A flame arrestor only does its job if it is in the right place, and there are really two places it can live. An end-of-line arrestor sits at an open vent to atmosphere, standing guard against a flame trying to get into the equipment from outside. An in-line arrestor sits buried inside a piped vapor system, stopping a flame from travelling along the pipe from one part of the plant to another. They protect against opposite directions of threat, they face different flame conditions, and they carry different installation rules. This page explains where each one goes, what it is defending against, and what to get right when you install it.
In-Line vs End-of-Line Flame Arrestor in one line: An end-of-line flame arrestor is mounted at an open atmospheric vent and stops an external flame from propagating back into the tank or vessel through the vent. An in-line flame arrestor is installed within a piped vapor, flare, or vapor-recovery header and stops a flame from travelling along the pipe between connected pieces of equipment. Placement is dictated by where the ignition can occur and which direction the flame would travel.
An end-of-line arrestor lives at the boundary between the process and the open air, typically screwed onto a tank vent, the outlet of a pressure-vacuum vent, or an atmospheric relief. The threat it addresses comes from outside: a lightning strike, a nearby fire, a static spark, or a flame from adjacent equipment could ignite the flammable vapor at the vent opening, and without an arrestor that flame could flash back through the vent into the tank's vapor space and ignite the whole ullage. Because ignition happens right at the opening, the flame arriving at an end-of-line arrestor has no pipe to accelerate in and presents as a mild deflagration, which is what these devices are built to quench.
An in-line arrestor lives inside the pipe, part way along a run that connects two or more pieces of equipment. Here the threat is internal and directional: a flame ignited at one end of a vapor system, say at a flare tip, a vapor recovery unit, or a thermal oxidizer, must be prevented from travelling back down the header toward the tanks feeding it, or vice versa. The arrestor is a firebreak in the middle of the piping. Because the flame may have run down a length of pipe to reach it, an in-line arrestor can face a more severe front and, depending on run-up distance, may need to be a detonation-rated rather than a deflagration-rated device.
Put simply, an end-of-line arrestor keeps a fire out of a vessel through its vent, and an in-line arrestor keeps a fire from spreading through connected piping. A single facility routinely has both: an end-of-line arrestor on each tank's individual atmospheric vent, and an in-line arrestor on the common vapor header that ties those tanks to a vapor recovery unit or flare. Each is placed and rated for the flame it can credibly see at its own position.
For an end-of-line arrestor, the two things installers get wrong most often are orientation and blockage. Many end-of-line vent arrestors are designed to sit vertically with the element clear, so that condensate, rain, and debris drain or fall away rather than pool in the element and block the passages. An arrestor whose channels are fouled with corrosion product, ice, insect nests, or polymerized vapor no longer flows freely, and a blocked vent arrestor can starve a tank of vacuum relief or overpressure relief just as surely as a stuck vent. Keeping it accessible for inspection and cleaning is part of specifying it, not an afterthought.
For an in-line arrestor, temperature is the extra concern. If a flame lodges against an arrestor element and keeps burning there instead of being knocked out immediately, the element heats up, and a sustained burn can eventually preheat the unburned side enough to let the flame through. In-line arrestors in systems where an endurance burn is credible are therefore often fitted with a temperature sensor on the element that trips a shutdown or isolation if the arrestor starts running hot. That sensor makes the difference between an arrestor that stops a flame and one that is being slowly defeated by a stationary fire against its face.
Both types share a few non-negotiables. The arrestor must be rated for the specific gas group it is protecting, since the passage size that quenches one fuel may pass another, and it must be installed with maintenance access because a flame arrestor that cannot be inspected will not be inspected. Pressure drop is designed in from the start, because an in-line arrestor that adds too much resistance chokes the vapor system and an end-of-line arrestor that clogs defeats the tank's breathing. Getting the position, orientation, rating, and access right at install time is what keeps the device a protection rather than a liability.
Neither an in-line nor an end-of-line arrestor produces a signal on its own, so their condition is inferred from the behavior of the system around them, which is precisely what a SCADA layer is already measuring. A fouling end-of-line arrestor shows up as a tank that no longer breathes cleanly: pressure that swings higher on filling and pulls lower on emptying than it should, because the restricted vent cannot pass air fast enough. Trending tank pressure against fill and draw activity turns a slow blockage into a visible pattern well before it forces a relief event.
An in-line arrestor is watched through the temperature sensor on its element, where fitted, and through the differential pressure across the vapor header it sits in. A rising element temperature is a direct warning that a flame may be sitting against the arrestor, and it is exactly the kind of point that should raise an immediate alarm and, in many designs, trip an isolation. A creeping differential pressure across the header points at the arrestor fouling and restricting flow. Both are ordinary process signals that a monitoring system can carry back from an otherwise unmanned site.
A cloud SCADA platform such as Merobix brings these tank pressures, header differentials, and arrestor temperature points together across a whole facility and historizes them, so an operator can confirm from a dashboard that every vent is breathing and every in-line arrestor is cool and flowing. The value for remote oil and gas operations is that a fouling arrestor or an endurance burn no longer waits to be discovered on the next site visit. The device stays passive and mechanical, as it should; the monitoring layer is what tells the organization it is still doing its job.
At an open vent to atmosphere, most commonly on a storage tank vent, the outlet of a pressure-vacuum vent, or an atmospheric relief. Its job is to stop an external flame at the opening from flashing back into the vessel's vapor space. Because ignition happens right at the vent with no pipe run to accelerate in, the flame it sees is a mild deflagration, which is what end-of-line arrestors are built to quench.
When you have piped vapor systems connecting equipment, such as a vapor recovery header, a flare or vent line, or a manifold tying multiple tanks together, and a flame ignited at one point could travel along the pipe to another. The in-line arrestor is a firebreak inside that piping. Because the flame may run down a length of pipe first, an in-line arrestor often has to be rated for a more severe front, potentially a detonation, depending on the run-up distance.
Because a flame can lodge against the arrestor element and keep burning there instead of being immediately extinguished. That endurance burn slowly heats the element, and if it goes on long enough it can preheat the far side and let the flame through. A temperature sensor on the element detects that heating and trips an isolation or shutdown, so the arrestor is not defeated by a stationary fire against its face. End-of-line arrestors, seeing only brief external flashbacks, usually do not need one.
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