19 Questions and Answers on Pressure Pipeline Knowledge

Thread Content

1. What materials are generally used to manufacture pressure pipelines? Pressure pipelines are generally made of steel pipes, concrete (precast, cast in place), and other composite materials. 2. Under what conditions are welding materials selected? The appropriate welding materials are selected based on the properties of the steel being welded; these welding materials mainly refer to electrodes, welding wires, and flux. 3. What are the requirements for inspecting circumferential welds in pressure pipelines? The inspection requirement for circumferential welds is that the number of inspections for each circumferential weld shall be no less than 10% of the total length of that weld. 4. What are the requirements for inspecting longitudinal welds in pressure pipelines? The inspection volume for longitudinal welds shall be no less than 25% of the total length of all longitudinal welds. 5. What welding method must be used for welding pressure steel pipes? Welding pressure steel pipes must use welding electrodes and arc welding methods that are compatible with the material of the pressure steel pipes. 6. What are the basic loads on pressure steel pipes? (l) Internal water pressure. (2) The axial force caused by water pressure at the points where the pipe diameter changes and at the bends in the steel pipe. (3) The self-weight of the metal structure of the pressure steel pipe, the weight of the water inside the pipe, and the self-weight of the abutments and supports. (4) The frictional resistance generated along the piers and expansion joints, as well as the frictional force exerted by water on the pipe wall, when the pressure pipe undergoes axial displacement. (5) The centrifugal force caused by the water flow inside the pipe at the bends of the steel pipe. (6) The force generated by the deformation of the steel pipe due to temperature changes. (7) The axial force resulting from deformation in the diameter direction under the water pressure inside the steel pipe. (8) Active earth pressure exerted by the soil on abutments or pier supports. (9) The force generated when the intermediate pier settles unevenly. 7. How many situations exist for external pressure acting on pressure pipelines? (1) When the water in the pressure pipeline is drained, a vacuum condition occurs inside the pipeline due to the failure of the vent valve; as a result, atmospheric pressure acts on the outside of the pipe walls, causing the pressure pipeline to be under external pressure. (2) For steel pipes buried underground, after the water inside the pipes is drained, the pipe walls are subjected to the pressure of groundwater or soil. (3) The sections of the pipe buried in concrete are subjected to the pressure of the uncured concrete during construction. (4) Grouting pressure. 8. What are the types of vibration phenomena in pressure steel pipes? (1) When vibration occurs in the pressure steel pipe, it appears only in certain sections of the pipe and not throughout its entire length. (2) The vibration mode of the pipe segment is essentially radial reciprocating deformation of the pipe wall, and the pattern of this deformation is similar to the wavy distortion that occurs when the steel pipe fails due to instability under external pressure. The entire circumference may have two or more wavy deformations, or even more. (3) In pressure pipe vibrations, vibrations in the direction of the pipe axis in the form of a continuous beam, that is, reciprocating deformations with the mid-span bending frequency as the amplitude, rarely occur. 9. What effective measures can be taken for vibrating pressure pipes to eliminate vibration? (1) A rigid ring is installed around the outside of the tube wall to improve the ellipticity of the original tube wall. (2) By installing a rigid member, the original natural vibration frequency of the steel pipe is altered, causing it to differ from the frequency of the pressure waves generated by the water flow inside the pipe; this prevents resonance and thus eliminates vibration. 10. What is the diameter range of the steel pipes used as the main load-bearing pipes in this design approach? Saddle-type supports are generally used in pressure pipelines with a pipe diameter of less than 1 meter. 11. What are the main principles for installing access holes on pressure steel pipes? (1) Manholes for pressure steel pipes are generally located above the downstream abutments of the pipe section. (2) Under normal circumstances, the insertion point into the cross-section of the steel pipe should be at an angle of 45 degrees below the horizontal diameter, and it must be located on the side with convenient access. (3) Under special circumstances, the manhole can be installed at the top of the steel pipe or at an angle of more than 45 degrees to the horizontal diameter. 12. What is the role of expansion joints in pressure pipelines? (1) The steel pipe can freely extend or contract along its axial direction. (2) When the steel pipe undergoes axial displacement, the frictional resistance at the expansion joint is very small. (3) The expansion joint not only accommodates axial displacement but also adjusts to the lateral displacement generated by the pipe section. 13. What components does a sleeve expansion joint consist of? It consists of a sleeve, an insertion tube, a water seal packing, and a packing compression ring. 14. Under what circumstances are end caps used for pressure steel pipes? (1) The installation of the capacity in a hydropower station requires phased construction, and the installed steel pipes need to be sealed at the inlet or end using end caps. (2) When a hydraulic test is required for the steel pipe during installation, a plug is used to seal the section of the pipe under test in order to conduct the hydraulic test. (3) Reserve branch pipes in the steel pipe for future projects or other facilities. 15. Why don’t pressure steel pipes have inlet and outlet valves (holes)? When draining water from the pressure pipe, air needs to be introduced from the outside to prevent a vacuum from forming inside the pipe; therefore, an air inlet valve (or port) must be provided. When flushing the pressure pipe, it is necessary to remove the air from inside the pipe, and an exhaust valve (hole) is provided at the highest point of the pressure pipe. 16. When the thickness of the welded part is less than 6 mm, what are the technical requirements for the weld joint? (1) When the thickness is greater than 2 mm, to ensure full penetration, a gap of 1–2 mm should be left at the joint. (2) When the thickness is less than 4 mm, welding is performed on only one side. (3) When the thickness is 5–6 mm, welding should be performed on both sides. 17. What is the typical range of steel plate thicknesses suitable for welding V-groove butt joints? The welded V-groove butt joint is suitable for 6–30 mm. 18. For the internal quality inspection of Class H welds in pressure steel pipes, what equipment should be used? What are the technical requirements for inspection? Class I and Class II pressure steel pipes should be inspected using instruments such as ultrasonic devices or radiographic methods. The technical requirements for inspection are as follows: (l) The inspection length using ultrasonic testing should account for at least 50% of the total length of the weld; for Class 1 welds, this percentage must be no less than 50% ； Type Th welds shall be no less than 30%. (2) The inspection length for radiographic testing shall account for no less than 20% of the total weld length for Class 1 welds ； Class II welds shall be no less than 10%. (3) For exposed steel pipes (i.e., pipes exposed to the atmosphere), if no hydrostatic test is conducted, the inspection length for ensuring that there are no defects in the welds is: 100% for longitudinal seams ； The circumferential seam shall be no less than 50%. 19. What is a suitable minimum turning radius for pressure steel pipes? Why? The turning radius of pressure pipelines is generally recommended to be 3 times the diameter of the pipeline. The turning radius is large; although the hydraulic losses are low, the construction workload is high, resulting in increased costs. The turning radius is small; although the investment is low, the hydraulic losses are high.