Step 1 of 3•3 minutes read
Conductivity is the measure of the ease at which an electric charge or heat can pass through a material. Materials and liquid products that are handled by tankers and terminals are classified as being non-conductive, semi-conductive, or conductive. Nowadays, in most electrostatic standards, the term ‘dissipative’ is now preferred to ‘semi-conductive’.
The basic unit of measurement of conductivity is the mho or siemens. Conductivity onboard vessels in relation to static electricity are measured in picoSiemens per meter (pS/m). The higher the pS/m of a material, the more conductive it is. The instrument used to measure conductivity is called an electrical conductivity meter.
These materials have such low conductivities that once they have received a charge they retain it for a very long period. Non-conductors can prevent the loss of charge from conductors by acting as insulators. Charged non-conductors are of concern because they can generate incendive brush discharges to nearby earthed conductors and because they can transfer a charge to, or induce a charge on, neighbouring insulated conductors that may then give rise to sparks.
Liquids are considered to be non-conductors when they have conductivities less than 50 pS/m (pico Siemens/metre). Such liquids are often referred to as static accumulators. Petroleum products, such as clean oils (distillates), frequently fall into this category with a conductivity typically below 10 pS/m. Chemical solvents and highly refined fuels can have conductivities of less than 1 pS/m. Reference should be made to a product’s (M)SDS to ascertain its conductivity. The solid non-conductors include plastics, such as polypropylene, PVC, nylon, and many types of rubber. They can become more conductive if their surfaces are contaminated with dirt or moisture.
Precautions are to be taken when loading static accumulator oils which will be explained in another lesson.
The liquids in this intermediate category have conductivities exceeding 50 pS/m and, along with conductive liquids, are often known as static non-accumulators. Examples of semi-conductive liquids are black oils (containing residual materials) and crude oils, which typically have conductivities in the range of 10,000-100,000 pS/m. The solids in this intermediate category generally include such materials as wood, cork, sisal, and naturally occurring organic substances. They owe their conductivity to their ready absorption of water and they become more conductive as their surfaces are contaminated by moisture and dirt.
However, when new or thoroughly cleaned and dried, their conductivities can be sufficiently low to bring them into the non-conductive range. If materials in the intermediate conductivity group are not insulated from the earth, their conductivities are high enough to prevent the accumulation of an electrostatic charge. However, their conductivities are normally low enough to inhibit the production of energetic sparks. For materials with intermediate conductivities, the risk of electrostatic discharge is small, particularly if safe practices are adhered to. The chance of electrostatic discharges from such materials being incendive is even smaller. However, caution should still be exercised when dealing with intermediate conductors because their conductivities are dependent upon many factors and their actual conductivity is not known.
In the case of solids, these are metals and, in the case of liquids, the whole range of aqueous solutions, including seawater. The human body, consisting of about 60% water, is effectively a liquid conductor. Many alcohols are conductive liquids.
The important property of conductors is that they are incapable of holding a charge unless insulated, but also that, if they are insulated, charged and an opportunity for an electrical discharge occurs, all the charge available is almost instantaneously released into the potentially incendive discharge.
Below is a table showing the typical conductivity of liquids carried onboard ships in tanks categorized into non-conductive, semi-conductive, and conductive.