Problems encountered during the use, installation and manufacturing of pipe elbows

Pipe ebows are common accessories in pipeline systems and are used to change the direction of pipelines. Some problems that may occur during the use, manufacturing and installation of elbow pipe fittings are mainly in the following aspects:

1. Corrosion problems

a. Local corrosion

In some complex media environments, such as fluids containing corrosive components such as chloride ions and sulfides, local corrosion is prone to occur at the elbow. Because the elbow changes the flow direction of the fluid, the flow state of the fluid at the elbow becomes complicated, and it is easy to form turbulence, vortexes, etc., which will lead to uneven concentration of corrosive media in local areas. For example, in a chemical pipeline system, stainless steel elbows that transport brine solutions containing chloride ions may experience pitting, that is, tiny corrosion pits appear on the surface of the elbow. These corrosion pits will gradually deepen and may eventually penetrate the elbow wall, causing leakage.

Crevice corrosion is also a common form of local corrosion of elbows. When an elbow is connected to other pipe fittings or pipes, if there is a gap in the connection, such as a gap formed by improper installation of the gasket during flange connection, the corrosive medium will accumulate in the gap, forming a local high-concentration corrosion environment, accelerating the corrosion of the elbow at the gap.

b. Overall corrosion

If the fluid in the pipeline system is highly corrosive, and the material of the elbow is not properly selected or the protective measures are not in place, overall corrosion may occur. For example, in some acidic environments, carbon steel elbows may suffer from overall corrosion, resulting in a gradual thinning of the elbow wall thickness. This corrosion may occur evenly over the entire elbow surface. Over time, the load-bearing capacity and sealing of the elbow will be affected, and may eventually cause safety accidents such as pipeline leakage.

2. Wear problems

a. Scouring wear

When the fluid flow rate in the pipeline is high, especially when it contains solid particles, the elbow part will suffer from scouring wear. As the fluid changes direction at the elbow, the solid particles will have a strong impact and friction on the inner wall of the elbow. For example, in the ash conveying pipeline of a thermal power plant, the fluid containing a large amount of coal ash particles will continuously scour the inner wall of the elbow when passing through the elbow, causing the elbow wall thickness to thin. If the material hardness of the elbow is not enough or the wall thickness design is unreasonable, the scouring wear will be more serious, and may even cause elbow perforation in a short time.

For some high-speed fluid delivery systems, such as some light oil delivery pipelines in petrochemicals, even if there are no solid particles in the fluid, the high-speed fluid will also have a scouring effect on the elbow, affecting the service life of the elbow.

b. Cavitation wear

In some special fluid systems, such as pipelines containing steam or gas-liquid mixed phases, cavitation may occur when the fluid passes through the elbow. When the pressure drops below the saturated vapor pressure of the liquid, the liquid will vaporize to form bubbles, which will suddenly collapse in the high-pressure area. The impact force generated when the bubbles collapse will damage the inner wall of the elbow, forming cavitation wear. Cavitation wear will cause defects such as pitting and pits on the surface of the elbow, reducing the strength and sealing of the elbow.

3. Stress problems

a. Installation stress

During the installation of the pipe elbow, if it is not installed properly, installation stress will be generated. For example, during the assembly of the pipeline system, if the elbow is forcibly connected to other pipe fittings without considering the thermal expansion and contraction characteristics of the pipeline or the manufacturing tolerance of the elbow itself, a large tensile stress or compressive stress will be generated at the elbow. These installation stresses will put the elbow in an unfavorable stress state before it is put into use. Once the pipeline system starts to operate, under the action of internal pressure and external loads (such as pipeline deadweight, vibration, etc.), the elbow is prone to deformation, cracking and other problems.

When installing elbow pipe fittings, if the joints cannot be firmly connected to the pipeline, problems such as water leakage, seepage, and falling off will occur.

b. Operation stress

When the pipeline system is in operation, the elbow will be subject to various stresses. In addition to the stress generated by the internal fluid pressure, it will also be affected by thermal stress. For example, in a thermal pipeline system, due to temperature changes, the pipeline will expand and contract. As the part where the pipeline changes direction, the thermal stress will be concentrated at the elbow. If the thermal stress exceeds the allowable stress of the elbow material, it will cause deformation, fatigue cracks and other problems in the elbow, seriously affecting the safe operation of the pipeline system.

4. Manufacturing quality problems

a. Dimensional accuracy problems

During the manufacturing process of pipe elbows, if the dimensional accuracy is not well controlled, it will affect its connection with other pipe fittings and pipelines. For example, if the curvature radius of the elbow is inaccurate, it may cause the pipeline system to be unable to dock smoothly during installation, requiring additional adjustments or even cutting, welding and other operations, which not only increases the installation cost, but also may affect the overall quality of the pipeline system.

The port size deviation of the elbow will also cause problems. If the diameter deviation of the port is too large, it will cause a loose seal when connected to the adjacent pipe fittings, which is prone to leakage.

b. Uneven wall thickness problem

Imperfect manufacturing process may cause uneven wall thickness of pipe elbows. For example, when casting elbows, due to defects in the casting process, the local wall thickness may be too thin. When the elbow with uneven wall thickness is subjected to internal pressure, the thin part will first bear greater stress and is prone to rupture, thus affecting the safety of the entire pipeline system.

Read more: 90 degree pipe elbow or Pipe elbow size selection