limits. The vapor mixture present above a liquid is only flammable if the liquid temperature exceeds the flash point. However, attention must be paid to the fact that, due to their small heat capacity, the temperature of liquid droplets in aerosols can rise very rapidly above the flash point. Although the explosion limits are known for gas mixtures, the corresponding data for dusts are largely unknown. The lower explosion limit for dusts lies normally in the range of 20 - 50 g/m3, though for highly ignition-sensitive dusts it may be around 5 g/m3. The upper limits are difficult to determine.

Hybrid mixtures (i.e. mixtures of combustible dust, flammable gas or vapour and air) may be flammable even if the concentration of both the dust and the vapour or gas are below the lower explosion limit valid for the individual component. The minimum ignition energy of a hybrid mixture lies somewhere between the minimum ignition energy of the dust and that of the gas or vapour. But for the time being, no further generally valid statements can be made.

In general, the probability of the simultaneous occurrence of an explosive atmosphere and a discharge with release of sufficient energy to cause ignition, is rather low. Therefore it is not uncommon that operations to which electrostatic charges are hazardous go on for years without difficulties before, all of a sudden, an accident occurs.


THE PROBABILITY OF IGNITION CAUSED BY ELECTROSTATIC DISCHARGES

Minimum
-
ignition energy in air
0.01-0.025
0.025-10
0.025 -10
>10*
 
mJ
 
Combustible substances
CS2,H2, C2H2
Hydro Carbons with
double
and
triple bonds
Saturated hydrocar bons and
derivative
s
Hybrid mixtures
Dusts
Types of Discharges
Spark
+
+
+
+
Lichtenberg
+
+
+
+
Brush
.
+
-
Corona
(+)
-
-
-

+ Ignition to be expected         - No ignition to be expected        • Some dusts have lower values

PROTECTIVE MEASURES
From the fundamentals explained on above, five principles for the application of protective measures are derived. In industrial operations, these principles have to be applied selectively, and commensurate to the prevailing circumstances. The most common applications are described in the practical section.

Principle: Earth all conductors
A large number of hazardous caused by electrostatic charging can be eliminated by careful earthing of all conductors

> Earth all pieces of equipment.
> Interconnect building structures with conductors.
> Earth isolated metal parts, e.g. flanges and metal sheeting on insulation material on plastic pipelines, cyclones etc.
> Place metal drums on conductive floors, or connect to earth.

Principle: Earth the personnel
The human body, which is a conductor with regard to static electricity, can carry charges around.

> Install conductive floor coverings.
> Wear conductive footwear.

Principle: Prevent and reduce charging by using conductive materials

> If possible, use aqueous solutions, alcohols, polar solvents.
> If possible, avoid highly insulating plastic containers and bags.
> Use conductive drive belts.
> Use conductive pipes.
> Use conductive filter materials for dust filters if flammable vapours may be present.

Principle: Keep Velocities low

> Limit the flow velocity of liquids and of gases containing dust and liquid particles.
> Avoid turbulence in chargeable liquids.

Principle: Avoid Explosive mixtures and aerosols

> Remove gases, vapours and dusts by suction at the source.
> Work with flammable liquids at temperatures at least 5 deg.C below their flash point (if combustible dusts or powders are present simultaneously, the phenomena explained must be considered).
> Charge flammable liquids to vessels by suction, and discharge (blow) by nitrogen pressure.
> Work with flammable liquids under inert gas.


It should be noted that working at concentrations above the upper explosive limit cannot be regarded as a protective measure, since the hazardous range between lower and upper limits must be passed through.

Measurement of various Electrostatic parameters in the Plant:
It is difficult to elaborate in details about the measurement of various electrostatic parameters in the plant in this paper. However some data is presented here as ready reference.

The various parameters like Voltage, Field Strength, Discharges, Resistances (general), Earthing resistance and Leakage Resistance (discharge resistance) are to be studied to take the right measures to avoid the hazard due to static electricity.

Surface resistance, specific resistance or conductivity, charging current/ magnitude of charge, lowest minimum ignition energy for flammable materials, chargeability of dusts and relaxation behavior of charged powder are studied and measured in laboratory.

The following examples are intended to indicate the order of magnitude of values found in practice.

