Grooved pipe couplings are some of the most important equipment in fire protection, allowing quick and water-tight connections without pipe threads, flanges, or welding. Instead, installers simply butt the ends of pipes together and seal them with a gasket and clamp.
But there are also two kinds of grooved couplings that serve different purposes. Rigid grooved couplings don&#;t allow much movement and serve as a standard pipe joint. But for situations where some give is needed&#;specifically, seismic protection of fire sprinkler systems&#;flexible grooved couplings are the solution.
This article reviews the basics of grooved pipe couplings and details the differences between a rigid and flexible coupling. We also explain when and how NFPA 13: Standard for the Installation of Fire Sprinkler Systems ( edition) requires using flexible couplings for seismic protection.
Shop our selection of rigid grooved couplings and other fittings for your pipe-joining needs. Looking for flexible couplings? Contact QRFS today for a quote or to place a special order.
Grooved couplings are the fastest way to join pipes. And the technology is simple. Earnest Tribe, the founder of Victaulic, patented the grooved pipe joint in after developing the tech to meet wartime needs. Grooved couplings avoid the need for welding or other joint types. Instead, they use three components: 1) pipe with grooved ends, 2) synthetic rubber gaskets, and 3) c-shaped clamps.
To form a joint, installers butt two pipes with grooves cut in the ends together. A lubricated gasket slips over the pipe ends so that it overlaps the joint. And a c-shaped clamp or housing fits into the grooves of the pipe, compressing and stabilizing the gasket. This video from Victaulic shows how the joints are made:
But as we mentioned, there are two kinds of grooved joints. So, what&#;s the difference between a rigid and flexible coupling?
From the outside, you can&#;t look at a grooved coupling and know whether it&#;s rigid or flexible; they look the same. The difference is on the inside of the joint in the way that the gasket, groove, and clamp fit together. A rigid clamp permits next to no separation, rotation, or angular movement. But a flexible coupling permits some movement and space without compromising the seal or damaging the pipe. NFPA 13 officially describes how much flexibility is allowed:
3.3.83 Flexible Coupling. A listed coupling or fitting that allows axial displacement, rotation, and at least 1 degree of angular movement of the pipe without inducing harm on the pipe. For pipe diameters of 8 in. (200 mm) and larger, the angular movement is permitted to be less than 1 degree but not less than 0.5 degree.
Key to this definition is the term &#;listed.&#; A testing laboratory such as UL or FM Global must verify that a flexible grooved fitting meets specific flexibility, seal, and strength requirements before it can be used in a fire sprinkler system.
This diagram from Bentley Communities helps illustrate the physical difference between the two types of joints:
Almost all grooved couplings use rigid joints because rigid, unmoving joints are desired for most situations. But flexible couplings, while less common, do play essential roles in seismic protection for fire sprinkler systems that require an earthquake-resistant design.
Some fire sprinkler systems require design measures to protect them from earthquake damage. In our overview of the principles of seismic protection, we explain the two basic techniques for protecting pipes from damage: adding rigidity and adding flexibility. Rigidity, in the form of seismic bracing, ensures that pipe moves with a building in an earthquake instead of shaking violently and breaking loose.
Flexibility, on the other hand, prevents damage to pipes caused by differential movement. When an earthquake&#;s powerful p-waves shake a building, the whole structure can move&#;but not always all at the same time. For example, the ground floor moves first in tall buildings, and the top floors lag behind. This wave-like motion that resembles a slinky is called &#;story displacement.&#; And the differential movement it causes in various onsite equipment can snap fire sprinkler pipes.
The solution to this problem is flexibility, which designers can achieve with flexible grooved couplings. The modest &#;give&#; allowed by these devices can prevent pipes from being damaged in an earthquake.
NFPA 13 requires grooved flexible couplings for pipes that are 2 ½&#; or larger (Section 18.2.1) in certain buildings that fall into specific design categories (SDC). And because story displacement significantly affects risers, most of these rules involve risers (18.2.3.1):
Flexible couplings are also required at horizontal tie-ins to the riser (18.2.3.2) and the connection of drop-pipes (18.2.4).
Flexible couplings allow flexibility when two structures move independently. But don&#;t forget that bracing, which ensures pipe moves with a building, is the other core technique of seismic design. Flexible couplings can work against this objective by reducing rigidity. So, to compensate, NFPA 13 says that flexible couplings should only coincide with structural separations (18.2.2). And where they are used outside the explicit instructions, extra sway braces are needed (18.2.3).
In our previous blogs, you can learn more about the principles of sway bracing and how cable sway braces work.
Flexible couplings can also be used to construct seismic separation assemblies. These pipe assemblies are used where horizontal pipe must cross an engineered gap, called a seismic joint, between two structures.
The &#;structures&#; can be two independent buildings or two freestanding wings of the same building. When a pipe crosses a gap like this, NFPA 13 requires seismic separation assemblies (18.3.1) to keep it from breaking when the two structures move during an earthquake.
