Piping and Pipeline Engineers often talk about pipe stress analysis or pipeline stress analysis which is a dedicated activity performed by stress engineers using some kind of software like Caesar II, Start-Prof, Autopipe, or Rohr-2. But to judge any piping or pipeline system, it is always good to know what is causing that stress in the piping or pipeline system. In this article, we will explain the answer to one of the most basic questions of pipe stress analysis; i.e., What causes the stresses in the pipe?

Classification of Stresses in Piping and Pipeline Systems

Let’s start the discussion by classifying the types of stresses in a piping system. In general, the Stresses generated in piping and pipeline systems can be broadly classified into three categories:

Primary Stresses:

These are stresses that result from external loads, such as internal pressure, dead weight, and external forces. They are generally steady and must be within the allowable limits to prevent failure.

Secondary Stresses:

These arise from displacement-controlled loads, such as thermal expansion or contraction. Secondary stresses are often self-limiting but can cause fatigue over time if not properly managed.

Peak Stresses:

These are localized stresses that occur at points of discontinuity, such as welds, fittings, and supports. Peak stresses are typically higher than the general stress in the system and can lead to crack initiation if not adequately addressed.

What Causes Primary Stresses in a Piping System?

1. Internal Pressure

One of the most significant contributors to stress in a piping system is the internal pressure of the fluid being transported. Internal pressure generates circumferential stress (also known as hoop stress) and longitudinal stress within the pipe wall.

Hoop Stress

This stress acts around the circumference of the pipe and is the result of the internal pressure acting outwards. It is the dominant stress in thin-walled pipes and is given by the formula: 𝜎_h=(PD/2T).

Here,

• 𝜎_h is the hoop stress
• P is the internal pressure
• D is the outside diameter of the pipe
• T is the wall thickness of the pipe

Longitudinal Stress

This stress acts along the length of the pipe and is generally lower than the hoop stress. It can be calculated using the formula: 𝜎_𝑙=PD/4T.

From the above, it is clear that

• 𝜎_h=2* 𝜎_𝑙 which means hoop stress is more significant than longitudinal stress in the piping and pipeline system. In most piping or pipeline design codes, this hoop stress equation is used as the base equation for pipe thickness calculation.
• Both hoop stress and longitudinal stress are proportional to the internal pressure, which means the generated stresses increase with an increase in pressure.
• The stress in the piping or pipeline system increases with a decrease in pipe wall thickness (T) which means the stress and thickness are inversely proportional.
• Also, with an increase in diameter, the generated stresses also increase.

2. External Loads-Weight

The weight load constitutes of the following loads:

• Self Weight of the Pipe: The weight because of the material’s mass.
• Liquid Weight: The weight of the amount of liquids that the pipe carries.
• Insulation Weight: The weight of the insulation material, if any
• Rigid Body Weights: Weight of Flanges, Valves, or any other rigid bodies.
• Weight of external attachments.
• Weight of Snow and Ice.

These loads induce bending stresses and axial stresses in the pipe.

Bending Stresses:

These occur due to the pipe’s weight and any external forces acting on it. The magnitude of the bending stress depends on the pipe’s span length, support conditions, and the distribution of the external loads.

Axial Stresses:

Axial stresses are caused by the pipe’s weight and any external forces acting along its length. They are also influenced by the pipe’s restraint conditions, such as whether it is fixed, anchored, or free to expand and contract.

3. Pressure Transients (Water Hammer)

Pressure transients, often referred to as water hammer or pressure surge, occur when there is a sudden change in the flow rate of the fluid within the pipeline. This can happen due to the rapid closing or opening of valves, pump starts or stops, or sudden changes in demand. The water hammer generates a pressure wave that travels through the pipeline, causing high transient stresses. These types of stresses are known as occasional stresses as they usually occur for very short period of time with respect to the design life of the piping or pipeline system.

Impact of Water Hammer:

The rapid pressure changes can induce severe axial and hoop stresses, which can lead to pipeline rupture, joint failure, or support damage. It is essential to design systems with surge protection devices, such as pressure relief valves and surge tanks, to mitigate the effects of water hammering.

4. Other Occasional Forces

There are various other occasional forces that also contributes to the initiation of pie stress. Some of the notable occasional forces are:

• Wind Forces
• Wave forces
• Accidental forces
• Reaction forces of sudden PSV/PRV discharge
• Vibration generated due to
  • High Flow velocities.
  • Acoustic behavior (predominantly in gas systems)
  • Equipment induced vibration
  • Any other external events like vortex shedding, pulsating flow, two-phase flow, etc.