  Person, walking on rubber soles Up to 1 kv
Person, walking with rubber soles on a carpet Up to 20 kv
Transmission belt, speed 3 to 15 m/sec Up to 80 kv
Surface of a light fuel in a large container that has been filled rapidly_ UptolOOkv
Flange on a glass line when Toluene flows through line Up to 15 kv
Flange of a steam ejector Up to 15 kv
Roll of paper, unwinding at a speed exceeding 10 m/s Upto150kv

B. CAPACITY (against earth)

  Single screws (bolts) Ca. 1 pF
Flange, 100 mm nominal size Ca. 12
Isolated cyclone (height = 1m, @ = 1 m) 30-50
Person   100-200
Road Tanker Ca. 1'000

c) LEAKAGE RESITANCE (discharge resistance)

Conductive footwear (10)4-(10)8ohms
Insulating footwear From(10)Bto(10)15
Human skin, dry (10)4 .
Human skin, wet Some 10
PVC floor without conductive additives (10)11
Concrete floor, dry (10)b
Wooden floor, untreated, dry (10)'
Flange on glass line (10)ii-(10)1'3

d) SPECIFIC RESISTANCE OF CERTAIN SOLIDS

Metals (10)-5to(1078ohms
Wood, dry Ca. 5 (10) 9
Powders Uptoca. (10)1J
Window glass Ca. (10) "
Pyrex glass Ca. 6. (10) "
Plexiglas Ca. 2. (10)12
PVC Ca. 2. (10)13
Teflon Ca. 2. (10) 1d
Makrofol (insulating foil) (10) 15

e) SPECIFIC CHARGE OF DUSTS AFTER PNEUMATIC TRANSFER
"Non conductive" liquids
In this category we find : aliphatic and aromatic hydrocarbons, ethers, esters of higher acids, C1 - derivates

Liquids Specific resistance
Carbon disulphide (10) 1b ohms
Carbon tetrachloride (10) 15
Diesel oils, petrol and white spirits cyclohexane, benzene, Toluene, Xylene, Mesitylene, Diethyl Ether (10) 13
1, 4- dioxane (10)12
Anisole ( 10) 11
Stearic acid dibutyl ester (10) 1U
Sebacid acid dibutyl ester bromobenzene, chlorobenzene dichloromethane chloroform propionic acid (10)8

"Conductive" Liquids
Examples of this category: hydrocarbons with "polar" groups like alcohols, aldehydes, ketones, acids, esters, nitrites, amides, amines, nitro compounds.

Liquids Specific resistance
1 ,2-dichloroethane benzoic acid ethyl ester (10)7 ohms
Methanol, ethanol n-propaol, n-butanol acetic acid ethyl ester cis-1 , 2-dichloroethylene (10)6
Acetic acid pyridine acetonitrite, propionitrite, benzonitrite acetone, butanone, cyclohexanone isobutanol (10)5
Isopropanol, t-butanol formic acid ethyl ester anhydrous acetic acid propionaldehyde nitrobenzene (10)4
Glycol, glycolmonoethyl ether dimethyl formamide acetaldehyde (10)3
Formic acid (10)2

f) FIELD STRENGTH

Dust cloud (at periphery) Ca. 400 kv/m
Dielectric strength of air Ca. S'OOO
Dielectric strength of oil Ca. 10'000

g) LOWEST MINIMUM IGNITION ENERGY IN AIR

Solvent vapours/ Gases Carbon disulphide 0.009 mJ
Hydrogen 0.011
Acetylene 0.017
Ethylene 0.07
Methanol 0.14
Diethyl ether 0.19
Cyclohexane 0.22
n-Heptane 0.24
i-Propyl alcohol 0.65
Acetone 1.15
i-Octane 1.35
Ethyl acetate 1.42
Dusts Red phosphorus 0.2
Sulfur (fine) Ca. 1
Sulfur 15
"normal" dusts >10

h) SENSITIVITY OF HUMANS AGAINST DISCHARGES OF STATIC ELECTRICITY

0.002 J Perceptible
0.010 J Distinctly perceptible
0.25 J Severe shock
10J Possibly lethal

i) RELAXATION TIMES FOR STATIC DISCHARGE OF SOME COMMON SOLVENTS

Solvent Relaxation Time Half-Time Value
Toluene 21 s 15s
Hexane 46 h 42 min 32 h 26 min
Heptane 0.18s 0.13s
Methanol 9x(10)-6s 6x(10)-6s
Ethanol 1.6x(10)-4s 1.1 x(10)-4s
Water 1.4x(10)-6s 1 x(10)-6s

The relaxation time, or amount of time needed to discharge static charge, varies widely as shown above. Heptane is preferred over hexane due to heptane's shorter relaxation time.

