Refineries built in the 1950s to 1970s used all welded piping for both process and utility services. The specifications used during that time are often applied to retrofits, expansions and turnaround projects. But this reliance on the traditional specifications does have a downside. Scheduling, safety and constructability are three major factors in turnarounds. They are also the impacting factors that affect welding activities. However, alternatives, such as grooved mechanical piping, have been slow to gain acceptance due to the perception that the joining method wont work and a reluctance to use a gasketed joint.
This article will compare the primary pipe-joining methodswelding, flanging, threading and groovedand discuss the advantages and disadvantages of each method as used in the hydrocarbon processing industry (HPI). The misconceptions over grooved piping will be explained and corrected, and how this method can speed project completion, and improve constructability and safety, making it an ideal pipe-joining method for plant utility services.
HISTORY OF HPI PIPE JOINING
During the wave of refinery construction in the 1950s to 1970s, over 99% of the HPI facilities elected to weld all piping. Everything was based on process piping. Even utility piping systems were designed to the same material class as process piping. Many engineers didnt differentiate between piping classes. Nonprocess piping was over-engineered and over-constructed for perceived safety reasons.
At present, during retrofits, expansions and turnarounds, the specifications of the mid-20th century are still used. There is a mindset of if it aint broke, dont fix it. Welding is certainly not broken. But when it comes to utility piping, it may not be the best choice, given this methods shortcomings as it relates to construction, maintenance and safety.
Reliability and maintainability needs
Several factors that are important during construction projects typical of existing refineries and chemical plants include:
- Constructability, reliability and maintainability of equipment and systems.
Whereas welding is typically very reliable when performed by an experienced and highly skilled welder, the method does not promote quick project completion. It lacks in the constructability and maintainability of piping systems, and is inherently unsafe, particularly in the presence of volatile, toxic and explosive chemicals. Its also simply unnecessary for low-risk, nonprocess oriented utility services such as domestic water, plant water, plant air and compressed air. Other major pipe-joining methods also have their challenges.
Other methods equally have their pros and cons in plant piping systems:
This pipe-joining method produces a high-strength, permanent joint, which is usually very reliable. With the ability to use the joining method on just about any piping service, welding has become the standard by which all other methods are compared. The strength and reliability of welded joints are essential for critical high-temperature, high-pressure process piping. However, for utility services, weldings disadvantages outweigh the advantages.
Safety. First, safety concerns are considerable during welding activities. Welding by its very nature is dangerous. It is one of the most dangerous industrial activities. When welding is done in a potentially volatile environment, the risks become even greater. Welding produces flames, sparks and fumes; all introduce the risk of fire or explosion. Welding requires a fire watch during and following the work, which can slow the construction schedule. Furthermore, welding exposes workers to noxious fumes and particulate matter, as well as potential burns and eye damage.
Time-consuming activity. Second, welding is a time-consuming process. Welders must cut, bevel and prepare the pipe lengths; align and clamp the joint; and then undertake two, three or more passes using the selected welding method at each joint. A single 4-in. carbon steel pipe joint can take up to 2.25 hours to weld; a 12-in. joint can take 4 hours or longer, based on values found in the Mechanical Contractors Association of Americas Labor Estimating Manual (Rev. 2/98). Once the weld is complete, an X-ray may be required for quality inspection. In the case of a failed X-ray inspection, the re-work increases facility downtime. The challenges, time and risk associated with welding galvanized pipe are even greater.
Complex method. Third, the maintenance of a welded system is difficult. Welded systems convert individual pipe sections into a single unit, making it much harder to access a specific point within the system. If not accessing a welded system at a flange, the pipe would have to be cut in place to provide access.
Skill shortage. Finally, the quality of welding is declining. Many of the highly skilled welders with years of experience are reaching retirement age. The need for welding is unlikely to drop as quickly as the number of skilled welders available to do the work. Result: A shortage of skilled labor is quite possible, which could affect the quality of work. Allocating skilled labor to critical process systems and using alternative joining methods for noncritical utility systems are strategies to mitigate this challenge.
This pipe-joining method is a mechanical method that uses a series of bolts and nuts to compress a gasket between two flat-faced, flanged pipe ends. Flanging also produces a strong and reliable joint. Unlike welding, it provides a means for system access, but requires more maintenance to support joint integrity.
Union maintenance. The bolts and nuts of a flanged union and gasket absorb and compensate for system forces. Over time, the bolts and nuts can relax due to surges, system working pressure, vibration and expansion and contraction. When the bolts lose tension, the gasket can slip, which can result in a leak. Flange gaskets can take on compression over time, also resulting in leakage. To prevent or stop leaks, routine bolt and nut tightening is required.
