Piping flexibility
As the pipe temperature changes from the installation condition to the operating condition, it expands or contracts. In the general term, both expansion and contraction are called thermal expansion. When a pipe expands it has the potential of generating enormous force and stress in the system. However, if the piping is flexible enough, the expansion can be absorbed without creating undue force or stress. Providing the proper flexibility is one of the major tasks in the design of piping system.
Piping is used to convey a certain amount of fluid from one point to another. It is obvious that the shorter the pipe is used the lesser the capital expenditure is required. The long pipe may also generate excessive pressure drop making it unsuitable for the proper operation. However, the direct shortest layout generally is not acceptable for absorbing the thermal expansion.
Figure 1 shows what will happen when a straight pipe is directly connected from one point to another. First, consider that only one end is connected and the other end is loose. The loose end will expands an amount equal to Δ = e L
However, since the other end is not loose, this expansion is to be absorbed by the piping. This is equivalent to squeezing the pipe to move the end back an ~ distance. This amount of squczzing creates a stress of the magnitude S = E (Δ/L) = E e
As the pipe temperature changes from the installation condition to the operating condition, it expands or contracts. In the general term, both expansion and contraction are called thermal expansion. When a pipe expands it has the potential of generating enormous force and stress in the system. However, if the piping is flexible enough, the expansion can be absorbed without creating undue force or stress. Providing the proper flexibility is one of the major tasks in the design of piping system.
Piping is used to convey a certain amount of fluid from one point to another. It is obvious that the shorter the pipe is used the lesser the capital expenditure is required. The long pipe may also generate excessive pressure drop making it unsuitable for the proper operation. However, the direct shortest layout generally is not acceptable for absorbing the thermal expansion.
Figure 1 shows what will happen when a straight pipe is directly connected from one point to another. First, consider that only one end is connected and the other end is loose. The loose end will expands an amount equal to Δ = e L
However, since the other end is not loose, this expansion is to be absorbed by the piping. This is equivalent to squeezing the pipe to move the end back an ~ distance. This amount of squczzing creates a stress of the magnitude S = E (Δ/L) = E e
The force required to squeeze this amount is F = A S = A E e
Take a 6-inch standard wall carbon steel pipe for instance, an increase of temperature from 70F ambient to 300F operating creates an axial stress of 42300 psi and an axial force of 236000 lbs in the pipe. These are excessive even though the temperature is only 300F. It is clear that the straight line direct layout is not acceptable to most of the piping, Flexibility has to be provided.
Expansion loop
Piping flexibility are provided in many different ways. The turns and offsets needed for running the pipe from one point to another provides some flexibility by themself. This inherent flexibility may or may not be sufficient depending on the individual cases.
Additional flexibility can be provided by adding expansion loops or expansion joints. In the straight line example discussed above, the stress can be reduced by loops installed as shown below. The idea is to provide some pipe perpendicular to the direction of expansion. In this way when the pipe expands it bends the loop leg first before transmitting any load to the anchor. The longer the loop leg the lesser the force will be created.
Piping flexibility are provided in many different ways. The turns and offsets needed for running the pipe from one point to another provides some flexibility by themself. This inherent flexibility may or may not be sufficient depending on the individual cases.
Additional flexibility can be provided by adding expansion loops or expansion joints. In the straight line example discussed above, the stress can be reduced by loops installed as shown below. The idea is to provide some pipe perpendicular to the direction of expansion. In this way when the pipe expands it bends the loop leg first before transmitting any load to the anchor. The longer the loop leg the lesser the force will be created.
The force created is inversely proportional to the cube of the loop length and the stress generated is roughly Hard Piping inversely proportional to the square of the loop length. The loop sometimes can take considerably more space and piping than what is available, or economically justifiable. This is especially true for large high temperature low pressure pipings.
In this case the better method is to use expansion joint. Expansion joints are more sophisticated than the pipe loops which are just extra lengths of the same piping. For this and other reasons, engineers tend to favor piping loops over expansion joints.
However, expansion joints can be used effectively in many applications when they are properly designed. One of the major requirements in the design of expansion joint system is to install sufficient restraints for maintaining the stability. This article deals mainly the loop approach.
In this case the better method is to use expansion joint. Expansion joints are more sophisticated than the pipe loops which are just extra lengths of the same piping. For this and other reasons, engineers tend to favor piping loops over expansion joints.
However, expansion joints can be used effectively in many applications when they are properly designed. One of the major requirements in the design of expansion joint system is to install sufficient restraints for maintaining the stability. This article deals mainly the loop approach.
The Critical Path❗️
In designing a plant, the piping is generally routed or laid-out by the piping designers then checked by the stress engineers.
