Are Pipe Nipples Always Threaded? 5 Types You Must Know in 2025

Abstract

This inquiry delves into the pervasive yet often misunderstood assumption regarding pipe nipples, specifically addressing the question of whether they are invariably threaded. A comprehensive examination of piping components reveals that while threaded nipples, conforming to standards such as NPT and BSPT, represent a significant and common category, they are by no means the exclusive configuration. The landscape of pipe fittings is far more diverse, encompassing non-threaded variants designed for specific, high-performance applications. This analysis explores the functional, mechanical, and material distinctions between threaded, weld, and grooved nipples. It posits that the choice of nipple type is not arbitrary but is a critical engineering decision dictated by system parameters like pressure, temperature, fluid medium, and the requisite for permanence versus maintainability. By exploring the contexts of fire protection, gas distribution, and HVAC systems, this work demonstrates that a nuanced understanding of nipple typology is fundamental for designing safe, efficient, and durable pipeline infrastructure. The conclusion is that the initial question, are pipe nipples always threaded, must be answered in the negative, opening a richer discourse on fit-for-purpose component selection.

Key Takeaways

• The answer to “are pipe nipples always threaded” is no; weld and grooved nipples are vital alternatives.
• Threaded nipples are ideal for low-pressure, maintainable systems like residential plumbing.
• Weld nipples provide permanent, leak-proof joints for high-pressure and high-temperature services.
• Grooved nipples offer rapid, secure assembly for systems like fire sprinkler lines.
• Material choice—steel, brass, iron—directly impacts a nipple’s durability and application.
• Proper selection depends on pressure, temperature, fluid, and required joint permanence.
• Understanding international standards like ASME, ASTM, and NFPA is vital for compliance.

Table of Contents

• The Fundamental Question: Are Pipe Nipples Always Threaded?
• Type 1: The Ubiquitous Threaded Pipe Nipple – A Deep Examination
• Type 2: The Weld Nipple – Forging Permanent, Leak-Proof Bonds
• Type 3: The Grooved Nipple – The Modern Approach to Pipe Joining
• Type 4: The Swage Nipple – Navigating Changes in Pipe Diameter
• Type 5: Specialized and Combination Nipples – Tailoring Solutions for Unique Challenges
• Material Science and its Influence on Nipple Selection
• Navigating International Standards and Certifications
• Practical Application: Choosing the Right Nipple for Your System
• Frequently Asked Questions (FAQ)
• Conclusion: Beyond a Simple Yes or No
• References

The Fundamental Question: Are Pipe Nipples Always Threaded?

To embark on an exploration of piping systems is to enter a world of precise engineering, where every component, no matter how small, plays a role of profound significance. Within this world, the pipe nipple—a short stub of pipe, typically male-threaded at both ends—is a foundational element, used to connect two other fittings. For many practitioners, particularly those whose experience is rooted in residential or light commercial plumbing, the image of a pipe nipple is inextricably linked with threads. This leads to a seemingly simple question, yet one with deep implications for system design and safety: are pipe nipples always threaded? The immediate, and perhaps surprising, answer is a definitive no. This response, however, is not an endpoint but a gateway to a more sophisticated understanding of fluid conveyance systems. To assert that all nipples are threaded is akin to claiming all fasteners are screws, ignoring the existence and specific utility of nails, rivets, and bolts. Each has its place, its purpose, and its domain of superiority. Similarly, the world of pipe nipples extends far beyond the familiar threaded form into specialized types designed for welding, grooving, and other joining methods. Recognizing this diversity is the first step toward moving from a competent technician to an adept system designer. The choice between a threaded, weld, or grooved nipple is not a matter of preference but a calculated decision based on a rigorous assessment of the system’s demands. Factors such as operating pressure, temperature, the nature of the fluid being transported, seismic considerations, and the need for future maintenance all weigh heavily on this choice. A threaded nipple that is perfectly suitable for a low-pressure residential fire sprinkler system could be a catastrophic point of failure in a high-pressure steam line, where a butt-weld nipple would be the only responsible choice. Therefore, the inquiry, “are pipe nipples always threaded,” forces us to confront the core principles of mechanical engineering and material science as they apply to pipeline integrity.

Deconstructing the Misconception

The common misconception that all pipe nipples are threaded arises from their prevalence in accessible, everyday applications. In hardware stores and plumbing supply houses catering to residential and light commercial construction, the vast majority of nipples on the shelf will indeed be threaded. These are the components used for connecting water heaters, extending gas lines for appliances, and assembling the familiar networks of pipes that serve our homes and offices. These systems typically operate at low pressures and ambient temperatures, conditions under which tapered pipe threads, when properly sealed, provide a reliable and cost-effective joint that can also be disassembled for repair or modification. This ubiquity creates a powerful cognitive bias. When a component is seen performing a task reliably in 90% of one’s personal experience, it is natural to extrapolate that this is its only form. However, the world of industrial piping—encompassing power plants, chemical refineries, pharmaceutical manufacturing, and large-scale fire protection systems—operates under a vastly different set of rules and stresses. In these environments, the potential for leakage is not merely an inconvenience but a significant safety hazard and a cause of costly downtime. The limitations of threaded connections become starkly apparent, compelling engineers to seek more robust solutions. This is the world where the non-threaded nipple comes to the forefront. Understanding why a weld or grooved nipple is superior in certain contexts requires a shift in perspective, from viewing a pipe joint as a simple connection to seeing it as a carefully engineered seal designed to withstand specific, often extreme, forces.

An Overview of Nipple Categories

To bring order to this discussion, we can categorize pipe nipples into three primary families based on their end connections: threaded, welded, and grooved. Each family represents a different philosophy of pipe joining. The threaded nipple embodies modularity and serviceability. The weld nipple represents permanence and ultimate joint integrity. The grooved nipple offers a modern hybrid, combining the speed of assembly with significant strength and flexibility. Within these families, further variations exist, such as the swage nipple, which changes pipe diameter, or the hex nipple, which incorporates a hexagonal nut for easier wrenching. The following table provides a high-level comparison to frame our deeper investigation into each type.

This table serves as our roadmap. By examining each of these categories in detail, we can build a comprehensive understanding that allows us to answer not just if pipe nipples are always threaded, but why and when they should not be. This knowledge is indispensable for engineers, fabricators, and installers who are entrusted with building the vital arteries of our industrial world. As we proceed, we will see that the humble pipe nipple is a microcosm of larger engineering principles, a component whose design reflects a deep history of trial, error, and innovation.

