Jun-2022

Understanding the value of arc flash management and mitigation for large electric process heaters

While larger medium voltage process heaters are relatively new in hydrocarbon processing, the safety issues surrounding medium-voltage equipment are well known.

Michael Jones
WATLOW

Article Summary

Arc flash events are a prime example of a rare but potentially deadly situation that can be mitigated with the right technology. As process heaters become a popular alternative for larger applications that have historically utilised gas-fired heaters, it is important to look at the ways in which they incorporate arc flash mitigation features.

When medium- or high-voltage equipment is used for industrial applications, it carries with it certain risks. Arc flash is an example of a well-known electrical risk that requires certain best practices to be followed to minimise damage and injury.

When a new piece of medium-voltage equipment is developed, special attention should be given to such risks. This is certainly the case for large electric process heaters, which are becoming more popular in hydrocarbon processing applications.

The Risk Posed by Arc Flash
Arc flashes typically occur when there is a short circuit in a system (due to, for example, a dropped tool, build-up of corrosion or conductive dust or the presence of pests). If the voltage is high enough, and if there is a path to ground or a lower voltage, the resistance of the air is overcome and results in an arc.

Arc flash events can result in significant damage. As the energy release increases, it can cause fire and injury. If the energy release is high enough molten conductor metal and high-pressure plasma energy can escape from the cabinet, posing a risk to personnel.

The potential arc flash energy is determined by several factors: Equipment voltage, available current and the duration of the event. As the equipment voltage increases, the potential for arc flash hazards also increases. Low voltage systems operating below 400 volts do not have the energy to cause a significant arc flash hazard. Equipment operating at voltages between 400 volts and 600 volts can cause arc flash hazards, but their capacity to cause massive energy release is limited. Equipment operating at medium voltage (above 600 volts) has a high capacity for energy release. The available current is determined by the feed equipment upstream.

While it may be practical in some cases to reduce the potential arc flash energy while limiting voltage or current, overall project cost can make this difficult. This makes reducing the duration of the event the most practical approach.

Reducing the duration of the arc event also reduces the area of damage. The area of impact is not limited to the explosive blasts. The arc flash produces intense heat and light energy. People in the vicinity of the blast can experience injury from these effects even if they are not exposed directly to the explosion. Equipment surfaces can absorb the energy and experience high temperatures from the radiant energy, causing damage and requiring repair.

Although arc flash incidents are relatively rare, their potential for damage, injury and death makes them a huge concern. Some estimates put the incidence of arc flash events roughly between five to 10 per day worldwide, or 3,500 per year. According to a 2021 article in the journal Safety, there were over 2,000 recorded hospitalisations in the U.S. due to electrical injuries, including arc flashes. Of these, 1,900 were non-fatal and 166 were fatal1. Although the article did not qualify fatalities in terms of whether they were due to arc flashes or other events, it is clear that arc flashes are on the more lethal end of the spectrum.

Are there special risks with regard to arc flashes when it comes to process heating? Yes and no. On the one hand, medium voltage process heaters are relatively new to many applications. Concerns about decarbonisation, automation and safety have driven many to replace gas-fired heaters with larger electrical heaters (> one megawatt)2; these heaters thus represent a potential new source of risk, and manufacturers of these heaters must be aware of that risk.

On top of that, many applications use arrays of large megawatt heaters, connected to multiple panels. This drives the need for a solution that can isolate the specific panel(s) where arc flash occurs, isolating faults from the main breaker/feeder where possible.

On the other hand, arc flash events are well understood, as are the strategies for reducing their effects. An understanding of these strategies should be “built in” to any technology for process heating.

Reducing the Effects of an Arc Flash Event
Generally speaking, there are three main strategies for reducing the effects of any potential arc flashes that could occur with a piece of equipment:
1. Increasing the distance from the potential source of an event
2. Reducing the available fault current
3. Decreasing the duration of the event (i.e., how long the arc itself exists)

Ideally, all could be used in combination to ensure maximum safety, but this is often not practical when considering overall project cost. That said, the duration of the event is the most practical influencer to reduce and has the largest impact on the total amount of energy released. Reducing the overall fault clearing time is often the best, most direct way to reduce arc flash hazards.

This point can be illustrated through a comparison between two current approaches to limiting the damage wrought by arc flashes: Arc-resistant cabinets and arc mitigation technologies. Mitigation technologies make more sense economically and from a safety perspective, and thus should be incorporated into designs for and around medium voltage process heaters and similar equipment.