Pipelines in Industry: Modern Practices of Design, Construction, and Operation

Pipelines in Industry: Modern Practices of Design, Construction, and Operation

  • Katarina Knafelj Jakovac

    February 1, 2024

Pipelines are used for the transfer of liquid and gaseous working fluids from one location to another. They consist of a system of pipes, elbows, flanges, various types of valves, supports, and other components of fittings.

Best practices encompass all phases of the lifecycle of a pipeline, from design, testing, and installation to operation and maintenance. This ensures an adequate return on investment in pipeline construction and optimal utilization of the structure.

The European Pressure Equipment Directive PED 2014/68/EU defines a pipeline as a system comprising all pipe components intended for fluid transfer, interconnected to form a pressure system. A pipeline specifically includes pipes or a system of pipes, fittings, expansion joints, hoses, or other necessary pressure parts. Heat exchangers consisting of pipes with the purpose of cooling or heating air are also considered pipelines.

Screenshot 2024-01-31 at 09.42.21.png

Pipelines for transporting working fluids in cogeneration plants, heating and cooling systems, geothermal water, and steam must meet the criteria of the ANSI/ASME B 31.1 standard during design and construction. Pipelines transporting working fluids in oil refineries, petrochemical plants, chemical factories, or the pharmaceutical industry are subject to the criteria of the ANSI/ASME B 31.3 standard. Pipelines for the transport of liquids between pumping stations, wells, boreholes, and terminals are constructed according to the requirements of the ANSI/ASME B 31.4 standard. In contrast, pipelines for the transfer of liquefied gases, gases produced as a byproduct of oil pumping, or gases between platforms are designed according to the requirements of the ANSI/ASME B 31.8 standard. The basic characteristic of metal pipes is the nominal diameter DN(mm). Pipe tables provide both the inner and outer diameter of the pipe and the wall thickness or schedule.


When designing a pipeline and related fittings according to the EN 13480-3:2017/A1:2021 standard for Metallic Industrial Piping, operational conditions, future maintenance needs, and inspection requirements must be considered. This depends on the planned operating parameters and prior agreements with personnel responsible for functionality and maintenance.

Additionally, future monitoring systems for pipeline operation, maintenance needs without compromising the integrity of components, access routes to the pipeline and fittings, media manipulation, and the required number of personnel and communication lines in daily operations should be considered.

Designing and constructing a pipeline ensures that the risk of material fatigue caused by vibrations in the pipes is adequately addressed.

Depending on the working medium transported by the pipeline, especially its chemical properties, flow regimes, temperature and pressure variations, the material selection for pipeline construction can be a fundamental challenge that determines the initial construction phase.

The material for industrial pipelines is most commonly carbon steel according to the API Spec. 5L, ANSI/ASME B 31.4, and ANSI/ASME B 31.8 standards.

Neglecting the impacts of the acidity of the working medium defined according to the NACE MR 0175 standard, the material for pipeline construction should be defined to be resistant to acidity, regardless of whether corrosion inhibitors are applied to the working media.

Carbon steel pipelines are used for mildly acidic working media where corrosion progression is less than 0.5 mm per year without the use of inhibitors but with a sufficiently large corrosion allowance, mandatory inhibitor dosing, and regular inspection tests, and operator supervision during operation.

When choosing steel for pipe manufacturing, weldability and welding procedure specification are essential, especially when there are requirements for preheating up to 300°C for unfinished welds before completing welding, and defined elongation ratios concerning strength.

Fittings on pipelines such as valves and dampers must be specified to match the internal diameter of the pipe. Valves with non-return flaps according to the API-6D standard are preferred.

Elbows, T-joints, and connectors must have a diameter that matches the pipe diameter and be made of the same or similar material. Flanges must have a welding neck and a flat face. Gaskets must correspond to the class and characteristics of the flange joint on the pipeline.

Different operating pressures, pipe diameters, flow rates, fitting positions, and construction materials define the behavior of the pipeline during its operational life.

The risk of unintentional release of the working medium must be minimized by specifying the type and content of the working medium and clearly marking the outlets of drainage safety pipes, indicating the positions, and marking the route of the underground and above-ground pipeline on the technical documentation.