What Causes Secondary Stresses in a Piping or Pipeline System?

1. Thermal Expansion and Contraction

Piping systems are often subjected to temperature variations due to the nature of the fluids they transport or changes in ambient conditions. Thermal expansion and contraction cause the pipe material to expand or contract, leading to secondary stresses.

Thermal Stress:

A change in temperature causes a pipe to expand or contract. This movement can not be contained. This expansion needs to be absorbed. If the pipe can move freely without any restriction, there will not be any stress. However, because of the closed nature of the piping system, free thermal movement without restriction is not allowed. If the pipe deforms with little restriction, a little thermal stress is generated. However, if the pipe is over-constrained, the pipe and supports will experience increased load and stress.

The stress induced by thermal expansion or contraction can be calculated using the formula: σ_t=E⋅α⋅ΔT

Where,

• σ_t is the thermal stress
• E is the modulus of elasticity of the pipe material
• α is the coefficient of thermal expansion of the pipe material𝑇
• ΔT is the temperature change

Thermal stresses are displacement-controlled and can cause significant movement in the piping system, leading to fatigue, stress concentration, and even failure if not adequately accommodated.

2. Thermal Gradient

A thermal gradient occurs when there is a temperature difference across the pipe wall or along the length of the pipe. This can happen in systems where hot and cold fluids are transported simultaneously, or where there are significant temperature differences between different sections of the pipeline.

Thermal Gradient Stresses:

The differential expansion caused by a thermal gradient can induce bending and axial stresses in the pipe. These stresses are particularly problematic in systems with rigid supports or where the pipe material has a low tolerance for thermal stress.

3. Pipeline Settlement and Ground Movement

In buried pipelines, ground movement due to settlement, earthquakes, or landslides can induce secondary stresses. These stresses arise from the differential movement of the pipeline relative to the surrounding soil or rock.

Settlement-Induced Stresses:

Differential settlement can cause bending and axial stresses in the pipeline. These stresses are particularly critical in long pipelines and can lead to buckling or rupture if not properly accounted for.

Seismic-Induced Stresses:

Earthquakes can generate significant ground movement, leading to high bending and axial stresses in pipelines. These stresses are often concentrated at points of restraint, such as bends, tees, and connections to rigid structures.

What Causes Peak Stresses in Pipe?

1. Discontinuities and Stress Concentrations

Peak stresses occur at points of discontinuity in the piping system, such as welds, fittings, flanges, and supports. These discontinuities create stress concentrations that are higher than the general stress in the pipe.

Welded Joints:

Welded joints are common sources of peak stresses due to the mismatch in material properties, geometry, and residual stresses from the welding process. Weld defects, such as cracks, porosity, or lack of fusion, can further exacerbate these stresses and lead to failure.

Fittings:

Fittings create geometric discontinuities that cause stress concentrations. For example, a sudden change in pipe diameter at a reducer fitting can induce peak stresses that are higher than the nominal stress in the pipe.

Supports and Restraints:

Pipe supports and restraints, such as clamps, hangers, and anchors, can create localized stress concentrations. The interaction between the pipe and the support can lead to high contact stresses, which can cause wear, fatigue, and eventually failure.

2. Corrosion and Erosion

Corrosion and erosion are common in piping systems that transport corrosive or abrasive fluids. These processes lead to material loss, which can create localized weak points and stress concentrations.

Corrosion-Induced Stresses:

Corrosion can cause thinning of the pipe wall, leading to a reduction in the pipe’s load-carrying capacity. Localized corrosion, such as pitting, can create stress concentrations that are much higher than the nominal stress in the pipe.

Erosion-Induced Stresses:

Erosion, particularly in high-velocity or turbulent flow conditions, can lead to material loss and the formation of localized pits or grooves. These defects act as stress concentrators, increasing the likelihood of crack initiation and propagation.

3. Fatigue

Fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. In piping systems, fatigue can be caused by fluctuating pressure, temperature cycles, and vibration.

Cyclic Loading:

Repeated cycles of loading and unloading, such as pressure fluctuations or thermal cycles, can cause fatigue in the pipe material. The fatigue life of the pipe depends on the magnitude and frequency of the cyclic loads, as well as the presence of stress concentrations.