STATIC ELECTRICITY
PART II

ORIGIN OF ELECTROSTATIC CHARGES IN PLANT


1. A non conducting solvent flows out of a metal pipe(separating effect).liquid is charged, excess charges on the metal pipe flow down to earth.

2. Powder pours out of a plastic bag (separating effect), and also the dust cloud. Charges drain away rather slowly

3. Transfer of non conductive liquid or powder in glass or plastic pipe (separating effect in wall zone).Pipe and contents are charged, also the flanges. Charges drain away slowly.


4. Plastic or paper film is unwounded(separating effect). The film and the roll are charged. Charges drain away slowly.


5. Liquid is atomized through a metal(separating effect when droplets are formed).The droplets of liquid and the nozzle are charged. The excess charge on the metal nozzle disputes via earth connection.


6. Stirring a non-conducting liquid in a vessel (separating effect on the wall). The liquid, the stirrer & the kettle are charged. The charges on the vessel& on the agitator flow down to earth.


7. A man walking with insulating sloes on a nylon carpet or plastic floor (separating effect between soles and floor). The man is charged. The charges cannot readily flow away because of the insulating soles.



8. Power flows through a plastic pipe. Powder and pipe are charged (separating effect between dust particles & pipe). The charges on the pipe will cause a separation of charges on the nearby metal housing, resulting in an accumulation of charges there. This process is called influence.

Non-homogenous liquids.
During crystallization process in non-conducting liquids, local zone with extremely high charges may be formed. Therefore, crystallization process in flammable, on-conductive liquids should always be kept under inert gas.


Improving the electrical conductivity of flammable liquids by means of additives
The specific resistance of white sprit of 2*1011 ohrn.m can be reduced to 2,52*109 ohm.m by adding 0.5% methanol. The specific resistance of the petrol is in the range of 1013 ohm.m. It can be reduced to approximately 108 ohm.m. by adding 1 or 2 grams of an antistatic additive per m3 (e.g. Teepol 530,Lissapol N, AerosolOT, ASA-3).


Practical measures
Keep velocities low. For only practically filled pipes or pipes which discharges into containers, the velocity is to be limited as follows (irrespective of the pipe material)
• For chargeable esters: max. 10 m/s
• For mineral oil products (e.g. gasoline, petrol, kerosene, paraffin, jet fuel)& for other
chargeable liquids. (Excluding carbon disulphide and ether), depending on line size.

Nominal
pipe diameter (mm)
<40
50
80
100
200
400
600
Velocity (m/s)
Max.7.0
6.0
3.6
3.0
1.8
1.3
1.0
Quantity (Its/mi n)
£600
800
1100
1600
3500
10000
17000

Rule of thumb for all homogenous liquids (except ether & carbon disulphide) & all pipelines: at velocities below 1m/s no dangerous charges will be generated.


Rule for ail type of flexible tubes or houses
The electrical resistance between the two couplings must not be higher than 106ohm. This resistance is to be measured at regular intervals.


Details on earthing
Earthing wires laid loosely around a non-conductive line for bonding & earthing of flanges will not create a hazard. ing of flanges!

An earthing wire should not be drawn through the interior of pipe. If the wire breaks, sparking is possible.
INCORRECT *


Charging or emptying of drums & small containers by gravity
Liquids with a specific resistance > 106ohm.m may become charged. A dangerous potential builds up on the surface of the liquid. When a conductive object (funnel, stirrer, hose) is approached, inductive brush discharges can occur

Measures
Take the filling pipe & tube or funnel right down to the bottom of the vessel, to avoid flashing, whirling up, spraying.
Conductive containers, funnels, nozzles, must be bonded & earthed.