Type 1: The Ubiquitous Threaded Pipe Nipple – A Deep Examination

The threaded pipe nipple is the patriarch of the nipple family, the most recognized and widely used type across a multitude of industries. Its design is a testament to the power of a simple mechanical principle: the helix. By cutting a spiral groove into the end of a pipe, we create a means of forming a strong, tight connection using rotational force. Yet, within this apparent simplicity lies a great deal of engineering nuance. The effectiveness of a threaded joint depends entirely on the precision of these threads, the material they are made from, and the manner in which they are assembled. A deeper look at the threaded nipple reveals the science behind this everyday component.

Anatomy of a Thread: NPT, BSPT, and the Language of Connection

Not all threads are created equal. The most common type in North America is the National Pipe Thread (NPT), defined by the ASME B1.20.1 standard. A critical feature of NPT threads is their taper. The threads are cut on a slight cone, suchthat as the male nipple is tightened into a female fitting, the flanks of the threads compress against each other, creating an interference fit. This wedging action is the primary source of the seal. However, due to imperfections in manufacturing, this metal-to-metal seal is not perfect. A helical leak path exists at the root and crest of the threads. This is why thread sealant—either Polytetrafluoroethylene (PTFE) tape or a liquid/paste pipe dope—is indispensable. The sealant does not act as a glue; rather, its purpose is to fill the microscopic voids in the thread path, blocking the potential for leaks. This is a crucial point: the strength of the joint comes from the mechanical interference of the threads, while the leak-tightness comes from the sealant.Another prominent standard is the British Standard Pipe Taper (BSPT). While also tapered, BSPT threads have a different angle (55 degrees versus NPT’s 60 degrees) and a different pitch in most sizes. NPT and BSPT are not interchangeable. Attempting to join them will result in a connection that may seem tight but will have poor thread engagement and will almost certainly leak, especially under pressure or vibration. For global projects, identifying the correct thread standard is a non-negotiable first step. Making an error here can lead to significant delays and costs. The choice of thread standard is a foundational aspect of system design, dictating compatibility across all components.

Common Materials and Their Properties

The material of a threaded nipple is just as important as its threads. The choice depends on the fluid, temperature, pressure, and external environment.

• Carbon Steel (e.g., ASTM A53/A106): This is the workhorse. Often referred to as “black iron” pipe, it is strong, inexpensive, and suitable for a wide range of applications, including natural gas, hot water heating (hydronic), and fire sprinkler systems. Its primary weakness is a susceptibility to rust in the presence of oxygenated water or a corrosive environment.
• Galvanized Steel: This is carbon steel that has been coated with a layer of zinc. The zinc acts as a sacrificial anode, protecting the steel from rust. Galvanized nipples are commonly used for potable water lines (though less so now with the rise of copper and plastics) and outdoor applications where they are exposed to the elements. However, they are not suitable for gas lines, as the gas can cause the zinc to flake off and clog downstream orifices.
• Stainless Steel (e.g., 304/316): The choice for corrosive environments or applications requiring high purity. Stainless steel is used extensively in the food and beverage, pharmaceutical, and chemical industries. Grade 316, with its added molybdenum content, offers superior resistance to chlorides and is often specified for marine or coastal environments. A significant challenge with stainless steel threaded nipples is their propensity for “thread galling,” where the surfaces seize and weld themselves together upon tightening. This can be mitigated with proper anti-seize lubricants and careful assembly techniques.
• Brass: An alloy of copper and zinc, brass is soft, easy to machine, and highly resistant to corrosion from water. It is commonly used in plumbing for potable water and in low-pressure instrumentation lines. Its relative softness makes it less suitable for high-pressure structural applications compared to steel.

The selection process involves a careful balancing act, weighing the material’s performance against its cost. Using a stainless steel nipple where a simple carbon steel one would suffice is wasteful, but using carbon steel in a corrosive service is a recipe for premature failure.

Applications and Limitations

Threaded nipples find their home in systems where pressures are generally moderate (typically below 300 psi for many common applications, though some classes of fittings go higher) and temperatures are not extreme. Their key advantage is the ease and speed of assembly using simple, readily available tools. This makes them ideal for:

• Residential and Commercial Plumbing: Connecting fixtures, water heaters, and distribution lines.
• Low-Pressure Gas Distribution: Supplying natural gas or propane to furnaces, water heaters, and appliances. Every joint must be meticulously leak-tested.
• General Utility Piping: Compressed air lines, low-pressure fluid transfer, and drainage systems.
• Fire Protection: While grooved pipe fittings dominate larger fire sprinkler lines, threaded fittings are still extensively used for smaller branch lines and for connecting sprinkler heads themselves.

However, the very nature of a threaded joint is also its primary limitation. The thread itself is a point of weakness. The process of cutting threads removes material, thinning the pipe wall. more importantly, the spiral leak path, even when filled with sealant, is a potential failure point. Threaded connections perform poorly under conditions of severe vibration, thermal cycling, or water hammer, as these forces can work to loosen the joint and break the sealant’s bond over time. For critical, high-pressure, or hazardous fluid systems, the risk associated with a potential thread leak is often unacceptable. This is the boundary where engineers must look beyond the threaded nipple and consider the more robust alternatives, leading us away from the initial assumption that pipe nipples are always threaded.

Type 2: The Weld Nipple – Forging Permanent, Leak-Proof Bonds

When the operational demands of a piping system transcend the capabilities of a threaded joint, we enter the realm of welding. The weld nipple represents a fundamentally different philosophy of connection. It is not about creating a separable, mechanical joint; it is about creating a continuous, monolithic structure. By fusing the metal of the nipple directly to the adjoining pipe or fitting, the joint effectively disappears, becoming an integral part of the pipeline itself. This approach offers the highest possible level of joint integrity, making it the standard for services where failure is not an option. The use of a weld nipple signals a commitment to permanence and an uncompromising stance on safety and reliability. Consequently, it helps to definitively answer that pipe nipples are not always threaded, especially in critical applications.

The Philosophy of a Permanent Joint

To choose a weld nipple is to accept a trade-off. You gain unparalleled strength and leak resistance at the cost of serviceability. A welded joint cannot be undone with a wrench. Disassembly requires physically cutting the pipe and grinding the weld away, a destructive and time-consuming process. This “design for permanence” is deliberate. It is employed in systems where the risk and consequence of a leak are so high that the possibility must be engineered out as much as possible. Consider a high-pressure steam line in a power plant. A leak could release superheated steam at immense force, posing a severe danger to personnel and equipment. In a chemical plant carrying toxic or flammable substances, any leak could be catastrophic. In these scenarios, the benefit of a completely sealed, permanent joint far outweighs the inconvenience of future modifications. The weld becomes the seal. There is no reliance on tape or dope, no helical leak path to worry about. The finished joint, when properly executed and inspected, is as strong and leak-proof as the parent pipe. This is the ultimate expression of pipeline integrity, a stark contrast to the mechanical nature of a threaded connection.

