How pipe-joining methods affect system operation
Equipment—such as pumps and chillers—is increasingly being designed with grooved inlets and outlets. Although the standard has been flanged in the past, manufacturers are recognizing the benefits of grooved pipe joining, not just for pre-sale equipment testing, but also for the performance and maintenance benefits that end users will realize. The grooved-pipe-joining method is a good choice for equipment connections and for utility and process piping systems within commercial and industrial facilities.
Grooved pipe joining employs a mechanical coupling—consisting of housings, an elastomeric gasket and nuts and bolts—to join grooved-end pipe, valves, fittings, equipment and accessories.
The pipe groove is made by cold forming (roll grooving) or machining (cut grooving) a groove into the ends of a pipe. To assemble the joint, two grooved pipe ends—or a pipe end and a valve or fitting, etc.—are abutted. The gasket is positioned around the joint. The housings are placed over the gasket, and finally, the bolts and nuts are tightened to secure the housings together. In the installed state, the coupling housings encase the gasket and engage the grooves around the circumference of the pipe to create a leak-tight seal in a self-restrained pipe joint.
Rigid couplings provide a mechanical and frictional interlock onto the pipe ends sufficient to result in a rigid joint with characteristics similar to those of welded or flanged joints, because all the piping remains in strict alignment and is not subject to deflection during operation. Flexible couplings permit controlled linear and angular movement, which can accommodate thermal expansion and contraction, deflection, seismic movement and vibration.
|The anatomy of grooved pipe joining|
When testing equipment, manufacturers need a joining method capable of connecting the unit to a test loop that is quick but secure. Grooved couplings enable the quick connection of equipment to run these tests. With just two bolts to secure each coupling—as opposed to the multiple bolts that are required for each flange—manufacturers can rapidly assemble the product and test it before it is sold to a client. With fewer bolts and typically no torque requirements, grooved piping is up to six times faster to install than flanging. This time savings is also beneficial when connecting the equipment and associated piping systems at the end user’s facility.
Once connected, many grooved systems are designed for visual inspection to ensure complete assembly. The joint is properly installed when the bolt pads meet metal-to-metal. Flanges, on the other hand, do not provide visual confirmation. The only measure to ensure proper assembly is to fill and pressurize the system and check for leaks.
Ease of Maintenance
The same characteristics of grooved systems that speed installation—fewer bolts and no torque requirements—also make system maintenance a quick task. To gain access to a pump or valve, for example, the two bolts of the coupling are loosened, and the housings and gasket are removed from the joint. In a flanged system, multiple bolts need to be removed. The same time-consuming bolt-tightening sequence required upon initial installation is also required upon reassembly of the flange.
Throughout the life of the system, flanging requires significant maintenance to sustain joint integrity. Pumps and other equipment frequently vibrate and are subject to noise in the piping system. For example, oscillatory forces can be generated due to the lack of balance of rotating components, such as in pump vanes. Although field balancing can reduce this, it requires specialized equipment, which can become difficult, expensive and impractical.
|Piping connected to a pump using the grooved-pipe method|
Flanged systems employ rubber bellows or braided flexible hoses to accommodate vibration. With time or overextension, these products can wear or fail, requiring replacement.
Grooved couplings accommodate vibration without the need for specialty products that require periodic repair or replacement. The resilient elastomeric gasket contained within both flexible and rigid couplings aids in the dampening of noise and vibration. The design of the coupling enables the gasket to seal against the pipe, while the housing provides both space for the elastomeric material to flex and containment to prevent overstretching.
Flexible, grooved systems allow the pipe to move and vibrate within the coupling, localizing the vibrations generated by equipment and reducing the amount of noise transmitted through the piping system. The design of the grooved joint permits this movement without wearing out the gasket.
Testing commissioned by a grooved couplings and fittings manufacturer and conducted by Seattle-based SSA Acoustics LLP shows that grooved couplings provide greater vibration isolation and sound attenuation characteristics than flexible metal or elastomeric arch-type connectors. The ideal arrangement for accommodating equipment vibration is the placement of three flexible couplings in close proximity to the vibration source. The use of additional couplings, whether flexible or rigid, will further reduce the transmission of vibration.
System vibration and other forces can lead to joint problems in a flanged system. Flanging employs a series of bolts and nuts to compress a gasket between two flat-faced, flanged pipe ends. The torque on the bolts employs a high compressive load on the internal gasket, which can cause the gasket to adhere to one or both of the flanges. When the joint is disassembled, the gasket can tear, resulting in failure upon reinstallation. Furthermore, the bolts and nuts of the flanged union absorb system forces. With time, they can relax due to surges, system working pressure, vibration, expansion and contraction. When the bolts lose tension, the gasket can “slip,” which can result in a leak. Flange gaskets can take on compression with time, also resulting in leakage. To prevent or stop leaks, routine bolt and nut tightening is required.
The ability of grooved couplings to accommodate system vibration reduces the risk of joint issues. A coupling holds the gasket in precise compression from the outside of the pipe joint. While the bolts and nuts hold the housings together, the coupling itself is what holds the pipe together. The gasket absorbs system forces, not the bolts and nuts. Throughout the life of the system, the nuts and bolts do not require regular retightening and will not slip or relax, therefore maintaining a leak-tight seal. The gasket of a grooved coupling has a C-shaped, cross section seal that is durable and can handle significant compressive and cyclical loading. A system can be pressurized and depressurized repeatedly without fatiguing the rubber.
|Grooved couplings accommodate vibration while flexible and rigid couplings dampen noise and vibration.|
The evolution from flanged to grooved inlets and outlets on mechanical equipment and pumps is occurring for several reasons, including the previously mentioned assembly speed of couplings. Another important reason is the difficulty associated with flange bolt-hole alignment. The bolt-hole index on the equipment inlets and outlets must line up perfectly with the flange on the piping to be connected to the unit. Flange alignment cannot be adjusted in the field, which can lead to misalignment.
Grooved systems do not have this problem. No bolt-hole pattern has to be lined up, and a coupling can be oriented at any angle around the joint. The coupling can be “free floated” around the pipe in the field to provide easy access to the bolts, and in turn, simplify access to the equipment. In addition to eliminating misalignment issues during installation, a coupling’s 360-degree orientation capability coupled with its lower profile, compared to a flange, makes the installation of grooved systems ideal for confined spaces.
In summary, grooved piping systems are ideal for equipment connections because of their speed of installation, ease of maintenance and simplified alignment. These characteristics, coupled with additional benefits—such as design versatility, safety and reduced total installed costs—also make grooved piping a good choice for joining utility and process piping systems.