There is a marked difference in the layout done by the experienced and the inexperienced designers. The experienced designers know the importance of the flexibility. However, they tend to provide too much flexibility in contrast to the inexperienced ones who tend to provide little flexibility. In either case, the resulL is an over priced project.
The layout done by an inexperienced designer is normally too stiff because the designer does not know how or too timid to add loops or offsets. If a piping system is too stiff, the stress engineer will almost certain to find it out.
The stress engineer will send the design, with recommended loops, back to the designer for revision. At this time, the designer have made some more layouts in the same area making the revision very difficulty. On the other hand, a layout done by an experienced designer often contains the loops which are excessive or not needed.
The excessive loops are normally maintained without revision, becuase it is a common prctice not to change something which works. The experienced one might have saved the manhour needed for the revision. The cost of the excessive loops can be prohibitive.
In designing a plant, the piping is generally routed or laid-out by the piping designers then checked by the stress engineers.
There is a marked difference in the layout done by the experienced and the inexperienced designers. The experienced designers know the importance of the flexibility. However, they tend to provide too much flexibility in contrast to the inexperienced ones who tend to provide little flexibility. In either case, the resulL is an over priced project.
The layout done by an inexperienced designer is normally too stiff because the designer does not know how or too timid to add loops or offsets. If a piping system is too stiff, the stress engineer will almost certain to find it out.
The stress engineer will send the design, with recommended loops, back to the designer for revision. At this time, the designer have made some more layouts in the same area making the revision very difficulty. On the other hand, a layout done by an experienced designer often contains the loops which are excessive or not needed.
The excessive loops are normally maintained without revision, becuase it is a common prctice not to change something which works. The experienced one might have saved the manhour needed for the revision. The cost of the excessive loops can be prohibitive.
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Where,
Δ = thermal expansion, in
L e = expansion rate, in/in
L = pipe length, in
s = axial stress, psi
F E = modulus of elasticity, psi
A = pipe cross section area, inZ
F = axial force, lbs
Δ = thermal expansion, in
L e = expansion rate, in/in
L = pipe length, in
s = axial stress, psi
F E = modulus of elasticity, psi
A = pipe cross section area, inZ
F = axial force, lbs
Acoustical Induced Vibration (AIV) & Flow Induced Vibration (FIV) studies
Vibration fatigue induced by acoustic phenomena and fluid flows is one of the major problems encountered in process piping.
At the initial design stage of a new plant or when modifying an existing one, we offer a 2-step preventive approach to resolve such vibration issues.
This approach involves a qualitative assessment and prioritisation followed by a qualitative assessment which results in practical recommendation
Vibration fatigue induced by acoustic phenomena and fluid flows is one of the major problems encountered in process piping.
At the initial design stage of a new plant or when modifying an existing one, we offer a 2-step preventive approach to resolve such vibration issues.
This approach involves a qualitative assessment and prioritisation followed by a qualitative assessment which results in practical recommendation
Step 1 – Qualitative assessment & prioritisation
Using a rigorous methodology, we carry out a qualitative assessment of all the main lines in a process system to identify the potential excitation mechanisms. We then produce a report prioritising and identifying the potential excitation mechanisms that need to be quantitatively analysed.
Using a rigorous methodology, we carry out a qualitative assessment of all the main lines in a process system to identify the potential excitation mechanisms. We then produce a report prioritising and identifying the potential excitation mechanisms that need to be quantitatively analysed.
Step 2 – Quantitative assessment & Solutions
Main lines: For each excitation mechanism that has been identified as potentially at risk, a quantitative assessment is carried out to determine the likelihood of vibration-induced pipe failure.
The potential excitation mechanisms that can be addressed are:
Flow induced turbulence
Mechanical excitation
Pulsation:
– Reciprocating/Positive displacement pumps and compressors
– Rotating stall
– Flow induced excitation
High frequency acoustic excitation
Surge/Momentum change due to valve operation Cavitation and Flashing
Main lines: For each excitation mechanism that has been identified as potentially at risk, a quantitative assessment is carried out to determine the likelihood of vibration-induced pipe failure.
The potential excitation mechanisms that can be addressed are:
Flow induced turbulence
Mechanical excitation
Pulsation:
– Reciprocating/Positive displacement pumps and compressors
– Rotating stall
– Flow induced excitation
High frequency acoustic excitation
Surge/Momentum change due to valve operation Cavitation and Flashing
Small Bore Connection (SBC): For each piping line that has been identified as a potential excitation source for SBC, a quantitative assessment is carried out to determine the likelihood of vibration-induced pipe failure.
Intrusive elements: The risk of coincidence between thermowell (or, intrusive elements) vortex shedding excitation frequency and the structural natural frequency is assessed.
For each of the above investigated excitation mechanism, when a situation is identified as potentially critical, a corrective action is suggested or an appropriate vibration analysis recommended.