Butt-Weld vs. Socket-Weld Nipples: A Tale of Two Welds

Within the family of weld nipples, two primary types exist: butt-weld and socket-weld. The choice between them depends on the pipe size, the specific application, and sometimes, the preference of the system designer or prevailing industry code.

• Butt-Weld (BW) Nipples: These are the most common type for larger pipe sizes (typically 2 inches and above) and for the most critical applications. A butt-weld nipple has ends that are beveled to a specific angle (usually 37.5 degrees). This bevel matches a corresponding bevel on the end of the pipe or fitting it is joining. The two beveled ends are brought together, leaving a small gap at the root. A skilled welder then fills this V-shaped groove with molten metal, pass by pass, until a solid, continuous weld is formed that penetrates the full thickness of the pipe wall. This creates a smooth, uninterrupted flow path inside the pipe, which is advantageous for minimizing turbulence and pressure drop. It is also easier to inspect with non-destructive methods like radiography (X-ray) or ultrasonic testing to ensure there are no hidden defects within the weld.
• Socket-Weld (SW) Nipples: These are typically used for smaller pipe sizes (generally under 2 inches). A socket-weld fitting has a recessed area—a socket—into which the plain end of the nipple is inserted. The welder then applies a fillet weld around the outside shoulder of the fitting, joining it to the nipple. Before welding, a small gap (typically 1/16 inch or 1.5 mm) must be left between the end of the nipple and the bottom of the socket. This gap is crucial; it allows for thermal expansion during the welding process and prevents stresses that could lead to cracking of the fillet weld. The primary disadvantage of a socket weld is this internal gap, which can create a crevice where solids can accumulate or corrosion can initiate. This makes them unsuitable for highly corrosive or erosive services or for systems where sanitary conditions are required. However, they are often considered easier to align and fit-up than butt-weld joints, potentially reducing installation time for small-bore piping.

The decision between butt-weld and socket-weld is a technical one. Butt welds offer superior strength and flow characteristics, while socket welds can offer easier fit-up for smaller pipes. Both, however, provide a permanent, welded connection that far exceeds the reliability of a threaded joint in demanding conditions.

When Strength is Non-Negotiable: High-Pressure, High-Temperature Scenarios

The domain of the weld nipple is defined by extremes. As pressures and temperatures rise, the forces trying to pull a joint apart increase dramatically. Threaded connections, relying on mechanical interference, become increasingly unreliable. Welding, on the other hand, creates a bond at the molecular level.

• High-Pressure Systems: In hydraulic systems, process chemical lines, or main steam piping operating at thousands of psi, a welded joint is the only acceptable option. The continuous metal structure can withstand these immense forces without the risk of a blowout that could occur at a threaded connection.
• High-Temperature Systems: Materials expand when heated. In a system with frequent temperature cycles, this expansion and contraction can cause threaded joints to loosen over time, a phenomenon known as thermal cycling fatigue. A welded joint, being part of the same continuous material, expands and contracts with the rest of the pipe, maintaining its integrity.
• Hazardous Fluids: When transporting substances that are toxic, flammable, or environmentally harmful, containment is the highest priority. The near-zero leak potential of a properly executed weld provides a level of safety that threaded systems cannot match.
• Vibration and Mechanical Stress: In systems connected to heavy machinery, pumps, or compressors, or in applications subject to seismic activity or water hammer, the rigidity of a welded joint prevents it from loosening under constant vibration.

In all these cases, the answer to “are pipe nipples always threaded?” becomes self-evident. For systems where reliability is paramount, the weld nipple is not just an option; it is a necessity. Its selection reflects a deep commitment to engineering safety and long-term performance, a commitment that is central to the work of a history of manufacturing excellence in the field of pipeline components.

Type 3: The Grooved Nipple – The Modern Approach to Pipe Joining

Standing as a remarkable innovation between the traditional threaded joint and the permanent welded connection is the grooved mechanical piping system. The grooved nipple is the corresponding component for this method, representing a leap forward in how pipelines are constructed, particularly in specific applications like fire protection and HVAC. This system offers a unique combination of strength, flexibility, and speed of installation that neither threading nor welding can fully replicate. It challenges the old dichotomy, providing a third way that has revolutionized large-scale pipe installation. The existence and widespread adoption of grooved systems is one of the most compelling arguments against the notion that pipe nipples are always threaded.

The Mechanical Joint Revolution

The concept of the grooved joint was patented in 1919 and commercialized by the Victaulic company. It was born out of a need for a rapid, reliable way to deploy pipelines for fuel and water during World War I. The core idea was brilliantly simple: instead of cutting threads onto a pipe or welding it, cut a groove near its end. A two-part coupling could then house a rubber gasket and clamp securely into the grooves of two adjoining pipe ends, creating a sealed and structurally sound joint. This “mechanical T” principle allowed for flame-free assembly that was significantly faster than welding and more reliable than threading for larger pipe sizes. Over the past century, this technology has been refined and expanded, becoming a dominant method in several key industries. It represents a paradigm shift, moving the complexity from the pipe end (as in threading) or the installation process (as in welding) to a purpose-built, factory-made coupling. This shift brings with it a high degree of quality control and predictability, as the performance of the joint is determined by the engineered coupling and gasket rather than the variable skill of a welder or the imprecise nature of a field-cut thread.

How Grooved Systems Work: The Coupling, the Gasket, and the Groove

Understanding the grooved nipple requires understanding the three components that make the system function:

• The Groove: A circumferential groove is cold-formed or machine-cut into the end of the pipe nipple. The dimensions of this groove—its diameter, width, and depth—are precisely controlled by standards set by organizations like the American Water Works Association (AWWA) and various system manufacturers. This precision is essential for the coupling to engage correctly.
• The Gasket: This is the heart of the seal. The C-shaped elastomeric gasket is designed to fit over the two pipe ends. When the system is pressurized, the internal pressure pushes the gasket outwards, reinforcing the seal against the pipe surface and the inside of the coupling housing. This pressure-responsive design means that the higher the internal pressure, the tighter the seal becomes. Gaskets are made from various materials (EPDM, Nitrile, etc.) to ensure chemical compatibility with the fluid being transported.
• The Coupling Housing: This is a two-piece (or sometimes multi-segment) metal housing that is placed over the gasket. The inner keys of the housing are designed to fit perfectly into the grooves on the pipe nipples. When the bolts and nuts of the coupling are tightened, the housings clamp down, securing the pipe ends, compressing the gasket to create the initial seal, and locking the entire assembly together.

