Pipe pressure test hoop stress

Each length of pipe shaFF be given a mill hydrostatic test that will produce In the pipe wall a hoop stress of 95 percent of the minimum specified yield strength. Test pressure shall be maintained for at least 10 seconds... 

A further variant is the testing of pipes by pressurizing, usually with water, and holding a constant pressure until the pipe bursts. The burst is usually a weeping leak. The pressure develops hoop stresses in the pipe, which can be calculated from a knowledge of the pressure and pipe dimensions. The type of curve that now emerges is as in Fig. 1.11. 

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-... 

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. 

When constmction is complete, the pipeline must be tested for leaks and strength before being put into service industry code specifies the test procedures. Water is the test fluid of choice for natural gas pipelines, and hydrostatic testing is often carried out beyond the yield strength in order to reHeve secondary stresses added during constmction or to ensure that all defects are found. Industry code limits on the hoop stress control the test pressures, which are also limited by location classification based on population. Hoop stress is calculated from the formula, S = PD/2t, where S is the hoop stress in kPa (psig) P is the internal pressure in kPa (psig), and D and T are the outside pipe diameter and nominal wall thickness, respectively, in mm (in.). 

Limitations on Design Pressure, P, in Para. PL-3.7.1 (a). The design pressure obtained by the formula in para. PL-3.7.1(a) shall be reduced to conform to the following P shall not exceed 85% of the mill test pressure for all pipes in the pipeline, provided, however, that pipe, mill tested to a pressure less than 85% of the pressure required to produce a hoop stress equal to the specified minimum yield, may be retested with a mill type hydrostatic test or tested in place after installation. In the event the pipe is retested to a pressure in excess of the mill test pressure, then P shall not exceed 85% of the retest pressure rather than the initial mill test pressure. It is mandatory to use a liquid as the test medium in all tests in place after installation where the test pressure exceeds the mill test pressure. This paragraph is not to be construed to allow an operating pressure or design pressure in excess of that provided for by para. PL-3.7.1(a). 

In industry, hydrostatic pressure testing of pipes is still widely used to assess their resistance to this type of failure. Typical results are shown in Fig. 6 [57, 58], where failure times of HDPE pipes are given as a function of the circumferential (or hoop) stress. At relatively high stresses, ductile failure is observed (stage I) although deformation is initially homogeneous, small local variations (arising from variations in the specimen thickness, for exam-... 

Therefore, substituting v = 0.4, the creep hoop strain will be smaller than that in a tensile creep test by a factor (1 — v ), if the hoop and tensile stresses are equal. Examining Fig. 7.6, the tensile stress to cause a creep strain of 3/0.84 = 3.6% after 50 years is approximately 4 MPa. For a 4 bar pressure to cause a hoop stress of 4 MPa, the pipe SDR = 21 by Eq. (14.2). This is the maximum SDR allowed. 

For a specified internal pressure and test temperature the measured times-to-failure exhibit a logarithmic normal distribution. To attribute a certain time-to-failure to the applied test condition (hoop stress and temperature) a fractile of this distribution is chosen, for example, the 5 % ffac-tile, i.e. 95 % of the pipes will have time-to-failure at the specified test... 

The pressure rating of thermoplastic pipe is mathematically calculated from the SDR and the allowable hoop-stress. The allowable hoop-stress is commonly known as the long-term hydrostatic design stress. This is the stress level that can exist in the pipe wall continuously with a high degree of confidence that the pipe wiU operate under pressure for at least 50 years with safety. The American Society for Testing and Materials (ASTM) and the Plastics Pipe Institute (PPl) has adopted... 

The testing process involves subjecting pipe samples to various hoop stress levels, and then recording the time to rupture. For some samples at some pressures, rupture will occur in as little as 10 hours. As hoop stress is reduced, the time-to- failure increases. At some hoop stress level, at least one of the tested specimens will not rupture until at least 10,000 hours (slightly more than 1 year). After the rupture data points (hoop stresses and times-to-failure) for this material have been recorded, the data points are plotted on log-log coordinates as the relationship between hoop stress and time-to-failure. (see Figure 13.3.) A mathematically developed best-fit straight line is correlated with the data points to represent the material s resistance to rapturing at various hoop stress levels. 

The standi test in ISO for measuring the resistance to SCO is the so called British Gas Notch Test that originated in ISO 138/SC4 gas committee. This test is done on a pressurized pipe with a notch depth that is 20% of the pipe thickness. Test temperature is 80 C and hoop stress is 4.6 or 4.2 MPa for PE 100 or PESO respectively. The minimum required failure time is 163 hr. The basic purpose of the test is to assess the resistance to SC3g of the resin. This test is inferior to the PENT test for the following reasons ... 

Elevated temperature sustained pressure tests are conducted to meet the requirements of ASTM F 1924 section 6.2.1. The assembled joints shall be tested in accordance with method D 1598 with the exception of the length of pipe between the joints. Failure of the test is constituted if two of the six joints leak. Tests were conducted using PE 3408 gas pipe. The hoop stress used to calculate internal pressure of the pipe and was 670 psi. The test pressure is calculated as follows P = 2S/DR-1 (P = test pressure, psig), (S = hoop stress), (DR = dimension ratio (OD/wall). The pipe and fitting assemblies are pressurized to 135 psig at 176°F for 170 hours. One-hundred eighty-six fittings have passed the elevated temperature test. 

In the present study we analyze the results of HDPE pipe sustain pressure test at elevated temperature and various hoop stress level (80 C 4.7 5.5 MPa). Some material properties are shown in Table 1. The failure time for each stress level is recorded, when the leak is observed. After the specimen failure, the fracture surface was inspected by optical and SEM microscopy to observe and characterize the fracture mechanism. All brittle fractures observed resulted from through cracks. A number of various size small cracks have also been developed at the time, when the largest crack(s) caused the le. In many cases, multiple through cracks were detected. It offers for analysis a set of cracks grown under the same controlled tenqierature and stress level. All the observed cracks have been originatod from inclusions. The size and location of an inclusion play an important role in failure time. A combination of random size and location of the inclusions results in a large scatter in failure time at the same applied stress, commonly reported for pipe failures [9]. A summary of plied stresses, failure time, inclusion size and location observed on the fracture surface is presented Table 2. The size of the inclusion is reported in wall thickness direction. The normalized apparent depth of inclusion location indicated in the Table 2, was measured as the ratio between the distance from the inclusion center to outer surface and... 

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