If the pipeline route is kilometers long, such as in the case of water, oil, and gas pipelines, break chambers and pump stations need to be considered.

The distance between pump stations and the capacities of pumps for pumping the working medium must also be planned and designed in consideration of other necessary infrastructure for the operation and maintenance of the pipeline.

A frequently overlooked factor is the interaction of the pipeline with the environment where it is located, e.g., near the sea or in a desert area because the environment affects the external walls of the pipe, fittings, and accompanying structures.

Protection of the internal walls of the pipeline from corrosion and erosion depends on process conditions such as the chemical properties of the working medium (e.g., a combination of water and carbon dioxide, hydrogen sulfide, or oxygen), variations in temperatures, pressures, and flow rates, and deposits of various particles.

Protection of external walls from corrosion differs from the protection of internal walls, so all external and internal influences on the material must be carefully considered during the design to ensure that the pipeline is in optimal condition throughout its life.

If there is a possibility of implementing different construction solutions during the design of the pipeline, then an economic analysis is carried out.

This checks the optimal solution that aligns specific requirements for pipeline operation with structural characteristics and minimizes financial costs.

Financial analysis considers all other influential factors and safety risks for the safety, health of workers, and environmental impact.

How to test a welded joint of 2 pipes?

After designing, the construction and building phase follows. The construction phase involves welding, connecting with flange joints, or both, laying on routes, and connecting with the necessary fittings.

Every time two or more pipes are welded in sequence to create a new or renewed part of the pipeline, and before the pipeline is installed in a specific position, it is necessary to test the executed welded joints.

Welding has certain advantages over other joining techniques, such as: the joint is impermeable, high-quality, and requires no maintenance; the process is fast and inexpensive, with no need for additional connecting elements and seals, making the pipeline lighter.

This type of joining metallic materials is always used for all pipes in process plants where greater security is required, and the joints are difficult to access for workers simultaneously.

Pipes must be professionally welded, welded only by qualified welders, and the welded joint must be 100% impermeable.

The standard EN ISO 15609 defines the content of the welding procedure specification and represents the basic proof of welding technology.

The welder's qualification is checked according to a set of requirements defined in the HRN EN ISO 9606 standard.

The lack of welding in pipes results in the rigidity of the pipeline, and for disassembly and dismantling, the pipes need to be cut. This is particularly dangerous and impractical when working media are flammable, corrosive, explosive, or a combination of all mentioned.

During the welding process, errors in joints can occur.

Shorter pipes are joined in the workshop, and during the installation of the pipeline, such pipes are welded together into pipelines.

Before putting the working medium and the pipeline into operation, the impermeability and strength of the welded pipes must be tested.

Testing methods for welded joints for process pipelines are non-destructive methods and include various types of controls applied before installing the pipeline in the plant.

The first method is the initial inspection performed by the welder after each stage of pipe fabrication (e.g., after welding flanges, connections, and branches, bending, and shaping).

This inspection includes checking the main dimensions and shape of the pipe according to workshop documentation, the quality, and accuracy of fabrication (cutting, welding, grinding) and shaping the pipe.

The final inspection includes checking the quality of fabrication (welding quality, grinding, and finishing) and detailed dimensional control.

First, a visual inspection is carried out to detect visible defects on the pipe, such as deformations and damages to the pipe, various defects in welded joints, etc.

During welding, deformations can occur when the weld pulls the pipe, resulting in deviations from the specified dimensions. Therefore, the second control involves checking the geometry of the processed pipe.

The pipe is placed on a horizontal surface, and then, using a level, square, and meter, the actual dimensions are compared to the dimensions on the measurement sketch or workshop drawing. Dimensions across the entire length of the profile, radii of the pipe curvature, all connection elevations, angles of welded branches, and the angle of attaching the flange relative to the pipe are particularly controlled.

After that, the inside of the pipe is inspected, and if no further work is required, the pipe is considered processed.

In shipbuilding, pipeline testing is divided into Classes I, II, and III.