The assembly is straightforward and fast. A worker lubricates the gasket, places it over the pipe ends, fits the two coupling halves over the gasket and into the grooves, and tightens the bolts with a simple wrench until there is metal-to-metal contact between the bolt pads on the coupling. No complex machinery, no open flame, and no long curing times are required.

Advantages in Fire Protection, HVAC, and Beyond

The unique characteristics of the grooved system make it exceptionally well-suited for certain applications, most notably fire protection. The National Fire Protection Association (NFPA) 13, the standard for the installation of sprinkler systems, widely recognizes and provides guidelines for the use of grooved piping systems.

• Speed and Safety of Installation: In large commercial buildings, warehouses, or industrial facilities, fire sprinkler systems involve thousands of feet of pipe. The ability to quickly assemble joints without welding (which requires fire watches, permits, and presents a fire hazard itself) dramatically reduces installation time and labor costs.
• Accommodation of Movement: Grooved couplings are available in two main types: rigid and flexible. Flexible couplings allow for a controlled amount of linear movement (expansion and contraction due to temperature changes) and angular deflection. This is invaluable in seismically active zones, as it allows the piping to move with the building during an earthquake without failing. It also helps to dampen vibration from pumps and equipment.
• Ease of Maintenance and Modification: Unlike a welded system, a grooved system can be easily disassembled. By simply removing the bolts on a coupling, a section of pipe or a fitting can be removed for maintenance, repair, or system modification. This accessibility is a major advantage over the life of the building.

Beyond fire protection, grooved systems are extensively used in HVAC systems (for chilled and hot water lines), water and wastewater treatment plants, mining operations (for dewatering and slurry lines), and other industrial applications where large-diameter pipes and the need for a reliable, fast, and maintainable joining system converge. The grooved nipple, therefore, is not a niche product but a mainstream component in modern, large-scale pipe construction. Its existence provides a clear and practical answer to our guiding question. To ignore the grooved nipple is to overlook one of the most significant developments in piping technology of the last century. When designing systems where speed, flexibility, and maintainability are key drivers, one must consider if threaded or welded options are truly the best fit, or if the modern grooved method holds the superior solution.

Type 4: The Swage Nipple – Navigating Changes in Pipe Diameter

In the intricate network of a piping system, it is rare for the pipe diameter to remain constant from start to finish. Systems often require changes in line size to manage fluid velocity, pressure, and flow rate. This is the specific challenge that the swage nipple is designed to address. A swage nipple is essentially a type of reducer, a short piece of pipe that connects a larger pipe to a smaller one. While it functions as a reducer, its form factor as a short, end-to-end connector firmly places it within the nipple family. Swages can have various end connections—threaded, beveled for welding, or plain—further demonstrating that pipe nipples are not always threaded and are, in fact, a highly diverse category of fittings.

The Functional Purpose of a Reducer

Why would a system need to change pipe size? The reasons are rooted in the principles of fluid dynamics. According to the principle of continuity, for an incompressible fluid, the product of the cross-sectional area and the fluid velocity must remain constant. Therefore, when the pipe diameter decreases, the fluid must speed up. Conversely, when the pipe diameter increases, the fluid slows down. Engineers manipulate this relationship for several purposes:

• Increasing Pressure: Reducing the pipe size ahead of instrumentation, like control valves or flow meters, can increase the fluid velocity and create the necessary operating conditions for those devices.
• Managing Velocity: In long pipe runs, it might be economical to use a larger diameter pipe to minimize pressure loss due to friction. However, at the point of use or at equipment connections, a smaller size may be required. A swage nipple facilitates this transition.
• Connecting to Equipment: Pumps, compressors, tanks, and other pieces of process equipment often have inlet and outlet connections of a specific size that may differ from the main piping header. Swage nipples are used to make these connections.

The swage nipple is the purpose-built component for these transitions. It is more compact and often more robust than a combination of a standard nipple and a separate reducer fitting, offering a streamlined solution.

Concentric vs. Eccentric Swages: A Critical Distinction

Swage nipples come in two fundamental geometries: concentric and eccentric. The choice is not aesthetic; it has significant functional implications, especially in horizontal pipe runs.

• Concentric Swage Nipple: In a concentric swage, the centerline of the larger end and the smaller end are aligned. The reduction occurs symmetrically around this central axis, forming a cone shape. Concentric swages are the default choice and are typically used in vertical pipe runs, where the symmetrical shape does not pose any operational problems. They are also common in applications where the potential for trapping substances is not a concern.
• Eccentric Swage Nipple: In an eccentric swage, the centerlines of the two ends are offset. One side of the swage is flat, while the other side tapers. This design is critically important for horizontal pipe runs. When handling liquids, the eccentric swage is installed with the flat side on the bottom (“FOB”). This prevents the formation of a dam that could trap water or debris at the point of reduction. When handling gases or steam, the eccentric swage is installed with the flat side on the top (“FOT”). This prevents the formation of a pocket at the top of the pipe where air or other non-condensable gases could become trapped, which could lead to corrosion or interfere with flow. A classic and vital application is on the suction line of a pump. An eccentric swage, installed flat-side-up, prevents an air pocket from forming at the reducer, which could cause the pump to lose its prime (cavitate) and suffer damage.

The ability to specify concentric or eccentric demonstrates the sophisticated level of control that engineers have over system design, and the swage nipple is the tool that enables this control.

End Connections and Material Considerations

The versatility of the swage nipple is further enhanced by its available end connections. A single swage can have different ends to join dissimilar piping systems. Common combinations include:

• PBE x PSE: Plain Both Ends / Plain Small End (for socket welding or other slip-on connections).
• TBE x TSE: Threaded Both Ends / Threaded Small End.
• BBE x BSE: Beveled Both Ends / Beveled Small End (for butt welding).
• PBE x TLE: Plain Big End x Threaded Little End (a common transition piece).

This variety reinforces our central theme. A single component, the swage nipple, can be threaded, weldable, or plain, depending on the specific need. The question “are pipe nipples always threaded?” finds a complex and multifaceted negative answer within this single category of fitting. The materials used for swage nipples follow the same logic as other piping components. They are specified to match the material of the adjoining pipe to ensure compatibility for welding and to maintain consistent corrosion resistance and strength throughout the system. Common materials include carbon steel (ASTM A234 WPB for weldable swages), stainless steel, and other alloys for specialized services. The manufacturing process, typically forging, ensures that the swage has a dense, strong grain structure capable of withstanding the pressures and stresses of the transition point. In essence, the swage nipple is a specialized but essential member of the nipple family, embodying the problem-solving nature of piping components and providing a clear illustration of the functional diversity that exists beyond simple threaded connectors.

