HorizonPSI Newsletter

January 2016 - Static Electricity Can Lead to Plant Explosions

January 2016 - Static Electricity Can Lead to Plant Explosions

Do you know how plant explosions occur? Besides injury and death, understanding how at risk your plant is for an explosion is important because it can lead to loss in production time causing your company money.

One of the main causes of a plant explosion is static electricity. This is particularly important this time of year, as the months from October to February are your greatest chance for explosion risk due to lower humidity, which means an increase in static electricity.

Static Electricity: The charge that builds on the surface of a material or object when it comes in contact with or rubs against another object.

It’s critical that you understand the importance of static electricity especially when it comes to your plant because static discharge can be a significant dust/vapor ignition hazard, and all equipment has the potential to generate a static charge.

All objects carry some charge relative to another object, and sometimes this can be as much as several (hundred) thousand volts. The current from a discharge is typically low unless a very large charge builds on a larger object. That is why lightning is a very large static discharge.

A static charge happens every time an object comes in contact with and moves against another object, thus generating a charge. One object will develop a negative charge, and the other will develop a positive charge. However, when objects separate they will still carry some of that charge with them. This can become dangerous because when a highly charged object gets close to a lower charged object, then the charge is transferred. This is known as a “ static discharge”. Dangers from static discharge include the following:

  • Ignition of flammable gases, vapors or dust clouds
  • Injury of operators
  • Damage to electrical equipment
  • Solid State Components (PLC’s, Level Probes, Scales, etc.)
  • Damage to mechanical equipment
  • Pinholes in coating
  • Bearing pitting

A charge relaxation happens over time when a charged object spreads its charge evenly over the object if it is slightly conductive. The object will also slowly give up its charge to the media (air, liquid, etc.) that it’s surrounded by. The relaxation rate is affected by how conductive an object is, its mass and other factors like humidity levels in the air.

Material Definitions

There are several types of discharges. The first is “ sparks between conductors”. This is the most dangerous and energetic type. This can happen when an ungrounded charged conductor gets near another ungrounded or grounded conductor. It becomes highly concentrated causing an almost complete equalization of charge between the conductors. Examples of sparks between conductors include:

  • Flange to flange around a gasket or silicone
  • Ungrounded wire in a flex hose near a metal convey line
  • Ungrounded cage to bag cup in a dust collector

The second type of discharge is a “ brush discharge,” which is between conductors and insulators. It occurs when a conductor gets near a charged non-conductive object. This type is lower energy than sparks between conductors. Examples of brush discharge include:

  • Plastic sheet or film moving in a production line coming near equipment
  • Emptying material from a poly container
  • Filling a conductive bin with non-conductive powder

The third type of discharge is “ propagating brush discharge,” which is on the surface of an insulator backed by a conductor. This combination acts as a capacitor, and it can be high energy. Examples of propagating brush discharge include:

  • Discharge on painted surface of metal objects
  • Liners inside metal drums
  • Conductive powders inside bulk bags
  • Surface of filter bags on ungrounded metal cages
  • Surface of cartridges with ungrounded internal metal parts
  • Unbonded flex hose spiral that doesn’t brush discharge at end of spiral

The last type of discharge is “ bulking brush/cone discharge,” which occurs on the surface of insulating powders while filling vessels. This means that the charge moves around the top of the pile and to the vessel walls. It can also be high energy. Examples of bulking brush/cone discharge include:

  • Filling silos, bins and containers with powders

There are powder hazard guidelines that should be followed to ensure safety and prevent static discharge or possible explosions. Materials that have a Minimum Ignition Energy (MIE) less than 30 mJ are susceptible to static discharge, which in turn causes flash fires and explosions. Examples of these materials would be starches or metal powders. Metal powders are highly hazardous since they are conductive and generally have a low MIE. (Figure 1)

Discharge Energies/MIE Ranges

Discharge Energies/MIE Ranges
Figure 1

Source: NFPA 77 Figure 5.5.1 Approximate Energies of Types of Discharges Compared with Minimum Ignition Energies (MIEs) of Typical Combustible Materials. (Adapted from H.L. Walmsley, “Avoidance of Electrostatic Hazards in the Petroleum Industry,” p. 26.)

As we mentioned earlier, understanding static electricity when it comes to your plant is crucial because all equipment has the potential to generate a static charge. This is why proper bonding and grounding of equipment is so important in order to prevent static discharges, which helps protect your equipment, facility and personnel.

Bonding ties objects together so that they have the same electrical potential. Objects with the same potential will not build a charge in relationship to each other, thus decreasing the chance of danger or explosion. Grounding ties an object(s) to an earth ground so that all charge is bled off to the earth. Both are necessary to prevent the accumulation and discharge of static electricity. (Figure 2)

In fact, bonding and grounding are so important that the National Fire Protection Agency (NFPA) is requiring ALL process equipment to be bonded and grounded per NFPA 77 – “the recommended practice offering guidance on identifying, evaluating and controlling static electric hazards for purposes of preventing fires and explosions.”

Bonding and Grounding
Bonding and Grounding
Figure 2

Source: NFPA 77 Figure 7.4.1 Bonding and Grounding p. 18

There are several areas of concern when it comes to properly bonding and grounding process equipment that need to be addressed and should be checked in order to prevent a static charge in equipment. You can read the document in its entirety here.

Besides bonding and grounding, you can also control static hazards by modifying your process. Slow down when it comes to filling and emptying processes. Add antistatic materials to the materials being handled or try increasing the humidity in your process. Another way to control static hazards is by using static neutralizer devices on plastic/paper web handling as well as on some dry handling processes.

During the winter months, it is especially important to think about static electricity, and the hazards it presents as the chances for an explosion are greatest during this period due to lower humidity. We hope you now have a better understanding of what static electricity entails by learning more about charges, discharges and how to properly bond and ground process equipment by focusing on the areas of concern we mentioned.

Horizon Systems provides end-to-end expertise that protects your products, employees and brand. If you have further questions on static electricity or plant safety, please contact our Technical Director, Todd Baker at (785) 856-8110 or toddbaker@horizonsystemsinc.com.

Source: NFPA 77 Recommended Practice on Static Electricity Document 2014 Edition