• Publication Date: 05/01/2020
  • Author(s):
    Grandaw, David
  • Organization(s):
    IEP Technologies
  • Article Type: Technical Articles
  • Subjects: Safety, Static electricity

David Grandaw | IEP Technologies

When making changes to powder and bulk solids manufacturing processes, there are many factors to consider, aside from the actual change itself. The effects of any change can go beyond the process and impact a facility’s existing explosion prevention and protection systems. This article explains the importance of complete process-change reviews, explains factors to consider, and provides some real-world examples of the potential consequences of incomplete reviews.

In today’s fast-paced, global business environment, the need for manufacturers to adapt and change to take advantage of new market opportunities and technology innovations is ever-present. For chemical, pharmaceutical, food, and other bulk solids processors, introducing new products, altering formulas, and modifying or upgrading a plant for increased production are all typical factors that might require changes to existing processes.

Before implementing any of these types of changes, however, a multidisciplinary, complete, start-to-finish review into all aspects of the change and the expected or potential consequences should occur. A critical focus in this type of review should always be a detailed look at any safety-related change implications. Often termed a hazard and operability study (HAZOP), these types of analyses take into account any impacts the proposed changes would have on the plant, the process, and the people working in the environment. The review must examine the likelihood of an explosion and how any unintended consequences might intensify and compromise the facility’s existing explosion prevention and protection (EPP) systems and their ability to safeguard the plant’s integrity. The process review in facilities that handle combustible materials such as loose solids, powder (dust), gases, and vapors is especially vital. In facilities that deal with materials that generate combustible dusts, a more specific review called a dust hazards analysis (DHA) is required by NFPA standards and other agencies. DHAs will be discussed later in this article.

When processing changes occur

Put simply, if correctly specified, a plant’s existing EPP system will have been designed to cope with the specific set of materials being processed, the operating conditions, and the plant’s equipment design, geometry, and layout. Alterations to any of these factors can greatly change the explosive characteristics of a process and render the current EPP system incapable of responding and protecting as intended.

The following examples highlight some real-world consequences of not considering the entire process when making these types of changes.

  • A bulk solids manufacturer handling a vitamin additive switched from conductive to nonconductive filter bags inside the dust collector, which led to electrostatic charge accumulation on the filters. These charges caused deflagrations in the dust collector over three consecutive days.
  • A chemical company changed the material being processed, switching to a material with more than triple the explosivity (KSt index) of the original material. This increased KSt meant that the facility’s original EPP system was now undersized for the explosion threat. Since the system was no longer suitable to mitigate a deflagration of this magnitude, structural damage to multiple vessels occurred during an explosion event.
  • A wood processing facility added an opening in the process line to create additional airflow. The opening adversely affected the explosion pressure detection function of the EPP system, enabling the explosion that did occur to propagate without the system initiating the explosion isolation system.

Process changes and explosion risk

These are just a few examples of the potential explosion-related consequences faced when implementing process changes. The following section discusses how changes in processed material, operating conditions, protected equipment, and interconnections can affect an existing explosion protection strategy within a process. This isn’t an exhaustive list, of course. Process-specific conditions and many other factors also could impact the effectiveness of an EPP system at a facility. Adequately evaluating these factors may require specialized knowledge and involve close dialogue with the EPP system supplier or working with a qualified third party in the explosion protection field.

Changes in material being processed. EPP systems are designed to operate based on specific factors such as: a material’s explosivity characteristics (KSt and Pmax), which respectively refer to a material’s rate of explosion pressure development during a dust explosion and the maximum achievable pressure generated during a dust explosion event; autoignition temperature (AIT), which is a material’s kindling point in a normal atmosphere without an external ignition; and minimum ignition energy (MIE), which is a material’s range from no ignition to the lowest energy value at which material ignition occurs.

Any material change, whether it’s in chemical composition, particle size, or moisture content, can have an adverse effect on the explosion control measures originally designed into an EPP system. The consequences of handling a material with a higher KSt than for what the system was originally designed, for instance, could mean an uncontrolled explosion if an event were to happen.

