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Earthing

This Technical Measures Document refers to codes and standards applicable to earthing of plant.

Related Technical Measures Documents include:

The relevant Level 2 Criteria are 5.2.1.11(64)f and 5.2.1.13.

General principles

Earthing can classified in two ways:

System earthing is essential to the proper operation of the system, whereas equipment earthing concerns the safety of personnel and plant. A key function of equipment earthing is to provide a controlled method to prevent the build up of static electricity, thus reducing the risk of electrical discharge in potentially hazardous environments. Generally, a resistance to earth of less than 106 W will ensure safe dissipation of static electricity in all situations.

Flammable liquids transfer

The major hazard involved with the transfer of flammable liquids is the build up of static due to charge separation with potential for discharge resulting in fire and subsequent loss of containment. Certain non-polar liquids can be charged, e.g. while flowing through pipelines. Detectable and hazardous charges must be expected if the specific resistance of the liquid exceeds 108 W.m.

The potential for accumulation of static charges may strongly increase if the liquid contains a non-miscible component or a suspended solid. Examples include:

With the presence of a second phase, velocities less than 1 m/s should be employed.

Measures that can be employed to reduce these hazards include:

Nominal pipe diameter, mm £40 50 80 100 200 400 600
Velocity, m/s 7.0 6.0 3.6 3.0 1.8 1.3 1.0
Quantity, l/min £600 800 1100 1600 3500 10000 17000

If these velocities are adhered to, no hazardous charges will be generated within homogenous liquids. But when suspensions of crystals in non-conductive liquids are conveyed, hazardous charges may always be generated, even at velocities below 1 m/s.

Powder transfer

Further information can be found in the Technical Measures Document on Explosion Relief.

Powder transfer can be carried out by several different methods:

There are two distinct types of pneumatic conveying used for powder transfer, namely low pressure / dilute phase or high pressure / dense phase. Low pressure / dilute phase systems tend to employ high system velocities ranging from 10 to 25 m/s, whereas high pressure / dense phase systems tend to employ low system velocities ranging from 0.25 to 2.5 m/s.

Intensive charging of the conveyed material and pipeline is possible during pneumatic powder transfer potentially resulting in:

Powders can be divided into three groups depending upon the volume resistivity of the material of which the particles are composed. These groups are:

Measures that reduce these hazards include:

Offloading

Stringent precautions are required to prevent accumulations of static electricity and to give protection against lightning. Standard copper strip (25 mm x 3 mm section or equivalent) is usually employed for the main earthing system. This should be connected to at least one copper-earthing rod that has been tested and shown to have a total resistance to earth of <10 ohms.

The operator should employ a bulk loading and offloading procedure. This should include written instruction that state when offloading flammable liquids, the driver must first connect the tanker to the earthing connection at the off-loading point. The electrically conducting discharge hose can then be connected to the liquid intake point on the storage. The electrical resistance between the two couplings on a flexible hose must not be higher than 106 ohms.

Temporary storage

Before temporary storage is brought on line for storage of flammable liquids or explosible powders, an assessment of earthing provision with associated earth testing should be undertaken. This should encompass the storage vessel and all supporting ancillary equipment.

Flexible pipelines

When flexible hoses are employed, measures that can be adopted include:

Status of guidance

Existing codes of practice provide comprehensive information with respect to best practice for earthing of plant and equipment.

Codes of Practice relating to earthing

Further reading material

Ebadat, Dr. Vahid, 'Reduce Electrostatic Hazards', Chilworth Technology, Chemical Engineering, 104, 7, p86, July 1997.

Ebadat, Dr. Vahid, 'Electrostatic Hazards in the CPI', Chilworth Technology, Chemical Engineering, 103, 7, p141, July 1997.

Case studies illustrating the importance of earthing

2010-03-11