Type 5: Specialized and Combination Nipples – Tailoring Solutions for Unique Challenges

Beyond the primary categories of threaded, weld, and grooved nipples lies a fascinating and highly practical world of specialized and combination nipples. These components are the problem-solvers of the piping world, designed to address very specific installation challenges, improve serviceability, or create transitions between entirely different types of conveyance systems (e.g., from rigid pipe to flexible hose). Their existence further enriches our understanding and provides a resounding “no” to the question, “are pipe nipples always threaded?” They show that the form of a nipple is fluid, adapting to meet the demands of the application with precision and ingenuity. These specialized fittings are a testament to a mature industry that has developed a unique tool for nearly every conceivable piping scenario.

The Hex Nipple: Adding a Wrench to the Works

At first glance, a hex nipple appears to be a simple variation of a standard threaded nipple. It is a short length of pipe with male threads on both ends. The key difference, however, is the inclusion of a hexagonal section, or “hex,” in the center. This is not a trivial addition. The hex serves the same purpose as the head of a bolt or a nut on a standard fitting: it provides a positive, non-slip surface for a wrench. When installing a standard threaded nipple (often called a “barrel nipple”), the only way to tighten it is by gripping the pipe itself with a pipe wrench. The serrated jaws of a pipe wrench are designed to bite into the pipe surface to gain purchase. While effective, this can mar the surface of the nipple, which can be undesirable in exposed or aesthetic applications. more importantly, it can be difficult to get a good grip on very short nipples or in tight spaces. The hex nipple solves this problem elegantly. A standard open-end or adjustable wrench can be used on the hex flats, allowing for controlled, high-torque tightening without damaging the nipple’s surface. This makes installation and removal easier and more precise. Hex nipples are particularly favored in instrumentation, hydraulic, and pneumatic systems where compact, reliable, and serviceable connections are paramount. They are almost universally threaded, but their unique form factor earns them a special place in the nipple catalog.

Close Nipples, Shoulder Nipples, and the Nuances of Length

The length of a nipple is not always a simple matter of inches or centimeters. In the world of threaded fittings, two special terms describe the shortest possible nipple configurations:

• Close Nipple (or Running Nipple): This is the shortest possible length of a pipe nipple. It is essentially all-thread; there is no unthreaded space between the two sets of male threads. When a close nipple is screwed between two female fittings, the fittings will practically touch. This is used when an absolute minimum of space is available to join two components. However, there is a significant drawback: because there is no unthreaded pipe surface to grip, a close nipple cannot be installed with a standard pipe wrench. It must be gripped on the threads themselves, which will inevitably damage them, or a specialized internal pipe wrench must be used. Often, one end is screwed fully into a fitting, and then the entire fitting is used as a handle to screw the nipple into the second fitting.
• Shoulder Nipple: A shoulder nipple is slightly longer than a close nipple. It has a very small, unthreaded space, or “shoulder,” between the threads. This shoulder is too small to be gripped by a pipe wrench but provides a small margin of separation between the fittings being joined. The primary purpose is to allow for a more complete thread engagement than a close nipple might permit, while still being extremely short.

These specialized short nipples highlight the detailed thinking that goes into piping design. They are solutions for extremely tight geometric constraints and demonstrate how even the length and thread coverage of a nipple are carefully considered variables.

Combination Nipples: Bridging the Gap to Flexible Hoses

Perhaps the most dramatic departure from the pipe-to-pipe connection is the combination nipple, also known as a hose nipple or a “King” nipple. This component is a hybrid, designed to connect a rigid piping system to a flexible hose. One end of a combination nipple is a standard pipe thread (NPT or BSPT) or is beveled for welding, allowing it to connect to the permanent piping infrastructure. The other end, however, is completely different. It consists of a series of “barbs” or “serrations.” A flexible hose is pushed over these barbs, and a clamp (such as a band clamp or a crimped ferrule) is tightened over the hose, compressing it onto the barbs. This creates a secure, leak-resistant seal between the rigid pipe and the flexible hose. Combination nipples are essential in countless applications:

• Connecting a pump to a temporary suction or discharge line.
• Providing flexible connections to vibrating machinery to isolate the main piping from the vibration.
• In water transfer, agriculture, and construction for connecting hoses to water sources or equipment.
• In pneumatic systems for connecting tools to a compressed air supply.

The combination nipple is a perfect illustration of a non-threaded (on one end) nipple. It is a transitional device, a bridge between two different worlds of fluid conveyance. Its existence fundamentally proves that the function of a nipple extends far beyond simply joining two pipes of the same type and that being threaded is only one of its many possible characteristics. These specialized nipples, from the practical hex nipple to the transitional combination nipple, showcase the adaptability and problem-solving depth of modern global pipeline system solutions.

Material Science and its Influence on Nipple Selection

The discussion of nipple types—threaded, welded, grooved—is inseparable from the science of the materials from which they are made. The choice of material is not a secondary consideration; it is a primary design decision that dictates the nipple’s strength, corrosion resistance, temperature limits, and ultimately, its suitability for a given application. An engineer or system designer must act as a practical material scientist, weighing the mechanical properties, chemical compatibility, and economic implications of each option. The material itself can influence which type of nipple is most appropriate, and a failure to match the material to the service conditions is a common cause of catastrophic system failure. This exploration into the core materials used for pipe nipples underscores the complexity behind what appears to be a simple component.

The Workhorses: Malleable and Ductile Iron

Malleable iron and ductile iron are foundational materials in the world of piping, particularly for threaded fittings used in fire protection, gas distribution, and general plumbing.

• Malleable Iron: Produced from a base of white cast iron, malleable iron undergoes a prolonged heat treatment (annealing) process. This process changes the brittle carbon structure (cementite) into more malleable nodules of graphite (temper carbon) within a steel-like matrix. The result is a material that is much tougher and more “malleable” than cast iron, meaning it can deform slightly under stress without fracturing. This makes malleable iron fittings, including nipples, resistant to the stresses of tightening and minor system vibrations. They are the standard for many black iron gas and fire sprinkler applications.
• Ductile Iron: Ductile iron is chemically different. Small amounts of magnesium or cerium are added to the molten iron, which causes the graphite to form into spherical nodules as it cools. This spherical shape is key; it eliminates the internal stress points found in the flake-like graphite of standard gray cast iron, giving ductile iron immense strength and ductility, approaching that of steel. Ductile iron is predominantly used for grooved fittings and couplings, where its high strength is needed to contain the forces exerted by the pressurized system on the coupling housings.