Earth operating personnel by means of conductive shoes & conductive floor.




P: insulating plastic material              M: Metal


Transfer of flammable liquids by means of drum pumps.

Hazard :

Turbulence in the pump & flow in the connecting pipe lead to charging of the liquid and of the pipe or tube material. Sparking is possible between metal parts e.g. between the drum pump & the drum, especially when the pump is going in or out.

Measures:
Connect the drum pump with a conductor both to the container being emptied & to the one being filled ,& earth it. The connections should not be clamped directly into the bung-hole or manhole, since combustible vapors are most likely to be present there.


The two balancing conductors are best fixed permanently to the drum pump so that the necessity for bonding & earthing is immediately apparent whenever the pump is used.



Operations at the open manhole of a reaction kettle containing a flammable liquid. Measures:
Avoid manipulations at the open manhole, use devices that permit sampling, pH & level checks on the closed vessel

If manipulations at the open manhole are unavoidable, use sample beakers(or at least rod of sample beaker) & measuring rods made of non-conductive material; do not use devices made of metal, measuring rods made of wood are acceptable, on-conductive measuring devices must not be wiped immediately before opening the manhole.

Charging of large glass vessels(Measuhnq vessels, charge tanks, separators, etc.)

Hazard:
• Charging of the liquid and of the vessel due to flow and whirling up. Discharge of sparks possible between conductive parts, e.g.
between metal flange and operating personnel.

Measures:
• Earthing of metal flanges larger than 50mm
• Inerting of glass vessel or
• Charging with low velocities or
• Charging by means of closed vacuum

Additional measures if possible:
- Take filling pipes right down the bottom of the vessel. If PTFE pipes (polytetrofluor ethylene, Teflon) are to be used, they should be of conductive quality and earthed.

vacuum

Loading & unloading of road tankers; filling the fuel tank of vehicle, filling plastic cans (chargeable liquids)

• Use conductive hose material.
• Wear conductive shoes.
• The material drain cock is earthed via petrol hose & must be firmly rest on the filler cap while the vehicle is being filled up, so that the latter is also earthed.
• Filling up plastic cans: Use only small plastic cans, when filling containers made of non-conductive material, charges accumulated in the liquid can drain off only very slowly. However on containers having a volume of up to 5 litres, no dangerous
charges will accumulate.



Handling of powders with flammable qasses/vapors
For the handling of powders in the presence of gasses or vapors, e.g. for
• Charging powders into flammable solvents
• Handling of solids being moist with solvents

The following applies:
• Due to the movement of the material during charging, handling, separation processes are taking place continuously, and he powder as well as the container (plastic bag) and pieces of equipment like pipelines, chutes, funnels, charge ducts will be charged; electrostatic charges already present in the powder from handling can even amplify this effect. Thereby discharges can occur. Without earthing, even
spark discharges can occur. Without earthing, even sparks jump between metal parts. Brush discharges occur where a plastic bag is shaken out, when a non conductor is pulled back from the funnel in the manhole.
• e.g. a charged surface area on a drum is touched with the hand. Since, during material charging operations, such discharges of static electricity will always occur near the filling part of the apparatus where explosive vapor/air mixtures are most likely to the present, such operations are particularly hazardous.
• Note: This type of handling in a chemical works is one of the most frequent causes of accidents, which are traced back to electrostatic charges!
• However, experience accumulated so far does not indicate that incendiary discharges can occur if conductive and earthed containers, funnels, charge ducts etc. are used exclusively.

Measures:
- Closed handling of the product, under an inert atmosphere.
As a matter of principle, inerting (i.e. primary explosion protection) is recommended since it is the most effective and most reliable measure. Bhut this requires closed handling so that an inert atmosphere can be maintained. Thereby it must be borne in mind that especially in powders of low bulk weight, considerable quantities of air and thus oxygen may be entertained.
- Closed charging of solids into an inerted apparatus (two possible alternatives)

 


for solids free from flammable solvents -all containers or packing materials permissible including' plastic btgs
for solids containing flammable solvents' only conductive, grounded containers without plastic bags or liners



For solvent free powders: any type of drums or charge container is permitted.

