Pressure threshold

The key to understanding dewatering by air displacement is the capillary pressure diagram. Figure 6 presents an example typical for a fine coal suspension there is a minimum moisture content, about 12%, called irreducible saturation, which cannot be removed by air displacement at any pressure and a threshold pressure, about 13 kPa. 

Volume of vessel (free volume V) Shape of vessel (area and aspect ratio) Type of dust cloud distribution (ISO method/pneumatic-loading method) Dust explosihility characteristics Maximum explosion overpressure P ax Maximum explosion constant K ax Minimum ignition temperature MIT Type of explosion suppressant and its suppression efficiency Type of HRD suppressors number and free volume of HRD suppressors and the outlet diameter and valve opening time Suppressant charge and propelling agent pressure Fittings elbow and/or stub pipe and type of nozzle Type of explosion detector(s) dynamic or threshold pressure, UV or IR radiation, effective system activation overpressure Hardware deployment location of HRD suppressor(s) on vessel... 

Correlation of Threshold Pressure with Propellant Burning Rate. P 449... 

Fig 17 Correlation of threshold pressure with propellant burning rate (5- x 40-inch rocket motor)... 

Threshold shock pressures to initiate detonation in several SP are shown in Table 55. In general, unless the SP contains solid HE ingredients such as HMX or RDX, the threshold pressures are considerably greater than for solid HE at a comparable degree of compaction. Even so, the pseudo-composites of AP/wax must be considered to be rather shock-sensitive provided their degree of compaction does not exceed about 90% of their theoretical max density (TMD), Figure 21 shows that up to about 90% TMD the shock sensitivity of an 80/20 AP/wax composite is very similar to that of TNT... 

Diameter Density Threshold Pressure Detonation Velocity ... 

HOPG cylinders showed that intercalation starts close to the basal planes and proceeds thence to the central layers. Cylinders capped at both ends with glass caps and grease are not intercalated. The layer system is split into a number of discs at a rate that increases with the degree of alignment of layer planes. For non-heat-treated PG, the threshold pressure increases with the number of layer planes in the sample. Similar results confirming this mechanism have been obtained with metal halides (H13). A mechanism of intercalation based on these results has been discussed by Hooley (H22). 

The problem asks for the partial pressure of N2 O4 that will make the decomposition spontaneous when T — 298 K and p of NO2 = 1 bar. The value of ZlG must be zero before it can become negative. Therefore, to find the threshold pressure of N2 O4 that makes the decomposition spontaneous, set A G = 0 and J N02 = 1 bar and then rearrange to solve for the partial pressure of N2 O4 ... 

Fig. 1.10 The Blake threshold pressure (1.17) and the linear resonance frequency (1.18) as a function of ambient radius...

For cavitation to occur in a liquid, it has to overcome the natural cohesive forces present in the liquid. Any increase in these forces will tend to increase the threshold pressure and hence the energy required to generate cavitation. In highly viscous liquids, severe attenuation of the sound intensity occurs and the active cavitating zone gets reduced substantially. Moholkar et al. [56] have confirmed this fact with experiments with different liquids and reported that for highly viscous liquids, cavitational effects are not observed. 

Adsorption of water is thought to occur mainly at steps and defects and is very common on polycrystalline surfaces, and hence the metal oxides are frequently covered with hydroxyl groups. On prolonged exposure, hydroxide formation may proceed into the bulk of the solid in certain cases as with very basic oxides such as BaO. The adsorption of water may either be a dissociative or nondissociative process and has been investigated on surfaces such as MgO, CaO, TiOz, and SrTi03.16 These studies illustrate the fact that water molecules react dissociatively with defect sites at very low water-vapor pressures (< 10 9 torr) and then with terrace sites at water-vapor pressures that exceed a threshold pressure. Hydroxyl groups will be further discussed in the context of Bronsted acids and Lewis bases. 

The butadiene reaction at high pressure has also been studied in the solid [429]. The reaction has been followed at several pressures ranging from 2.1 and 6.6 GPa and has been monitored by infrared spectroscopy. It has been found that below 4.0 GPa, only vinylcyclohexene is formed with trace amounts of the polymer. Above this threshold pressure, the amount of polymer formed is not... 

The fact that the pressure is negative implies that a negative pressure must be applied to overcome the cohesive forces of a liquid and produce a bubble of radius R. Writing P] = Pjj — Pg allows the estimation of Pg, (known as Blake threshold pressure), which is the negative (or rarefaction) pressure which must be applied in excess of the hydrostatic pressure (Pj ) to create a bubble of radius R. e.g. for large bubbles (i.e. 2a/R, Ph)... 

The strong dependence of Tc and [M]c on pressure can lead to the observation of a threshold pressure—a pressure below which polymerization does not occur for a particular monomer concentration at a particular temperature. 

In hydrodynamic cavitation, the tensile stress is of relatively long duration and plenty of caviration nuclei are usually available. As a result, cavitation occurs when the total net pressure, or the stagnation pressure, becomes approximately equal to the vapor pressure of the liquid. In acoustic cavitation, the cyclic pressure required to produce cavitation is a function of the frequency, the partial pressure of any dissolved gas, and the population of cavitation nuclei. For frequencies above about 200.(XX) Hz. the threshold pressure for cavitation increases with the square of the frequency and is almust independent of the degree of gas saturation. For frequencies below 200,000 Hz. the threshold pressure is a funelion of the partial pressure of the dissolved gas. In saturated liquids at sound pressures less than a... 

A readily apparent differentiation between the shock sensitivity of homogenous and heterogenous expls is the much greater shock pressures required to initiate the former. Examples of these high pressures (not necessarily threshold pressure but probably close to threshold) are shown in Table 4... 

The most detailed studies of the shock initiation of Tetryl are those of Iindstrom (Ref 40) and those made at NOL (Refs 30, 32,41 42). Iindstrom used large diameter, unconfined charges and plane-wave shocks. Threshold pressures and particle velocities to just initiate detonation (computed by the writer from Lind-Strom s data in Ref 42a) are shown in Table 4 Note that threshold pressures decrease rapidly as packing d decreases, but particle velocities stay essentially constant (with the possible exception of Tetryl at 1.7g/cc, which is 98% crystal d)... 

Obviously the variation of threshold pressure or particle velocity in gap tests is analogous to that observed with plane-wave shocks... 

The data in Refs 32 41 lead to the following tabulation of the effect of diameter of confined 1.47g/cc Tetryl charges on threshold pressure (in the ex pi at its interface with the barrier) ... 

They found a deflagration threshold pressure of 14kb as compared to 18kb for the detonation threshold. Bartels (Ref 50) investigated the shock behavior of coarse Tetryl at 1.65g/cc in a Plexiglass tube of 2lmm ID. He claims that no reaction occurs below 7.2kb and reaction is observed regularly above 12.1kb. With recrystd Tetryl of a finer particle size, the no-reaction level shifts to 9.1kb, while the reaction level is still 12.1kb... 

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