Vibration-Induced Fatigue:

Vibration, particularly in systems with rotating equipment or turbulent flow, can induce cyclic stresses that lead to fatigue. Vibration-induced fatigue is often concentrated at points of discontinuity, such as welds, fittings, and supports.

Methods for Mitigating Stresses in Piping and Pipeline Systems

Mitigating the stresses in piping and pipeline systems is essential for ensuring their longevity, reliability, and safety. There are several methods and best practices that can be employed to reduce the impact of these stresses:

A. Proper Design and Material Selection

Design Codes and Standards:

Adhering to industry codes and standards, such as ASME B31.3 for process piping or API 1104 for pipeline welding, ensures that the piping system is designed to withstand the expected loads and stresses.

Material Selection:

Choosing the right material for the pipe, considering factors such as strength, toughness, corrosion resistance, and thermal properties, is crucial for minimizing stress-related issues. For example, selecting a material with a high thermal expansion coefficient can help reduce thermal stress in high-temperature applications.

Wall Thickness:

Increasing the wall thickness of the pipe can reduce the hoop stress induced by internal pressure. However, this must be balanced with the need to keep the system economical and manageable in terms of weight and installation.

B. Stress Analysis and Simulation

Finite Element Analysis (FEA):

FEA is a powerful tool for analyzing the stresses in complex piping systems. It allows engineers to simulate various loading conditions, including thermal expansion, pressure transients, and external loads, to identify areas of high stress concentration and potential failure points.

Piping Flexibility Analysis:

Flexibility analysis helps ensure that the piping system can accommodate thermal expansion and contraction without excessive stress. This involves calculating the pipe’s flexibility, considering factors such as support locations, pipe routing, and material properties.

C. Stress Mitigation Techniques

Expansion Loops and Joints:

Incorporating expansion loops or joints into the piping design can help absorb thermal expansion and reduce stress. These components provide the necessary flexibility to accommodate thermal movement without inducing excessive stress on the pipe or its supports.

Proper Support Design:

Properly designed and placed supports are critical for minimizing stresses in piping systems. Supports should be positioned to prevent excessive sagging, bending, and axial movement. Additionally, using spring supports or hangers can help accommodate thermal expansion and contraction.

Surge Protection:

Installing surge protection devices, such as pressure relief valves or surge tanks, can help mitigate the effects of pressure transients (water hammer). These devices absorb the energy of pressure waves, reducing the stress on the piping system.

D. Regular Inspection and Maintenance

Non-Destructive Testing (NDT):

Regular NDT, such as ultrasonic testing, radiographic testing, and magnetic particle testing, can help identify areas of stress concentration, corrosion, or fatigue before they lead to failure.

Corrosion Protection:

Applying protective coatings, using corrosion inhibitors, and implementing cathodic protection systems can help mitigate corrosion-induced stresses, extending the life of the piping system.

Fatigue Monitoring:

Implementing fatigue monitoring techniques, such as vibration analysis and thermal cycle monitoring, can help detect early signs of fatigue and allow for timely maintenance or replacement of affected components.

On a broader view, the following can be introduced to reduced stresses generated in a piping system:

• To reduce pressure stresses, increase the pipe thickness, if economically feasible.
• To reduce weight stress, add more support by reducing the support span.
• To reduce expansion stresses, add more flexibility to the system.
• To reduce occasional stresses, add more guides and axial stops in the piping system
• To reduce vibrational stresses, increase the system rigidity by adding supports.

Online Courses on Pipe Stress Analysis

If you are looking for an online course to learn pipe stress analysis then visit the following:
Complete Pipe Stress Analysis using Caesar II Online Course

While “piping” and “pipeline” may sound similar, they refer to different applications and contexts within the broader field of fluid transportation. This distinction affects how wall thickness is considered for each. Here’s a breakdown of the differences between piping wall thickness and pipeline wall thickness:

1. Context and Application

• Piping Wall Thickness:
  • Context: Piping typically refers to systems used within facilities such as refineries, chemical plants, power plants, and other industrial settings. These systems transport fluids and gases over short distances within a controlled environment.
  • Application: Piping systems are used for processes, utilities, and distribution within a plant. They often involve complex networks with numerous fittings, valves, and pressure vessels.
• Pipeline Wall Thickness:
  • Context: Pipelines refer to long-distance transportation systems, often spanning hundreds or thousands of kilometers, and are used to transport oil, gas, water, or other fluids between different locations, such as from a production site to a refinery or from a water treatment plant to a city.
  • Application: Pipelines are used for transporting fluids across large geographical areas, typically underground or underwater, with minimal intervention between the start and endpoints.