There are two ways to build a seismic separation assembly (18.3.2). One approach uses flexible pipe arranged in a u-bend. The other involves a zig-zag pattern of flexible grooved elbows. Either way, the assembly should accommodate a wide range of motion in three directions: the complete closing of the gap between the structures, a doubling of the gap, or lateral movement equal to the size of the gap.
You need a stiff joint for the vast majority of sprinkler pipe connections. That&#;s why most grooved pipe joints use rigid couplings. But sometimes, flexibility is necessary for seismic design, and the extra range of motion provided by flexible couplings can prevent earthquake damage.
Be sure to check your local codes, including whether a building falls within a seismic design category (SDC) that requires special equipment to keep fire protection systems safe. A qualified system designer must make this determination. Regardless of what you need, QRFS can provide:
If you have questions or need help finding the part you need, call us at (888) 361- or .
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Flexible Coupling: Basic Concepts and Application Insights
Flexible coupling is a widely used sub-category of couplings, offering distinct characteristics and advantages compared to rigid couplings. Common examples of flexible coupling include diaphragm couplings, jaw couplings, spring couplings, and so on. There is a broad range of flexible coupling types available. Therefore, to facilitate a quicker and more convenient selection of the required coupling, engineers can refer to this guide for purchasing.
Flexible coupling is a device that transmits torque between two shafts using elastic elements. This design provides the flexible coupling with strong vibration absorption, the ability to compensate for significant misalignments, and effective noise reduction. In situations involving high vibration, heavy impact, or the need for high precision, flexible coupling are often the preferred choice. Hence, flexible coupling is commonly used in motors, pumps, fans, and transmission systems.
Jaw couplings are widely used and feature a design-oriented appearance. They consist of metal jaws on both sides with an elastomeric element in the middle. This structure enhances torque transmission and centering tolerance. It allows for some degree of misalignment, while also being easy to install and maintain. Jaw couplings are typically well-suited for high-speed rotation and high-precision transmission. However, their ability to handle significant misalignments in the transmission system is somewhat limited.
Fig. 1 Jaw Coupling
Diaphragm couplings can be further subdivided into different types, but here we&#;ll focus on the standard diaphragm coupling. The principle behind diaphragm couplings is the use of the elastic deformation of a metal diaphragm to transmit torque. This design offers high torsional stiffness and low inertia, making it ideal for precision equipment that requires precise alignment.
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Fig. 2 Diaphragm Coupling
The principle of spring couplings is similar to that of diaphragm couplings, but instead of a diaphragm, a spring is used to improve the ability to compensate for axial movement and to resist vibration and shock. Compared to other flexible coupling, spring couplings are better suited for mechanical systems that experience high deformation, heavy loads, and high pressures.
Fig. 3 Spring Coupling
Gear couplings transmit torque through meshing gears, offering high load capacity and durability, making them ideal for heavy-duty and high-torque applications. Their design allows them to accommodate significant axial and angular misalignments, making them widely used in high-power and industrial heavy-duty mechanical systems.
Bellows couplings use the flexibility of bellows to compensate for shaft misalignment and offer excellent resistance to high temperatures, corrosion, and vacuum conditions. They are ideal for applications requiring high-precision alignment and operation in extreme environments, with the ability to accommodate large axial and angular misalignments.
Fig. 4 Bellows Coupling
1. Vibration Damping: Elastic couplings can effectively absorb vibrations and shocks in mechanical systems, reducing wear on components and noise.
2. Misalignment Compensation: They can compensate for axial, radial, and angular misalignments, providing greater installation tolerance and adapting to imperfect alignment conditions.
3. Noise Reduction: The elastic design helps reduce noise and vibration during operation, enhancing the system's stability and comfort.
4. Equipment Protection: By mitigating vibrations and impacts, elastic couplings protect connected equipment from excessive loads and potential damage, thereby extending the equipment's service life.
5. Flexible Connection: They can provide a flexible connection, allowing the mechanical system to operate smoothly even when there are minor movements or misalignments between shafts.
6. Durability and adaptability: Elastic couplings typically use high-performance materials such as rubber, polyurethane, or metal diaphragms. These materials offer excellent wear resistance and chemical stability, making the couplings highly durable and adaptable to a wide range of working environments.
When selecting a flexible coupling, it&#;s important to consider the torque, speed, and accuracy requirements of your equipment. These factors above will determine the appropriate type and standard of the coupling you choose.
After setting your performance goals, you also need to consider external factors such as temperature, humidity, and corrosiveness in the operating environment. Finally, assess whether the coupling is easy to install and maintain.
If this article helps you better understand, select, or apply flexible coupling, we invite you to stay connected with us at JLCMC. We can offer high-quality, cost-effective mechanical parts procurement and supply services tailored to your needs. There is currently a $9 newcomer no-threshold coupon event to help every customer have a better shopping experience.
If you want to learn more, please visit our website Ductile Iron Flexible Coupling.