Class I includes testing the welded joints along the full length when it comes to longitudinal pipe joints and testing up to one-half length (1/2) when it comes to transverse joints.

Class II includes testing welded joints up to one-quarter length (1/4) for both transverse and longitudinal joints.

Class III includes testing welded joints according to the assessment of the ship registry and is performed after the assembly of the pipeline on the ship, unlike Classes I and II.

For testing the quality of welded joints, pressure tests are applied to pipelines.

Pipes for assembling pipelines of Classes I and II, as well as steam, compressed air, feedwater, and ship fuel pipelines with a design pressure greater than 3.5bar, after fabrication and final processing, before painting, must undergo a hydraulic test pressure that is 50% higher than the design pressure for a specific pipeline.

The design pressure represents the highest working pressure of the fluid that can occur in the pipeline system, and it must not be less than the highest pressure for which any safety valve or relief valve on the same pipeline line is set.

Pipes for assembling Class III pipelines, which do not need to be pressure-tested in the presence of a ship registry representative, are pressurized directly on the ship after the assembly of the pipeline.

Due to internal control conditions, pipes of such classes can also be pressurized in the workshop, eliminating the possibility of leakage of welded joints of pipes during their assembly and commissioning.

Leakage of the working medium is the most common drawback in pipelines. The second most common drawback is thermal losses.

The leakage test is often conducted at the installation site of equipment in process plants. Each test has prescribed pressure testing methods applied in the workshop.

The leakage test of the pipes is performed by pressurizing the pipeline with water in a specially prepared system in the metalworking workshop for pipe fabrication.

The pressure is 10% to 15% higher than the design-specified pressure.

The image shows a schematic of pipe connections and a finished joint ready for leak testing.

Image: Pipeline Tightness Testing, 1-Testing System, 2-Pump, 3-Flexible Connecting Pipe, A, B, C – Various Pipes Welded Together (Source)

Manufactured pipes are connected to each other using flanges, regardless of the later sequence of pipeline assembly. They are then simultaneously subjected to hydraulic pressure to test the tightness and strength of the welded pipe joints.

There should be no water leakage at the welded joints while the pipes are under pressure, with continuous visual observation.

After completing the tightness control, the results are recorded on standardized forms and stored in the archive. Leakage indicates that the welded joint has defects and is unsuitable for use.

As the pipeline is in operation for an extended period, various influences come into play. On one side, there is the influence of the working medium on the walls, while on the other side, the environment also affects the external surfaces of the pipe or the insulation in the case of an insulated pipeline for the transport of cold or hot working media.

Leakage of the working medium is the most common flaw that occurs in pipelines. The second most common flaw is thermal losses.

Do you have thermal losses in the pipeline?

The following practical example explains how to calculate the thermal losses of an insulated pipeline.

The image shows the cross-section of an insulated heated steel pipe. Steel has a thermal conductivity k = 45 W/mK.

The internal radius of the pipe r1 is 150 mm, the external radius r2 is 155 mm, and the radius of the pipe with insulation r3 is 255 mm.

Group 11.png
Image: Cross-section of an insulated pipe (Source: I created the image in AutoCAD)

The pipe is insulated with a layer of insulation with a thickness of 100 mm, which has a thermal conductivity k=0.06 W/mK. Air heated to a temperature Ti = 60°C flows through the pipe.

The convective heat transfer coefficient from air to the inner walls of the pipe is hi = 35 W2/mK. In the working environment of the pipe, the air is at a temperature of 15°C, with a heat transfer coefficient from the external surface of the pipe to the air in the surroundings of hou = 10 W2/mK.

The task is to calculate the thermal losses for this pipeline if its total length is 50m.

