2. The middle graph shows the soil settlement distribution. The horizontal axis is the same as above, the vertical axis (length dimension) is the middle axis as the scale and position of the graph differ from the first graph.
3. The lower graph shows the combined maximum stress distribution along the pipeline. Horizontal axis again as above, vertical utmost left axis (stress dimension) shows the stress scale. The red line above the graph is the allowable stress.
The overall maximum stress occurs at the middle of the roadcrossing due to pipe bending as a result of the maximum soil settlement
The overall maximum stress occurs at the middle of the roadcrossing due to pipe bending as a result of the maximum soil settlement
🆕 Keep pipe analysis records
Most people believe that a computer printout is a sufficient record of a pipe stress analysis. This a big mistake that can be avoided with little effort. Creating a record of your work is about more than keeping a hard copy or PDF of the computer-aided pipe stress analysis. It means documenting a trail of all inputs, not just the drawings used to create the piping geometry. Items that could be included are the piping and instrument diagrams, system parameters, load cases, and any corresponding external forces applied to the piping system, pipe-support locations, and type of pipe support used. Most pipe stress analysis records will fill a three-ring binder.
As most consulting engineers have internal quality assurance/quality control procedures, develop a standard list of the inputs commonly used and corresponding reference for the information. This would provide the checker of a calculation a place to sign off, indicating they concur with the input and acknowledge the source of the input. In the end, your documentation should tell a complete story.
Most people believe that a computer printout is a sufficient record of a pipe stress analysis. This a big mistake that can be avoided with little effort. Creating a record of your work is about more than keeping a hard copy or PDF of the computer-aided pipe stress analysis. It means documenting a trail of all inputs, not just the drawings used to create the piping geometry. Items that could be included are the piping and instrument diagrams, system parameters, load cases, and any corresponding external forces applied to the piping system, pipe-support locations, and type of pipe support used. Most pipe stress analysis records will fill a three-ring binder.
As most consulting engineers have internal quality assurance/quality control procedures, develop a standard list of the inputs commonly used and corresponding reference for the information. This would provide the checker of a calculation a place to sign off, indicating they concur with the input and acknowledge the source of the input. In the end, your documentation should tell a complete story.
Pipe stress analysis computer models are a series of 3-D beam elements that create a depiction of the piping geometry. Three-dimensional beam elements are the most efficient way to model the piping system, but not necessarily the most accurate; and without complex finite element models, it is nearly impossible to account for everything. However, it is known from historical empirical testing that these methods and 3-D beam computer models demonstrate enough behavior that they are a good approximation. In addition, piping codes, such as ASME B31, have safety margins that allow for approximation. That being said, there are some pitfalls with modeling piping systems that one should avoid:
The computer models are only as good as the information entered into them. It is important when developing a pipe stress analysis, as with any finite element analysis (FEA) model, to also understand the physics and boundary conditions of the model.
Elements used to model the piping system have their limitations. One-dimensional beam elements are great for straight pieces of piping, but not so good with pipe fittings (elbows, tees, reducers, etc.). Therefore, ASME has developed stress-intensification factors (SIFs) for piping fittings through empirical testing. They allow for greater approximation without using complex FEA models with shells, plates, and brick elements.
It is important to make sure these limitations are considered when developing a pipe stress analysis. Most pipe stress analyses do not perform like a high-powered FEA software package.
The computer models are only as good as the information entered into them. It is important when developing a pipe stress analysis, as with any finite element analysis (FEA) model, to also understand the physics and boundary conditions of the model.
Elements used to model the piping system have their limitations. One-dimensional beam elements are great for straight pieces of piping, but not so good with pipe fittings (elbows, tees, reducers, etc.). Therefore, ASME has developed stress-intensification factors (SIFs) for piping fittings through empirical testing. They allow for greater approximation without using complex FEA models with shells, plates, and brick elements.
It is important to make sure these limitations are considered when developing a pipe stress analysis. Most pipe stress analyses do not perform like a high-powered FEA software package.
⬇️⬇️ Three-dimensional beam element
The 3-D beam element behaviors are dominated by bending moments. As mentioned above, it is efficient for most analyses and sufficient for system analysis. However, there are downsides to using a 3-D beam element:
1. No localized effects will be seen on the pipe wall.No second-order effects.
2. No large rotation.
3. No accounting for a large shear load.Wall deflection occurs before bending failure. Short, fat cantilever versus long and skinny.
