Static electricity in explosive atmospheres
Every object on earth has a certain electrical current. Electrically conductive objects (such as most metals) easily build up an electrical charge and discharge it just as easily. The opposite applies for non-conductive objects (most plastics).
As soon as two charged objects come into contact with each other, the charge on the two objects is equalised. This can cause a brief release of energy known as static discharge.
A static charge can build up on non-conductive materials or on insulated materials. Because a static charge can ignite an explosive atmosphere under the right conditions, static electricity must be taken into account in the explosion protection risk assessment.
The risk assessment evaluates whether a static charge can both exceed the product’s minimum ignition energy and discharge fast enough. If that is the case, it is a potential ignition source and countermeasures must be taken.
|Figure 1: An electrostatic discharge|
STATIC ELECTRICITY AS IGNITION SOURCE
Static charges can build up in various ways. For instance, static charge can easily build up through:
- friction of product in a pneumatic transport system;
- contact between product and the silo wall during filling or emptying of the silo;
- friction between a V-belt and a pulley in a conveyor belt system.
Built up static electricity can be discharged in several different ways. How the static electricity is discharged is largely dependent on the environment, the materials, the operation, the product, the shape of the conductor, and the process characteristics. Common forms of electrostatic discharge include:
|Puntontlading:||a sharp, conductive object builds up a charge and ionises the surrounding air. This causes discharges with a low energy density (+/- 0.03 mJ)|
|Brush discharge:||a spherical object builds up a charge and ionises the surrounding air. This causes discharges with a low energy density (+/- 3 mJ)|
|Propagating brush discharge:||a brush discharge which continuously repeats itself. Can be caused by things such as the friction of a conveyor belt. This causes discharges with a high energy density (+/- 1,000 mJ)|
|Cone discharge:||a charge which builds up on a silo cone. This causes discharges with a high energy density (up to 10,000 mJ)|
|Arc discharge:||a discharge between two conductive objects. Because both objects are conductors, the charge flows across very quickly. Energy densities can easily reach over 10,000 mJ.|
A product’s minimum ignition energy depends on the particle size, humidity, and environmental conditions. The ignition energy is usually at around 300 mJ, but there are exceptions with a lower value. If you relate the above values to your product’s minimum ignition energy, you will have an indication of whether static discharge can ignite your product.
If the explosion protection document indicates that static discharge can ignite the explosive atmosphere, immediate countermeasures must be taken. The most effective measure is connecting conductive system components (potential equalisation), preventing potential differences from building up. The system components are also grounded.
It is said that a ground resistance value of 1M Ω is sufficiently low in most cases. However, 1M Ω is actually very high! In a good ground connection, the ground resistance value is often no higher than 10 Ω. By adding monitoring points between various system components and the ground point, you can use periodic measurements to identify “trends” and intervene if necessary if a connection is at risk of getting worse.
A sleeve or flexible hose can build up its own charge and insulate conductive system components, and therefore also requires additional measures such as the use of a version with a lower resistance, or connecting system components. The same applies to internal system components such as sensors, valves, screen decks, filter frames, and filter media.
|Figure 2: potential equalisation of a filter frame|
WORK CLOTHING AND PERSONAL PROTECTIVE EQUIPMENT
Employees can also produce and hold a static charge through things like friction between work clothing and the body. The static electricity is discharged as soon as the employee comes into contact with a conductive object, such as a door handle or a machine frame. Precautions must therefore be taken if an employee or third party can come into contact with an explosive dust atmosphere.
For example, they must be provided with anti-static footwear. All safety shoes rated type S1 (or higher) have anti-static properties. There is a lot of ambiguity (and sometimes even disagreement) on providing anti-static work clothing. From an explosion protection perspective, it is not usually necessary to provide anti-static work clothing. This is evident from the technical standard NPR-IEC/TS 60079-32-1 (see table 1). However, the choice is entirely up to the employer. An ignition sources risk assessment can aid in the decision.
Static electricity is an intangible idea for many people. One of our customers has experienced an explosion caused by an insulated level sensor. An electrical technician had removed the level sensor from the system for inspection, and used the wrong sealing ring when replacing it. The static charge building up on the sensor’s measurement tube was unable to dissipate, resulting in an explosion which caused extensive damage. Fortunately, nobody was injured and we can learn from such incidents.
As employer, you are expected to evaluate the risks in your company and implement a policy which reduces the risks to an acceptable level. For example, operators must be educated and instructed, work instructions must be created, inspections and maintenance must be planned, supervisors must be appointed, a work permit system must be established, the correct work equipment must be purchased and, if necessary, measures must be taken to prevent the buildup of static charges.
Written by Frank de Jager, Senior Consultant at D&F Consulting B.V. Frank is an ATEX expert at D&F’s Business Unit Process Safety.