The air passing through the pipe transfers a quantity of heat to the walls of the pipe, calculated by the formula:

Q=2πr1Lhin(TinT1)Q=2\pi r_{1} L h_{in} (T_{in}-T_{1})

The amount of heat conducted through the steel pipe is calculated by the formula:

Q=2πLKsteel(T1T2)/ln(r2r1)Q=2\pi L K_{steel} ( T_{1} - T_{2}) / ln( r_{2} - r_{1})

The amount of heat conducted through the insulation is calculated by the formula:

Q=2πLKinsulation(T2T3)/ln(r3r2)Q=2\pi L K_{insulation} ( T_{2} - T_{3}) / ln( r_{3} - r_{2})

The amount of heat convected from the outer surface of the insulation to the air in the surroundings is represented by the formula:

Q=2πr3LHout(T3Tout)Q=2\pi r_{3} L H_{out} ( T_{3} - T_{out})

Connecting the expressions for the amount of heat, we get:

Q=2πL(T1T0)(1r1hin)+(ln(r2/r1)ksteel)+(ln(r3/r2)kinsulation)+(1r3hout)Q = \frac{2\pi L (T_{1}-T_{0})}{({{1}\over{ r_{1}h_{in}}})+ ({{ln{(r_{2}/r_{1}}) }\over{k_{steel} }}) + ({{ln{(r_{3}/r_{2}}) }\over{k_{insulation} }}) + ({{1}\over{r_{3} h_{out} }} )}

When we substitute the existing values, we have:

Q=2π×50×(6015)(135×0.15)+(ln(0.155/0.150)45)+(ln(0.255/0.155)0.06)+(10.255×10)Q = \frac{2\pi \times 50 \times (60-15)}{({{1}\over{ 35 \times 0.15}})+ ({{ln{(0.155/0.150}) }\over{45 }}) + ({{ln{(0.255/0.155}) }\over{0.06 }}) + ({{1}\over{0.255\times 10 }} )}

Q=1592WQ = 1592 W

The thermal resistance of insulation is greater than the resistance of steel and the working medium passing through the pipe or in the surroundings.

Increasing the thickness of insulation does not guarantee a reduction in heat loss. Adding insulation increases the surface area from which heat escapes into the surroundings.

If the surface area increases more than the thermal resistance, then there will be a greater heat loss.

The critical half-thickness radius of insulation is the largest radius at which increasing insulation will lead to increased heat loss.

The critical radius is calculated by the formula:

rcrit=kis/kextr_{crit} = k_{is} / k_{ext}

For This Example, the Critical Half-Thickness Radius of Insulation Will Be:

rcrit=kins/hextr_{crit} = k_{ins} / h_{ext}

rcrit=0,06/10r_{crit} =0,06 / 10

rcrit=6mmr_{crit} = 6 mm

For a radius r > 6mm, any subsequent increase in insulation thickness will lead to increased heat loss.

Best Practices for Pipes in Mechanical Systems

When it comes to the assembly, connection, and inspection of pipes and smaller pipelines in mechanical systems, typically within the context of units or facilities with fewer pieces of equipment such as a food production factory, there are several guidelines for good engineering practices.

Let's go through them:

  • Drainage connections must be at the lowest points of the system.
  • Flushing the pipeline will solve the potential problem of sediment buildup on the walls and corrosion.
  • Forced circulation of the working medium will also prevent potential corrosion.
  • If cooling is required in the system, design it so that it does not disrupt the pipeline route and avoid compromising integrity.
  • Continuously check for leaks in the system.
  • When designing, lay out the lines to completely avoid or minimize losses in the pipeline.
  • Use larger-diameter elbows where applicable.
  • Position outlet connections tangentially.
  • Ensure that the pipe material is compatible with the working medium in the system and is in compliance with applicable industry standards.
  • Use reductions where necessary and applicable.
  • Before commissioning, check whether all connections are correctly attached to the respective pipelines for the flow direction of the working medium.
  • Pipe slopes must be gentle and gradual (40 mm/m). Sharp slopes cause increased flow resistance and losses in the pipeline.
  • Before starting the system, vent and drain the pipeline.
  • Before starting the system, check the values at which all pressure and level alarms are set on all pipelines and equipment.

Pipelines are complex structures without which it is not possible to realize any processing or transport system. Without adequate design and adherence to professional rules during assembly and operation, the full potential of the pipeline cannot be utilized.

Pipelines have a significant impact on connected equipment, as explained in the next example. The interaction of pipelines contributes to the profitable operation of a centrifugal pump.

