High Volume Submillisecond Pressure Relief Emergency System Sigmar 1. K. Wittig’ Institut f u r Allgemeine Mechanik, Rheinisch Westfalische Technische Hochschule Aachen, Aachen, Germany
A new design i s described for a fast opening emergency relief system for high pressure vessels such as refinery blow down systems or nuclear reactor boiler rooms. By a simple electronic triggering circuit for timing, in conjunction with a double diaphragm arrangement and the exploding wire technique, very large pressure ranges can be covered. Experiments show that b y choosing the proper influence parameters a submillisecond opening system of high reproducibility i s obtained. The simplicity of the design allows many modifications and finds wide fields of application in safety systems of industrial facilities as well as in experimental equipment.
C o n t r o l l e d high speed valves for large flow rates and fast opening devices are required for many industrial and research facilities such as high pressure vessel safety systems, refinery blow down systems, light gas guns, and shock tubes. Some of the attendant problems were discussed in detail by Billington and Glass (1960), Walters (1967), White (1968), and Wittig (1969). I n addition to high speed performance, good reproducibility of the opening process is often a necessity. For example, in refineries in chemical plants and in nuclear power plants where pressures can range from subatmospheric to hundred atmospheres and higher, pressure relief systems are necessary for the safety of personnel and equipment. In the event of emergencies such as process upset and resulting explosions, fire, or equipment failure, only by the rapid opening of a relief duct can disastrous consequences be avoided. Another example is the shock tube technique and in particular the single-pulse shock tube introduced by Glick et al. (1955), where relatively large cross sections a t different positions have to be opened in very short times defined by the experimental conditions. In this case the shape and duration of the temperature pulse and the cooling rate effects depend on proper timing. The high pressures often involved, the high forces caused by high pressures, and the inert masses of the parts of the valve make a verification of short, defined, and reproducible opening times extremely difficult. Results of Inoue and Uchida (1968) show that opening times below 1 msec cannot be expected with conventional magnetic, pneumetic, or hydraulic driven valves when the cross sections exceed some mm’ and pressure differences across the valve are above a few atmospheres. Mechanical arrangements with compressed springs are also very slow and the reproducibility is above 100 wsec. Glick et al. (1955) describe an auxiliary shock tube driving plungers to pierce the disks dividing a high pressure from one or two low pressure reservoirs. However, this method ’ Present address, School of Mechanical Engineering, Purdue University. Lafayette. Ind. 47907
is applicable only for relatively thin diaphragms, i.e., low pressure differences. Long delay times between triggering and opening are required, and the reported reproducibility is about 100 wsec. The use of ramset guns and of primacord explosives attached to the face of the valve panels, a method applied by Billington et al. (1962) and used commercially for some time by Fenwal Inc., Ashland, Mass., brings the same problems. The detonation velocity of the explosives (about 7000 m/sec), in addition to the time required for electrical initiation, yields opening times of above 2 msec in combination with the disc rupturing. The arrangement described here is a new concept which cop-bines a double-diaphragm technique with the exploding wire effect. Thus, an emergency opening system is obtained which can be applied under widely varying influence parameters-especially for extensive pressure ranges. The conditions of short time behavior are conserved, and the arrangement can be used in many modifications. Experimental Study
Beylich (1964) used an exploding wire to rupture thin Cellophane and Hostaphan (a polyester product comparable to Mylar) membranes. To investigate whether this method could be made applicable for strong diaphragms, experiments were performed with multilayer Hostaphan and with aluminum disks in a tube of 82 mm id. The pressure difference across the diaphragm varied from 2 atm with thin plastic membranes up to 35 atm with aluminum diaphragms of 2 mm thickness. The wires to be exploded were glued to the diaphragms with small Hostaphan counterplates. A great number of influence parameters were found, such as pressure difference across the diaphragm, the ratio of the pressure difference to the natural bursting pressure where rupturing by pressure only occurs, wire material and dimensions, voltage, and capacitor charge. With a capacitor bank of 20 capacitors (40 pF each, 2.5 kV), which could be added independently and with Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 4, 1970 605
