In this type of support arrangement, pipe is fixed with reference of directions other then the direction in which weight of pipe and containing fluid is acting. It must satisfy an imposed strain pattern rather than being in equilibrium with Rap and sex external load. This type of Process pipe models act throughout the life cycle of pipe. No accounting for a large shear load. Pipe material must withstand these stresses at the prescribed temperature and pressure conditions. Several disconnected and independent piping segments can be created in one file See Knowledge Base. Movement in downward vertical direction, mainly ppipe to Prpcess weight of pipe and containing fluid, is not allowed. By taking advantage of our services to produce 3D piping models you will:.
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Learning objectives. Pipe stress analysis is an analytical method to determine how a piping system behaves based on its material, pressure, temperature, fluid, and support. Pipe stress analysis is not an accurate depiction of the piping behavior, but it is a good approximation.
The analytical method can be by inspection, simple to complex hand calculations, or a computer model. The computer models can vary from 1-D beam elements to complex, finite element models. For instance, if it is a water system with no outside forces applied to the piping system, inspection or hand calculations are usually sufficient. Understanding pipe stress analysis software does not make for a solid foundation of pipe stress analysis. There are many piping codes and standards that could be used during a pipe stress analysis depending on the application power, process chemical, gas distribution and location country or local jurisdiction.
The physics of pipe stress analysis does not change with piping code. Pipe stress analysis should be done primarily to provide safety to the public, whether you are designing a building heating system or a high-pressure gas line in a refinery. Public safety is paramount. On a horrible day, someone is killed. Another reason a pipe stress analysis is performed is to increase the life of piping.
Both have moving parts and must be designed and maintained properly to ensure a proper life. Pipe stress analysis also is used to protect equipment, because a pipe is nothing more than a big lever arm connected to a delicate piece of equipment.
If not properly supported and designed, it can have devastating effects on that equipment. There are several common reasons that could warrant a pipe stress analysis, in addition to those above. They include:. 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. 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. 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 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:. There are five primary piping stresses that can cause failure in a piping system: hoop stress, axial stress, bending stress, torsional stress, and fatigue stress.
Hoop stress is the result of pressure being applied to the pipe either internally or externally. Because pressure is uniformly applied to the piping system, hoop stress also is considered to be uniform over a given length of pipe.
Note that hoop stress will change with diameter and wall thickness throughout the piping system. Hoop stress is most commonly represented by the following formula:. Axial stress results from the restrained axial growth of the pipe.
Axial growth is caused by thermal expansion, pressure expansion, and applied forces. If a pipe run can grow freely in one direction, there is no axial present—at least in theory. When comparing axial growth caused by pressure, steel-pipe growth is minimal at over ft and can be ignored. Composite piping such as fiber re-enforced pipe FRP or plastic pipe will exhibit noticeable growth, as much as 2 to 3 in.
The primary reason for the difference in growth rates under pressure is related to the modulus of elasticity. Steel has a modulus of elasticity of approximately 30 x psi, whereas composites will be 2 to 3 orders of magnitude or less.
Axial stress is represented by the axial force over the pipes cross-sectional area:. Bending stress is the stress caused by body forces being applied to the piping. Body forces are the pipe and medium weight, concentrated masses valves, flanges , occasional forces seismic, wind, thrust loads , and forced displacements caused by growth from adjacent piping and equipment connections.
Body forces create a resultant moment about the pipe, for which the stress can be represented by the moment divided by the section modulus:. Torsional stress is the resultant stress caused by the rotational moment around the pipe axis and is caused by body forces.
However, because a piping system most likely will fail in bending before torsion, most piping codes ignore the effects of torsion. Fatigue stress is created by continuous cycling of the stresses that are present in the piping. For example, turning a water faucet on and off all day will create a fatigue stress, albeit low, because of the pressure being released and then built up. Fatigue stress results in a reduction of allowable strength in the piping system and is commonly caused by cycling of:.
Piping codes, such as those published by ASME, provide an allowable code stress, which is the maximum stress a piping system can withstand before code failure. A code failure is not necessarily a piping failure. This is because of safety factors built into piping codes. ASME codes consider three distinct types of stress: sustained stress, displacement thermal or expansion stress, and occasional stress.
Sustained or longitudinal stress is developed by imposing loads necessary to satisfy the laws of equilibrium between external and internal forces. Sustained stresses are not self-limiting.
If the sustained stress exceeds the yield strength of the piping material through the entire thickness, the prevention of failure is entirely dependent on the strain-hardening properties of the material. Displacement stress is developed by the self-constraint of the piping structure. It must satisfy an imposed strain pattern rather than being in equilibrium with an external load. Displacement stresses are most often associated with the effects of temperature; however, external displacements, such as building settlements, are considered a displacement stress.
As a pipe stress analyst, it is critical to understand how wall thickness is determined. If the pipe wall is too thin, it will not matter how the pipe is supported; it will fail. Typically, the engineer designing the system also will determine the wall thickness; however, the wall thickness is also verified during the pipe stress analysis.
