Static Electricity

Static electricity is an omnipresent and insidious hazard in many chemical industries.To comprehend the threat posed by static electricity, it is necessary to understand the fluids that are being handled in the process. The ability of a fluid to retain an electrostatic charge depends on its electrical conductivity, and in this connection 50 pico siemens /cm (same as pico mhos/cm or Conductivity Units – C.U.) is the magic number that process engineers should remember. Fluids which have electric conductivity below 50 pico siemens /cm are called accumulators, and those having values above 50 are non-accumulators. In non-accumulators, charges combine as fast as they are separated and hence electrostatic charge generation is not significant. 50 pico siemens /cm is the recommended minimum value of electrical conductivity for adequate removal of charge from a fluid.

Another important term is relaxation time, which is calculated as 18 divided by electrical conductivity. Thus a fluid that has an electrical conductivity of 1 pico siemens /cm has a relaxation time of 18 seconds. 4 to 5 times the relaxation time is the time required to be given to a fluid for the charges to dissipate itself. 90 seconds for the fluid in this example.

Flow of poorly conducting hydrocarbon liquids at high velocities in pipelines is an important source of generation of static charges. This is more so with pipe diameters larger than 8 inches and not so much with smaller diameter pipelines. Static charge generation in such flow systems is best controlled by limiting the velocity. The British standard BS PD CLC/TR 50404:2003 (formerly BS-5958-Part 2) Code of Practice for Control of Undesirable Static Electricity recommends that the product of the velocity (in metres per second) and the pipe diameter (in metres) be less than 0.38 for liquids with conductivities of less than 5 pico siemens/m and less than 0.5 for liquids with conductivities above 5 pico siemens/m. The presence of water in the hydrocarbon makes a big difference and in this case the velocity should be limited to 1 m/sec.

Bonding and earthing are the usual ways by which charge separation can be prevented. For fluids with electrical conductivity below 10 pico siemens/cm, bonding and earthing are not adequate for charge dissipation. Antistatic additives would be required here.

FLUID	Electrical Conductivity (C.U.)
Highly Purified Hydrocarbons	0.001
Light Distillates	0.01 - 10
Jet fuel	150 - 300
Crude Oil	1000 – 100,000

Process engineers concerned with static electricity would do well to read any or all of the following standards:

1. BS PD CLC/TR 50404:2003 Code of Practice for Control of Undesirable Static Electricity
2. NFPA 77 (2007) Recommended Practice on Static Electricity
3. API RP 2003 (1998) Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents
4. OISD 110 (1999) Recommended Practices on Static Electricity

General overviews on the subject are available on the internet at

http://www.fiberglasstankandpipe.com/handlingpetrol.htm#I

http://www.osh.govt.nz/order/catalogue/pdf/staticelectricity.pdf