Wet flashback arrestors: how a hydraulic seal stops a flame and how to keep the water level that makes it work

A wet flashback arrestor stops a flame with water. Gas is led in below the surface of a water column and rises through it as bubbles; a flame travelling back from downstream has to pass through that water, and it cannot. It is the oldest arrestor design still in widespread use, it is the natural choice for the wet, low-pressure gas leaving an acetylene generator, and it has one overriding weakness: it works only while the water level is right. Everything about maintaining one follows from that.

The principle is the same quenching physics as a sintered element, by other means. The gas stream is broken into bubbles small enough that a flame cannot propagate from one to the next, and the surrounding water absorbs heat far faster than a bubble of burning gas can generate it. Hydraulic flash arrestors had already been in use for half a century when S. A. Miller surveyed them in 1965, and the user-end hydraulic back-pressure valve he described let gas bubble through water, shed entrained water into a reservoir, and leave through a side outlet. Its relief valve was there for ordinary pressure upsets; it was not able to relieve explosion pressures. That distinction still holds: a relief valve on a wet arrestor is for process upsets, not for explosions.

The Commission Permanente Internationale de l'Acétylène set out the family in 1968 and noted that even the names were confused. Its terms are still the clearest:

In the low-pressure hydraulic valve, the pipe bringing gas in dips deeper into the water than the discharge pipe. On a blowback of oxygen or a flashback, water is driven up both pipes; as soon as the bottom of the shorter discharge pipe is uncovered, the gas escapes outside through it, while the deeper inlet stays sealed. The discharge pipe must be long enough for its water column to hold the generator pressure, a baffle at its end recovers expelled water, and the inlet must be fitted with a water-retaining device, or set higher than the top of the discharge pipe, so the valve cannot siphon back into the generator. The C.P.I. was blunt: efficiency depends essentially on pipe arrangement, submersion depths and the quantity of water.

The submerged non-return valve has no discharge pipe, so there is no pressure compensation and the gas from a blowback stays inside. The C.P.I. made draining the valve and the hose to the torch absolutely indispensable after a blowback, because that trapped mixture could produce high pressures in a following flashback. Experience showed its real efficiency depended on how evenly the gas stream was distributed through the water, so the distribution devices need periodic inspection. Some are fitted with a bursting disc, and the body must be strong enough to withstand a flashback with the disc taken into account.

At the outlet. IS 8471:2003 cl 12.8 requires the generator's flash-back arrestor to be designed, made and tested to IS 11006, cl 12.2 sets its steel sheet at not less than 2.0 mm, and cl 11.1 requires generators holding 50 kg of carbide or more to carry two automatic reliefs, one on the generating chamber and one on the hydraulic back-pressure valve. IS 15190 (Part 1):2002 cl 4.12.1.2 then asks for arrestors at the outlet of the generation plant, at the entry and exit of any gasholder, at each booster exit, at branch points and at each consuming unit. ISO 5175-1:2017 does not reach this position, because its scope begins downstream of a regulator.

On KIGL's carbide-to-water plants, the KIGL plant operation manual places two water-filled arrestors in parallel in the generator outlet line, with non-return valves to stop gas backing into the generator, stainless-steel wire mesh and baffles inside, and a water level check valve. A safety valve on each arrestor doubles as a manual vent.

Sludge is the generator-specific enemy: lime carried over from the reaction can fill the outlet pipe, the arrestors and the non-return valve, as the troubleshooting guide describes.