As a process safety engineer/specialist, from time to time, I get questions from colleagues at work or ex-colleagues to help or asking my opinion on a topic/design or a regulation issue.
During studying at the university, I learned the habit of looking for information, gathering and keeping them for later use. In fact, my experience is that a good engineer (irrelevant to her/his field of study) is the one who can look for information, where to find them, read it and could summarize it.
When I was studying, there was no internet and having reference books and going to library to search for data and information was normal. These days there are many internet sites that offer information. So, bookmarking will be a good idea.
Recently, I was asked about electrostatic electricity hazard and connection to earth system to avoid electrostatic discharge and hazards associated with it.
Electrostatic electricity hazard
The principal hazard of electrostatic electricity is a spark discharge which can ignite a flammable mixture. Electrostatic electricity accumulates in liquids as they move in a pipeline. They accumulate proportional to their electric conductivity constant. The less conductive a liquid is, the less they can discharge the electrical charge gathered to a ground system.
In other words, the more conductive a liquid, the more they generate electrostatic charges and because they are good conductive, the faster they release the electrostatic charge to the ground and less hazard can arise.
Most of the time, the static electricity generated in a liquid will instantaneously release to the ground because it is a good conductor liquid.
The high refined substances like kerosene or jet fuels, they are good accumulator of static electricity (least conductivity), and it means that they can catch fire if they generate a mist upon entering a tank or leaking from a flange because the accumulated charge will discharge through a connection to the ground.
This highlights the fact that the less conductive a liquid is, the greatest danger they bring.
Exploring the Hazards of Electrostatic Electricity and the Importance of Conductivity
The primary risk associated with electrostatic electricity is the potential for spark discharges, which can ignite flammable mixtures. As liquids flow through pipelines, they accumulate electrostatic charges in proportion to their electric conductivity. Less conductive liquids have a lower ability to discharge the accumulated electrical charge to a ground system.
In contrast, highly conductive liquids generate more electrostatic charges and can rapidly release them to the ground, resulting in reduced hazards. Generally, static electricity in liquids tends to promptly discharge to the ground due to their good conductivity.
However, highly refined substances like kerosene or jet fuels, which have low conductivity, serve as significant accumulators of static electricity. If these liquids form a mist when entering a tank or leaking from a flange, the accumulated charge can discharge through a connection to the ground, posing a fire hazard.
Understanding Conductivity and its Role in Electrostatic Safety
Conductivity refers to the capacity of a liquid to generate, conduct, and discharge static electricity to the ground. It is a physical characteristic inherent to the liquid. Conductivity is typically measured in Pico siemens per meter (pS/m).
According to a rule of thumb, if the conductivity of a liquid surpasses 50 pS/m (with a resistivity lower than 2x10E12 Ohm-cm), the accumulation of electrostatic charge is considered insignificant. Similarly, when equipment or containers are properly earthed or grounded, the impact of electrostatic charge becomes negligible as well.
This emphasizes that liquids with lower conductivity pose a greater danger in terms of electrostatic hazards.
Guidelines for Equipment Earthing and Bonding in Relation to Conductive Materials
To ensure safety when dealing with conductive materials like kerosene, it is important to establish certain guidelines, particularly concerning the velocity of the liquid flowing through pipes. The following list provides minimum examples of equipment within the industry that should be earthed or bonded. However, it is essential to prioritize local/national regulations or client standards, which take precedence:
- Conductive tanks should be earthed, with a maximum earth resistance of 10 Ω.
- External and internal floating roofs of tanks should be earthed.
- Pumps, filter housings, and other relevant equipment should be earthed.
- Above-ground pipes must be conductive or dissipative, ensuring electrical continuity and proper earthing.
- When plastic pipes rely on inner linings for conductivity, the design and installation should ensure reliable electrical connections across joints.
- Below-ground pipes may be insulative, but short sections should protrude above the ground to facilitate connections to conductive pipes. All conductive fittings must be properly earthed, and caution should be exercised to prevent isolation of sections of conductive pipe or hose connected to insulative pipes.
- Road tankers and rail cars during loading or unloading should be bonded to the loading/unloading structure.
- Rail tracks in the loading/unloading area should be isolated from the main track and bonded to the loading/unloading structure.
- Loading arms, hoses, and probes should be bonded to the loading structure.
- All loading/unloading structures and facilities should be earthed.
- Ship and barge loading/unloading gantries should be earthed, while incorporating an insulative flange in the pipework or a single section of insulated hose in each hose string to isolate the ships and protect against galvanic currents.
It is common practice to adhere to the model code of safe practice EI(IP)21 when implementing these guidelines.
Start within your project to see what materials you are handling or using within your facility. Investigate and gather the conductivity data of these materials, decide if they are conductive or not. You may consult the MSDS and if you could not find it on MSDS, contact the vendor.
It is always a good idea to discuss with the client and see what codes and standards they follow on the issue of earthing and electrostatic electricity. Make a list of equipment that needs earthing and clearly document it with fluid handled and their conductivity.
For more information, help on electrostatic electricity or even a tailor-made training for you or your organization please don’t hesitate to contact us!