Tightness Testing and Research Laboratory

Transcription

1 Tightness Testing and Research Laboratory ROOM TEMPERATURE CRUSH RESSTANCE TGHTNESS TESTS ON THE SELCO SEAL 4" cl 150 b SS 316/GORE- TEX GASKET STYLE Report prepared for Mr Mel Lowry SEALNGCORPORATON Mr Nitin R. Parekh OLN CHEMCALS and Mr Jerry Waterland W.L. GORE & ASSOCATES, NC. by Hakim Bouzid and Michel Derne May 1st, 1996 TESTED MATERALS A SELCO Seal 4" c 150 b SS 316/GORE- TEX gasket style was tested at the Tightness Testing and Research Laboratory of the Ecole Poly technique. The gasket style is a NPS 4 c 150 b with a corrugated 316 SS metal insert with a strip of Goretex on both sides. The gaskets were provided by SEALNG CORPORATON for testing. Our laboratory test code for this material is SSO1. TEST PROGRAM Three room temperature CRUSH resistance tightness tests were conducted on the gasket samples. The CRUSH test is performed after the ROTT (ROom Temperature Tightness) test has already be conducted on the specim. The ROTT test results are documted in a separate report.

2 2 PROCEDURE The CRUSH test consists of applying very high compression loads on a gasket specim in order to evaluate both its mechanical and leakage resistance to excessive compression. The maximum applied compressive stress is approximately psi (nominal). The procedure consists of cycling the gasket from a minimum stress of 1025 psi (S 1 stress level of the ROTT procedure) up to the required maximum stress of psi in incremts of 5000 psi with Helium at a pressure of 400 psig. Gasket deflection and leakage are measured after every 5000 psi stress incremt. The CRUSH test procedure, wh performed as a continuity of a ROTT test, is as follows: (1 ) At the d of the ROTT test, the gasket is at the S 1 stress level and the pressure is at 800 psig. The pressure is set to 400 psig and a leakage measuremt is tak. (2) The gasket specim is th loaded to a stress level of S5 ('" psi for soft procedure and for standard procedure) psi. Gasket deflection and leakage measuremts are th tak. (3) The gasket specim is unloaded to a stress level of 1025 psi. Gasket deflection and leakage measuremts are recorded. (4) The compressive stress is th increased to the value of the previous highest stress level incremted by 5000 psi (For example, if the previous highest stress level was psi in step 2, the gasket stress is increased to psi.) Gasket deflection and leakage are recorded. (5) Steps 3 and 4 are repeated up to the required maximum gasket stress (40000 psi wh possible). TEST SPECFCS Dimsions of specims The SELCO Seal gasket samples were provided with the dimsions of an NPS 4 class 150 b in accordance with the definitions specified in the proposed ASTM Draft No.9 for ROTT testing of non-sheet gasketing materials: 4.5 in. D x in. 00. Computation of gasket stress Gasket stress is computed based on the initial gasket area (before loading and defined below). The increase in gasket area due to Poisson's effect of the gasket materials is not tak into account in the computation. Gasket stress values in the ROTT/Crush test are computed based on the initial gasket area (before loading). For this gasket type the stress calculations were based on a loaded area of 8.44 in2. Since the initial loaded area of the SELCO Seal gasket specim is limited by the plat raised faces which are at a diameter of 6.19 in., the approximate gasket area is therefore 8.44 in2 (computation based on 5.25 in. D and 6.19 in. OD).

3 3 Based on this area, loads of 8650 b up to b were applied which correspond to the S 1 stress level and the maximum psi stress level respectively. TEST RESULTS Gasket deflection, leakage and tightness results are prested graphically, for the test, according to the following:.. A plot of Gasket Stress, Sg, vs Gasket Deflection (gasket thickness loss), Dg, on a linear scale.. A plot of Gasket Stress, Sg, vs the Tightness Parameter, Tp, on logarithmic scales (this type of plot is called an Sg-Tpgraph). See appdix 1 for the definition of the Tightness Parameter, Tp The CRUSH test sequce has be idtified (from step 1 to step 11) on all Sg-Tp plots. A rapid and direct comparison of the results betwe two tests can be done by superimposing the corresponding plots. ANALYSS OF RESULTS.. The Sg-Dg graph shows that the gasket specim resisted the crush test... The three Sg-Tp graphs indicate that the gasket specims maintained tightness at high stresses during the crush testing. n effect, from these same graphs, points 2,4,6,8 and 10 show that tightness under high loads is effectively maintained up to the maximum compressive stress of psi. Also, passed the psi stress level, the specims tightness improved considerably and the specims maintained their tightness during the unloadings to S 1 stress levels (1 024 psi).

4 Appdix 1 -Tightness Parameter, Tp: Definition Any specific measure of mass leak rate is strongly correlated to the applied fluid pressure. To permit the comparison of the leakage behaviour of gaskets tested at differt fluid pressures, the effect of pressure must be tak into account. This has led to the definition of a non-dimsional Tightness Parameter, Tp' which relates a giv measured value of leak rate to the pressure at which this leak rate was actually obtained: Tp :: Lrm. ~ p. ( Lrm) 0.5 where: P p+ Lrm Lrm+ = Fluid pressure (MPaor psig) = Referce pressure (0.101 MPa or 14.7 psig) = Mass Leak Rate (mg/s) = Unit Mass Leak Rate (1 mg/s for a 150 mm OD gasket in a joint). n other words, Tpwas developed so that at a giv state of condition of a gasket, the same tightness value be obtained whether leakage is measured with fluid pressurized at 200, 400, 800 psig or any other pressure. Tp is proportional to pressure and inversely proportional to the square root of leak rate. More precisely, Tp can be thought of as the number of atmospheres required to cause a helium leak of 1 mg/sec for a 150 mm gasket OD in a joint. Since this is about the same as the OD an NPS 4 in. joint, the pressure to cause a leak of 1 mg/sec of helium for that joint is its tightness. A higher value of Tp means a tighter joint. Because of the square root, a joint that is 10 times tighter leaks 100 times less.

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