Additionally, material changes can bring with them different bulk resistivity and charge relaxation characteristics, which can lead to an increased risk of electrostatic charge accumulation. This accumulation could, in turn, result in an uncontrolled static discharge with enough ignition energy to cause an explosion. Even a change in specific material suppliers warrants a review to determine if the material characteristics differ from what the original EPP system was designed to accommodate.

Changes to process operating conditions. Any modification to the operating airflow, temperature, or pressure can also impact explosion protection measures. For example, explosion isolation barriers often are determined based on a combination of factors including airflow. Increased airflow velocity may mean that an isolation barrier is too close to the protected vessel to prevent flame propagation under an explosion condition. Increased operating temperature may mean that instead of a standard temperature detector, a high-temperature explosion detector is necessary. Modifications to operating pressure could mean that changes to explosion detection styles or settings are warranted or that explosion vents have an increased risk of either being pulled in or prematurely failing.

Changes to protected equipment. Modification to or even replacement of an existing protected vessel can have an impact on the protection measures originally designed in a system. For instance, if a mill is replaced with another mill that provides a finer grind than the original unit, the result is likely to be an end product with a higher KSt and lower AIT — along with the added possibility that the existing EPP system is no longer suitable for the process. New penetrations into a protected vessel, such as newly installed inspection doors for example, must be evaluated to ensure that they can withstand, at minimum, the worst-case pressure (known as the Pred) that might develop in an enclosure during a vented or suppressed explosion. The addition of mechanical shock, vibration, or sonic devices to minimize material bridging inside the vessel also need to be discussed with the EPP system provider to determine what testing, if any, should be conducted to help ensure that these devices won’t inadvertently activate the system.

Changes to interconnections. Often, a plant may need to add a duct, pipe, or chute to a protected vessel for production reasons. These additions are potential flame propagation pathways that must be considered for explosion isolation. The mechanical integrity of these ducts also must be considered, as they must be at least capable of handling the Pred from the deflagration protection. The addition of flexible ducting should always be reviewed to ensure that adequate grounding or bonding is in place to prevent a static charge from accumulating on isolated conductive parts, which could lead to potential static discharge ignition.

Grounding is the safest way to discharge built up static charge. Grounding, also called earthing, connects the process equipment to the earth via a grounding rod or electrode stuck in the ground. As the electrons are produced, they transfer between the equipment and the earth, which drains the static charge. Bonding connects two or more pieces of conductive equipment using wire, cable, or connectors to equalize the equipment’s static charge so that sparks can’t occur. The need for grounding and bonding must always be considered when adding an interconnection to a vessel handling a combustible dust or vapor.

Rules and regulations

One example of how the change process is covered in terms of industry standards, regulations, and safety guidelines can be seen in NFPA 652: Standard on the Fundamentals of Combustible Dust. This standard is aimed at facilities that handle materials that generate combustible dust and requires that before these facilities implement a change, they must conduct an assessment, or DHA, to review the safety implications of the proposed change. Typically, the EPP system supplier can be consulted to determine if the proposed changes will impact the efficacy of the system as currently designed and installed. Even temporary changes must be considered for the potential impact on the safety systems installed.

In general terms, a manufacturing process change may be viewed in just the same way as any business change management project. The need to review the process as a whole is pertinent, and there are a wide range of change-management models and toolkits used by consulting firms to assist companies in finding successful outcomes.

Any proposed process change or modification, however minor it may seem, necessitates a thorough review of the existing EPP measures. The review will help to ensure the new process characteristics don’t render the existing system incapable of providing the required assurance level of safe operation and working conditions.

PBE


For further reading

Find more information on this topic in articles listed under “Safety” and “Static electricity” in the article archive.


David Grandaw (331-212-5003) is vice president of sales for IEP Technologies. He has a degree in fire protection engineering technology and has been with the company for 34 years. He sits on two NFPA technical committees, including as an alternate on NFPA 652.

IEP Technologies • Marlborough, MA
855-793-8407 • www.ieptechnologies.com

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