Most engineers are more concerned with mass flow and pressure drop, therefore the effects of pipe size and wall thickness may be lost on them.
Going to a thicker pipe wall or a larger pipe size may be worth the material costs, versus facing design issues and added pipe-support costs in labor and materials. Hoop stress simplified is PDo4tn. ASME codes apply a safety factor of two when determining wall thickness based on hoop stress, yielding:. The safety factor is to account for the additional stresses caused by bending and axial stresses to be applied later.
Through basic algebraic manipulation, the code equation for wall thickness is:. A is the additional thickness added to the pipe corrosion, erosion, and wear during normal operation. However, most people consider 0. The minimum will thickness actual shown above is based on the internal diameter ID of the piping. The main difference in the two wall thickness equations is the simplified version is more conservative, quicker, and easier to calculate for scheduled pipe.
The actual version is closer to the measured hoop stress. Most stress analysis programs default to calculating hoop stress based ID. Please note that when factoring in the There are a couple of reasons why. First, recommended pipe support spans are governed by deflection, and not by allowable stress, to ensure proper flow and drainage. The second is from the discussion above, the wall thickness is based on a safety factor of two, which is removed from the sustained-stress equation.
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. As mentioned above, the sustained-stress equation is based on nominal wall thickness, with extra wall thickness for milling and corrosion.
Because there is extra wall thickness, the pipe has extra strength available to resist deflection. Furthermore, to achieve pipe failure from deflection, the supported pipe spans would be at least three to four times greater in length than the recommended MSS SP spans. In most cases, if displacement or expansion stresses are perceived to be a concern e. 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. Occasional stresses in the piping system are caused by short-term events, such as seismic, wind, and relief-thrust loads. These three loads comprise most of the possible occasional load combinations. Because occasional stresses are short-term, most piping codes allow for increased pipe stresses for a brief period.
ASME codes typically allow an increase of:. If occasional stresses are perceived to be a concern or are complex in nature, a computerized pipe stress analysis is warranted. However, in most cases. 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.
The actual version is closer to the measured hoop stress. The other framework in use is Guidelines of Modeling GoM  based on general accounting principles include the six principles: Correctness, Clarity deals with the comprehensibility and explicitness System description of model systems. Typically, this recommendation is met by ensuring the equipment connection loads are within published allowable code stresses through adding flexibility to the piping system. Prentice Hall, Hommes quoted Wang et al.
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A pipe support is a designed element that transfer the load from a pipe to the supporting structures. The load includes the weight of the pipe, the content that the pipe carries, all the pipe fittings attached to pipe, and the pipe covering such as insulation. The four main functions of a pipe support are to anchor, guide, absorb shock, and support a specified load. Pipe supports used in high or low temperature applications may also contain insulation materials. This type of load act throughout the life cycle of pipe.
These loads cause bending, and the bending moment is related to normal and shear stresses. In horizontal pipes, this load is taken care by placing pipe support on regular span to avoid sagging of pipe. A pipe used for transporting fluid would be under internal or external pressure load.
A pipe with high pressure fluid is under net internal pressure whereas pipe with vacuum conditions or jacketed pipe covering may be under net external pressure. Internal or external pressure induces stresses in the axial as well as circumferential Hoop Stress directions.
The pressure also induces stresses in the radial direction, but these are often neglected. This type of load is usually taken care during the selection of material for pipe. Pipe material must withstand these stresses at the prescribed temperature and pressure conditions.
Piping which are located outdoors and above a certain elevation are exposed to wind and will be designed to withstand the maximum wind velocity expected during the plant operating life.
Wind pressure for various elevations is used to calculate wind force. Seismic load is one of the basic concepts of earthquake engineering which means application of an earthquake-generated agitation to a structure. It happens at contact surfaces of a structure either with the ground, or with adjacent structures, or with gravity waves from tsunami.
Water hammer is a pressure surge or wave caused when a fluid usually a liquid but sometimes also a gas in motion is forced to stop or change direction suddenly momentum change. Water hammer commonly occurs when a valve closes suddenly at an end of a pipeline system, and a pressure wave propagates in the pipe. Steam hammer, the pressure surge generated by transient flow of super-heated or saturated steam in a steam-line due to sudden stop valve closures is considered as an occasional load.
Though the flow is transient, for the purpose of piping stress analysis, only the unbalanced force along the pipe segment tending to induce piping vibration is calculated and applied on the piping model as static equivalent force.
Reaction forces from relief valve discharge is considered as an occasional load. The reaction force due to steady state flow following the opening of safety relief valve in an open discharge installation can be calculated in accordance with ASME B Secondary loads are caused by displacement of some kind.
For example, the pipe connected to a storage tank may be under load if the tank nozzle to which it is connected moves down due to tank settlement. Similarly, pipe connected to a vessel is pulled upwards because the vessel nozzle moves up due to vessel expansion.
