Pressure-Vacuum Purging

In some cases both pressure and vacuum are available and are used simultaneously to purge a vessel. The computational procedure depends on whether the vessel is first evacuated or pressurized. 

At this point the remaining cycles are identical to pressure purging and Equation 7-6 applies. However, the number of cycles j is the number of cycles after the initial pressurization. 

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Use seven minimum, perhaps use eight, for assurance that purging is complete. Note that the above relationships hold for vacuum purging. Keep in mind the relationships between high and low pressure of the system and use mmHg for pressure if more convenient. For sweep-through purging, see Ref. [29]. 

Vacuum purging is the most common inerting procedure for vessels. This procedure is not used for large storage vessels because they are usually not designed for vacuums and usually can withstand a pressure of only a few inches of water. 

Use a vacuum purging technique to reduce the oxygen concentration within a 1000-gal vessel to 1 ppm. Determine the number of purges required and the total nitrogen used. The temperature is 75°F, and the vessel is originally charged with air under ambient conditions. A vacuum pump is used that reaches 20 mm Hg absolute, and the vacuum is subsequently relieved with pure nitrogen until the pressure returns to 1 atm absolute. 

One practical advantage of pressure purging versus vacuum purging is the potential for cycle time reductions. The pressurization process is much more rapid compared to the relatively slow process of developing a vacuum. Also, the capacity of vacuum systems decreases significantly as the absolute vacuum is decreased. Pressure purging, however, uses more inert gas. Therefore the best purging process is selected based on cost and performance. 

The number of purge cycles is j = 5.6. Thus six pressure purges are required, compared to four for the vacuum purge process. The quantity of nitrogen used for this inerting operation is determined using Equation 7-7 ... 

Equation 7-12 is used in place of Equation 7-6 for both pressure and vacuum purging. 

After weighing, the munitions are released to a four-unit vacuum purge station. Automatic vacuum/purge nozzles make a vacuum/pressure seal on top of the empty munitions, and a three-way automatic valve... 

Vacuum Purging. The vessel is evacuated and then is increased to atmospheric pressure using the chosen inert gas. This procedure is repeated until the desired oxygen concentration is reached. The number of vacuum purges, n, required to achieve the desired LOC can be calculated by using the following equation ... 

Seeondary containment for process gas lines can be operated in three different modes pressurized, vacuum, and purge. In the pressurized mode the coaxial outer line is sealed at both ends, inside the gas cabinet and inside the tool. The aimular space is pressurized to a pressure greater than the process gas line. If there is a leak in the process tube, there will be a net flow of pressurization gas (nitrogen, argon, or helium) into the process tube with a reduction of pressure in the armular space. A pressure transducer in the annular space will signal this condition. 

The vacuum plate drwer is provided as pari of a closed system. The vacuum dryer has a cylindrical housing and is rated for fiill-vacuum operation (typical pressure range 3-27 kPa absolute). The exhaust vapor is evacuated try a vacuum pump and is passed through a condenser for solvent recovery. There is no purge-gas system required for operation under vacuum. Of special note in the vacuum-drying system... 

While some video display screens such as liquid crystal, gas plasma or vacuum fluorescent displays do not present the same charged screen hazards as CRTs, this does not imply that they are safe for use in hazardous locations. This requires special design and certification for use with a given flammable atmosphere. Non-certified equipment used in locations classified as hazardous under Article 500 of NFPA 70 National Electrical Code require a purged or pressurized enclosure to control ignition hazards as described in NFPA 496 Standard for Purged and Pressurized Enclosures for Electrical Equipment. The screen in this case is located behind a window in the enclosure. 

Beyond the complexities of the dispersive element, the equipment requirements of infrared instrumentation are quite simple. The optical path is normally under a purge of dry nitrogen at atmospheric pressure thus, no complicated vacuum pumps, chambers, or seals are needed. The infrared light source can be cooled by water. No high-voltage connections are required. A variety of detectors are avail-... 

To determine the number of purge cycles and achieve a specified component concentration after j purge cycles of pressure (or vacuum) and relief [29] ... 

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