Resealing pressure

Resealing Pressure the pressure after valve opening under pressure that the internal static pressure falls to when there is no further leakage through the pressure relief valve. See Figure 7-7A. 

Resealing pressure The value of decreasing inlet static pressure at which no further leakage is detected after closing. The method of detection may be a specified water seal on the outlet (API 527) or other means appropriate for this application (see Section 4.2.3). 

Chapter 10 presents the revised empirical static bubble point model for cryogenic liquids. Model dependencies are systematically presented to explain the trends in the data. Chapter 11 echoes Chapter 10 by presenting a refined reseal pressure drop model, using cryogenic data to build the model. Chapter 12 presents a new steady state quasi-3D... 

Analogous to the bubble point pressure, the reseal pressure can be defined from a simplification of the general 3D YT.E for the pressure drop across a curved L/V interface embedded within the 3D space of the mesh. Consider the L/V interface formed within the LAD mesh screen as shown in Eigures 3.16 and 3.17. Retaining assumptions 1-4 from the bubble point model in Section 3.2.2, the following additional assumptions are required to solve the reseal pressure ... 

Equation (3.70) is the simplified room temperature reseal pressure model. Equations (3.16) and (3.70) represent the breakthrough and reseal pressure models for screen channel LADs operating in room temperature storable liquids, respectively. Comparing the two equations, the measured breakthrough and reseal pressures are thus different on account of differences between advancing and receding contact angles as well as differences between the size of the pore throat and pore mouth. 

The same IBP test configuration used to measure the bubble point is the ideal test configuration for measuring the reseal pressure for a given screen. Each controlled bubble breakthrough test allows for a controlled reseal pressure test. Details of the inverted reseal pressure test are reserved for Chapter 4 along with the IBP test configuration. 

Effective reseal diameters can be estimated using the three methods outlined in Section 3.2.5 for specifying the effective pore diameters. Reseal diameters can be determined through room temperature reseal pressure experiments with a liquid of known surface tension using a modification of Equation (3.70) (Method 1) ... 

Clearly the optimal mesh for a particular mission requires trading all of the aforementioned influential factors against one another. Space flight requirements which include mass flow rate, acceleration level and direction, and thermal environment dictate selection of the screen for a particular mission. The primary performance parameters governing screen channel LAD design are the bubble point (and reseal pressure) and FTS pressure drop, while secondary parameters are the wicking rate, screen compliance, and material compatibility. 

The purpose of this chapter is to present the LAD performance experiments carried out in room temperature liquids. Bubble point and reseal pressure tests for a 325x2300, 450 X 2750, and 510 x 3600 Dutch Twill screen are conducted in storable liquids, methanol, acetone, IPA, water, and binary methanol/water mixtures of various methanol concentrations. First screen pore diameters are estimated based on analysis from scanning electron microscopy and historical data. Experimental results are used to compare methods for determining effective pore diameter. Next, contact angles are measured for both pure and binary mixture fluids using a modified version of the Sessile Drop technique. Then, the equation of state analysis from Neumann and Good (1979) is used to determine the critical Zisman surface tension for stainless steel LAD screens, which... 

FIGURE 4.4 Raw Differential Pressure Transducer Signal, Bubble Point, and Reseal Pressure as a Function of Time. 

Bubble point tests were then repeated numerous times at identical conditions to ensure quality results, repeatability, and consistency and to null out the possibility of defects by ensuring bubbles would break through various locations on the screen. A sample test run is depicted in Figure 4.4 where the pressure differential at break through and at reseal is superimposed on the raw DPT signal. Note that the reseal pressure is always lower than the breakthrough pressure. 

Room temperature reseal pressure data was collected simultaneously with bubble point data. The same inverted bubble point test configuration used to measure the bubble point was used to measure reseal pressures. A bubble point test ceases once pressurant gas breaks through the wetted pore. To commence a reseal point test, the pressurant gas flow rate beneath the screen is slowly reduced in fixed quasi-static increments to slowly encroach upon the differential pressure across the screen at which the screen reseals. Eventually the screen rewets/reseals itself as evident in visualization of no more bubbles... 

Figure 4.4 illustrates this effect for a room temperature bubble point and reseal test where the corrected reseal pressure is superimposed on the raw DPT signal. As shown, the reseal pressure is always lower than the bubble point pressure. Unlike bubble point data reduction, the DPT across the screen carmot alone be used to determine screen reseal. Rather, time synchronization with the visualization system is required to determine the exact differential pressure across the screen at reseal. Sole reliance on visualization makes reseal point data inherently noisier than bubble point data. 

