Air overpressure

Pressure Zero shift, air leaks in signal lines. Variable energy consumption under temperature control. Unpredictable transmitter output. Permanent zero shift. Excessive vibration from positive displacement equipment. Change in atmospheric pressure. Wet instrument air. Overpressure. Use independent transmitter mtg., flexible process connection lines. Use liquid filled gauge. Use absolute pressure transmitter. Mount local dryer. Use regulator with sump, slope air line away from transmitter. Install pressure snubber for spikes. 

In ballasted cycles the increase in pressure within the headspace of the container is counterbalanced with an internal chamber air overpressure. Figure 7 illustrates an idealized type of ballasted cycle the autoclave operates as for a normal fluid load cycle up to the point at which the exposure tempeature is attained (in this case 121 C or one bar [gauge] steam pressure), and then com-... 

An insufficient specific charge (weight of explosive per unit rock volume broken) can increase vibration by delaying and reducing the effect of reflected tensile waves from free faces (du Pont Handbook, 1977). Excessive specific charge will increase throw and flyrock and may increase air overpressure. The substantial increase of vibration that can occur from insufficient specific charge is difficult to counteract by other measures such... 

Ramulu, M., Chakraborty, A. K., Raina, A. K., Reddy, A. H. (2004). Influence of burden on the intensity of ground vibrations and air overpressure in opencast bench blasting. JnProceedings of the ISEE s 30 Annual Conference on Explosives and Blasting Techniques, New Orleans, LA (pp. 465-478). 

Sames, F. (1999). The influence ofblastparameters on vibration and air overpressure. In Proceedings ofthe C Int. Symposium on Rock Fragmentation by Blasting (FRAGBLAST-6), Johansbeig, South Africa (pp. 149-154). 

Han and Park (1975c) experimentally studied flow instabilities in film blowing by introducing disturbances in the air overpressure or the take-up speed. They found that HDPE and LDPE are more sensitive to take-up speed disturbances... 

The most overlooked hazard and contaminant is water (99). Water reacts with isocyanates at room temperature to yield both ureas and large quantities of carbon dioxide. The presence of water or moisture can produce a sufficient amount of CO2 to overpressurize and mpture containers. As Httle as 30 mL of water can result in 40 L of carbon dioxide which could result in pressures of up to 300 kPa (40 psi). For these reasons, the use of dry nitrogen atmospheres is recommended during handling. If a plant air system must be used, purification equipment, such as oil traps and drying beds, should be installed between the source and the isocyanate vessel. 

FIG. 26-9 Incident overpressure vs. scaled distance, surface hurst. The t points are from Kingery and PanniU, Memo Report ISIS BRL. Adapted fr am Department of Army, Navy, and Air- For ce TM5-1300, NAVFAC P-397, AFM 88-22.)... 

Vapor-air deflagration inside dryer thermal decomposition resulting in vessel overpressure or rupture. 

Self-initiated reaetions, e.g. pymvie aeid on storage ean beeome oxidized by air (or airborne yeasts) to form suffieient gaseous earbon dioxide to overpressurize the eontainer ... 

Now let us consider utility failure as a cause of overpressure. Failure of the utility supphes (e.g., electric power, cooling water, steam, instrument air or instrument power, or fuel) to refinery plant facihties wiU in many instances result in emergency conditions with potential for overpressuring equipment. Although utility supply systems are designed for reliability by the appropriate selection of multiple generation and distribution systems, spare equipment, backup systems, etc., the possibility of failure still remains. Possible failure mechanisms of each utility must, therefore, be examined and evaluated to determine the associated requirements for overpressure protection. The basic rules for these considerations are as follows ... 

Normal Individual Process Unit Basis for PR Sizing Considerations - The following single contingencies should be considered as the normal basis for evaluating overpressure that can result from an instrument air failure ... 

Flash fire The eombustion of a flammable vapor and air mixture in whieh flame passes through that mixture at less than sonie veloeity, sueh that negligible damaging overpressure is generated. 

Do not allow nitrogen or air supplies to overpressure tanks or vessels. Tanks and vessels could be designed to withstand the air and nitrogen header pressure. Another solution is to install a pressure relief valve downstream of a pressure reducing station sized to relieve the entire flow on failure of the station. 

The receiving gauge in the control room works on the transmitted pneumatic pressure, 15 psi giving full scale, but has its dial calibrated in terms of the plant pressure that it is indicating. The Bourdon tube of such a gauge is capable of withstanding only a limited amount of overpressure above 15 psi before it will burst. Furthermore, the material of the Bourdon tube is chosen for air and may be unsuitable for direct measurement of the process fluid pressure. 

Low-energy ignition of unobstructed propane-air and natural gas-air clouds does not produce damaging overpressures. 

The results in Tables 4. la and 4. lb demonstrate that in the absence of obstacles, the highest flame speed observed was 84 m/s, and it was accompanied by an overpressure of 60 mbar for hydrogen-air in a 10-m radius balloon (Schneider and Pfortner 1981). For all other fuels, flame speeds were below 40 m/s and corresponding overpressures were below 35 mbar. Hence, weak ignition of an unconfined... 

Hjertager (1984) reported overpressures of 1.8 bar and 0.8 bar for propane and methane-air explosions, respectively, in a 0.5-m radial disk with repeated obstacles. 

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