Both materials offer an excellent balance of strength, pressure-tightness, and cost-effectiveness, making them the default choice for a vast range of non-corrosive, moderate-temperature applications. A wide selection of pipe nipples is available in these reliable materials.

The Champions of Resistance: Stainless Steel and Brass

When the fluid being transported or the external environment is corrosive, iron-based materials are no longer suitable. This is where stainless steel and brass come to the forefront.

• Stainless Steel: Stainless steel is not a single material but a family of iron-based alloys containing a minimum of approximately 10.5% chromium. The chromium forms a passive, invisible, and self-healing oxide layer on the surface of the steel. This layer is what gives stainless steel its renowned “stainless” quality, protecting it from rust and many chemical agents. The two most common grades are:
  • Type 304: The most common general-purpose stainless steel. It offers excellent corrosion resistance in a wide variety of atmospheric and chemical environments.
  • Type 316: Contains added molybdenum. This addition dramatically increases its resistance to corrosion from chlorides (like salt water) and other industrial chemicals. It is the preferred choice for marine applications, pharmaceutical production, and food processing where aggressive cleaning agents are used.

  When selecting stainless steel nipples, especially threaded ones, one must be mindful of thread galling, a form of cold welding that can occur under pressure. Proper lubrication and careful tightening are essential.

• Brass: An alloy of copper and zinc, brass offers excellent corrosion resistance, particularly against water. It does not rust like iron. This property, combined with its ease of machining, has made it a traditional favorite for plumbing fittings, valves, and nipples used in potable water systems. One consideration with brass is dezincification, a process where the zinc can be selectively leached out of the alloy in certain water conditions, weakening the fitting. Modern, high-quality brass alloys are designed to resist this phenomenon.

The Protective Layer: Galvanized Steel

Galvanizing is not a material itself, but a process applied to carbon steel. The steel nipple is dipped into a bath of molten zinc, creating a bonded coating. The zinc provides protection in two ways:

• Barrier Protection: The zinc coating physically isolates the steel from the environment.
• Sacrificial Protection: If the coating is scratched and the steel is exposed, the zinc, being more electrochemically active, will corrode preferentially, “sacrificing” itself to protect the steel. This is known as cathodic protection.

Galvanized steel nipples have long been used for potable water lines and outdoor piping (e.g., fences, handrails). They offer much better corrosion resistance than bare carbon steel at a lower cost than stainless steel. However, their use is restricted. They are not suitable for underground burial unless further protected, and as mentioned earlier, they are prohibited for natural gas lines because the gas can cause the zinc to flake and obstruct downstream equipment. The rise of alternative materials like copper, PEX, and CPVC has reduced their use in modern residential plumbing, but they remain a viable option for many industrial and water-based applications.

Ultimately, the material selection process is a critical dialogue between the system’s needs and the properties of the available materials. A nipple is not just a shape; it is a carefully chosen material formed into that shape for a specific purpose. Understanding this relationship is fundamental to designing systems that are not only functional but also safe and durable over their intended lifespan.

Navigating International Standards and Certifications

In a globalized marketplace, where a pipe nipple manufactured in one country may be installed in a fire protection system in another, the importance of standards and certifications cannot be overstated. These codes and approvals are the common language of engineering, ensuring that components are safe, reliable, and interoperable, regardless of their origin. For a manufacturer, adherence to these standards is a mark of quality and a prerequisite for market access. For an engineer or installer, specifying and using certified products is a professional and ethical obligation. Navigating this landscape of acronyms—ASME, ASTM, NFPA, UL, FM—is essential for anyone involved in the design, construction, or maintenance of critical piping systems. These standards directly impact the design of all types of nipples, governing everything from thread dimensions to material chemistry, and provide yet another layer to the answer of whether pipe nipples are always threaded.

The Authority of ASME and ASTM in North America

In the United States and much of North America, two organizations form the bedrock of piping standards:

• ASTM International (formerly American Society for Testing and Materials): ASTM develops and publishes technical standards for a vast range of materials, products, systems, and services. When you see a pipe nipple specified as being made from ASTM A53 pipe, it means the raw material meets a specific ASTM standard that defines its chemical composition, mechanical properties, and manufacturing method. ASTM standards are concerned with the “what”—the properties of the material itself. For example, ASTM A197 specifies the requirements for malleable iron used in threaded fittings.
• ASME (American Society of Mechanical Engineers): ASME develops standards for the design, construction, and inspection of mechanical devices. In the piping world, their influence is immense. The ASME B1.20.1 standard, for instance, is the definitive document that specifies the exact dimensions, taper, and gauging for NPT threads. The ASME B16 series of standards covers flanges, fittings, and valves. ASME standards are concerned with the “how”—the design, dimensions, and pressure-temperature ratings of the components.

Together, ASTM and ASME create a comprehensive framework. ASTM ensures the steel (or iron, or brass) is of a known and reliable quality, while ASME ensures that the nipple made from that material has the correct dimensions to connect properly and safely within a system.

The Seal of Approval: UL and FM in Fire Protection

For components used in life-safety systems, particularly fire protection, an even higher level of scrutiny is applied. General adherence to ASTM and ASME standards is not enough. Components must typically be tested and certified by a third-party laboratory. The two most prominent in this field are:

• Underwriters Laboratories (UL): UL is a global safety certification company. When a pipe nipple or fitting bears the UL mark, it means that the component has been rigorously tested by UL to meet specific safety and performance standards relevant to its application. For a grooved coupling, this might involve hydrostatic pressure tests far exceeding its rated pressure, as well as tests for vibration, assembly, and long-term durability.
• FM Global (Factory Mutual): FM Global is a major commercial property insurer that conducts its own research and testing to minimize property loss risks for its clients. Products that pass their stringent tests are “FM Approved.” For many industrial and commercial clients, FM Approval is a requirement for components used in their fire protection systems. FM’s standards are often considered among the most demanding in the world.

A UL listing or FM approval is a powerful statement. It tells the specifying engineer, the local fire marshal (the Authority Having Jurisdiction, or AHJ), and the building owner that the component has been independently verified as fit for purpose in a critical life-safety system. For manufacturers, achieving these certifications is a major investment but is essential for competing in the fire protection market.

A Global Perspective: EN, ISO, and Other Regional Standards

While North American standards are influential, they are not the only ones. A global supplier must be conversant in a multitude of international and regional standards to serve markets in Europe, Asia, and beyond.

