Pipe spools are an essential element of piping systems. They are pre-assembled and fabricated offsite which involves detailed processes such as cutting, bevelling, forming, and welding, to enhance construction efficiency and quality.

1. Introduction to Pipe Spools

Pipe spools refer to the pre-assembled piping components, encompassing pipes, flanges, and fittings. These elements are assembled during fabrication before being transported to the construction site. The pre-mounted state of these spools facilitates easy assembly using hoists, gauges, and other tools. 

The spools connect long pipes with flanges at the ends which allows them to be securely bolted to another pipe. Before pouring the concrete, pipe spools are embedded into concrete walls to ensure proper positioning. This also allows to withstand the weight and force of the concrete. 

In applications such as the construction of power plants and petroleum refineries, where extensive piping is essential for fluid and gas transport, the demand for efficient fabrication and assembly arises. Due to space constraints and the urgency to complete construction swiftly, these industries heavily rely on offsite fabrication and assembly. This involves two key stages: pipe spool production and site installation.

During pipe spool fabrication, a variety of raw pipes and fittings, including flanges, elbows, and tees, are employed. This fabrication occurs offsite. The raw pipes being pre-cut to specific sizes and temporarily fitted together with other components before undergoing welding. 

The pipe spooling process is categorized into two main stages: spool roll fitting and welding. In roll fitting, a rolling machine turns the main pipe without requiring the fitter to change position, and spool permanent position fitting. Welding involves the fitter moving around the main pipe to complete the process. Welding tends to be a longer process compared to the roll fitting. Minimizing the number of spool position fitting and welding instances is a key objective in the sequencing of pipe spool fabrication.

2. Pipe spooling materials

Depending on the application, pipe spooling can be done using these materials:

  • Carbon steel
  • Galvanized steel
  • Stainless steel
  • Aluminium alloys
  • Copper

Carbon steel and stainless steel are the most frequently used materials. Stainless steel fabrication of the pipe spools requires a dedicated stainless steel workshop.

3. Advantages of Pipe Spooling

Construction companies engaged in the development of facilities with intricate pipe networks find several benefits in employing pipe spooling. Here are some of the pipe spooling:

  1. Offsite assembly of piping ensures the creation of a high-quality product. It minimizes the risk of delays due to the need for reworks.
  2. Fabricating pipes within an enclosed facility eliminates potential delays caused by adverse weather conditions.
  3. The combination of fabrication and installation times is significantly reduced in comparison to utilizing pipe spooling work directly on-site.
  4. Quality assurance testing, including radiography, magnetic particle inspection (MPI), hydro testing, and ultrasonic testing, becomes more manageable in a controlled environment.
  5. Stringent regulations govern pipe spools, ensuring consistent adherence to optimal quality standards, 

The use of pipe spooling allows the creation of complex systems through sophisticated and demanding processes. It is often not feasible to achieve similar quality results in on-site conditions.

4. Pipe Spool Fabrication

The fabrication of pipe is the bending, cutting, forming, and welding of individual pipe lengths and piping components to each other.

Carbon steel is the most commonly used material of construction used for process piping systems, followed by stainless steel and various alloys. Many nonmetallic materials also are used. 

Material are selected according to their corrosion resistance to the fluid and ability to handle the design temperature and design pressure. Also considering environment and service conditions are critical in material selection.

The material used for fabrication must conform to a relevant ASTM, API, or other recognized standard that guarantees the predictability of

  • Chemical Composition
  • Mechanical Properties
  • Physical Properties
  • Heat Treatment

The major fabrication steps are;

4.1. Cutting

The primary fabrication process, predominantly applicable to pipe lengths, is a fundamental step in the construction workflow. This is particularly true for pipes, as fittings come in standard dimensions. The cutting techniques commonly employed in this process are either mechanical or thermal. Mechanical methods, referred to as cold cuts, employ tools such as saws, abrasive discs, and specialized pipe cutting machines. On the other hand, thermal methods, also known as hot cuts, involve flame cutting utilizing either gas or electric arc cutting.

Photo 1: Pipe Cutting

4.2 Bevelling

Upon cutting a pipe to length, the end of the pipe is positioned at a 90-degree angle to the axis of the pipe for preparation before welding can start. Bevelling is the essential process employed to shape the end of a pipe correctly. It enables a seamlessly connection to another pipe or piping component to create a groove suitable for welding. 