With this type of handling, solvent free powder may be poured even out of non-conductive containers or out of plastic bag into the charging de-

For powders containing flammable solvents: only conductive, earthed containers are permitted.
Powders containing solvents must not be poured out of non-conductive plastic containers or out of plastic bags into the charging device.

"Open" charging of powders into flammable liquids or "open" charging of powders containing flammable liquid into an empty apparatus, when and inert atmosphere cannot be maintained(see hybrid mixtures page....)

Hazard:
Decisive for the hazard situation and consequently for the type of measures to be taken are the following criteria:

  • The flash point of the flammable liquid is pre-charged to the apparatus or being present in the powder.
  • The working temperature, i.e. the temperature of the flammable liquid during the charging operation or the highest temperature it can reach during that operation
  • The combustibility and the minimum ignition energy of the solid being charged.

CASE-A

  • The powder is either not combustible or its minimum ignition energy (measured for the fine fraction of the powder) lies in the range abouve 50mj*, and (*This limit was arbitrarily decided for the time being to verify it, extensive investigations are being carried out)

  • The working temperature
  • Is, with certainty, at least 5 deg. C below the flash point (closed up) of the flammable liquid (normal scatter of flash point determinations is +/- 5 deg. C); in addition, possible quality variations of the liquid have to be considered, they could cause flash point fluctuations of several deg C.

Measures for Case-A

  • The choice of materials of construction for the containers, funnels chutes, etc. is not restricted, plastic bags are also permitted

  • Any parts made of conductive materials must be bonded and earthed.

CASE-B

  • The powder to be charged is combustible and its minimum igintion energy (measured for the fine fraction of the powder) is in the range <= 50 mj or unknown.
  • AND/OR
  • Adherence to the flash point/temperatuere condition stated for the case A cannot be guaranteed.

Measures for Case-B

  • Drum or container, funnel, chute, etc. must be of conductive material* and must be earthed.
    ("Conductive" plastic is acceptable only if the conductivity has been verified by measurement.)

The floor must be conductive for static electricity & operation personnel's must wear conductive shoes.

  • The use of containers (Whether conductive or not) with plastic liners or of containers, packing, etc. with non-conductive coating is forbidden.


All external ignition sources (viz. Open flames, sparks from grinding or percussion) must be excluded.
Shovels used for charging must either be of wood and have a wooden handle, or be of
metal and earthed.
The charging of bulk quantities must be done in lots of not more than 50 kgs, with an
interval of approximately 1/2 minute between charges.
Open charging in the presence of carbon disulphide, hydrogen, and acetylene is
forbidden.
This procedure makes great demands on the reliability of the operating personnel and places upon the supervision and the manager a particularly high degree of responsibility.


SPECIAL PROBLEMS PERTINENT TO CERTAIN TYPES OF EQUIPMENT

Entrainment of electrostatic charges

Hazard:

- When highly charged product is filled into a conductive container, the latter will also be charged if it is not earthed.
When an earthed object is approached - this may be an operator wearing conductive shoes, a charge can jump. If the operator handling such a conductor wears non-conductive shoes, sparking is still possible when he touches an earthed conductive object (e.g. earthed pipeline).




MEASURES:
Use transport containers with conductive wheels.
- Wear conductive shoes.
- Keep wheels and shoes clean, check conductivity periodically by measurement

PNEUMATIC TRANSFER Hazards:
Intensive charging of the conveyed material as well of the pipeline due to the separation processes constantly occurring at high speeds between the conveyed material and the pipeline.

Sparks between conductive parts, e.g. between metal flanges and a part of the steel structure of the building. Entrainment of considerable charges into receiving Containers (silos).
Measures:
Wherever possible have pipeline used for pneumatic transfer made of metal # throughout and ensure good earthing.
# (Plastic material is acceptable only when sufficient conductivity has been verified by measurement.)
No insulating coatings on the inner surfaces of metal containers and metal pipelines.
For plastic transfer lines, plastic flanges should used. If metal flanges are used, they must be earthed. Individual bolts on plastic flanges need not be earthed.

On pipelines constructed of insulating material, no coating or sheathing (e.g. for reinforcement) of conductive material should be applies, since this will lead to an increased accumulation of charges on the inside; berg discharges may occur.
If possible, pneumatic transfer lines should not be directly connected to containers of silos; instead, the powder should be separated first in a cyclone of conductive material (charges will drain off, velocities will be reduced).


