Effects Of Hydrostatic Test On The Quality Of Spiral Welded Pipes

The spiral welded pipe (SSAW) hydrostatic testing, by filling the water into the test pipe, pressure, and pressure maintaining process to detect pipe at a predetermined pressure whether meets the design requirements, is the most intuitive detection means that close to the actual usage of pipeline. Inspection standard clearly requires each spiral welded pipe should be tested by hydrostatic testing, and at a specified level or pressure, the weld seam and pipe body should without broken or leak. Test pipe under hydrostatic pressure loads and residual stresses inherent in the superimposition will lead to some plastic deformation, thereby affecting the steel pipe dimensions, mechanical properties and the distribution of residual stress. These three aspects have intimate connection with the safe operation of pipeline. So the study of influence for the hydrostatic test to spiral welded pipe shape size, mechanical properties, the residual stress distribution is significant.This article describes the hydrostatic testing workflow, as well as the hydrostatic test pressure requirements of the main line standard. Study analyzed hydrostatic test in standard tests under pressure 0.90SMYS spiral welded shape and size, mechanical properties, residual stress changes. Then analyzed the impact of the quality by using the high test pressure of 0.95 SMYS.In the pipe shape and size, this paper analyzed changes in pipe diameter and out-of-roundness based on extensive experimental data before and after the hydrostatic test results, distributed the pipe diameter distribution in the longitudinal direction and out-of-roundness law. After 0.95 SMYS pressure test, the study established the relations according to the ratio of different pipe circumference deformation and the thickness. By verified, the calculated value and actual value error is 6.7%, which can be taken as reference to the engineering application.In terms of mechanical properties, this paper illustrates the stress-strain process of making the pipe. Due to the micro plastic deformation in the hydrostatic test process, under the action of Bauschinger effect and deformation strengthening, the pipe body yield strength will be improved. To validate the conclusion, this paper conducted a number of test, test data shows that the yield strength has increased by 6%-8%.At the same time, the toughness of the pipe body test is conducted before and after the hydrostatic test, the test data show that the hydrostatic test had no significant effect on spiral welded pipe body toughness.At the field of residual stress, this paper analyses the generate mechanism of residual stress in pipes, and introduced the residual stress measurement methods at engineering application.This paper also expounds the influence mechanism of hydrostatic pressure test for pipe residual stress distribution: hydrostatic test can produce trace deformation to release body micro structure stress, macro performance is the steel pipe residual stress reduced after water pressure. Theoretical analysis and practical verify the high test pressure of 0.95 SMYS hydrostatic test is more beneficial to eliminate residual stress.In this paper, through theoretical analysis and the related test, hydrostatic test checked pipe quality, and at the same time, is optimized for the pipe size, mechanical properties and residual stress, and points out the 0.95 SMYS test pressure is better for the spiral welded pipe delivery quality, and is feasible in engineering application.