2. Design Standards and Codes

• Piping Wall Thickness:
  • Design Codes: Piping wall thickness is usually governed by standards such as ASME B31.3 (Process Piping), ASME B31.1 (Power Piping), or API 570 (Piping Inspection Code). These standards provide guidelines for calculating wall thickness based on internal pressure, material, temperature, and other factors.
  • Thickness Consideration: In piping systems, wall thickness is often designed to handle higher pressure differentials, due to the more complex internal flow paths, higher temperatures, and diverse fluid properties.
• Pipeline Wall Thickness:
  • Design Codes: Pipeline wall thickness is generally governed by standards such as ASME B31.4 (Pipeline Transportation Systems for Liquid Hydrocarbons and Other Liquids), ASME B31.8 (Gas Transmission and Distribution Piping Systems), or API 5L (Specification for Line Pipe).
  • Thickness Consideration: For pipelines, wall thickness is more influenced by external factors such as ground conditions, potential mechanical impacts (e.g., from excavation equipment), human occupancy, and long-term environmental exposure. The design must ensure the pipeline can withstand the pressure over long distances with minimal maintenance.

3. Piping and Pipeline Wall Thickness Calculation Equation

The pipe wall thickness calculation for piping and pipeline systems is provided in Fig. 1 below:

To learn more about piping wall thickness calculation visit Pipe Thickness Calculation as per ASME B31.3 | Pipe Thickness Calculator and Pipe Thickness Calculation of Straight Pipe under External Pressure/ Vacuum Pressure Condition. However to know the steps for pipeline wall thickness calculation visit Steel Pipeline Wall Thickness Calculation With Example and Steps for Pipeline Wall Thickness Calculation with Case Study.

4. Pressure Considerations

• Piping Wall Thickness:
  • Pressure Levels: Piping systems within facilities typically handle a wide range of pressures, including very high pressures, depending on the process requirements. Therefore, the wall thickness is often calculated with higher precision to manage these variable pressures.
  • Pressure Drop: Piping systems experience more frequent pressure drops due to the complexity of the network, with numerous bends, valves, and fittings. This variability in pressure requires careful consideration of wall thickness.
• Pipeline Wall Thickness:
  • Pressure Levels: Pipelines generally operate under a more consistent pressure, which is determined by the distance and the elevation changes along the pipeline route. The wall thickness is designed to manage the steady state pressure over long distances, ensuring durability and integrity.
  • Pressure Drop: Pressure drop in pipelines is usually managed over long distances using booster stations. The wall thickness is selected to handle the maximum pressure expected in the system, considering factors like terrain elevation and flow rate.

5. Piping and Pipeline Thickness Values

Piping thicknesses after calculating the minimum required thickness are always selected from ASME B36.10 or ASME B36.19. So, all pipe thicknesses in the piping industry are mostly standard thicknesses (Also known as pipe schedules). However, as pipelines run for several kilometers, tonnage cost matters a lot. In this respect to reduce cost, pipeline thickness is selected as a non-standardized value. For example, let’s assume in your pipe thickness calculation, you got the calculated thickness as 4.8 mm for an 8″ pipe size. In general, for a piping system the selected thickness will be 8″, Sch 20 means 6.35 mm whereas for pipeline system the selected pipe wall thickness will be 4.8 mm.

6. Material and Corrosion Allowance

• Piping Wall Thickness:
  • Material: Piping systems often use a wider range of materials, including carbon steel, stainless steel, alloy steel, and non-metallic materials like PVC or HDPE. The choice of material influences the wall thickness, especially when dealing with corrosive or high-temperature environments.
  • Corrosion Allowance: In industrial settings, where pipes may be exposed to harsh chemicals or corrosive fluids, a significant corrosion allowance may be added to the wall thickness. This ensures that the pipe remains safe over its expected service life, even as it experiences material degradation.
• Pipeline Wall Thickness:
  • Material: Pipelines are predominantly constructed from carbon steel, with specialized coatings and cathodic protection systems to prevent corrosion over long distances. Material selection is crucial for managing both internal corrosion (from the fluid being transported) and external corrosion (from the environment).
  • Corrosion Allowance: For pipelines, the corrosion allowance is often integrated into the initial design, with additional protective measures like coatings and cathodic protection to ensure longevity. The wall thickness must be sufficient to last for decades with minimal maintenance.

