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.

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

Figure 2. The two types of groove configuration. The roll groove (above) differs slightly from the cut groove (below).

Cut and roll grooving

Some ask how the pipe groove is formed and how that affects the pipe’s performance in different applications. There are two types of grooving: roll grooving and cut grooving (Figure 2).

Used in approximately 90% of grooved piping applications, roll grooving is the most common method. Roll grooving was developed more than 50 years ago for light or thin-wall pipe, which opened the door to a fast, economical method for mechanically joining such pipe. Roll grooving is now used on a wide variety of pipe sizes and wall thicknesses because it’s a fast, efficient and clean pipe end preparation technique without sacrificing pipe joint performance.

Roll grooving displaces a small portion of the pipe wall radially in a cold forming process. The pipe end is placed between the roll set of a grooving machine. The roll set closes, the pipe is compressed and rotated, and a groove is formed around the outside diameter, leaving a circumferential recess on the outside and an indent on the inside.

Roll grooving can be used on Schedule 5 through ANSI standard wall thickness carbon steel and stainless steel, copper, and aluminum pipe or tubing, and up to schedule 80 PVC plastic pipe. Roll-grooved systems range in diameter from 3/4-in. to 60 in. in some materials.

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Figure 4. The mechanical joint introduces only a small disturbance to the flow profile.

Flow testing has confirmed that the joints exhibit good flow characteristics. Testing involved grooved and plain-end 4-in. Type K copper tubing, and 2 in. to 8 in. Schedules 10 and 40 carbon steel pipe at flow velocities of 4, 8, 12 and 16 ft./sec. at ambient water temperature.

Pressure taps located 10 ft. apart on a straight section of pipe (or tube) established the baseline measurement at the tested flow rates. Then, the pipe/tube was cut into four segments to insert three roll-grooved pipe joints between the pressure taps. After the new pressure losses were recorded, the baseline loss was subtracted out to determine the insertion loss. The average loss of each grooved coupling joint was about 1/2 equivalent feet of pipe.

Additional independent testing by Factory Mutual Research Corp. supports these findings. Factory Mutual’s Loss Prevention Data Technical Advisory Bulletin 2-8N stipulates that one equivalent foot of pipe be added for each roll-grooved joint on any pipe size. This is conservatively higher than actual test values, yet still relatively low compared to the values established for components such as valves and fittings. These values further attest to the low loss characteristics of roll grooved joints.

Roll grooving pressure drop is consistent and can be accounted for during system design. Although, in theory, welded systems produce little to no loss at the joints, variables of craftsmanship can lead to welding material entering the pipeline, interfering with flow and, in some cases, dislodging and causing a system blockage.

Pipe stress

Another concern is the ability of a grooved joint to perform adequately under load. Regardless of pipe joint type, a pipe under load exhibits two forms of stress: longitudinal and hoop. Longitudinal stress is a tensile stress, tending to stretch the pipe axially. A failure from longitudinal stress produces a circumferential fracture. Hoop stress is “ballooning,” a radial expansion, and the potential failure mode is a lengthwise split. The calculations for determining stress also show that the hoop stress will be twice longitudinal stress:

Hoop stress = (P x OD) / (2 x Tw)
Longitudinal stress = (P x OD) / (4 x Tw),

P is the line pressure, OD is the outside diameter and Tw is the wall thickness. This means that overstress failures are most likely to occur along the length of the pipe — in a weld seam, for example — not on the pipe circumference.

Everything else being equal, a decrease in wall thickness results in an increase in hoop stress. In a grooved joint, the coupling housing, which engages the groove, prevents diametric expansion and reinforces the pipe. This suggests the grooved technique doesn’t produce greater hoop stress and, therefore, doesn’t weaken the pipe. Any potential increase in pipe hardness, reduction in tensile strength or reduction in elongation the roll grooving process produces has no effect on the pressure capability of the joint, and pipe material changes are comparable to any other cold-forming manufacturing operations.

Cut grooving reduces the wall thickness by removing a narrow circumferential strip of material from the outside surface. The hoop stress remains approximately the same because the groove is narrow and reinforced by the full wall thickness of pipe on either side of the groove. The groove also is reinforced by the coupling key engaged in the groove, preventing it from expanding diametrically. However, the longitudinal stress increases proportionally with the decrease in the wall thickness. Therefore, if one half of the original wall thickness remains, longitudinal stress is doubled or approximately equal to the hoop stress.

Because the cut groove depth in standard wall thickness pipe removes only about one-third the original pipe wall thickness, the hoop stress remains larger than the longitudinal stress. Any over-stress failure continues to occur along the length of the pipe, not at the groove, demonstrating that the groove area isn’t weaker than the longitudinal barrel of the pipe. Again, this means that the groove doesn’t compromise joint strength.

The pressure rating on a grooved mechanical pipe joint is determined in consideration of all the components involved. Grooved pipe has no rating without the corresponding coupling, and coupling ratings are a function of the piping material and wall thickness. Every manufacturer’s published pipe joint rating is calculated or tested on pipe that contains a groove, meaning that any potential effect of the groove on the strength of the pipe is incorporated in to the coupling’s performance rating.

Rigid versus flexible systems

Another misconception about grooved mechanical pipe joining is that couplings can’t produce rigid joints and require extra supports to prevent system sagging. The housing on a rigid coupling positively clamps the pipe to produce a rigid joint, providing system behavior characteristics similar to those of other rigid systems. The piping remains aligned and isn’t subject to axial movement or angular deflection.

Systems using rigid couplings need support techniques identical to those of welded systems when designed and installed according to the hanger spacing requirements as noted in the ASME B31.1 Power Piping Code, ASME B31.9 Building Services Piping Code and NFPA 13 Sprinkler Systems Code.

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