Space qualification of the ISO cryogenic rupture discs

Space qualification of the ISO cryogenic rupture discs E. Ettlinger, H. Riidiger and M. W a n n e r Linde TVT, AKW Department, 8023

H611riegelskreuth, FRG

Space cryostats, like the model to be used in the Infrared Space Observatory (ISO), require safety components to protect the satellite, the launcher and the personnel against overpressure in the helium system. The ISO cryostat, which carries 2250 dm 3 of liquid helium, will be equipped with a rupture disc as the ultimate safety component in case of loss of the insulation vacuum. Because it will have to operate under conditions of zero gravity and low pressure drop, the rupture disc has to be located directly on the helium tank and may thus release up to 5 kg s -1 of helium at a differential pressure of 2.6 bar* directly into the insulation vacuum space. The selection and design of the rupture disc, as well as the test and qualification philosophy, are described in this paper. Qualification has been based on a batch of 20 discs to give a high level of reliability for this critical safety component.

Keywords: space cryogenics; helium; safety components; cryogenic rupture discs

Although superfluid space cryostats normally operate at pressures of only a few millibar they may be pressurized by failures during ground handling. Consequently, they have to be equipped with safety components. The Infrared Space Observatory (ISO) funded by the European Space Agency (ESA) will be launched in 1993 and will perform astronomical infrared measurements. For the ISO cryostat, which carries 2250 dm 3 of He II at 1.8 K, a three-stage safety system was chosen ~. The first stage allows small amounts of gas, which may be trapped in the vent system, to be released by an external safety valve. Increased helium evaporation, caused for example by excess heat input to the tank due to operational failures of the heaters or a helium leak into the insulation vacuum, is vented in a second stage through a cold safety valve and released to the atmosphere via the liquid helium filling line of the ISO. The most critical failure, however, would be the loss of the insulation vacuum with subsequent condensation of air on the cold He II tank. According to published data2, up to ~ 4 W cm -2 of heat load would be expected in the case of completely unrestricted air flow. For the ISO cryostat this would imply a maximum vent rate of 12 000 g s -1. Since the maximum penetration into the vacuum vessel expected would be from a broken electrical feedthrough with a diameter of 34 mm, the corresponding maximum air leak reduces the ultimate liquid helium vent rate expected to 5040 g s -1. Any vent system in the cryostat would cause an unacceptable pressure drop for such a high flow rate. On the other hand, the He II tank must be completely enclosed, because under zero gravity all inner surfaces will be wetted by the liquid helium. Therefore, the rupture disc must be located directly on the He II tank (see Figure 1). Once the rupture disc has burst, two safe-

ty valves, each of 100 mm diameter and located at the cryostat vacuum vessel, will open at a pressure difference of 0.4 bar, in order to protect the insulation vacuum vessel from internal overpressure.

"1 bar = 10 s Pa

Figure 1 Photograph showing location of rupture disc on He II tank

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Requirements for rupture disc Based on the safety system of the ISO cryostat, the following requirements have been specified for the rupture disc: set pressure (at liquid helium temperature), Ap = 2.6 bar; set pressure tolerance band, + 10% ; maximum flow rate at T = 4.9 K, >5030 g s-l; maximum operation

ISO cryogenic rupture discs: E. Ettlinger et al. temperature, 353 K; minimum operation temperature, 1.6 K; helium leak tightness, < 10 -s mbar dm 3 s-l; and vibration loads, 22.5 g (sine) and 5.6 g t.m.s. (random). The set pressure of the rupture disc is a compromise between the tolerable pressure of the He II tank and a safety margin added to all normally expected pressure stages in the helium system. On the other hand, it has to be considered that lowering the set pressure of a rupture disc usually increases the tolerance band and the minimum cross-section required. Since rupture pressure varies considerably with temperature, care has to be taken that at ambient temperature the rupture pressure will not fall below the set pressure of the next lowest safety component. For the ISO cryostat the set pressure of the cold safety valve is 1.6 bar, which gives a sufficient safety margin for the rupture pressure. The need for the tolerance band is based on manufacturers' recommended standard tolerance band of 4-5 % for ambient temperature discs. The tolerance band for the cryogenic discs was doubled to take into account liquid helium temperature effects. The required flow rate of 5030 g s -1 is the maximum value, and can only occur with a 100% full helium tank at a temperature of 4.9 K. These thermodynamic points are determined by the minimum heat of vaporization, a constant pressure, p = 2.6 bar. Under all other conditions the helium will reach the 2.6 bar limit at temperatures other than 4.9 K. Therefore, v-l(dh/dv)p and hence the mass flow will be lower in general. The temperature requirements are determined by the lowest expected temperature of 1.6 K during system tests, as well as by the fact that the ISO cryostat shall be baked out at + 80°C prior to evacuation to enhance outgassing of the insulation system. The high helium leak tightness is usual for cryostats, to ensure a proper insulation vacuum, and the standard has to be met, especially under superfluid conditions. To facilitate exchange of the rupture disc in the helium system and to be able to test and qualify the unit in its original configuration, the rupture disc should be easily exchangeable. Finally, mechanical stability against vibrations is a mandatory requirement for all space components.

