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).
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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.
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.
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.
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.
Grooved pipe couplings and fittings are used to join pipes in both wet and dry system. They provided some flexibility in terms of movement and deflection to the piping system. The concept of grooved coupling was invented in when Victaulic designed the first mechanical coupling. Though many other companies make mechanical grooved couplings, grooved couplings are still known mostly as Victualic pipe couplings or Victaulic style pipe fittings.
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Grooved pipe fittings consist of grooved couplings, grooved fittings, grooved outlets, grooved valves and grooved replacement parts. Typical application of grooved fittings are in sprinkler risers, pipe feeds passing from one building area to another, locations prone to earthquakes, in discharge line above pump suction, in air or water fire services lines, piping systems subject to excessive vibration, and installation in tight spaces or where pipe alignment is difficult.
Grooved pipe fittings are limited to use with rolled or cut groove pipe, valve and fittings. These fittings are rated to 175 psi ( kPa) as a minimum. Pipe grooves are made according to ANSI/AWWA C606 specification "Grooved and Shouldered Joints".
These couplings are FM approved and can be used in underground services. The maximum ambient temperature for these couplings are 225 deg F (107 deg C)
These couplings are sold by nominal pipe sizes and will fit the corresponding nominal pipes ,valves and fittings.
A grooved coupling or a Victualic style groove coupling has four main components:
A grooved piping system is the most versatile, reliable and economical piping system. It is more reliable than a threaded or flanged connection and often results in 2x-4x faster installation. The system is designed to work with rolled or grooved pipes. Flexible grooved pipe couplings offer the flexiblity needed for vibration dampening or installation in earthquake prone areas.
Grooved piping systems offer up to 30% cost savings. It offers fast assembly in tight systems, cleaner systems with no welding slag, contamination or hot work permit. More predictable cost estimates for contractors, resulting in faster and cheaper installation.
A simple system with 3 components only Each pipe joint is a union Rolled or cut pipe grooveEach pipe joint is a flexible joint and removing the couplings allows removal of pipe for easy cleaning, servicing and maintenance. It also allows the flexibility to change the piping system and add / remove equipment.
Grooved pipe couplings require creating either a rolled groove or a machine cut groove. A rolled groove creates an indentation in the ID of the pipe whereas the machine cut groove is smooth on the ID. A rolled grooved is mostly done on thinner wall pipe whereas the machine cut groove is more common on thicker wall pipes. Grooved couplings are commonly installed on pipe schedules 5 to schedule 40.
A grooved pipe is much easier than a welded pipe joint, requiring no X Ray, hot work permit and associated hazards of welding a pipe.
Use of grooved pipe coupling isolates the noise and vibration. Resilient gasket absorbs noise and vibration resulting in a quieter system. Provides superior vibration attenuation and eliminates the need of noise suppression devices.
A grooved pipe joint allows for expansion and contraction of the pipe joint without compromising the integrity of the connection. It eliminates the need for costly expansion joints. Grooved pipe joints also eliminate pipe stresses for buried and underground pipes.
A groove is made in the pipe by cold forming and machine cutting a groove into the end of pipe. See figure below for difference between rolled and cut groove. A gasket is housed in the coupling housing and is wrapped around the pipe ends. The groove in the pipe mates with tongue in the pipe and creates a restraint that prevents pipe pull out. Gasket / sealing element creates the pressure barrier. The entire assembly is tightened with bolts and nuts.
Flexible grooved pipe coupling
There are two types of basic grooved couplings:
Grooved pipe fittings were introduced in by Victaulic. Since then several manufacturers have entered the market. Notable among them are: