Piping pressure testing

Purge points shall be fitted at section isolation valves, at the end of pipe runs and at other suitable positions to facilitate the correct purging of pipes. Pressure test points should be fitted at the outlet of each section-isolating valve. 

Figure 2. Oxidation induction time (OIT not the log-scale) for samples taken at a certain depth (0.9-1.0 mm from the inner wall from PB pipes pressure-tested at 90 C with internal chlorinated water (0.5, 1.0 and 1.5 ppm Cl) as a function of the exposure time. The line is a linear fit of the data. The insert figure shows the results of extrapolation to 01T= 1 min based on OIT data from exposures shorter than tfnax- From Lundback et al.(12) and with permission from Elsevier, UK.

Stress Crack Test Pipe Pressure Test and NCTL Test... 

In the pipe pressure test a pipe section is exposed to a specified internal test pressure. For this an end closure is attached to the section and the specimen is filled with water. Then the open end is closed and attached to a pressurising device and the whole section is immersed in a water bath having a specified test temperature. After some conditioning a pressure is applied and the time until the pipe section fails, the so-called time-to-failure, is measured. Failure is defined as loss of internal pressure. 

Fig. 3.14b. Stress-rapture data from pipe pressure testing of various HDPE resins used for geomembranes...

Sufficient data for HDPE geomembrane materials are not usually available from pipe pressure tests which would enable a direct extrapolation of the stress-rupture curve at 23 °C or 40 °C according to the instmctions of ISO 9080 for small stresses and longer times. The emphasis is on the brittle branch of the stress-rupture curve which results from brittle failures and determines the failure behaviour at small stresses and extremely long service lifetimes. However, one can use the experience accumulated with HDPE materials over many decades (Schulte 1997 Krishnaswamy 2005) and fall back on the extrapolation factors of DIN 16887 or ISO 9080 which were checked by measured stress-rupture data of polyethylene pipes. These factors Ke give the time limits of a permissible extrapolation of the stress-rupture curves in the following sense. Let us assume that hoop stress versus times-to-failure data are measured at several higher test tempera-... 

Figure 3.25 illustrates typical displacement vs. time plots measured in the test on geosynthetic clay liners (Thies 2002 Muller et al. 2004). As shown by the graphs, an initial phase of rapid deformation (primary creep) in response to the compressive load is followed by a stable phase of gradual, continuous deformation (secondary creep). Shortly before failure, the speed of deformation increases sharply (tertiary creep or materials degradation) and failure occurs. Similarly to the pipe pressure test, these tests on textured geomembranes are carried out at 80 °C over a minimum of 10,000 h. 

Durability tests performance based long-term tests under accelerated test conditions pipe pressure tests (Gebler 1989) and long-term shear strength test (Seeger et al. 2000) discussed in Sects. 3.2.13 and 3.2.18 can serve as illustrations... 

In the pipe pressure test, NCTL test and long-term tensile test, a well-defined load condition (internal pressure and tensile force) is specified under which the stress crack develops. In reality, however, geomembranes are predominantly stressed by imposed deformations. Therefore, methods which characterise stress crack resistances under constant strain have also been used for geomembrane tests. 

It is well known from pipe pressure tests that immersion in water enhances antioxidant migration in comparison to air, as was observed in our experiments. Smith et al. attributed this to the influence of carbon black (Smith et al. 1992). Antioxidants are adsorbed at the earbon black surface, whereby the migration process is retarded considerably. During immersion in water some moisture is also present in the HDPE bulk material and this is preferentially adsorbed by the carbon black aggregates. The adsorption of water thereby supersedes that of the antioxidants and decreases the retardation coefficient and increases the apparent diffusion coefficient of the latter. In addition, it was suggested that the antioxidant forms loosely bonded clusters within the dry PE bulk material which may disperse slowly in contact with diffusing water thus enhancing the antioxidant diffusion coefficient (Le Poidevin 1977). 

From this, however, no conclusion can be made on the quality of the seam and on its long-term strength in comparison to the base material. In the pipe pressure test the longitudinal component of stress, i.e. the stress perpendicular to the plane of the seam of the butt-welded seam, is only half of the hoop stress. The weld seam is thus exposed to a much smaller tensile stress than the base material in planes outside the seam. 

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