• EN (European Norms): These are the technical standards for the European Union. For example, the standard for BSPT threads is EN 10226-1. Pipe materials are often specified by an EN standard rather than an ASTM one.
• ISO (International Organization for Standardization): ISO develops and publishes international standards to facilitate global trade. The ISO 9001 standard for quality management systems is perhaps the most famous, but ISO also publishes thousands of technical product standards. For instance, ISO 7-1 is the international standard governing pipe threads where pressure-tight joints are made on the threads.
• Other National Standards: Many countries also maintain their own national standards, such as DIN (Germany), BS (United Kingdom), JIS (Japan), and GB (China). While there is a global trend towards harmonization with ISO and EN standards, these national standards remain highly relevant in their respective regions.

For a company providing pipeline system solutions to a global customer base, mastering this complex web of standards is not optional. It is the foundation of their business, ensuring that a nipple sold for a project in Dubai meets the required British Standards, while another sold for a project in Texas meets the necessary ASME and UL requirements. This global perspective reinforces that the world of piping is one of precision, defined by universally understood rules that ensure safety and reliability across borders.

Practical Application: Choosing the Right Nipple for Your System

Having explored the theoretical landscape of nipple types, materials, and standards, we now arrive at the most important part of the inquiry: the practical application of this knowledge. The selection of a pipe nipple is not an academic exercise; it is a concrete decision with real-world consequences for the safety, efficiency, and longevity of a piping system. Answering the question “Are pipe nipples always threaded?” is only the first step. The more crucial skill is to be able to answer, “Which nipple is correct for this specific situation?” To illustrate this decision-making process, let’s walk through a few simplified case studies, examining how a system’s parameters dictate the choice of component.

Case Study 1: A Commercial Fire Sprinkler System

• The Scenario: We are designing the fire sprinkler system for a new five-story office building. The system involves a main riser, large cross-mains running down the corridors, and smaller branch lines feeding the individual sprinkler heads in the offices.
• The Analysis:
  • Mains and Cross-Mains (3″ to 6″ pipe): For these larger pipes, speed of installation and accommodation of building movement are major factors. Welding would be slow, expensive, and create fire hazards during construction. Threading pipes of this size is cumbersome and less reliable. This is the ideal application for a grooved system. We would specify grooved nipples (or grooved-end pipe) connected with FM-approved/UL-listed rigid and flexible couplings. The flexible couplings would be strategically placed to accommodate thermal expansion and potential seismic movement, as required by NFPA 13. The material would likely be standard schedule 10 or schedule 40 carbon steel pipe (ASTM A53).
  • Branch Lines (1″ to 2″ pipe): For the smaller lines branching off the mains, threaded connections are often more economical and practical. The volume of fittings is high, and the ease of assembly with standard tools is an advantage. Here, we would specify standard threaded malleable iron fittings and threaded carbon steel nipples.
  • Sprinkler Head Connection: The final connection to the sprinkler head itself is almost always a threaded joint. We would use a short, threaded nipple or a specialized threaded reducer to connect the branch line to the sprinkler head.
• The Conclusion: In a single fire sprinkler system, we have used both grooved and threaded nipples. The choice was dictated by pipe size, installation efficiency, and established industry practice (NFPA 13). This clearly demonstrates that even within one project, the answer to “are pipe nipples always threaded?” is no.

Case Study 2: A High-Pressure Steam Manifold

• The Scenario: A power generation facility needs a manifold to distribute superheated steam at 800 psi and 750°F (400°C) to several pieces of equipment. The pipe size is 4 inches.
• The Analysis:
  • Pressure and Temperature: The operating conditions are extreme. The pressure is far too high for standard threaded fittings, and the high temperature would cause sealant to fail and could lead to creep and relaxation in a mechanical joint.
  • Safety: The fluid is superheated steam, which represents an immense safety hazard if it leaks. The integrity of the joints must be absolute.
  • The Unambiguous Choice: This is a textbook application for a butt-weld system. Any nipples used in the construction of this manifold must be butt-weld nipples. The ends must be beveled to match the pipe and fittings. The material would need to be a high-strength carbon steel or alloy steel specifically rated for high-temperature service, such as ASTM A106 Grade B or a chrome-moly alloy steel (e.g., P11, P22). All welds would require 100% radiographic or ultrasonic inspection to ensure they are free of defects.
• The Conclusion: In this critical service, there is no alternative. Threaded and grooved nipples are completely unsuitable and would represent gross negligence if used. The permanence and proven integrity of a welded joint are non-negotiable.

Case Study 3: A Pharmaceutical Purified Water Loop

• The Scenario: A pharmaceutical manufacturing plant requires a piping loop to circulate highly purified water (WFI – Water for Injection) to various points of use. The system must remain free of any contamination and must be able to be sanitized.
• The Analysis:
  • Purity and Cleanliness: The primary design driver is hygiene. The internal surface of the piping must be as smooth as possible to prevent microbial growth. Crevices, such as those found in threaded joints or the gap in a socket weld, are completely unacceptable as they can harbor bacteria.
  • Material: The material must be highly corrosion-resistant and must not leach any substances into the water. The industry standard is Type 316L stainless steel. The “L” designates low carbon content, which improves weldability and reduces the risk of corrosion at the weld site.
  • The Joining Method: Given the prohibition of crevices, the only acceptable joining method is welding. Specifically, butt-welding. However, the standard is even higher than in the steam plant. The welds must be performed using an orbital TIG (Tungsten Inert Gas) welding process. This automated process produces extremely smooth, consistent, and clean internal weld beads that are easy to clean and sanitize. Any nipples used would be stainless steel 316L with ends prepared for orbital butt-welding.
• The Conclusion: Here, the choice of a butt-weld nipple is driven not by pressure, but by the demand for sanitary conditions. It shows that the rationale for choosing a specific nipple type can come from many different engineering requirements. The threaded nipple is not even a consideration in this context.

These case studies reveal that pipe nipple selection is a logical process of elimination and optimization. By systematically evaluating the system’s pressure, temperature, fluid, safety requirements, and installation logistics, the correct component choice becomes clear. The professional does not ask “are pipe nipples always threaded?” but rather, “what are the forces and conditions this joint must withstand, and which nipple is engineered to withstand them?”

Frequently Asked Questions (FAQ)

1. What is the fundamental difference between a pipe nipple and a pipe coupling?

A pipe nipple and a pipe coupling are opposite but complementary fittings. A pipe nipple is a short piece of pipe with external (male) threads, designed to connect two components with internal (female) threads. A coupling is a short fitting that has only internal (female) threads on both ends, designed to join two male-threaded pipes or nipples together. Think of them as male and female connectors: the nipple goes into a fitting, while the coupling goes over a pipe.

