Piping | In the groove — Common myths about mechanical pipe joints

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).

Solid Product Page

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.

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.

Pipe couplings play an important role in repairing water and sewer piping systems. Due to the fact that couplings offer a fast method for connecting pipes of all sizes and materials, they can be used in virtually any pipe joining conditions, from cooling return pipes in power plants to gas and water supply systems.

If you are looking for more details, kindly visit Restrained Flexible Coupling.

Whether you are looking for a suitable pipe coupling for water and wastewater systems in Rochester or anywhere else in the country, you should learn as much as you can about the different coupling types as well as their specific applications in the industry. Read on.

How does a pipe coupling work?

In water and wastewater piping systems, pipe couplings are used to connect two or more sections of pipe with the aim of maintaining continuity between them. They can be used to join pipes of different or equal sizes, as well as connect more than just two pipes if they are cross or T-shaped.

Flexible couplings allow for efficient and speedy piping repair. This is especially important when repairing municipal water and sewer lines since prolonged service interruptions are usually not permissible. There are numerous different types of couplings used in the water and wastewater industry, providing an economical and effective means of joining various pipe materials and sizes.

How do you use a pipe coupling?

There is no single method for using pipe couplings in water and wastewater settings. This is because different piping repair or replacement projects involve the use of different types of pipe repair clamps or couplings which may have specific installation requirements. In general, pipe couplings can be used for:

• Connecting pre-fabricated pipe systems: couplings can be used to connect a wide range of pre-manufactured pipes. Some couplings are designed to join pipe sections of the same O.D. diameters, whereas others can be modified to a number of different pipe sizes.

• Performing fast piping modifications: in many circumstances, using a pipe coupling or connector is the most efficient and economical way to modify or replace pipe sections in water and wastewater systems.

• Connecting pipe sections of different materials: pipe couplings are generally made from similar or identical materials as that of the tube or pipe to be joined. For instance, copper couplings would be used for copper pipes, although brass and bronze are also common.

• Performing reliable and quick piping repairs: in many cases, couplings offer the most economical solution for repairing leaky, damaged, or deteriorated piping. This is especially true if the deterioration is extensive and replacing the damaged section of pipe is the most viable option.

When using couplings for pipe repair, it is also important to consider the internal pressure and axial stresses present in the pipework. The axial loadings or stresses have to be locally restrained during the repair procedure in order to prevent the pipe components from separating.

Where is plumbing rubber coupling used?

While a plumbing rubber coupling can be used for any of the aforementioned applications, they are primarily used to connect two sections of the pipe together in drainage, wastewater, or ventilation systems, either as a permanent or a temporary solution.

Their flexibility and adaptability make them especially effective at joining pipes made of different materials (e.g. PVC to cast iron, HDPE to steel, etc.), as well as connecting pipes with slight diameter variations.

For these reasons, rubber plumbing couplings are commonly found in sewer and (underground) drainage infrastructures, as well as in systems where slight movements may occur, primarily due to soil shifting.

What is a drain coupling?

A drain pipe coupling is a type of plumbing fitting designed to connect two pieces of piping with the same or approximately the same diameter (i.e. ±10mm or ~0.4”). These fittings typically consist of a rubber “sleeve” or “body” and several clamp bands, allowing for the creation of a fast and easy connection with only the basic tools.

This simple, yet effective design and incredible ease of installation make drain pipe couplings a popular choice for a wide variety of applications, including drainage systems for sinks, showers, bathtubs, toilets, and a whole host of other fixtures.

What is the advantage of rubber coupling?

We already mentioned many features that make rubber plumbing couplings stand out among similar fittings. Now, it’s time to see how those features give them an advantage over their rigid counterparts:

• Flexibility: Quality rubber drain pipe couplings are made out of allows for slight movements, shifts, or contractions of the conjoined pipes – without risking leaks.
• Versatility: The rubber’s ability to accommodate different materials and pipe sizes makes it applicable in a wide variety of projects and settings.
• Vibration absorption: In settings where vibrations may occur (e.g. HVAC, high rise plumbing, industrial plumbing, etc.), the rubber’s natural ability to absorb shocks provides a safeguard against piping becoming misaligned or cracking at the joints.
• Installation ease: Rubber pipe couplings are incredibly easy to install, as they do not require any specialized equipment or methods, making them an excellent choice in standard and emergency situations.
• Durability: Resistance to corrosion, chemicals, elements, and temperature variations (to a degree) makes rubber fittings long-lasting even in harsh environmental conditions.

How long do plumbing rubber couplings last?

The average lifespan of a plumbing rubber coupling depends on the quality of the materials, conditions of use, and environmental factors, and can range anywhere from several years to several decades. As it stands, this is a massive discrepancy, so it demands a bit more clarification.

As an example, standard low-cost low-quality plumbing rubber couplings that can be bought in any hardware store by a bucketload have a nominal lifespan of 5 – 10 years. They are acceptable as a temporary fix only.

However, if the issue requires a more permanent solution, then going for top-quality drain pipe couplings, such as Fernco or Smith-Blair, is the only feasible choice. These (and similar) manufacturers create fittings that can easily last upwards of 50 years – with regular, everyday use and minimal maintenance.

Do flexible couplings meet code?

In short, yes – flexible drain pipe couplings generally do meet plumbing codes, especially if they’re made by a recognized manufacturer. However, different coupling types are intended for different applications and, therefore, may be subject to slightly different local code requirements.

As such, it is important to ensure that the fitting is specifically rated for its intended use. This is typically denoted with a certification by the relevant organizations (e.g. American Society for Testing and Materials (ASTM)). Still, it is always advisable to verify compliance with the relevant local authorities prior to installation, just to be on the safe side and avoid future issues.

Looking for a durable pipe coupling in Rochester or across the nation?

At Blair Supply Corp, we carry high-quality piping repair materials, including top-of-the-line drain pipe couplings and plumbing rubber couplings, for a wide range of applications. In fact, we are the leading supplier in the utility distribution industry, offering everything from heavy-duty water mains tools to sturdy clamps for industrial water pipe repair. We carry products from a great number of reputable suppliers, including Fernco, Krausz, Smith-Blair, Integrity Fusion, and more.

For more wholesale gibault joint for pvc pipeinformation, please contact us. We will provide professional answers.