Photo 2 : Pipe Beveling

This bevelling process may involve a single bevel or, in the case of thicker-walled pipes, a compound bevel at two or more angles. The bevelling operation can be carried out through either mechanical or thermal methods.

4.3 Forming

Forming in piping fabrication covers various techniques such as bending, extruding, swaging, lapping, and expanding, all aimed at creating a component for a connection. While the standardization of welded pipe fittings has reduced the necessity for this fabrication process, it remains an option permitted by ASME B31.3.

4.4 Bending

Cold or hot bending of straight pipes presents an alternative and more frequently employed fabrication method compared to forming. Small-bore piping, typically less than 2″ in diameter for utility services, can be bent using an approved procedure. Additionally, large pipelines that need to be pigged for cleaning or batching purposes necessitate long-radius bends, often three to five times the outside diameter, to ensure a smooth passage for the pig. Such bends can be achieved through the bending of pipes.

When considering pipe bending, three critical dimensional limitations must be carefully addressed;

  • Thinning
  • Buckling 
  • Ovality

A comprehensive bending procedure is essential, and an appropriate bending method should be selected to manage these factors effectively.

Photo 3: Pipe Bending

Thinning is a crucial consideration because, during the bending process, the outer edge undergoes stretching, while the inner edge experiences compression. This stretching results in a thinning of the wall thickness in the outer section, and it must be closely monitored to ensure it does not exceed the allowable tolerance for the pipe.

Buckling is also a significant concern, as the bending operation has an opposing effect on the inner wall thickness, tending to compress it. However, this compression doesn’t always lead to thickening of the wall section. There is a tendency, at a certain stage of compression, for the inner edge to buckle.

Ovality is another key dimension to be mindful of because, during the bending process, the cross-section of the bend can take on an oval shape. The degree of ovality is determined by the difference between the major and minor axes divided by the nominal diameter of the pipe. Managing thinning, buckling, and ovality is crucial for ensuring the integrity and dimensional accuracy of the bent pipes.

4.5 Welding

Welding plays a pivotal role in the fabrication of process piping systems, emphasizing the necessity of employing correct procedures and qualified welders to ensure the integrity and reliability of the welded joints.

Photo 4: Pipe Welding

A well-defined Welding Procedure Specification (WPS) that clearly outlines the base metal, filler material, shielding fluxes/gases, positions, and heat treatment can result in a welded joint that possesses the necessary characteristics and is free from leaks. The specific parameters employed during welding are documented in a Procedure Qualification Record (PQR).

Fabrication tasks should be carried out by personnel who are qualified to work in accordance with the relevant Welding Procedure Specification (WPS). Following the guidelines set by ASME Section IX, a qualified welder who has not performed welding in a specific WPS within a specified period must undergo requalification. This ensures that the skills and qualifications of the welders remain up-to-date and in compliance with the established standards.

Defective welds requiring repair must be ground back to the base metal. The repair weld must adhere to the correct Welding Procedure Specification (WPS), considering that the contour surface may differ in profile and dimensions from the original. Preheating and heat treatment should match the specifications outlined for the original welding.

In both fabrication shops and site environment, two common types of welds are butt-weld and socket weld are used. However, due to the fabrication shop’s access to more equipment and a controlled environment, it is typically the preferred location for completing a weld, if possible. 

The butt-weld requires special end preparations and can be applied to pipes of all commercial sizes. On the other hand, the socket weld is intended for use only on pipes up to NPS 4 (DIN 100)

Preheating before the welding process serves to decelerate the cooling rate of the weld joint. This deliberate slowing of cooling contributes to a heightened level of ductility in both the final weld and the heat-affected zone (HAZ). Preheating facilitates the more effective diffusion of dissolved hydrogen. It helps the reduction of shrinkage, distortion, and potential cracking induced by residual stresses in the welded structure.

5. Conclusion

Pipe spooling emerges as a pivotal technique in the construction and maintenance of intricate piping systems, particularly in industries where precision and efficiency are paramount. The advantages of pipe spooling, including quality control, reduced construction time, and adherence to stringent regulations, underscore its significance in modern construction practices. The controlled, off-site fabrication of pipe spools not only ensures high-quality outcomes but also addresses challenges like adverse weather and on-site space constraints. As industries continue to evolve, the role of pipe spooling in maintaining the integrity and functionality of extensive piping networks remains indispensable. 

Novelty Steel offers contract pipe spool fabrications for various industries.

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