High velocity vent valve

Schampel and Steen (1975) describe experimental equipment and tests carried out at the Physikalisch-Technische Bundesanstalt (PTB) in Germany on high velocity vent valves. Also, conditions for a sufficient air entrainment and dilution of the vented flammable vapors are discussed. 

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The International Maritime Organization (IMO) published a standard in 1984 for the design, testing and locating of deflagration and detonation flame arresters and high velocity vent valves for cargo tanks in tanker ships (IMO 1984). This was amended in 1988 and reissued as Revision I (IMO 1988). A new revised standard was issued in 1994 (IMO 1994). 

There are test procednres for liqnid prodnct flame arresters, high velocity vent valves, flow controlled apertnres (velocity flame stoppers), and hydranlic flame arresters, as well as specific reqnirements for testing flame arresters nsed with compressors, fans, blowers, and vacnnm pnmps, which are not covered in other standards. 

Test procedures for liquid product detonadon arresters, hydranlic flame arresters, high velocity vent valves, and flow controlled apertures (velocity flame stoppers) allow the rise of these kinds of flame arresters as an alternative to standard static fixed element dry type flame arresters. This corild be especially risefnl in high flow applica-... 

The IMO standard MSC/Circ. 677 (1994) provides testing procedures for end-of-line deflagration and in-line detonation flame arresters, as well as high velocity vent valves for use on cargo tanks in tanker ships. Its development is discussed in Section 2.3.4. 

High Velocity Vent Valve A device to prevent the passage of flame in the reverse direction, consisting of a mechanical valve which adjnsts the opening available for flow in accordance with the pressnre at the inlet of the valve in snch a way that the efflnx (exiting) velocity cannot be less than 30 m/s. 

Safe flow velocities in the described device depend on the diameter of the pipe and the explosion properties of the gas involved. The location of installation has to be considered, for example, at an open outlet (e.g. high velocity vent valve) or enclosed, e.g. at the burner inlet of an incinerator, where the heat flux from the surroundings has an influence. In both cases turbulence shall not be increased, because that would accelerate the combustion process. This is achieved by the high velocity vent valve shown in Fig. 7.13. 

For conventional relief valves venting to a closed system, the line is sized to keep the back pressure within 10% of the relief valve set pressure. This is subject to a maximum velocity of 0.75 Mach. API 521 recommends a velocity up to 1 Mach. However, proper analysis of piping integrity is required for a velocity close to Mach 1 such high velocity should not be used for the design of a new system. Outlet line sizing for balanced-bellows and pilot-operated relief valves is based on a maximum velocity of 0.75 Mach. 

When the relieving scenarios are defined, assume line sizes, and calculate pressure drop from the vent tip back to each relief valve to assure that the back-pressure is less than or equal to allowable for each scenario. The velocities in the relief piping should be limited to 500 ft/sec, on the high pressure system and 200 ft/sec on the low pressure system. Avoid sonic flow in the relief header because small calculation errors can lead to large pressure drop errors. Velocity at the vent or flare outlet should be between 500 ft/sec and MACH 1 to ensure good dispersion. Sonic velocity is acceptable at the vent tip and may be chosen to impose back-pressure on (he vent scrubber. 

Rotary/star valve use amps as guide to solids throughput. Keep air velocity high enough to prevent plugging of the air-vent line. 

Vent line valves This is a deceptively easy duty, butif it is on ahigh pressure blow tank handling fly ash or cement, the valve will have to operate in a very harsh environment. The air velocity will be very high, albeit for a very short period of time, but a lot of abrasive dust is likely to be carried with the air. If the material is abrasive then the choice is between a pinch valve and a dome valve. If the material is non-abrasive, a diaphragm valve could be used. 

In those conflgurations shown above requiring a vent line, it is almost always necessary to install one or more block valves so as to help set the vent gas flow and for isolation purposes. If the solids are erosive, one must limit the velocity within the vent line to acceptable levels (generally under 30 m/s) so as not to erode the vent line piping. Vent line elbows, valves and any flow restriction orifice that may be present are especially vulnerable to erosive attack. Special erosion resistant, double-block valves may be necessary under highly erosive conditions. 

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