Also, a pipe may vibrate due to vibrations in the rotating equipment it is attached to. A pipe may experience expansion or contraction once it is subjected to temperatures higher or lower respectively as compared to temperature at which it was assembled. The secondary loads are often cyclic but not always. For example load due to tank settlement is not cyclic.
The load due to vessel nozzle movement during operation is cyclic because the displacement is withdrawn during shut-down and resurfaces again after fresh start-up. Rigid supports are used to restrict pipe movement in certain direction s without any or limited flexibility in that direction. Main function of a rigid support can be:. In this type of support arrangement, pipe is fixed with reference to the supporting structures. Movement in any direction is not allowed.
This can be achieved by welding or bolting the support with supporting structure. In this type of support arrangement, pipe is fixed with reference to vertical downward direction. Movement in downward vertical direction, mainly due to the weight of pipe and containing fluid, is not allowed.
This support is sometimes also referred as sliding support. In this type of support arrangement, pipe is fixed with reference of directions other then the direction in which weight of pipe and containing fluid is acting. Limited flexibility can be provided with the provision of guide gap gap between pipe outer surface and guide plate inner surface. Spring supports are used to support a load and allow simultaneous movement. Spring supports use helical coil compression springs to accommodate loads and associated pipe movements due to thermal expansions.
The critical component in both the type of supports are Helical Coil Compression springs. They are broadly classified into,. Variable effort supports also known as variable hangers or variables are used to support pipe lines subjected to moderate approximately up to 50mm vertical thermal movements. Variable effort supports are used to support the weight of pipe work or equipment along with weight of fluids while allowing certain quantum of movement with respect to the structure supporting it.
For pipes which are critical to the performance of the system or so called critical piping where no residual stresses are to be transferred to the pipe it is a common practice to use CES. In a constant effort support the load remains constant when the pipe moves from its cold position to the hot position.
Thus irrespective of travel the load remains constant over the complete range of movement. Therefore it is called a constant load hanger. A CES unit does not have any spring rate. Valery, the man built the plants before the creation of finite elements theory. Codes have been developed to rule these issues. We are engineers, not physicists. A simplified theory with a safety margin is better than a precise but too time-consuming one. A theory based on experience results is also acceptable even if not supported by theoretical calculations in its entirety.
So We have code by rules B So We can consider a Pipe as a Beam, the important is to know when a shell problem can arise, but codes help to avoid them. For thin-walled beams, when one transverse dimension is much larger than the other, estimates of the relative orders of magnitude of the normal and tangential stresses cease to be valid, the hypothesis of flat sections loses force and the Saint Venant principle becomes unacceptable.
Therefore, for almost all cases — the pipeline should be considered as a shell construction. In the process of loading the pipeline, the cross section ceases to be perfectly round — the curvature of the pipeline changes, both radially and in the longitudinal direction — this can not be taken into account by the theory of beam finite elements. The beam theory of finite elements excludes the possibility of REAL simulation of the contact of pipelines with supports.
And since in the process of loading the area of contacts and their location change — the beam theory of finite elements can not give REAL values of stresses that arise in contacts. You are mistaken in saying that the pipelines have small deformations. Almost always — pipelines have large deformations and almost always pipelines have geometric nonlinearity. Applying the theory of beam finite elements it is impossible to correctly define the boundary conditions that simulate the supports — since the boundary conditions are set to points located on the axis of symmetry of the pipeline.
Occasional Loads Wind Load Piping which are located outdoors and above a certain elevation are exposed to wind and will be designed to withstand the maximum wind velocity expected during the plant operating life.
Seismic Load Seismic load is one of the basic concepts of earthquake engineering which means application of an earthquake-generated agitation to a structure. Water Hammer Water hammer is a pressure surge or wave caused when a fluid usually a liquid but sometimes also a gas in motion is forced to stop or change direction suddenly momentum change.
Safety Valve Reaction Force Reaction forces from relief valve discharge is considered as an occasional load. Types of Pipe Support Rigid Support Rigid supports are used to restrict pipe movement in certain direction s without any or limited flexibility in that direction. Main function of a rigid support can be: Anchor or 3 Dimensional Stop In this type of support arrangement, pipe is fixed with reference to the supporting structures. Rest or Sliding Support In this type of support arrangement, pipe is fixed with reference to vertical downward direction.
Guide In this type of support arrangement, pipe is fixed with reference of directions other then the direction in which weight of pipe and containing fluid is acting.
Spring Support Spring supports are used to support a load and allow simultaneous movement. They are broadly classified into, Variables Effort support Variable effort supports also known as variable hangers or variables are used to support pipe lines subjected to moderate approximately up to 50mm vertical thermal movements.
June 12, at PM. Valery Anpilov says:. June 4, at PM. Subscribe via Email Name. The Process Piping Facebook Page. Blog Statistics , Users. Sorry, your blog cannot share posts by email.