7. Environmental and Mechanical Loads

• Piping Wall Thickness:
  • Environmental Loads: Piping within facilities is generally protected from environmental conditions like extreme temperatures, UV radiation, and mechanical impacts. However, thermal expansion, vibration, and support loads are critical considerations.
  • Mechanical Loads: Piping systems must account for mechanical loads from supports, thermal expansion, and vibration. These loads can influence the required wall thickness, especially in high-stress areas like bends or connections to equipment.
• Pipeline Wall Thickness:
  • Environmental Loads: Pipelines are exposed to a variety of environmental conditions, including temperature extremes, soil movement, and potential mechanical impacts from external sources (e.g., digging, landslides). The wall thickness is often designed to withstand these external threats.
  • Mechanical Loads: In addition to internal pressure, pipelines must withstand mechanical loads from ground movement, external impacts, and installation processes. This requires a robust wall thickness to prevent failures in harsh environments.

8. Inspection and Maintenance

• Piping Wall Thickness:
  • Inspection: Piping systems within facilities are regularly inspected, with wall thickness measurements taken periodically to assess corrosion and wear. This frequent inspection allows for adjustments or replacements before significant failures occur.
  • Maintenance: Maintenance in piping systems is more frequent and accessible, given the proximity and accessibility within industrial plants. Wall thickness considerations must account for the ease of repair or replacement.
• Pipeline Wall Thickness:
  • Inspection: Pipelines, due to their length and remote locations, are inspected using methods like pigging (pipeline inspection gauges) and remote sensing technologies. Wall thickness is designed to minimize the need for frequent inspections.
  • Maintenance: Pipeline maintenance is more challenging and costly due to the remote and inaccessible nature of many pipeline routes. As such, the wall thickness must be designed for long-term reliability with minimal intervention.

Piping and Pipeline Diameter Chart

The following table provides a piping and pipeline diameter chart. Note that, the outside diameter (OD) of steel pipe in piping and pipeline applications is constant for the same pipe size. However, the Internal diameter will vary depending on the thickness. To give an example, the OD of a 10″ steel pipe will be 273.05 mm whether it is used in a piping or pipeline application.

Sr No	Pipe Nominal Size (NPS)	Pipe Outer Diameter (mm)	Pipe Outer Diameter (inches)
1	1/2	21.34	0.84
2	3/4	26.67	1.05
3	1	33.40	1.32
4	1 1/4	42.16	1.66
5	1 1/2	48.26	1.90
6	2	60.33	2.37
7	2 1/2	73.03	2.87
8	3	88.90	3.50
9	3 1/2	101.60	4.00
10	4	114.30	4.50
11	5	141.30	5.56
12	6	168.28	6.63
13	8	219.08	8.63
14	10	273.05	10.75
15	12	323.85	12.75
16	14	355.60	14.00
17	16	406.40	16.00
18	18	457.20	18.00
19	20	508.00	20.00
20	22	558.80	22.00
21	24	609.60	24.00
22	26	660.40	26.00
23	28	711.20	28.00
24	30	762.00	30.00
25	32	812.80	32.00
26	34	863.60	34.00
27	36	914.40	36.00
28	38	965.20	38.00
29	40	1016.00	40.00
30	42	1066.80	42.00
31	44	1117.60	44.00
32	46	1168.40	46.00
33	48	1219.20	48.00
34	50	1270.00	50.00
35	52	1320.80	52.00
36	54	1371.60	54.00
37	56	1422.40	56.00
38	58	1473.20	58.00
39	60	1524.00	60.00

This table provides the nominal pipe sizes (NPS), and the corresponding outer diameters in both millimeters and inches for steel pipes ranging from 1/2 inch to 60 inches.

In summary, the differences between piping wall thickness and pipeline wall thickness stem from their distinct applications, operating conditions, design codes, and design requirements. While both are critical for ensuring the safety and integrity of fluid transport systems, the considerations for each are tailored to the specific challenges and environments they face. Understanding these differences is essential for engineers and designers to select the appropriate wall thickness for their specific needs.