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Design considerations A reverse buckling type of rupture disc was selected because the rupture disc must also withstand evacuation of the He II tank in an ambient environment during tests. This type of rupture disc consists of a thin membrane which is prebulged towards the inlet side and therefore needs no evacuation support. When the inlet pressure exceeds the set pressure of the disc, the membrane buckles to the opposite side, whereby it is cut by a three-fold knife and the whole cross-section is opened (see Figure 2). A further advantage of the reverse buckling rupture disc is that it may be subjected to pressures of up to 90% of its nominal set pressure wi%out degradation. The rupture disc layout was according to German regulations (ADMerkblatt A1). The required minimum cross-section is given by A = 0.1791 x

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kmol ~); T = absolute temperature (4.9 K); Z = real gas factor (0.22); ~b = exhaust function (0.863); ~x = flow coefficient (0.85); and p = inlet pressure = 2.6 bar. For these specified flow conditions, a minimum area of 891 mm is required which gives 23% of margin with respect to the selected cross-section of 1100 mm 2, corresponding to a free diameter of 40 ram. Special consideration has been given to selection of the membrane material. Nickel alloy was chosen as the membrane material, as it shows a small variation in modulus of elasticity with temperatures between 295 and 4.2 K. To keep stresses on the ~ 50 t~m membrane to a minimum, the membrane was attached using laser spot welding, performed by the manufacturer (Rembe GmbH, Brilon, FRG). Special emphasis was given to the detailed design and manufacture of the welding zone because these factors can have considerable influence on the rupture pressure. The stainless steel housing is sealed against the 5083-aluminium alloy He II tank using Kapton foil.

Qualification philosophy So far, no cryogenic rupture discs have been used for space cryostats in Europe. Therefore, a qualification philosophy had to be established and agreed upon with the ESA. Since verification of the main property of a rupture disc, i.e. its set pressure, can only be done in a destructive way, a batch qualification system based on 20 cryogenic units has been chosen. The number of units chosen, i.e. 20, is a compromise between an acceptable level of test effort and a batch size which allows the simulation of all relevant environmental conditions with at least four units. Prior to performing the main qualification steps, the disc manufacturer had begun a qualification system using selection of membrane thickness and its curvature in a clamped configuration without spot welding. After these pretests, 12 test units were manufactured in their final configuration. These units were subsequently ruptured at 295, 77 and 4.2 K, in order to determine the detailed manufacturing parameters and measure the temperature dependence of the rupture pressure. The actual qualification batch of 20 cold units was divided into a prebatch of six units and a main batch of 14 units. The prebatch was eventually used to perform fine adjustments in the manufacturing process, as necessary according to test results. From the main batch,

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ISO cryogenic rupture discs: E. Ettlinger et al. four units were statistically selected and were integrated into the He II structural and thermal model (STM) tank and the flight model (FM) tank, respectively, to be stored as flight spares (FS). In addition to the 20 cold burst discs delivered some additional 10 units have been manufactured and burst by the manufacturer at 295 and 77 K to verify the manufacturing process. To subject the discs to all the specified environmental conditions a test sequence was established, as shown in Figure 3. The following environmental conditions were simulated (in no cases were rupture pressures outside the tolerance band accepted):

Whereas the leak, pressure, thermal cycle and burst tests at 4.2 K were performed in a standard glass cryostat, a specially designed shake-proof cryostat was used to cool the components to liquid helium temperature and perform the required vibration test.

