Flame Arresters: The Last Line of Defense

January
,
2018

Flame arresters are designed to prevent catastrophe. How do they work, and which type is right for your application?

The unassuming process-safety devices installed in flare lines, on flammable-liquid storage tanks, in vapor-control systems, and on processing vessels at your plant are called flame arresters. They may even be hiding in plain sight every day on your commute. At your local gas station, for example, the small threaded components at the ends of the underground gas tank-venting lines are likely end-of-line atmospheric deflagration flame arresters rated for National Fire Protection Agency (NFPA) Group D vapors.

Flame arresters comprise a diverse array of safety devices, ranging from small, end-of-line units with threaded connections to massive, custom-built flanged behemoths weighing several tons. They are available with different element designs, housing shapes, and materials of construction to handle various deflagration and detonation explosions, vapor (or explosion) groups, installation positions, operating pressures and temperatures, and allowable burn times. The sheer magnitude of options and configurations can be daunting to the novice user or rookie engineer. However, all flame arresters have the same purpose: to protect people and property from the impact of a safety event that is already in progress, and to serve as the last line of defense against a tragic accident.

Regular maintenance, including frequent inspections, cleaning, and occasional part replacement, is critical to ensure the long-term functionality of flame arresters. Despite their rugged appearance, the guts of flame arresters can be quite fragile. Maintenance staff must be especially careful to avoid damaging internal elements. A dropped wrench or hammer can irrevocably damage a flame arrester.

While many engineers are exposed to the basics of relief valve sizing, combustion, explosions, and other process safety fundamentals, flame arrester theory and application criteria are not typically included in courses or training. To novice engineers, flame arrester functionality and application principles can seem like a mysterious black box.

Because engineers and other personnel are often unfamiliar with flame arrester fundamentals, misapplication is quite common. This article covers the basics of flame arresters to help prevent misapplications, which can have catastrophic results.

What is a flame arrester?

Flame arrester is an umbrella term that covers numerous subdivisions of devices that are used in various applications (Figure 1). Simply using the general term when procuring or specifying a flame arrester may not provide enough detail to define the necessary unit. Several definitions have been proposed to describe a flame arrester, including:

  • a device that prevents the transmission of a flame through a flammable gas/air mixture by quenching the flame on the surfaces of an array of small passages through which the flame must pass (1)
  • a passive device designed to prevent propagation of gas flames through a pipeline (2)
  • a device fitted to the opening of an enclosure or to the connecting piping of a system of enclosures and whose intended function is to allow flow but prevent the transmission of flame from either a deflagration or detonation (3).
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Figure 1. When specifying a flame arrester, use the appropriate terminology to ensure you purchase the correct arrester type.

In general, a flame arrester is comprised of an outlet housing or weather hood, an inlet housing, and a center section, which contains the flame-extinguishing elements (Figure 2). Design and construction of these elements includes an effective quenching diameter, typically expressed as the hydraulic diameter, as well as a quenching distance, or length that a flame would travel within the element matrix. Quenching diameter and quenching distance influence the arrester’s flame quenching ability and resistance to flow.

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Figure 2. The flame arrester element is the heart of the device. It has a large surface area that absorbs heat and lowers the temperature of the gas flow to prevent an accident.

The flame arrester element (Figure 3) can be manufactured using various methods, depending on the element structure. The most common element structure is a repeating pattern of spiral-wound, crimped metal ribbons made by weaving alternating layers of corrugated metal ribbon and flat metal ribbon around a mandrel to form a cylindrical assembly of regularly spaced triangular gaps of uniform size. The height and width of the triangular cells can be varied to provide the required quenching diameter, and the assembly can be manufactured to tight dimensional tolerances.

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Figure 3. The large surface area of the element helps to absorb and distribute heat. Image courtesy of PROTEGO.

Other element types include metal mesh, gauze, and shot; ceramic balls; stacked parallel plates; and perforated plates; as well as packed beds that can contain metal pebbles, Raschig and Pall rings (named for their manufacturers), or other packings. Each of these element types has its own advantages, disadvantages, and application limitations, but all arresters containing these element structures can be loosely categorized as static dry flame arresters, which are the focus of this article.

The article does not cover hydraulic (liquid) seal arresters, high-velocity vent valves (or dynamic flame arresters), and flame arresters that are combined with breather valves. These types have unique purposes, applications, and functioning principles, and their application in industry is much less common than static dry flame arresters.

How do flame arresters work?

Flame arresters are a simple and elegant process safety solution that prevents flame propagation within pipes or into tanks. The passive devices have no moving parts and do not require external power, sensors, logic controllers, wiring, or manual operation to function.

Combustion reactions require fuel, oxygen, and an ignition source (Figure 4). If a rapid, self-propagating,...

Author Bios: 

Matthew B. Barfield, P.E.

Matthew B. Barfield, P.E., is an explosion-protection test engineer at Fike Corp. (Blue Springs, MO). He regularly performs explosion, fire, and pressure relief tests to aid research and development of explosion-­suppression systems, rupture discs, fire-suppression systems, and other process safety products. He previously worked as a junior consulting process engineer for PROTEGO USA (Charleston, SC), a conservation vent and flame arrester manufacturer. Barfield holds a BSE in chemical engineering from the Univ. of South Carolina. He is a licensed professional engineer in South Carolina...Read more

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