4. No shell/wall effects can be seen
The 3-D beam element behaviors are dominated by bending moments. As mentioned above, it is efficient for most analyses and sufficient for system analysis. However, there are downsides to using a 3-D beam element:
1. No localized effects will be seen on the pipe wall.No second-order effects.
2. No large rotation.
3. No accounting for a large shear load.Wall deflection occurs before bending failure. Short, fat cantilever versus long and skinny.
4. No shell/wall effects can be seen
🛠 Manufacturers Standardization Society (MSS) SP-58:
Pipe Hangers and Supports—Materials, Design, Manufacture, Selection, Application, and Installation recommends support spans to be based on deflection criteria of approximately 0.125 in. or less between supports. The deflection criteria assume a simply supported beam. However, a supported piping system is a continuously supported beam that reduces reaction and moments at each support, further reducing the deflection between supports. This negates the bending moments between supports and reduces the bending moment term of sustained stress.
Pipe Hangers and Supports—Materials, Design, Manufacture, Selection, Application, and Installation recommends support spans to be based on deflection criteria of approximately 0.125 in. or less between supports. The deflection criteria assume a simply supported beam. However, a supported piping system is a continuously supported beam that reduces reaction and moments at each support, further reducing the deflection between supports. This negates the bending moments between supports and reduces the bending moment term of sustained stress.
⬇️ Standard span guidelines
Below are some general thoughts on standard pipe spans to consider:
1. Fluid has a greater impact as the pipe size becomes larger. Water weight is more than pipe weight for 12 in. nominal pipe size (NPS) for standard wall thickness (STD), or greater.
2. When concentrated loads, such as flanges, valves, and piping specialties, are present between pipe supports, the recommended span should be reduced to account for them.
3. A pipe support should be placed within one-third the recommended span of a rotating equipment connection to minimize vertical load and moments at connection. In most cases, this support should be a variable spring to help with adjustment and reduce translation vibration.
4. When piping changes horizontal direction, the recommended span between pipe supports shall be reduced by 25%.
Below are some general thoughts on standard pipe spans to consider:
1. Fluid has a greater impact as the pipe size becomes larger. Water weight is more than pipe weight for 12 in. nominal pipe size (NPS) for standard wall thickness (STD), or greater.
2. When concentrated loads, such as flanges, valves, and piping specialties, are present between pipe supports, the recommended span should be reduced to account for them.
3. A pipe support should be placed within one-third the recommended span of a rotating equipment connection to minimize vertical load and moments at connection. In most cases, this support should be a variable spring to help with adjustment and reduce translation vibration.
4. When piping changes horizontal direction, the recommended span between pipe supports shall be reduced by 25%.
⬇️⬇️Displacement stresses
In most cases, if displacement or expansion stresses are perceived to be a concern (e.g., elevated temperatures), then a computerized pipe stress analysis is required. If a computerized analysis is performed, displacement stresses should be kept at 80% to 90% of what the code allows.
Typically, this recommendation is met by ensuring the equipment connection loads are within published allowable code stresses through adding flexibility to the piping system. Flexible piping systems typically have low displacement stresses because the piping can grow freely.
In most cases, if displacement or expansion stresses are perceived to be a concern (e.g., elevated temperatures), then a computerized pipe stress analysis is required. If a computerized analysis is performed, displacement stresses should be kept at 80% to 90% of what the code allows.
Typically, this recommendation is met by ensuring the equipment connection loads are within published allowable code stresses through adding flexibility to the piping system. Flexible piping systems typically have low displacement stresses because the piping can grow freely.
⬇️⬇️ Jacketed Pipe
Jacketed piping can be idealized as two piping systems running along coincident lines in space. The two piping systems are of two different sizes, so the smaller runs inside of the larger. The internal pipe contains the piped fluid, while the outer pipe (the jacket) is used for protection or to carry a heated fluid to warm that in the inner pipe.
Jacketed piping can be idealized as two piping systems running along coincident lines in space. The two piping systems are of two different sizes, so the smaller runs inside of the larger. The internal pipe contains the piped fluid, while the outer pipe (the jacket) is used for protection or to carry a heated fluid to warm that in the inner pipe.