a spark gap as a switch, best results were obtained with copper wires of 0.5-mm diameter a t 140-mm length. Satisfactory results were not obtained with wires of diameters smaller than 0.2 mm or larger than 1 mm. A major concern is the portion of the bursting time of the wire as defined by Nash and Olsen (1962) in the total rupturing time of the diaphragms. The method for measuring the current and voltage across the wire as a. function of time is shown in Figure 1. The oscillogram (Figure 2) shows the typical hehavior of an exploding wire when the response time of the ignition circuit (2 rsec) is subtracted. When we apply Maninger’s, (1964) method of transformation time integrals, Muller’s (1959) analysis to separate the different steps of the process, or the simplified relation used by Nash and Olsen (19621, results show considerable shorter explosion times for the wire than the present experimental data (Figure 2). Gluing the wire to a surface, in addition to the relatively low voltage, appears to be responsible for the long explosion times. However, the reproducibility of the wire bursting time
is 2 rsec or less as in numerous wire explosions under the conditions stated above. This is satisfactory for the system. The hehavior of one- and two-layer Hostaphan diaphragms (each layer 356 pm thick) and of thin 1.5mm and 2.5-mm aluminum diaphragms, bursting after exploding wire initiation, was investigated by recording the rarefaction wave originated after rupturing of the diaphragms. Fast resporise pressure transducers were used for monitoring the waves. The arrangement is shown in Figure 1. The strong A1 disk (3 in the sketch) was not inserted when the rupturing of diaphragm 2 alone was to be tested. The triggerpulse initiates the ignition of the spark gap through special electronics and triggers the oscilloscope. The wire explodes and the diaphragm, 2, prestressed by the gas on the high pressure side, bursts. A rarefaction wave then travels into the high pressure chamber. By subtracting the time which the rarefaction fan needs to reach the transducer, the bursting time can he determined. The results are shown in Figure 3. For polyester memhranes the bursting’times are in good agreement with the times found by Beylich (1964) for thin membranes using optical analysis. The prestressing ratio, i.e., the ratio of the actual pressure difference t o the natural bursting pressure difference, is most important. The diagram shows that the average deviation for the whole process a t a prestressing ratio of 0.9 is below 40 rsec-a value which originally could he expected only for very thin diaphragms. - Polyesterfilm A Aluminuml.5mm 0 Aiuminum 2.5mm
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Figure 1. Measurement of wire explosion and diaphragm opening times 1, middle pressure chamber; 2, Hortophon (polyester) diaphragm: 3, strong aluminum dirk; 4, fast response pressure transducer; 5, fort response current probe; 6, fast response voltage probe
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Figure 2. Current and voltage measurement of an exploding
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I t is also possible to rupture metal, especially aluminum, diaphragms of a maximum of 2.5-mm thickness at 82-mm diameter with the exploding wire method. However, the reproducibility is not so good as with Hostaphan diaphragms (Figure 3). The metal diaphragms are scrihed along two perpendicular diameters for good opening characteristics. Inaccuracies m scribing are the main reason that the prestressing ratio cannot he determined exactly, and therefore it is difficult to obtain good reproducibilities. The results described were unexpectedly satisfactory. However, the main goal in the sequence of experiments was to find a fast, submillisecond, reproducible opening mechanism for almost unlimited pressure differences. In the present stage, it seems to he impossible to burst metal diaphragms thicker than 3 mm with the exploding wire under the previously described conditions. A solution was found by introducing a small doublediaphragm chamber. Figure 1 shows the principle. The natural bursting pressure of thick aluminum diaphragms could he determined within the limits of + 3 atm. By introducing a middle-pressure chamber in which the pressure is so high that this uncertainty is counterbalanced and which is separated from the low pressure side by a plastic diaphragm, a fast opening device for high pressure vessels was devised. As the opening of the strong metal diaphragm is always the slowest part of the process, best results were obtained when the ratio of the actual pressure on the high pressure side to the natural bursting pressure was relatively high. Figure 4 shows the results for an experiment where the process was initiated by rupturing the plastic diaphragm by the gas pressure in the middle pressure chamber. Subtracting the time needed by the rarefaction wave to travel through the chamber and from the A1 diaphragm to the pressure transducer, the bursting time for the A1 diaphragm is 860 psec. This is in good agreement with results obtained by Drewry and Walenta, (1965). If we apply higher pressures and prestressing ratios above 1.3 for low pressures (Le., thin (