Note: The AIV & FIV assessment methodology is based on the “Guideline for the Avoidance of Vibration Induced Fatigue Failure in Process Pipework” published by Energy Institute.
For each of the above investigated excitation mechanism, when a situation is identified as potentially critical, a corrective action is suggested or an appropriate vibration analysis recommended.
Note: The AIV & FIV assessment methodology is based on the “Guideline for the Avoidance of Vibration Induced Fatigue Failure in Process Pipework” published by Energy Institute.
Centrifugal compressors coincidence Analysis (AIV & FIV)
We offer a preventive approach to identify and mitigate the risk of Acoustical and Flow induced vibration in centrifugal compressors :
Support arrangement is assessed and design optimized against flow induced turbulence and when process condition cannot avoid pulsation generated by rotating stall.
A coincidence analysis is carried out for each no-flow side branch that can be subject to flow induced pulsation caused by periodic vortex shedding in T-joint.
The fatigue generated by high frequency acoustic phenomena is assessed by calculating the sound power level in each pressure reducing devices (e.g. Valve or high capacity & press. drop devices) and discontinuity.
Small bore connection (SBC) design is assessed against potential vibration excitation sources.
Coincidence analysis of intrusive elements (or thermowell) structural natural frequency with vortex shedding excitation frequency is carried out.
We offer a preventive approach to identify and mitigate the risk of Acoustical and Flow induced vibration in centrifugal compressors :
Support arrangement is assessed and design optimized against flow induced turbulence and when process condition cannot avoid pulsation generated by rotating stall.
A coincidence analysis is carried out for each no-flow side branch that can be subject to flow induced pulsation caused by periodic vortex shedding in T-joint.
The fatigue generated by high frequency acoustic phenomena is assessed by calculating the sound power level in each pressure reducing devices (e.g. Valve or high capacity & press. drop devices) and discontinuity.
Small bore connection (SBC) design is assessed against potential vibration excitation sources.
Coincidence analysis of intrusive elements (or thermowell) structural natural frequency with vortex shedding excitation frequency is carried out.
PRG FEATools™ improves the quality of Intergraph CAESAR II® users’ analysis for critical service lines by incorporating finite element analysis (FEA) and other empirical sources into the evaluation process.
By using Intergraph® CAESAR II in combination with FEATools, analyzed systems are neither over- nor under-designed, but designed with consistent safety factors, which also saves time and money.
By using Intergraph® CAESAR II in combination with FEATools, analyzed systems are neither over- nor under-designed, but designed with consistent safety factors, which also saves time and money.
⬇️ Higher Accuracy, Lower Costs
Piping analysts know that that properly qualified FEA presents the greatest opportunity to produce the most accurate analysis results. However, FEA can be extremely time-intensive and require more technical expertise than is needed for the majority of pipe stress problems. An ideal solution would allow FEA results to be easily and seamlessly incorporated within traditional code-based pipe stress analysis, so that your jobs benefit from the accuracy of FEA and the practicality of code-based analysis. CAESAR II with FEATools provides this solution.
Piping analysts know that that properly qualified FEA presents the greatest opportunity to produce the most accurate analysis results. However, FEA can be extremely time-intensive and require more technical expertise than is needed for the majority of pipe stress problems. An ideal solution would allow FEA results to be easily and seamlessly incorporated within traditional code-based pipe stress analysis, so that your jobs benefit from the accuracy of FEA and the practicality of code-based analysis. CAESAR II with FEATools provides this solution.
⬇️⬇️Addressing Code Limitations
There are well-known limitations in piping code accuracy when it comes to piping branch connections. Performing a complete FEA of a piping system can be prohibitively expensive. Instead, using FEA data for branch intersections is one of the most effective and pragmatic uses of FEA technology for code-based pipe stress analysis.
There are well-known limitations in piping code accuracy when it comes to piping branch connections. Performing a complete FEA of a piping system can be prohibitively expensive. Instead, using FEA data for branch intersections is one of the most effective and pragmatic uses of FEA technology for code-based pipe stress analysis.
⬇️⬇️FEA Results & Piping Codes
Piping codes such as ASME B31.3 Appendix D state that, in the absence of more directly applicable data, the engineer should use the stress intensification factor (SIF) and flexibility factor (k factor) data from Appendix D of the code. FEATools uses the results of the latest analysis, research, and testing to supply this “applicable data” to CAESAR II, and it does so seamlessly and intuitively.
Piping codes such as ASME B31.3 Appendix D state that, in the absence of more directly applicable data, the engineer should use the stress intensification factor (SIF) and flexibility factor (k factor) data from Appendix D of the code. FEATools uses the results of the latest analysis, research, and testing to supply this “applicable data” to CAESAR II, and it does so seamlessly and intuitively.