SPRAY DRYING, FLUID BED DRYING
Hazard:
High charging of the substances (powders and possibly granulates) owing to the high speed separation of the particles in very dry atmosphere both in the drier itself and in the dust separator (cyclone) or filter. Charging of parts of apparatus that come into contact with the powder, spark discharges with ignition, especially if flammable dust and flammable vapors or gases are present simultaneously. Flammable gases may be ignited with 1 /100 to 1 /10 of the energy required to ignite dusts.
Mixtures of combustible dusts with flammable gasses or vapors in air may be explosive even when the concentration of the individual components is below their respective lower explosion limits.
Measures:
All metal parts of the casing of the apparatus must be. bonded and earthed; internal metal installation such as sieves, prop rings, spray nozzles, filter support baskets, receiving containers and transfer troughs must be earthed. Individual bolts and nuts need not be earthed.

If conductive filters are used (necessary only if combustible dust and flammable vapours or gases can be resent simultaneously) the filter cloth must contain the metal fibres (> 5 % in homogenous distribution) which are spun into the individual threads of the fabric. Filter cloth with separate leads or wires is dangerous if such a lead or wire breaks, a spark gap is created.
Inerting of dries. If, for inerting purposes, combustion gases are used instead of nitrogen, their composition (oxygen concentration, concentration of flammable components) must be monitored and control by suitable instruments and safeguards.

MOVING PARTS ON MACHINES AND APPARATUS
Drive belts, conveyor belts, rollers, cylinders, etc.; foils cloth.
Hazard:
- The continuous separation of two touching surface generates electrostatic charges depending on the material breadth and speed of the rotating parts, which can lead to discharges with sparks.

Measures:

Use drive belts, belt pulleys, conveyor belts, cylinders, bearing pulleys, etc. made of conductive material.

Notes:
On stirred reaction vessels, kneaders, driers, etc. as a rule the shafts and drive pulleys need not be earthed since they are sufficiently connected to earth via bearings and stuffing boxes. Lubricating agents with admixtures of graphite or molybdenum help to improve the conductance. In doubtful cases check the moving parts for earthing (measure resistance to earth).

Charges on paper, textiles or plastic webs are best conducted away by means of ionized air, by applying points, metal reeds, conductor brushes, metal wedges, or by greatly humidifying the air locally.

It is most unlikely that explosive mixtures will be ignited when electrostatic charges are drained off by means of earthed points or reeds.





grounded conductive wedge                    discharge brushes



MAN AS A CARRIER OF ELECTROSTATIC CHARGES Hazard:
Charging of the human body; discharge with sparks when earthed object is approached (e.g. reaction kettle metal pipeline).
Measures:
Wear conductive shoes. (Resistance through soles < 108 ohms Correct: Safety shoes with conductive soles; shoes with leather soles.
Wrong:
Plastic shoes, crepe-rubber soles. Have conductivity of footwear measured periodically.

Note:
When a piece of clothing made of synthetic fibre is removed, high charges and sparks may be caused (separation effect). Therefore, clothes must not be changed when an explosive atmosphere is present.

Hints for planning & construction
Floors Hazard:
On a non-conducting floor persons can become charged by walking, owing to the constant separation effect between their shoes & the floor. A discharge with sparks is possible on approaching an earthed object.

Measures:
Keep floors made of asphalt or plastic material damp or wet.

In special case, a floor can be made sufficiently conductive by washing it with aqueous glycerine (1%glycerine in water). However, such treatment will have to be repeated every few weeks.

When a floor is to be made of epoxy resin, the antistatic araldite floor material (long lasting antistatic effect) with araldite BY 157 & hardener HY2987 or 355 should be used. *

Humidity of the air
The conductivity of the air is scarcely improved by being greatly humidified. Static charges cannot be sufficiently dissipated via moist air. Increasing the humidity of the air, as such, is therefore not a recognized protective measure. However ,the surface resistance of solid insulating materials can be reduced by if the relative humidity of the air is above 65%,because the surface of such materials always has a certain layer of impurities on which a conductive film of liquid will be formed the humidity of the air has a great influence on the chargeability of substances such as textiles, paper etc. The humidification is best applied locally & specifically, e.g. by spraying or moistening (e.g. steam) the paper or fabric webs. General humidification of rooms above 65% is expensive, especially in ventilated rooms, and can also cause the atmosphere to become intolerable for the personnel.

