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What are the advantages and disadvantages of grooved flexible coupling?

Grooved flexible coupling is a new pipeline connection technology, which is mainly used in pipeline systems of different media such as liquid, gas and solid. Compared with the traditional pipe connection method, grooved flexible coupling has many advantages and some disadvantages. The following will introduce the advantages and disadvantages of grooved flexible coupling from several aspects.

Advantages:

(1) Easy installation

Compared to traditional pipe connections, grooved flexible couplings are much easier to install. All that is required is to fit the grooves at each end of the pipe to the fitting bayonet and bolt it in place without welding or cutting. This not only saves time, but also reduces the labor intensity and safety hazards of field workers.

(2) Good flexibility

Grooved flexible coupling joints have good flexibility and can absorb deformation in different directions and sizes in the piping system, effectively reducing the deformation and vibration of the piping system caused by temperature, vibration and earthquakes, thus protecting the safe and stable operation of the piping and equipment.

(3) Easy maintenance

Trench type flexible coupling pipe joints have a long service life, but they can also be damaged or worn out during use. Compared to traditional pipe connections, grooved flexible coupling joints are easier to maintain by simply removing the bolts and replacing or repairing them without removing the pipe or performing complicated maintenance operations.

(4) Good sealability

The grooved flexible coupling has good sealing, which can effectively prevent the leakage of liquid, gas and other media. The rubber gaskets on the joints' jaws can be fully sealed, thus ensuring stable and safe operation of the piping system.

(5) Wide applicability

Grooved flexible coupling is suitable for pipeline systems with different media, diameters and pressure levels, including pipeline systems in water, chemical, metallurgical, electric power, paper making and other industries, and has wide application prospects.

As a kind of pipe joint, grooved flexible coupling has many advantages in the process of use, but there are also some disadvantages, the following are some of the more common ones:

(1)

Restricted by temperature

The material of grooved flexible coupling pipe joints has a certain limitation in withstanding temperature and cannot be used under extreme temperature. If the temperature exceeds its tolerance range, the joint may be deformed or damaged, thus affecting its normal use. For example, some grooved flexible coupling joints made of rubber materials are prone to aging and deformation at high temperatures, which affects their service life.

(2)

Restricted by pressure

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The flexible material of grooved flexible coupling joints also has certain limitations when it comes to pressure, and its ability to withstand pressure is relatively low and cannot withstand a large amount of pressure. Therefore, in some special working conditions, it is necessary to choose the appropriate type and material of the coupling to meet the requirements of the pressure it can withstand.

(3)

Vibration and noise

Due to the flexible materials used in grooved flexible coupling fittings, they have good flexible connection performance in piping systems, but they are also susceptible to vibration and noise. Under high speed flow and high vibration environment, noise and vibration will be generated, which will affect the use effect and stability.

(4)

High installation space requirement

Because the connection method of grooved flexible coupling pipe joints is different from that of traditional joints, a certain space needs to be reserved in the piping system to meet its installation and disassembly requirements. Therefore, in some piping systems where the space is relatively small, its installation may be restricted to a certain extent and requires targeted design and selection.

(5)

Vulnerable to chemicals

Grooved flexible coupling pipe joints are susceptible to chemical substances in some special media, causing corrosion or aging of their materials. Therefore, proper material selection and regular inspection and maintenance are required to ensure its proper operation.

In summary, as a new pipeline connection technology, grooved flexible coupling has many advantages, such as easy installation, good flexibility, easy maintenance, good sealing and wide applicability. However, there are also some disadvantages, such as temperature and pressure restrictions, vibration and noise, and high installation space requirements. Therefore, when using grooved flexible coupling joints, it is necessary to choose the appropriate type and material according to the specific working conditions and requirements to ensure its safe and reliable operation. With the continuous development and improvement of technology, the application prospect of groove type flexible coupling will be more broad in the future.

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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