Vent valve

Most small Hquid helium containers are unpressurized heat leak slowly bods away the Hquid, and the vapor is vented to the atmosphere. To prevent plugging of the vent lines with solidified air, check valves of some sort are included in the vent system. Containers used for air transportation are equipped with automatic venting valves that maintain a constant absolute pressure with the helium container in order to prevent Hquid flash losses at the lower pressures of flight altitudes and to prevent the inhalation of air as the pressure increases during the aircraft s descent. Improved super insulation has removed the need for Hquid nitrogen shielding from almost all small containers. 

Another component of the TDH is the Hp, pre.ssure head. VVe can see in Figure 8-7 that both tanks have vent valves. These two ve,ssels are exposed to atmospheric pressure, which is the same in both tanks. So by simple observation, pressure head doesn t exist. AHp = 0. 

Caution should be taken to prevent excessive acceleration. One solution is to provide for opening of the compressor discharge vent valve to increase the compressor flow and increase blower horsepower. This is effective with a centrifugal compressor, however, not with an axial compressor unless the compressor is provided with adjustable stator vanes that are reliable. This can be seen from the performance curve. The head versus flow curve for a given vane setting is extremely steep and opening the vent valve is ineffective. However, if the vanes operate fully open and the vent valve opens, the combined effect is satisfactory. The vent valve must not be oversized. 

The key components in the fuel vapor control system include the fuel tank, vapor vent valves, vapor control valve, vapor tubing, the activated carbon canister, and the engine vapor management valve (VMV) [25,26], During normal vehicle operation, fuel tank vapor pressure is relieved through the use of vapor vent valves installed in the vapor dome of the fuel tank. The vent valves are designed to allow for the flow of fuel vapor from the tank, and to assure that liquid fuel does not pass through the valve. 

The vapor vent valves are connected to the tank vapor control valve, and ultimately to the carbon canister by tubing that is resistant to swelling in the presence of fuel vapors. The tubing material must also have a low HC permeation rate, so that the evaporative emissions are not increased due to release of HC molecules. The tank vapor control valve connects the carbon canister to two fuel tank vapor sources the vapor vent valve lines and a refueling vent tube. 

At high points in piping, vent valves are required to remove air for hydrotesting and for purging the system. At low points, drain valves are required to drain liquids out of the system to perform maintenance. Normally, vent and drain valves are 14-in. or i-in. ball valves. 

Whenever it is necessary to control the process level, pressure, temperature, etc., a control station is installed. A control station may be as simple as a single control valve or it may contain several control vahes, block valves, bypass valves, check valves, and drain or vent valves. 

Without consulting the manufacturer, the owner of a set of hot tapping equipment made a small modification he installed a larger vent valve to speed up its use. As a result the equipment could no longer withstand the pressure and was violently ejected from a pipeline operating at a gauge pressure of 40 bar (600 psi) [22]. 

In 1984, an explosion in a water pumping station at Abbeystead, UK, killed 16 people, most of them local residents who were visiting the plant. Water was pumped from one river to another through a tunnel. When pumping was stopped, some water was allowed to drain out of the tunnel and leave a void. Methane from the rocks below accumulated in the void and, when pumping was restarted, was pushed through vent valves into a valvehouse, where it exploded [18J. 

The hose was disconnected before the pressure had been blown off, sometimes because there was no vent valve through which it could be blown off. 

In addition, the vent valve was repositioned at the foot of the ladder [6]. 

A suspended catalyst was removed from a process stream in a pressure filter. After filtration was complete, the remaining liquid was blown out of the filter with steam at a gauge pressure of 30 psi (2 bar). The pressure in the filter was blown off through a vent valve, and the fall in pressure was observ ed on a pressure gauge. The operator then opened the filter for cleaning. The filter door was held closed by eight radial bars, which fitted into U-bolts on the filter body. The bars were withdrawn from the U-bolts by turning a lai ge wheel, fixed to the door. The door could then be withdrawn. 

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). 

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. 

NFPA 30 recognizes that a conservation vent valve (pressnre-vacnnm valve) is an alternative to a flame arrester nnder certain circnmstances. This recognition is based on tests hegnn in 1920 and is snpplemented hy many years of experience. 

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. 

Conservation Vent Valve A device designed to maintain pressure within preset limits in a liquid-containing vessel for the purpose of emissions reduction. It also provides protection against excessive pressure or vacuum. 

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. 

---