This cross-section view shows the internal design of the Hoerbiger Explosion Relief Valve Type EVS.

With the introduction of new safety rules described in the M 66 directive, from July 1, 2008, all engines installed onboard a ship or any type of floating vessel (FPSOs included) or offshore oil platforms must include explosion relief valves capable of withstanding more than one explosion.

Hoerbiger is a specialist in this market sector — its EVN explosion relief valves are presently being manufactured at a rate of 40 000 per year. The company has issued the EVS type, which includes double flame arresters capable of withstanding two or more explosions. Large engine crankcases and those of a reciprocating compressor or a gearbox — where the atmosphere is filled with oil mist capable of ignition by an accidental spark — must include a set of such explosion relief valves.

In case of an oil mist explosion, these valves have the function of relieving the pressure from the crankcase, thus avoiding destructive damage while at the same time preventing the flame from coming into the engine room, which could cause damage to surrounding equipment and injure personnel.

The construction of these valves is very simple. A valve plate is pressed by a spring toward its seat, and in the case of overpressure (only 0.05 bar), opens to relieve the pressure. The spring will then immediately push the valve plate back against its seat, so that external air cannot rush inside the case — this could potentially facilitate a second explosion.

During the pressure relief function the flame resulting from the explosion also tends to escape. This is prevented by a flame arrester in the EVN-type valve and now by two arresters in the new EVS-type valve. On both valve types, a specially shaped valve cover directs the hot gas escaping the valve toward the engine to prevent injuring personnel within the engine room.

The pressure relief valve itself does not need to be replaced. It is designed to withstand several explosions without suffering deformation or other damage. However, according to the rules of classification societies flame arresters must be replaced. In order to facilitate this operation without dismantling the entire valve, the new EVS valves allow lifting of the valve cover (after removal of every other screw)to access the flame arresters for replacement. equested by classification societies, this is a simple and inexpensive operation that is to be carried out after an explosion.

The flame arrester steel sheet is designed to withstand a flame temperature of 1300° to 1400°C. However, the thermal shock can weaken its resistance. The two flame arresters of the EVS valves can withstand at least two explosions. Tests have demonstrated their ability to perform this job, and Germanischer Lloyd, as well as all other main classification societies, have granted a type approval to the new EVS line.

As well as on the covers of the main engine crankcases, these valves can be used on the exhaust lines of the engine to prevent damage in case a nonallowed manual starting procedure is performed after a faulty start. In this case the opening pressure is set at very low values. On the intake line, between the engine and the turbocharger, explosion relief valves are needed to absorb the return pressure wave due to misfiring — preventing damage to the turbocharger. In this case the opening pressure has to be set somewhat higher than the turbocharger compression ratio. Both the EVN and EVS pressure relief valves are available in different measurements according to the size of the relief area required.

The diameter of the external cover of the EVS type, featuring a double flame arrester is larger than that of the EVN type. Differences are more accentuated on small sizes and become negligible on the larger ones. The 735 EVN has a cover of 1160 mm, which compares with 1050 mm of the 735 EVS cover. The difference is in the range of only 10%.

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