Difference Between Rigid Vs Flexible grooved pipe Coupling

There are many types of grooved pipe fittings, including rigid couplings and flexible couplings for pipes, open-hole mechanical tees, crosses and grooved tees, crosses, single-piece flanges and grooved flanges, etc., which have similar functions, but the usage occasions are different. Before installation, you should have an in-depth understanding of the performance and service conditions of various couplings and accessories, so that you can choose them correctly. The most important of these is to distinguish the difference between a flexible joint and a flexible joint.

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In engineering construction, due to insufficient technical disclosure and quality monitoring, workers often do not distinguish between rigid couplings and flexible couplings, and use them at will, which brings hidden dangers to the reliable operation and normal maintenance of the entire pipeline system.

For example: in the domestic hot water system, the temperature difference between the system operating temperature and the pipe installation temperature can reach 60°C, and the steel pipe is 6m long, and the resulting thermal expansion stress will reach kg/mm2. If the pipe is rigidly connected, it will cause deformation of the bracket and cause damage to the pipe system. Therefore, the butt joint fittings should be distinguished and selected reasonably.

The following typical situations reveal the difference between rigid and flexible couplings:

1. Domestic hot water pipes, air-conditioning water pipes, and open-air pipes, in order to effectively compensate for the pipe displacement caused by thermal expansion and contraction, the above three types must use flexible couplings. If the displacement of the telescopic section exceeds the total compensation amount of the flexible couplings, should also consider installing metal expansion couplings.

2. Pipes with little change in temperature difference, such as indoor fire-fighting pipes, can use a large number of rigid couplings. However, a certain number of flexible couplings should be installed in the middle section of the long straight pipeline to compensate for a small amount of thermal expansion of the long straight pipeline section to ensure that the entire pipeline system is in a safe operating state.

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3. For bent pipes and zigzag pipes with temperature compensation function, flexible couplings shall be installed on the pipe sections where angular displacement occurs.

4. It is recommended to use flexible couplings for pump room pipelines to reduce noise and weaken vibration transmission.

5. Flexible couplings should be used for buried pipelines, which can effectively deal with local deformation of the pipeline system caused by uneven settlement.

6. Flexible pipe couplings must be used for the connection of small arc pipes. It can be assembled according to the straight pipe first, and then bend the pipe to make it reach the predetermined corner. But under no circumstances should the angle of rotation exceed the value specified in the instructions for grooved pipe fittings.

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We previously have introduced different types and combinations of threaded fittings - which have been around for more than a century.

Here we're introducing another common way to join pipe; using grooved fittings. 
An attic sprinkler system using a grooved elbow with couplings.  Use of "mechanical" couplings that could allow faster joining of pipe came to life in by Lieutenant Ernest Tribe. Just a few years later the Victory Pipe Joint Company renamed itself to Victaulic (a combination of "victory" and "hydraulic"), and grew to expand the technology worldwide. 

Today, Victaulic and other manufacturing leaders provide grooved fittings that are often used for pipes in fire sprinkler systems. It is not uncommon for both mains and branch lines to be grooved today.

What are common grooved fittings, and how do they work? Let's introduce them.  An in-rack sprinkler with a branch line using (starting with the sprinkler) a groove x thread reducing elbow
​with a grooved coupling, a grooved piece of pipe, and a grooved tee (connection not shown). PIPE
Let's start with the pipe. In order to give grooved fittings an opportunity to "grip" the pipe and remain in place, they need an opportunity to resist the pressure of the water that is trying to "pull away" the pipe from the fittings which join them together. A grooved coupling about to connect two grooved-end pipes. Note the loose nut and bolt on the right-hand side, allowing the coupling to be expanded and "slip" over the pipe on the left. In order to create a groove in the pipe, steel can either be "roll groove" or "cut groove". Roll groove pipe involves pressing an indentation into the pipe near the end of the pipe. This allows a grooved fitting to slip over the end of the pipe and fit into the groove. Roll groove pipe has the advantage of not reducing the pipe thickness, so it can have more tolerance for corrosion than thinner pipe, similar pipe with threads, or pipe with cut grooves.

Pipe which is cut groove involves cutting into the pipe rather than pressing it. This cutting removes a portion of the pipe wall, making a thinner but smooth interior pipe wall. This thinner wall makes it more susceptible to corrosion, however, for pipe systems with a minor slope, the smooth inside of the pipe does not create a ridge where water can sit and corrode the pipe. Roll Grooved Pipe (top) and Cut Grooved Pipe (bottom). Note the ridge on the inside of the pipe wall for roll groove pipe, and the thinner pipe wall along the cut groove pipe. A tape measure with a "go" or "no-go" measurement to determine if the groove is within manufacturer tolerances. ELBOWS & TEES
Let's start with the basics. Elbows allow bends of 90-degrees (most common), 45-degrees, 22-1/2 degrees, and 11-1/4 degrees.

Why not every possible angle? What if I need to have a 60-degree bend because of my building?

First, it wouldn't be economical to make a fitting of every bend. Second, is that using just two 90-degree elbows back-to-back we're able to create a "swing joint" and make any angle we could want, just by changing the elevation of the pipe that's being joined. Victaulic "FireLock" Grooved Fittings; 
90-Degree Elbow #001 (left), 45-Degree Elbow #003 (center), and Standard Tee #002 (right) One notable specialty with the grooved elbow is a "Drain Elbow", which has the elbow except it includes a drain outlet at the bend of the elbow. This is used all the time with fire department connections which come down a wall and need to be capable of being drained (to avoid having water-charged pipe freeze and burst). This is also called a "Drain-El" or is a Victaulic #10-DR. A wall-mounted fire department connection that is away from the riser, here showing the "Drain Elbow" with a ball drip below. The portion upstream of the check valve is intended to be dry unless the FDC is actively being used in order to avoid freezing water inside. COUPLINGS
Nice sketches, Joe, but that's not how things look in the field!

That's because unlike threaded fittings, the actual pipe joining is by a grooved coupling. The coupling has malleable iron bumps that grip the indent of one groove (pipe/fitting) and connect it to the second groove (the other pipe/fitting).  A grooved coupling (here a Victaulic #009N shown). OTHER FITTINGS
There are a host of other fitting types. Grooved Reducing Tees? Yep. Less common. Less common can equate to more expensive, or at least that's what I hear from contractors familiar with all the pricing nuances.

What other grooved fittings do I often see? 

Reducing fittings, which is a concentric, single-cast piece of metal that has a large groove on one end and tapers down to a smaller groove on another end. One note of caution is using these in the vertical orientation; I've heard it is much better, more stable, and stronger to use a reducing-fitting as opposed to a reducing-coupling when in a vertical orientation. One of my clients goes so far to say to not use reducing couplings at all (where the coupling itself has two different groove sizes). I wouldn't have the expertise to gauge that myself. A flange x groove reducer (left) and a grooved cap (right). There are also reducing adapters, than can accept a flange connection and convert it to a reduced groove connection. 

Crosses are also available, as are caps (like the Victaulic #006 shown above on the right) which can terminate the end of a branch line. These caps even have 1-inch threaded opening options for easy auxiliary drains. 
Many manufacturers have equipment and components with grooved ends that can readily attach to pipe and fittings.