Experimental Execution

The preparation of test setup was done in the same way for every single test series.

After build-up a leak test was done to avoid any leakages at the tests setup. That test had to be repeated after every modification, regeneration or maintenance of test setup.

A daily inspection of the setup was done after startup to ensure the functionality of all parts. Afterward, the heating devices were switched on and the test temperature of T = 40 °C was adjusted. The evacuation of the whole setup was necessary to avoid any influences of air resided in setup and the formation of explosive atmospheres when hydrogen is discharged.

5.3.1 Determination of Burst Pressure

The investigation of pressure resistance of hollow fibers made from different glass compositions with different ratios of diameter to wall thickness comprehended the determination of the burst pressure of tested single fibers.

The surface of prepared single hollow fibers was investigated for obvious defects and afterwards connected to the setup. Hereafter the test sample had to be evacuated.

When connecting and evacuation were finished the buffer volume was charged with gas by keeping the valve between buffer and supply line closed. At a specific inner pressure the pneumatic valve was opened as well as the pneumatic valve to the first test sample.

At the point of collapse of the fiber, the gas flow had to be interrupted immediately.

Consequentially, the procedure had to be repeated for each single of the four connected test samples. When all four burst pressure tests were carried out, the pressure in the complete setup was released.

The maximum pressure of the test set-up was p = 150 MPa, limited by the compressor.

A constant pressure increase in the test samples was able to be controlled by a needle valve up to a pressure of about p = 100 MPa. From that pressure, a compensation of the pulsation of the compressor outlet and an appeasing of the gas flow was not realizable even with the enlargement of buffer volume. In that case, the pressure increased stepwise due to compressor strokes to burst pressure.

5.3.2 Burst Pressure after Storage in Different Air Humidity

The knowledge regarding glass corrosion including interaction with air moisture led to the investigation of the influence of different air humidity on pressure resistance of hollow glass fibers. The prepared single fibers were stored for seven weeks at controlled conditions in atmospheres with different air humidity. Therefore, two different sated saline solutions were confected and filled in two separate closed containers. Due to the saturation of the solution, in the atmosphere above the saline solution of a closed system defined constant air moisture ensues dependent on temperature and dissolved salt. Here the vapor pressure of the solvent is decreased as well as the relative air humidity above solution [155], [156]. Thus, two separate solutions, first of magnesium chloride (MgCl₂) and second of sodium chloride (NaCl) in water, were confected. The relative air moisture adjusted above the liquid phase is given in Table 9 for a storage temperature T_storage = 35 °C.

Table 9: Literature and measured values of relative air humidity above confected saline solutions

Salt dissolved in water

Literature value of relative air humidity ϕ

[%]

Measured relative air humidity ϕ [%]

MgCl2 32.05 [157] 30.9

NaCl 74.87 [157] 75.8

The hollow fibers hung free in the atmosphere without any contact to the other fibers completely surrounded by the humid air. After finishing the storage time the burst pressure of the single glass fibers were determined in the final stage. Thereby, the tests were carried out with the same procedure described in chapter 5.3.1.

5.3.3 Influence of Cyclic and Static Pressure Load

Cyclic Pressure Load

After preparation of the test samples as described in chapter 5.2 the single fibers were connected to the test setup. The test samples were evacuated and the buffer volume filled with pressurized gas. In the case of the cyclic pressure procedure, all four fibers were loaded at the same time up to a predefined pressure of p_cyclic = 50 MPa. When pressure was reached the buffer volume was separated from the supply line and refilled with gas. The pressure inside the hollow fibers was released to the exhaust and fibers were relaxed. Thereafter, the exhaust was closed and the hollow fibers filled again with gas up to p_cyclic = 50 MPa. That procedure was repeated up to a pre-defined number.

The different numbers of cycles n_c were n_c = (10; 30; 50; 100) and valid for all test series. After reaching the number of cycles the pressure was increased till the collapse eventuated and the burst pressure was reached. That was done for every test sample particular to ensure the correct measurement of burst pressure.

Static Pressure Load

After connecting the test samples to the setup the test plant was evacuated. The buffer volume was filled with gas and afterwards gas was piped to test samples. All connected fibers were loaded with pressure of p_static = 50 MPa. The pressure load was continuously held on that pressure for a pre-defined period t_static and were t_static = (300; 1800; 3600) s.

At the end of the specified time, the pressure inside the hollow fibers was increased up to burst pressure. The single hollow fibers had no time to relax first. Again, it was done for every single fiber particular to ensure the measuring of the correct pressure value at the point of collapse.

6 Results and Discussion

The experimental tests were carried out to determine the burst pressures of single test samples. Thereby, a single hollow glass fiber was loaded with increasing inner pressure until a collapse of the hollow fiber eventuated. This pressure value of every single fiber was defined as burst pressure. The information of the pressure resistance is a safety-related substance data, an important component property and a mechanical characteristic for glass structures. Pre-tests were carried out to investigate the influence of different glass types on the pressure resistance of the hollow glass fibers. Based on the data, hollow glass fibers with the highest pressure resistance were chosen to determine the effect on the burst pressure of several parameters, such as air humidity, different diameter and wall thicknesses, cyclic and static pressure load as well as the effect of coatings. The test results were evaluated by using the Weibull distribution as the statistical instrument.