Pressure decay

When pressure-decay rates less than critical are employed, the gas-phase combustion zone is removed from the propellant surface and extinguished, but not the ignition from within the condensed phase. Therefore, the temperature of the surface material will be above the autoignition temperature, and steady-state combustion will eventually be initiated. This mechanism is consistent with the observation that the luminosity of the combustion zone can vanish without combustion having been completely terminated. 

Actually, the system capacitance and the pressure-decay rate measure the rate of capacitance discharge of the system. Because the combustion process is known to be controlled by pressure, the pressure-decay rate will disturb the combustion process. If the decay rate is greater than the intrinsic pressure-growth rate of the controlling reaction, the combustion process will not recover. This suggests that the pressure-decay rate dP/dt is the intrinsic term, rather than the capacitance term (L ). 

Gholami Y., Azin R., et al. Prediction of carbon dioxide dissolution in bulk water under isothermal pressure decay at different boundary conditions. 2015 Journal of Molecular Liquids 202 23-33. 

Sheikha H., Pooladi-Darvish M., et al. Development of graphical methods for estimating the diffusivity coefficient of gases in bitumen from pressure-decay data. 2005 Energy Fuels, 19, 2041- 2049. 

This is the most common test method employed to qualify the leak characteristics of a new seal material. The test method involves applying the seal between two ceramic discs or between a ceramic and a metal disc, pressurizing the cavity formed by the seal and monitoring the pressure decay as a function of time.22 Alternatively, a metal tube and a ceramic disc can also be used [34], Typically, the cavity is pressurized to about 2 psi and the leak rate is determined by the pressure decay as a function of time. These tests can be done at room temperature or elevated temperatures. Similar test arrangement has also been used to test a plastically deformable brazed metal seal between fuel cell anode material and Haynes 214 washer [35], The cavity is pressurized to measure the rupture strength of the seal material. 

A thin zirconia membrane is sealed to a stainless cup of about one inch diameter (Figure 5.9). The stainless cup has a feed tube welded to it. The inside cavity is evacuated to about 5 psi and sealed off. One can calculate the leak rate by monitoring the pressure decay as a function of time (see Section 5.2.1). 

The arrangement is then placed in a furnace, heated to 800°C, and the pressure decay test is repeated. The setup can be subjected to thermal cycles with pressure decay tests at room temperature and at 800°C. A good hermetic seal, however, will not show any pressure decay at a low pressure differential. The results of one series of tests using a glass composite material that has a glass transition temperature (Tg) of about 700°C, at 5 psi are shown in Figure 5.10. The combination of pressure and thermal cycle tests showed that the seal is capable of withstanding the pressure... 

Test methods described in the literature inelude the bubble method, helium mass speetrometiy, liquid tracer (dye), head spaee analysis, vacuum and pressure decay, weight loss and gain, and high voltage leak deteetion [6]. 

The inventory of contained liquid and gas can be calculated between isolation points to determine the amount of material that can be released for pro-cess-related scenarios. The time frame to close manual valves—depending on accessibility, operating conditions, and availability of personnel—is typically 10-30 minutes. For remotely operated valves actuated from the control room, 60-90 seconds is used for isolation. The release rate will decrease based on the pressure decay of the system once the system has been isolated. 

Fig. 5.7 shows scanning electron microphotographs of a TAGN surface before combustion (a) and after quenching (b). The quenched surface is prepared by a rapid pressure decay in the strand burner shown in Appendix B. The quenched sur-... 

Fig. 7.29 Scanning electron microphotographs of quenched AP composite propellant burning surfaces without LiF (a) and with 0.5% LiF (b), obtained by a pressure decay from 2 MPa to 0.1 MPa the width of each photograph is 0.60 mm.

Since the burning rate of a propellant is dependent on the burning pressure, the mass balance between the mass generation rate in the chamber and the mass discharge rate from the nozzle is determined by the pressure. In addition, the propellant burning rate in a rocket motor is affected by various phenomena that influence the mass balance relationship. Fig. 14.4 shows typical combustion phenomena encountered in a rocket motor, from pressure build-up by ignition to pressure decay upon completion of the combustion. 

Pressure test Proper functioning check valve Pressure decay <75psi/min No leak from the pump 6 months... 

Note It is generally assumed, and empirically established, that, over ranges of interest, the shock wave pressure decays exponentially to about one time constant after that the pressure decays more slowly. The solid curve is a truly exponential pressure decay... 

The measurable underwater shock wave parameters, namely peak overpressure, pressure decay and shock velocity were defined in Fig 2. Actual pressure time records are similar to the idealized sketch of Fig 2, but unfortunately they are rarely as neat . Peak overpressures and time constants can be read directly from such records. Impulse (/pdt) and Energy Flux Density (const x /P2dt) require either analytical or graphical integration. Shock velocity is obtained from arrival times, ie, the time between firing of the expl charge and the start of the steeply rising pressure pulse... 

It is known from Fig. 7.2-3 that the pressure should be reduced to 100 or 50 MPa to avoid decomposition. When a relief valve is activated the rate of pressure-decay should be fast, so... 

For the comparison of waves of different amplitudes at different gas densities, it is convenient to characterize the rate of pressure decay by a dimensionless quantity, the logarithmic derivative dlnp/dlnt = (t/p)(dp/dt) or, equivalently, by the value of the exponent in the formula p a t g. 

The law of pressure decay /(f/r) is restricted by the obvious condition that the pressure at the piston must decrease faster than the pressure amplitude in the self-similar solution itself, i.e., faster (with a greater absolute value of the negative exponent) than i-4/5. 

In each half-space we take Sedov s solution, which satisfies the condition of zero velocity at the coordinate origin, and impose on them the condition of pressure equality. Since the law of pressure decay with time in a strong explosion is independent of the gas density (in the plane case this law is p this joining of the two solutions is possible. At equal pressures,... 

For input pressures below threshold only a slow pressure decay is observed. The pressure-time profiles of Fig 23 were obtained at various distances within the expl. They also show the input shock and a progressively shorter delay for subsequent pressure increase with increase in distance from the impact face. The pressure decay and further pressure rise after the above pressure increase is a gage artifact... 

Figure 4. Left Schematic of pressure profile reported by Fairweather et al40 Right Capillary pressure curve obtained from pressure profile showing pressure decay at constant saturation. Dotted line shows envelope of peak pressures corresponding to pressure-controlled injection2 53,54 while dashed fine shows envelope of decayed pressures corresponding to the data of Fairweather et al4,3...

Figure 4.4. Typical oscillograms of pressure-jump experiments. Relative change in conductivity for pressure-jumps of 13.1 MPa in solutions of 0.05 M InCI, pH = 3.25. (a) At 383 K showing only pressure decay, (b) At 300.5 K, r = 50 15 /us. (c) At 273.7 K. r = 215 10 jj.s. (d) Solution of 0.10 M a-ketoglutaric acid, pH 1.69. at 274 K, t = 25.8 s. [From Knoche and Wiese (1974), with permission. ...

Pressure release after the dwell period should be very slow until the initial expansion and relaxation has taken place. Typically, this is done with a bleeder valve or a capillary. Sudden pressure decay can result in microcracks or visible cracks as the still entrapped air expands. 

After all drug is dissolved, the osmotic pressure decays and the beads shrink to their equilibrium swelling value, as observed for beads with high water swelling (11,12) during this second release phase normal Fickian diffusion kinetics becomes more important, characterized by a very low rate which depends on the hydro-philicity of the polymer. 

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