2. Can you weld a threaded pipe nipple into a system?

While physically possible, it is extremely poor practice and generally prohibited by piping codes for pressure systems. Welding on threads is unreliable. The threads create an uneven surface with potential voids and stress concentrations, leading to a weak and potentially leaky weld. Furthermore, the material of a standard threaded nipple (like malleable iron) may not be suitable for quality welding. The correct approach is to use a nipple specifically designed for welding, such as a butt-weld or socket-weld nipple made from a weldable grade of steel (e.g., ASTM A106).

3. Why are some pipe nipples galvanized while others are black?

“Black” refers to the dark iron oxide scale left on a steel nipple after manufacturing. These are standard carbon steel nipples. A “galvanized” nipple is a carbon steel nipple that has been coated with a layer of zinc. The purpose of galvanizing is to provide corrosion resistance, primarily for use with water or in outdoor/damp environments where the nipple would otherwise rust. Black nipples are used for systems where rust is less of a concern, such as closed-loop heating systems or in gas piping (where galvanizing is prohibited).

4. How do I know what size pipe nipple I need?

Pipe nipple sizing is based on “Nominal Pipe Size” (NPS), which is a North American set of standard sizes. It’s important to note that for sizes up to 12 inches, the NPS is not the same as the actual outside diameter of the pipe. For example, a 1″ NPS nipple has an actual outside diameter of 1.315 inches. You must match the NPS of the nipple to the NPS of the pipe and fittings you are connecting. The size will be stamped on the fittings (e.g., “1/2 NPT”). You also need to specify the length, which is measured from end-to-end.

5. Are grooved pipe fittings a better choice than threaded ones?

“Better” depends entirely on the application. For large pipes (typically 2.5″ and up), grooved fittings are often superior because they are much faster to install, require no welding, and can accommodate movement and vibration. They are the dominant choice in commercial fire sprinkler systems. For smaller pipes (2″ and under), threaded fittings are often more cost-effective and simpler, as they don’t require a special grooving tool. For a small residential plumbing repair, a threaded nipple is the better choice. For the main riser of a skyscraper, a grooved system is superior.

6. What does the term “close nipple” mean?

A “close nipple” is the shortest possible version of a threaded pipe nipple. It is fully threaded from end to end with no unthreaded space in the middle. This design is used when two female fittings need to be connected with the absolute minimum separation between them. Their main drawback is that they are difficult to install, as there is no surface for a pipe wrench to grip without damaging the threads.

7. Why are eccentric swage nipples so important for pumps?

An eccentric swage nipple is used on the horizontal suction line of a pump to prevent air from becoming trapped. If a concentric (cone-shaped) reducer were used, an air pocket could form at the top of the pipe at the transition point. This air bubble could then be drawn into the pump, causing it to lose prime and suffer from cavitation—a destructive phenomenon where vapor bubbles rapidly collapse, creating shockwaves that can severely damage the pump’s impeller. By using an eccentric swage with the flat side on top, a smooth, level path is created for the fluid, eliminating the possibility of an air pocket.

Conclusion: Beyond a Simple Yes or No

We began with a straightforward question: Are pipe nipples always threaded? The journey through the various forms, materials, and applications of this fundamental component has revealed that a simple “no” is an insufficient answer. The response is far more textured and profound. To state that pipe nipples are not always threaded is to acknowledge the existence of a rich and varied technological landscape, one shaped by the relentless pressures of safety, efficiency, and environmental demands. The threaded nipple, while ubiquitous, is but one member of a diverse family, each with its own purpose and philosophy of connection.

The weld nipple embodies a commitment to permanence and absolute integrity, a necessary choice when dealing with the immense energies of high-pressure steam or the dangers of hazardous chemicals. It represents a joint that is forged, not just fastened. The grooved nipple stands as a monument to modern ingenuity, a brilliant compromise that offers strength and flexibility with unparalleled speed of installation, revolutionizing industries like fire protection. The swage nipple demonstrates a mastery over fluid dynamics, allowing engineers to precisely control the flow within a system. And the array of specialized nipples, from the humble hex to the transitional combination nipple, showcases a mature industry’s capacity to create a specific tool for every unique challenge.

Ultimately, the choice of a pipe nipple is a microcosm of the entire engineering discipline. It is a process that demands a holistic understanding of the system’s purpose, a rigorous analysis of the forces at play, a deep knowledge of material science, and a firm grasp of the codes and standards that ensure public safety. A true professional in the field of piping does not rely on assumptions. They understand that every component choice is a deliberate act of design, backed by reason and experience. The pipe nipple, therefore, is not merely a piece of pipe. It is the physical manifestation of a solution to an engineering problem. The fact that it is not always threaded is not a trivial detail; it is a testament to the sophistication and adaptability of the technologies that form the vital circulatory systems of our modern world.

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References

1. American Society of Mechanical Engineers. (2018). ASME B1.20.1-2018: Pipe Threads, General Purpose, Inch. ASME.
2. National Fire Protection Association. (2022). NFPA 13: Standard for the Installation of Sprinkler Systems. NFPA.
3. Nayyar, M. L. (Ed.). (2000). Piping handbook (7th ed.). McGraw-Hill.
4. ASTM International. (2018). ASTM A53/A53M-18: Standard Specification for Pipe, Steel, Black and Hot-Dipped, Zinc-Coated, Welded and Seamless. ASTM International. https://doi.org/10.1520/A0053A0053M-18
5. International Organization for Standardization. (2004). ISO 7-1:1994: Pipe threads where pressure-tight joints are made on the threads — Part 1: Dimensions, tolerances and designation. ISO. https://www.iso.org/standard/4261.html
6. ASTM International. (2020). ASTM A234/A234M-20: Standard Specification for Piping Fittings of Wrought Carbon Steel and Alloy Steel for Moderate and High Temperature Service. ASTM International. https://doi.org/10.1520/A0234A0234M-20
7. The American Welding Society. (2020). AWS D1.1/D1.1M:2020: Structural Welding Code — Steel. AWS.
8. Victaulic. (2023). Piping Design Manual I-100. Victaulic Company. https://www.victaulic.com/assets/uploads/literature/I-100.pdf
9. Engineering ToolBox. (2005). NPT – National Pipe Taper Threads. 
10. ASTM International. (2019). ASTM A197/A197M-19: Standard Specification for Cupola Malleable Iron. ASTM International. https://doi.org/10.1520/A0197A0197M-19