Stresses and Loading Conditions in Pipes

Because of the inertia effect, the pipe system will resist the force by creating stress through an internal resistance force. So, first and foremost, we must understand the pressures that cause these stresses. Pipe stress can come from many different places. Hence, it is vital for hot and cold pipes to combine the effects of all forces involved to ensure safe operation, especially for the industry experts like the piping stress analysis consultants.

Correction of pipe stress problems is usually the most difficult to undertake an action plan since it is expensive, exposes the plant to potential safety issues, and shuts down the processing unit. To help you out, in this article, I will talk about the various pipe loading conditions and the types of piping stresses.

Types of Loading Conditions

The following are some of the several types of loads that cause stress in a pipe system. These are:

Sustained Loads

Pressure - An internal or external pressure load applied to a fluid transport pipe. A pipe carrying a high-pressure fluid is under net internal pressure, whereas a pipe under vacuum or with a jacketed pipe covering is under net exterior pressure. This internal piping pressure usually results in tensions in the pipe wall rather than loads on the pipe supports. This is because the pipe wall tension balances the pressure forces, resulting in zero pipe support loadings. Internal and external design and operating pressures, as well as hydro test pressure, are included in this type

.

Weights - It is the pipe's weight, including the weight of the contained fluid, pipe fittings, and other inline components. This type of load is present throughout the pipe's life lifetime. Bending is caused by these loads, and the bending moment is proportional to normal and shear stresses. This load is taken care of in horizontal pipes by adding pipe support on a regular span to prevent pipe sagging. This load is taken care of in vertical pipes by providing enough load-bearing support in the horizontal section right before/after the vertical section or lug support in the vertical section.

Occasional Loads

Steam Hammer - Steam hammer is defined as a pressure surge caused by the brief passage of superheated or saturated steam in a steam line caused by rapid stop valve closures. Even though the flow is transitory, only the unbalanced force along the pipe segment that tends to generate piping vibration is estimated and applied to the piping model as a static equivalent force for piping stress analysis.

Reaction Force of the Safety Valve - Safety valve discharge reaction forces are treated as a one-time load. In an open discharge installation, the reaction force due to steady-state flow following the opening of a safety relief valve can be computed following ASME B31.1 Appendix II and applied to the piping model as a static equivalent force.

Wind Load - Piping that is outside and above a specific height is exposed to the wind and must be constructed to resist the maximum wind velocity expected for the plant's operating life. The wind force is calculated using wind pressure at various heights.

Seismic Load - One of the fundamental ideas in earthquake engineering is seismic load, which refers to applying earthquake-generated agitation to a structure. It occurs at a structure's contact surfaces with the earth, neighboring structures, or gravity waves from a tsunami.

Water Hammer - When a fluid, typically a liquid, but sometimes also a gas in motion, is suddenly forced to stop or change direction, it causes a pressure surge or wave (momentum change). When a valve at the end of a pipeline system closes abruptly, a pressure wave propagates through the pipe, causing a water hammer. Hydraulic shock is another name for it.

Snow loads in locations where snowfall is common, sand loads in areas where sand storms are common, and rail and truckloads for buried pipework are also included in this category that cannot be neglected by piping stress analysis consultants.

Types of Piping Stresses

A piping system generates the following types of piping strains.

Normal Stresses - Normal stresses act in the direction of the material structure's face. Normal stresses can be tensile or compressive, and depending on the application and type of load, they can be delivered in multiple directions. In a plumbing system, there are three types of normal stresses. They are as follows:

Axial or longitudinal stress - Axial Stresses, also known as Pipe Longitudinal Stresses, are normal stresses parallel to the pipe centreline axis's longitudinal axis. Axial stresses emerge in the pipe when external loads are applied in the axial direction.

Hoop or circumferential stress - Hoop Stress is a type of normal stress that acts perpendicular to the axial or circumferential direction. Internal pressure causes hoop tension. This tension develops tangentially to the cross-section under internal pressure loading. Because hoop stress is twice that of longitudinal strains, it is critical.

Bending Stress - Bending stresses emerge in a pipe when loads are applied in a plane parallel to the pipe's axis. Temperature, pipe weight, contents weight, snow and ice, wind, and earthquakes are all possible causes.

Longitudinal Stress - This stress is generally applied to the pipe's cross-section.

Radial Stress - It is a normal stress-induced by internal pressure parallel to the pipe radius. Internal design pressure at the interior pipe surface varies from atmospheric pressure at the outside pipe surface.

Shear Stress - It acts in a plane parallel to the face of the material crystal structure's plane. This stress causes the slipping tendency of one plane against the other. This is ignored because the shear stresses induced by shear forces in a piping system are minor.

Thermal Stress - When the pipe's free thermal movement is hindered, thermal or expansion stress is created in the system. The pipe is installed at room temperature or atmospheric temperature, and it transports fluids of various temperatures throughout the operation. As a result, the length of the pipe changes as the temperature changes.

Conclusion

To comprehend and build a pipe system that will last for years, you must look into the total forces and each one individually. These stressors differ greatly from one another, and they behave in a dynamic, ever-changing manner as situations change.

Judith Morrison is an expert in the field of industrial engineering and writes articles related to piping, civil, equipment engineering related articles.