Why choose rigid or flexible couplings for fire sprinklers and other fire protection systems?

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.

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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.

Rigid coupling vs flexible coupling: similar parts but different roles

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.

The flexible coupling in fire protection: a seismic safeguard

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.

Flexible couplings prevent damage to pipes by allowing movement

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):

• Within 2 feet of the top and bottom of each riser, with some exceptions for short risers
• Within 1 foot above each floor in multistory buildings (except roofs)
• Within 2 feet below each floor in multistory buildings
• Within 1 foot of concrete or masonry walls, with exceptions made where adequate clearance is provided
• Within 2 feet of building expansion joints

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).

You will get efficient and thoughtful service from CNG.

In our previous blogs, you can learn more about the principles of sway bracing and how cable sway braces work.

Flexible couplings in seismic separation assemblies

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.

Couplings: a small part with a crucial role in fire protection

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:

• Rigid grooved couplings and pipe fittings ' in stock and online.
• Flexible couplings on request; contact us today to place a special order.

If you have questions or need help finding the part you need, call us at (888) 361- or .

Grooved FAQs

Grooved FAQ Document August

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Q:          Is Reliable grooved compatible with other manufacturers' grooved products?

A:           Reliable grooved products use the same AWWA C606 grooved standard common with most grooved products.  Any grooved product manufactured to this common standard should be compatible with Reliable grooved products.

Q:          Is Reliable grooved qualified on proprietary Sch 7 sprinkler pipe?

A:           Yes, we have worked with UL and FM to expand qualifications for coupling products beyond just Schedule 10 and Schedule 40 pipe to include:

• Youngstown Tube Fire-Flo
• Wheatland Tube Mega-Flow
• Bull Moose Eddyflow
• Nucor Hydroflow

Our product-specific technical bulletins contain details on pipe qualifications for every gasketed fitting. Reliable's bulletins are constantly updated to reflect changing industry trends, evolving customer needs, and expanded 3rd party qualifications to ensure we deliver what our customers expect and need.

Q:          Are the coupling gaskets pre-lubricated?

A:           There are two types of pre-lubricated gaskets; a surface treatment applied to finished gaskets prior to coupling assembly (typically a white powder) or embedding a lubricant in the rubber mixture itself before molding so that it 'blooms' out naturally. Reliable grooved couplings use both types of pre-lubrication, providing a double advantage to users of Reliable Grooved.

Q:          How do the Reliable grooved elastomer options compare to competitor products?

A:           We focus on using a standardized single elastomer type that is the most common in fire protection products, EPDM rubber. Most EPDM rubbers are very similar, and the specific chemistries provided by our manufacturing partner are fit-for-purpose for a grooved fire protection system application. The temperature range for our EPDM rubber is annotated in our product-specific technical bulletins; an end-user should evaluate the specific requirements of their application and municipality regulations to determine if our UL / FM qualified temperature range is appropriate. All gaskets are qualified by UL and FM just like our competitor's products, so you can be assured that our grooved products will perform the same.

Q:          Are the takeout dimensions of grooved fittings and couplings the same as other grooved manufacturers?

A:           Some products (depending on the model number and NPS) use identical takeout dimensions, while others are slightly different. Most of the takeout dimensions that are not identical across the various brands are close enough that they should not impact installation; however, a qualified and experienced installer should review the takeout dimensions and assess any compatibility issues with Reliable grooved products.

Q:          Can I use two halves of a mechanical tee to make a mechanical cross?

A:           FM and UL require specific qualifications to utilize these products in this way; at this time, RASCO is not qualified for this configuration. While the products are physically capable of being assembled in this configuration due to the design of cast components, they lack FM and UL qualification in this configuration should be considered against industry and municipality requirements before installing in this configuration; and is not recommended.

Q:          Are u-bolt mechanical tees approved for use on branch lines, or just sprigs and drops?

A:           This is a complicated question to answer. FM allows the use of u-bolt mechanical tees with 1' outlets for connecting branch lines, but UL requires additional testing that has not been performed for Reliable u-bolt mechanical tees. If an installer wishes to use a u-bolt mechanical tee with a 1' outlet to connect a branch line, the lack of UL-qualification in this application should be considered by a competent technical authority. Reliable's recommendation is that a conventional mechanical tee (MTT2 or MTG1) be used for branch line connections, as these products in all sizes are qualified for this type of use.

Q: My 1-1/4' x 1-1/2' RCD Reducing Couplings do not have a metal ring.  Is this a problem?

A:  Most sizes of RCD Reducing Couplings use a metal reinforcing ring to provide additional support to the gasket that allows it to resist pressure and maintain a seal.  The need for this reinforcing ring is dependent on the size difference between the two sides of the reducing coupling.  For the 1-1/4' x 1-1/2' size, the outlets are so close in size that a metal reinforcing ring is not necessary, and all UL / FM qualification testing has been performed without one.  The product is fit-for-purpose without the metal ring and can be installed just like any other grooved coupling.

Contact us to discuss your requirements of Flexible Grooved Coupling. Our experienced sales team can help you identify the options that best suit your needs.

Q:          I'm having difficulties with Reliable grooved installation; what do I do?