Hydrostatic pressure testing procedure

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.). 

For a given test system it will be necessary to define the test pressure, the duration and the test fluid. Where the test includes both piping and items of equipment, specified test pressures of both must be reviewed before deciding on the value to be chosen for the test. The majority of tests will be hydrostatic with suitably treated water. However, there will be instances where the entry of water will not be allowed, in air instrument piping for example. In these cases the test will be pneumatie, usually with air. The procedure followed must incorporate the necessary safety precautions, with prior formal clearance by the site safety department, to take account of the energy that would be released in case of rupture. Pressure testing is normally a hold point. 

The ASME Codes recommend that hydrostatic tests be run at a temperature that is usually above the nil-ductility temperature of the material. This is, in effect, a pressure-temperature treatment of the vessel. When tested in the relatively ductile condition above the nil-ductihty temperature, the material will yield at the tips of cracks and flaws and at points of high residual weld stress. This procedure will... 

When the seal maximum pressure is such that the gas test must be conducted well below rated discharge pressure, then another test, or substituted test, may be invoked in which the seals are removed. It may require removing the rotor however, with this procedure the joint makeup still remains a problem when the rotor is replaced. However, without the seals, the test described earlier is performed, using the rated discharge pressure rather than the maximum seal pressure. The acceptance test is the same also no leaks should be detected. The latter test should help to still controversy about minor joint leaks on a hydrostatic test, if the casing is proven at rated pressure on gas. 

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