Room temperature reseal pressure data is shown in Figure 4.15 for a 325x2300, 450 X 2750, and a 510 x 3600 screen in IPA, methanol, acetone, and water. Also plotted in Figure 4.15 is a best fit line to the data. The exact same trends in bubble point pressure are seen in reseal pressures. Comparing Figures 4.6-4.15, all reseal pressures clearly lie below the corresponding bubble points. For all data collected here, reseal pressures are approximately 90% of the bubble point data. 

Trends in room temperature reseal pressure data mirrors trends in bubble point pressure data. All reseal pressures collected here are about 90% of the corresponding bubble point values. Operationally, this implies that only a ->10% reduction in differential pressure across the screen is required to reseal the screen and prolong the point of total LAD failure to yield a higher overall expulsion efficiency. Wicking rate test results performed in IPA align nicely with historical trends as coarser meshes outperform finer meshes. 

For any LAD mesh type, a pore throat defines the point within the pore where cross-sectional area is a minimum for a gas or vapor bubble to pass through the wetted screen, while the pore mouth defines the point where area is a maximum. The effective pore diameter, and thus bubble point pressure, is related to the pore throat. Statistically, the screen breaks down when a gas bubble passes through the largest pore throat, and thus path of least resistance, of the LAD screen. Likewise, the largest pore mouth controls the screen reseal pressure. 

The simplified reseal pressure model presented in Chapter 3 predicts reseal pressures well when reseal diameters are based on reference fluid tests (Method 1) or through summation of historical data (Method 2) ... 

If reseal diameter is known, the reseal pressure equation can theoretically be used to determine the reseal point of any fluid with a known surface tension. However, the same problem arises with cryogenic reseal data as with the cryogenic bubble point data. The room temperature prediction value matches neither the non-condensable or autogenous pressurant gas case. In addition, the room temperature model cannot be used to predict reseal pressures of subcooled cryogenic liquid states or elevated pressurant gases. Therefore, the new model must therefore address the following three discrepancies that exist between cryogenic reseal pressure data and simplified room temperature model. These are ... 

As mentioned in Section 3.9.2, there are only three previous studies where reseal data was reported. Reseal pressure data was collected alongside all bubble point test data from both room temperature as well as cryogenic bubble point tests for a 200 x 1400, 325 x 2300, 450x2750, and 510x3600 Dutch Twill LAD screen in IPA, methanol, acetone, water, LH2, LN2, LQX, and LCH4. A total of 4836 reseal pressure data points, of which 4815 were new points, were collected, processed, and analyzed to develop this model. 

The room temperature reseal diameters from Section 3.9 can be updated based on inclusion of room temperature data from Chapter 4 into the historical set of data. To determine the reseal diameter, the value was essentially back calculated from the plot of reseal pressure versus the product of surface tension times the cosine of the receding contact angle for all fluids. Uncertainties for reseal diameters were estimated in the same way as effective pore diameters as outiined in Section 10.3.1. 

FIGURE 11.1 Ratio of Room Temperature Reseal Pressure to Bubble Point Pressure versus the Fineness of the Screen. 

A similar relationship can be derived using the number of warp wires per square inch instead. Therefore both the pore and reseal diameters obey an inverse dependence on the shute to warp diameter ratio, and not the number of pores. Equation (11.3) can be used to determine the reseal pressure for any new LAD screen through interpolation. 

Thus the simplified room temperature reseal pressure model can be modified to account for differences in performance between different pressurant gases at cryogenic temperatures ... 

Fitting various functions to the NBP data revealed that nsat,Rs for reseal pressure could again be best fit using a simple constant for each screen/liquid/pressurant gas triplet. Tables 11.2 and 11.3 list the fitting parameters used for computing cryogenic reseal points in normally saturated liquid states using the non-condensable and vapor pressurants, respectively. 

Trends are as follows First, for all three screens and both pressurization schemes, examination of both tables reveals that, for a given mesh such as the 325 x 2300 screen, sat,RS decreases with decreasing temperature. This indicates reseal pressure always... 

FIGURE 11.3 Model Generated 325 x 2300 Reseal Pressure Ratio as a Function of the Reduced Temperature for Normally Saturated Liquid Taken Over the Range of Conditions of the Data. 

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