Jacketed piping systems are modeled by running the jacket elements directly on top of the core elements where the two are concentric. Internal supports (spiders) offer negligible resistance to relative bending and axial displacement, so rigid restraints should be placed between the inner and outer pipe (for example between node points 15 and 1015) only in the local Y- and Z-directions. The end caps connecting the core to the jacket of the pipe are usually much stiffer than either the core or the jacket. For this reason at end cap locations (node points 10 and 25), the inner and outer pipes should share the same node point (i.e., node points 1010 and 1025 should not be used) — this ensures that the rotations and displacements are identical for the two pipes at these locations.
The +Y support acting on the jacket at node point 1020 does not cause any restraint to be inserted between nodes 20 and 1020. Node 20 is included in the model so that interference with the outside diameter can be checked at the 20-1020 cross section. Should there be concerns about interference, a restraint with a gap equal to the clearance between the inner and outer pipes can be entered. If a load develops at that restraint, this indicates an interference.
The +Y support acting on the jacket at node point 1020 does not cause any restraint to be inserted between nodes 20 and 1020. Node 20 is included in the model so that interference with the outside diameter can be checked at the 20-1020 cross section. Should there be concerns about interference, a restraint with a gap equal to the clearance between the inner and outer pipes can be entered. If a load develops at that restraint, this indicates an interference.
The specific modeling process is fairly simple. The inner pipe is modeled first, with the user taking care to place a node point at each location where there is an internal spacer support. Next the inner pipe is duplicated using the element block copy feature (accessed with the List hot key from the input spreadsheet). The entire run of the inner pipe should be copied, with a suitable node increment to ensure that no nodes are duplicated between the two copies.
The second copy becomes the outer pipe. It is necessary to first change the diameter and wall thickness (and possibly the fluid density and temperature) of the pipe on the first screen of the outer pipe; these changes propagate through. Next the user must go through and change the bend radii of each of the elbows in the outer pipe. The first and last node numbers of the outer pipe should then be changed to the same node numbers as those of the first and last point of the inner point — this serves to connect the inner to the outer pipe, a fact that can be confirmed by using CAESAR II's PLOT option.
Finally, the internal supports are modeled by placing guides (and vertical supports on horizontal runs) at each of the support points on the inner pipe, with CNODES to the corresponding points on the outer pipe. Any pipe restraints are then placed on the outer pipe only.
Pipe support design
The supports shall be designed to meet all static and operational conditions to
which the piping system and connected equipment will be subjected. These
conditions shall include hydrostatic test loadings, thermal expansion and
contraction, seismic shock loading, impact, vibration, overloading, spring failure,
erection loading, pressure thrust and fluid momentum (for open discharge
systems), and (for out-door locations) wind, snow, and ice loadings. Code,
seismic and safety classifications, and other necessary data will be defined in the
Project Specification. For supports specified to be designed in accordance with
ASME B31.1, the loads should not be based on increasing the allowable stress
level in hanger components by 20%, as allowed for piping in Paragraph
121.1.2.A.1 of the code.
The supports shall be designed to meet all static and operational conditions to
which the piping system and connected equipment will be subjected. These
conditions shall include hydrostatic test loadings, thermal expansion and
contraction, seismic shock loading, impact, vibration, overloading, spring failure,
erection loading, pressure thrust and fluid momentum (for open discharge
systems), and (for out-door locations) wind, snow, and ice loadings. Code,
seismic and safety classifications, and other necessary data will be defined in the
Project Specification. For supports specified to be designed in accordance with
ASME B31.1, the loads should not be based on increasing the allowable stress
level in hanger components by 20%, as allowed for piping in Paragraph
121.1.2.A.1 of the code.
The materials and fabrication of all supports that are to be furnished by Contractor
and that are detailed on the Purchaser drawings shall be in strict conformance
with such details. However, Contractor shall check design data furnished to him
and shall indicate on the drawings returned to the Purchaser all modifications
recommended by the Contractor. Contractor shall fill in the bill of material
tabulation in the spaces provided, indicating weight of support, correct total
loading, and any/all other necessary data, where not complete by the Purchaser.
and that are detailed on the Purchaser drawings shall be in strict conformance
with such details. However, Contractor shall check design data furnished to him
and shall indicate on the drawings returned to the Purchaser all modifications
recommended by the Contractor. Contractor shall fill in the bill of material
tabulation in the spaces provided, indicating weight of support, correct total
loading, and any/all other necessary data, where not complete by the Purchaser.