Pipe walls erosion

T = pipe wall thickness (measured or minimum per purchase specification) t — pressure design thickness, as calculated in accordance with para. IP-3.2.1 for internal pressure or as determined in accordance with para. IP-3.2.2 for external pressure tm — minimum required thickness, including mechanical, corrosion, and erosion allowances W = weld joint strength reduction factor per para. IP-2.2.10... 

The mechanical erosion of a solid surface such as a pipe wall in a gas—solid flow is characterized by the loss of solid material from the solid surface due to particle impacts. The collisions of the particles either with other particles or with a solid wall may lead to particle breakup, known as particle attrition. Pipe erosion and particle attrition are major concerns in the design of a gas-solid system and during the operation of such a system. The wear of turbine blades or pipe elbows due to the directional impact of dust or granular materials, the wear of mechanical sieves by the random impact of solids, and the wear of immersed pipes in a fluidized bed by both directional and random impacts are examples of the erosion phenomenon in industrial systems. The surface wear associated with the erosion phenomenon of a gas-solid flow has been exploited to provide beneficial industrial applications such as abrasive guns, as well. 

The pipe wall thickness is selected to resist the internal pressure, with allowances for corrosion, erosion, and other mechanical allowances for pipe threads, etc. Process pipes can normally be considered as thin cylinders only high-pressure pipes, such as high-pressure steam lines, are likely to be classified as thick cylinders and must be given special consideration (see Chapter 13). 

Line sizes based on velocity limitations are calculated only in special cases where corrosion, erosion or deposits on the pipe wall have to be accounted for or where critical flow conditions exist. 

Truck-tire pipes were made by cutting the bead and sidewall from heavy truck tires and stacking and compressing beads/sidewalls [13]. The bead and sidewall are cut from heavy truck tires to make pipes for roadway cross drainage by stacking and compressing the bead/sidewalls to 2.4 m, held in place with rebars wrapped length-wise around the pipe walls and welded [13]. Sidewalls and faces cut from scrap tires can also be used on erosion control structures. 

Corrosion of the pipe wall can occur either internally or externally. Internal corrosion occurs when corrosive fluids or condensates are transported through the pipelines. Depending on the nature of corrosive liquid and the transport velocity, different forms of corrosion may occur, namely, uniform corrosion, pitting/crevice corrosion, and erosion-corrosion. Figure 3.8 shows an example of internal corrosion that occurred in a crude oil pipeline because of high levels of salt water and carbon dioxide (CO2). 

Water containing 0.01 wt% sand is flowing through a pipe at a flow velocity of 60 m/s. Describe how the annual erosion depth increment in the pipe wall can be calculated on the basis of short duration tests in a similar pipe where the sand concentration in the water is varied in the range 0.1-10 wt% and the flow velocity in the range 20—40 m/s. Assume in the first instance that the deterioration process is mainly erosion and only little corrosion. 

Effect of erosion corrosion on pipe wall thickness. 

NUREG-1344, "Erosion/corrosion induced pipe wall thinning in US nuclear power plants," April 1989. 

In the insertion of any metering device in a pneumatic conveying line, the possibility of erosion of the device as well as the changes in the flow patterns can severely affect the pipe with high erosion rates because of jets of particles being targeted at the pipe wall. Instrumentations should be nonintrusive for best operation. 

In fig, 4 local corrosion by erosion is shown in a pipe with a bore of 100 mm behind a welding. In this case only the nominal wall thickness of the pipe is known (6.3 mm). To calibrate the obtained density changes into wall thickness changes a step wedge exposure with a nominal wall thickness of 13 mm (double wall penetration in the pipe exposure) and the same source / film system combination was used. From this a pcff = 1-30 1/cm can be expected which is used for the wall thickness estimation of the pipe image according to equation (4). 

Fig. 5 Erosion pit inside a reducing pipe fitting, projection technique at 160 kV, profile plot with optical densities of the digitised film. The varying background caused by the geometrical set-up prevents a wall thickness calibration as in fig. 4...

Flow in bends and elbow fittings is more turbulent than in straight pipe, thus increasing corrosion and erosion. This can be countered oy selecting a component with greater radius of curvature, thicker wall, or smoother interior contour, but this is seldom economical in miter-elbows. 

Sentinel holes are used as a simple form of thickness testing. A small hole of about I - 6 mm diameter is drilled from the outer wall of the piece of equipment to within a distance from the inner wall (in contact with the corrodent) equal to the corrosion allowance on the equipment (Fig. 9.11). The technique has been used even in cases where the corrodent spontaneously ignites on contact with the atmosphere. The philosophy is that it is better to have a little fire than a big one which would follow a major leak from corrosion through the wall. When the sentinel hole begins to weep fluid a tapered plug is hammered into the hole and remedial maintenance planned. Siting the sentinel holes is somewhat speculative although erosion at the outside of a pipe bend is often monitored in this way. 

One report stated thickness measurement a short distance away from the rupture showed the line was a nearly full design thickness. Investigators concluded the line failure was the result of the thinning of the Schedule 120 carbon steel 90-degree elbow due to long-term erosion/corrosion. [21] Another story stated the piping was originally or nominally 0.625 inches thick, but had worn down to 0.085 inches. [27] That represents an 86-percent wall loss. 

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