Test results The helium leak rates were continuously controlled and met the specifications throughout all the liquid helium tests. The 4.2 K rupture pressure for the test units, the prebatch and the main batch, as well as the results of the accompanying tests at 295 and 77 K, are graphically illustrated in Figure 4. The vertical bars indicate the minimum and maximum burst pressures for the appropriate test and are not centred at the average burst pressure. As can be seen from the graphs, the burst pressure increases by a total of 37-44% between 295 and 4.2 K. This behaviour can be qualitatively explained by the change in the modulus of elasticity. The exact details of the manufacturing process (e.g. radii of curvature, welding

1 after the initial incoming quality inspection all ddivered rupture discs were subjected to a pressure cycle test at 90% of the nominal 295 K burst pressure; 2 all units had to pass a helium leak test at ambient temperature; 3 a thermal cycle between 295 and 353 K at 1 bar differential pressure represents the bake-out conditions during ISO tests; 4 the He II leak tightness was measured at 1.8 K for all units including the FM and FS discs; 5 thermal cycle tests between 295 and 4.2 K were performed to check the behaviour after repeated cool-down; 6 pressure cycle tests were performed five times at liquid helium temperatures at up to 80 % of the nominal cold burst pressure; 7 as an additional confidence test, this pressure cycle test was also performed once with the four STM/FM/FS units to reduce the risk of premature rupture within the system; 8 vibration tests at 4.2 K were performed with four discs; 9 for the burst test all 16 units were individually cooled down to 4.2 K until thermal equilibrium was established and the pressure was slowly increased at a rate of l bar rain -~, while the differential pressure was continuously recorded until rupture of the burst discs.

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Cryogenics 1990 Vol 30 March

ISO cryogenic rupture discs: E. Ettlinger et al. parameters, local stresses and inhomogeneities of the membrane) obviously also contribute to the temperature dependence. This effect is clearly seen when comparing results of the prebatch with those of the main batch. Both batches were manufactured according to the same procedure, as shown by the close agreement of the 295 K burst pressures. At 4.2 K, however, the average rupture pressure of the main batch is 20 % higher than that of the prebatch. A significant observation made during all cryogenic rupture tests performed so far is the increase in spread of the burst pressures at 4.2. K. Whereas at 295 K typical tolerance bands are as small as +3.3%, the low temperature tolerance band increased to ± 6.5 % for the ISO rupture discs investigated (which, however, is still within the required bandwidth). No correlation could be found between the rupture pressures and the different sequences of environmental tests performed on individual discs. The average rupture pressure of the main batch was determined to be 2.80 bar, which exceeds the specification by 0.2 bar. This specification must be met with respect to decreasing the set pressure because of the risk of premature opening (overlap with other safety components). The specification is somewhat arbitrary with respect to maximum opening pressure, as there is a significant margin in the tank strength, and so this higher value was accepted.

Conclusions Extensive investigations have been performed with the rup-

ture discs for the He II tank of the ISO cryostat. The reported results for the ISO rupture discs are confirmed by additional qualification tests for other space cryostats and with discs from a different manufacturer. Common to all the cryogenic tests is that both the absolute burst pressure, as well as the tolerance band at liquid helium temperature, cannot be predicted theoretically with sufficient confidence. Therefore these figures have to be evaluated by statistical tests within a batch qualification programme for each type of disc. In any case, the structural design of the cryogenic pressure vessel should exhibit a sufficient margin to allow for a certain level of variation in the actual burst pressure of the rupture discs.

Acknowledgements This work has been performed under MBB-subcontract No. R3141-0545R. The ISO prime contractor of ESA/ESTEC is Aerospatiale. The authors acknowledge helpful discussions with F. Kesting from Rembe GmbH.

References 1 Seidel, A., Wanner, M. and Davidson, A. Proc ICEC 12 Buuerworths, Guildford, UK (1988) 669 :2 Lehmann, W. and Zahn, G. Proc ICEC 7IPC Science and Technology Press, Guildford, UK (1978) 569

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