Suggested values for Surface & Resistance to earth
For the prevention or dissipation of charges, the following should be adhered to:
The earthing resistance of individual conducting parts should be less than 106ohms(smaller parts up to 108ohms).
The surface resistance of insulating materials should be less than 1011 ohms.
The resistance of drive belts(ohms) multiplied by their supporting width(m) should be less than 105ohms Practical rules.

• Iron grids of the building structure count as earthed
• Metal pipes which are clamped to the building structure count as earthed.
• Completely(also on the outside)enamelled valves & flanges have to be bridged by an earthing conductor & must be earthed
• Apparatus such as agitator vessels, receivers, measuring vessels, box filters etc. made of metal which are firmly connected to metal pipes as earthed.
• Covers of apparatus (agitator vessels etc.) which are firmly screwed to the lower part, even if there is an insulating gasket (e.g. Teflon ring) in between, count as earthed. In the case of enamelled vessels, it is advisable to check by measurements.
• Agitators, thermometer tubes, kneading arms inside apparatus require no special

measures.
• Metal storage tanks with fixed metal pipes count as earthed. Use conductive material for hoses.
• Drive belts (flat belts & V-belts) must be conductive. In cases of doubt have them checked.
• Transmission has the earthing resistance checked.
• In metal silos & pipes for products in powder from do not apply an insulating coating inside.
Glass pipes: Plastic flanges for small glass pipes, despite the somewhat higher purchase price compared with metal flanges, work out cheaper since they do not have to be earthed. Metal flanges, valves, flaps throttles etc. having a nominal diameter above 50 mm (pipeline above 2") are to be earthed.
• Apparatus made of glass and porcelain (stills, stirrer vessels, steam ejectors): Metal flanges and metal parts are to be earthed. Often they are already earthed via the metal supports.
• Plastic apparatus and pipes: Metal parts, i.e. flanges with a nominal diameter above 50 mm (pipeline larger than 2"), valves, flaps, supporting shells, protective pipes etc. are to be earthed. Individual metal bolts need not be earthed since their capacity is too small to generate a spark capable of being an ignition source.
• Conductive outer coating or casings on plastic pipes or apparatus increase the accumulation of charges on the opposite side of the plastic wall and are thee fore to be omitted.

Notes:
Temporary cables, which are used only occasionally for earthing movable apparatus can be accommodated, to save space, with the help of cable rollers that wind up automatically.

The paint on metal pipelines can have an insulating effect, if the individual parts are completely painted prior to assembly.
Approved conductive paints are available.

SPECIFICATIONS FOR FIXED EARTH CONNECTIONS
In order to prevent the earthing in the plant to become ineffective by damage, the conductor must have an adequate cross-section. The wire must be adequately fastened or supported and where necessary, inserted locally in a protective tube.

Recommendation for earthing conductors:

Copper wire or braid, cross section 1, 5 to 2, 5 mm2, colour of insulation as specified in local rules or regulations.
- Bare copper, diameter not less than 4 mm.
Bridge connectors across flanges should be made of flat copper, 10X1 mm and firmly screwed to one flange so that they will not get lost during inspections.
Connecting terminals on flanges, valves etc. have to be in perfect contact with the metallic object that is to be earthed:
On enameled parts, if necessary, the enamel is to be ground off a small area, or a hole is to be drill to the metal body.
On painted or powder-coated parts, fan-type lock should be used on bolts and nuts for installation the part.
For large pipelines (e.g. ventilation lines) the use of bare copper rod as earthing conductor is recommended.
Metal flanges on glass line: It is not advisable to connect a large number of flanges for earthing purposes. If the earthing conductor is broken or torn off, the total capacity of the flanges lined up in parallel) can assume dangerous values. For each pair of flanges it sufficient to earth one flange, provided that
The flanges are painted with conductive paint or
- Special washers with points penetrating through paint are used.

______________________________________________________________________________________________________________