Effect of Pump Eccentricity on Stress in Suction and Discharge Pipelines

After the installation of the pump and electric motor, and before starting it, it is mandatory to perform centering or alignment of the entire pump unit.

Eccentricity of the pump unit leads to stress in the suction and discharge pipelines at the flange joints, resulting in internal stress in the pump bearings, excessive wear of the mechanical seal, motor bearing wear, and a shorter lifespan of the coupling.

It is not excluded that the pump will achieve lower than required operating parameters.

The image shows a pump unit for pumping the working medium from the tank to the exchanger in a part of a technological process.

The pump unit consists of an electric motor, coupling, and a single-stage centrifugal pump located on a common foundation.

Screenshot 2024-01-31 at 16.08.53.png
Image: Pump unit connected to suction and discharge pipelines

Flange joints are highlighted in yellow on the image. On the suction and discharge flange of the pump, flanges of the suction and discharge pipelines are connected, where potential points of concentration of the greatest stress exist. To avoid harmful stress, certain guidelines must be followed when assembling the pump unit and connecting the suction and discharge pipelines:

  1. Flanges of pipelines that connect to pump flanges must be mutually parallel and aligned, with a gap between them not exceeding the thickness of 2 gaskets or the gap between flanges not exceeding dimensions recommended by the pump manufacturer, such as the joint shown in the image.

Group 13.png
Image: Properly assembled flange joint (Source: Image from AutoCAD)

  1. Screws and nuts mounted on flanges must be mounted without jamming or forced adjustment by pulling.
  2. When aligning the pipeline flange and the pump flange, do not use crowbars, bars, and manual hoists. Use only suitable non-sparking hand tools from verified manufacturers.
  3. It is essential that all work performers follow company procedures (if any) during the assembly of flange joints to avoid stressing the pipeline.
  4. A laser alignment of the pump and motor shafts should be performed within specified manufacturer limits, company standards, and good engineering practice.

Image: Laser alignment of the pump unit (Source: Laser Alignment Shaft Centrifugal Pump Check Stock Photo 1185219934 Shutterstock)

  1. Then, release the suction and discharge flanges from the suction and discharge flange on the pump, remove gaskets and all screws with nuts.
  2. Laser-check the alignment of the pump and motor shafts again. Now, you will have one of two situations: a) There is no change in the alignment of the pump unit, proving that there is no stress on the pipelines. b) There is a change in the alignment of the pump unit. It is proven that stress in the pipelines is present, and it needs to be identified and eliminated as soon as possible. Afterward, realign the pump unit.
  3. Create a measurement protocol on alignment, confirming that the driving and driven machines are correctly aligned, certify it, and hand it over to the user or equipment owner.

Stress in suction and discharge pipelines is not at all harmless and entails numerous negative effects on the machine.

Commonly, failures of pump or electric motor bearings occur as a result of stress in pipelines, manifesting as:

  • Material wear caused by leakage on seals
  • Part wear caused by vibrations
  • Machine load in a stationary state
  • Corrosion due to inadequate lubrication from excessive load and resulting leakage on seals
  • Material peeling on surfaces, caused by misalignment and excessive load

Every technical system requires adequate care and monitoring throughout its life cycle as a basic prerequisite for achieving a reliable and profitable operation.

Pipelines are seemingly simple mechanical constructions; however, it is little known how much effort and attention they need throughout their working life, starting from the design phase, construction, welding inspection, assembly, and operation, and their impact on connected and accompanying equipment.

Only in this way can optimal functionality, a high level of reliability in operation, and a satisfactory return on investment in the process plant be achieved.

Katarina Knafelj Jakovac
Katarina Knafelj Jakovac social media icon
January 31, 2024

Katarina Knafelj Jakovac holds Master degree in Mechanical engineering with long term work experience in Oil industry. She is Certified Reliability Leader specialized for mechanical equipment and operational excellence. Author of blog Strojarska Radionica (Mechanical Workshop) where she shares professional knowledge and personal experience in maintaining various rotating machines, machine systems and process equipment. Adores mechanics, thermal engineering and internal combustion engines. She is dedicated to the continuous improvement of machine maintenance and quality management of physical assets.