Galvanization. Joint integrity may also be affected by the galvanization process. Under normal process conditions, galvanization may result in a zinc buildup on the flange, thus producing a flange face that is no longer flush. Such conditions can cause the flange to be more prone to leaks.
Although flanges provide system access, performing maintenance can be a time-consuming process because each of the bolts needs to be loosened and removed. In some cases, the gasket needs to be scraped off the flange and then replaced. The same bolt-tightening sequence required upon initial installation is also required upon reconnection of the flanges.
Welding issues. Finally, because flanges are typically welded onto the pipe ends, this method carries the same issues associated with welding, including safety risks and lengthy installation time.
In threading, a process that is typically used to join small-diameter pipe involves cutting conical spiraling male or female channels into the inside or outside of pipe or mating components. The joint is quick and simple to assemble. However, it is the least reliable compared to the other pipe-joining methods.
Threaded joints are notorious for leaks, which can result from improper initial installation and ongoing plant operations that weaken the threaded seal. System vibration can compromise the thread tape or sealant, resulting in a leak. Poor thread cuts can also cause leaks. In a threaded system, the leak is usually fixed by tightening the joint. The problem with this solution is that tightening one end of the threaded joint ultimately loosens an adjacent joint, so fixing one leak may lead to a new one.
Threading joints can present alignment issues with branches and elbows. In addition, the joints are difficult to repair. Over time, the joint may become fused, making system access more challenging. Many refineries have experienced problems with threaded small-diameter galvanized piping. They are replacing these systems with stainless steel (SS) systems.
The final method is the grooved mechanical piping. It is widely known and highly regarded in the upstream oil and gas industry. However, grooved mechanical piping is relatively unknown and, in some cases, misunderstood on the downstream side.
ANATOMY OF A GROOVED JOINT
A grooved mechanical joint is formed with grooved-end pipes, fittings or valves, and a coupling, as shown in Fig. 1. The coupling comprises three elements: gasket, housings, and nuts and bolts.
Fig. 1. A grooved mechanical joint is formed
with grooved-end pipes, fittings or valves and a
Grooved mechanical piping does not require special pipe. Standard, off-the-shelf pipe is fabricated by cold-forming or machining a groove into the pipe ends. There are two types of grooving: roll and cut grooving. Roll grooving is far more common, and is the preferred method for most utility services. To form a roll groove, the pipe end is placed between the roll set of a grooving machine. As the roll set closes, the pipe is compressed and rotated, which radially displaces a small portion of the pipe wall to form a groove around the outer diameter of the pipe that is recessed on the outside and indented on the inner pipe wall, as shown in Fig. 2. Unlike threading, roll grooving does not remove any material from the pipe. A fast and clean technique, roll grooving is used on a variety of pipe sizes and wall thicknesses, from Schedule 5 through ANSI standard wall thickness carbon steel (CS) and SS, copper and aluminum pipe. Roll-grooved systems range in diameter from ¾ in. up to 60 in.
Fig. 2. A small portion of the pipe wall is
displaced to form the groove around the outer
diameter of the pipe.
To seal the joint, a resilient, pressure-responsive elastomer gasket seals around two abutted grooved pipe ends. The nitrile gasket, which is common in most water with oil vapor applications, is injection-molded to precise tolerances and is resistant to aging, heat and oxidation.
Housings, bolts and nuts
The coupling housings fully enclose the gasket, and the key sections of the housings engage the grooves. The housings are typically constructed from ductile iron (painted or with engineered coatings), SS or aluminum. While the housings are exposed to the external environment, they are insulated from the system media by the coupling gasket that contains the fluid within the interior of the pipe. The bolts and nuts, which hold the housings together, are tightened with a socket wrench or an impact wrench.
In the installed state, the coupling housings encase the gasket and engage the groove around the circumference of the pipe to create a leak-tight seal in a self-restrained pipe joint. With the availability of rigid and flexible couplings, a grooved joint can be completely rigid, like a welded joint, or offer flexibility to accommodate thermal expansion and contraction, deflection, seismic movement and vibration.
The housings of a rigid coupling positively clamp the pipe to create a rigid joint, resulting in system behavior characteristics similar to other rigid systems. The piping remains strictly aligned and is not subject to axial movement or angular deflection during operation. For this reason, systems installed with rigid couplings utilize 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 B31.3 Process Piping Code, ASME B31.9 Building Services Piping Code and NFPA 13 Sprinkler Systems Code.
Flexible couplings provide controlled linear and angular movement that may be used to accommodate linear movement due to thermal changes. It may be used at system changes in direction to provide stress-free offsets, or it may be used on traditional expansion loops, resulting in loops one-half to one-third the size of a loop of welded construction.
Couplings localize vibration within the pipeline, dampening the vibration of the system. Grooved piping systems do not require rubber bellows or a braided flexible hose, which can wear out and require replacement.
MISCONCEPTIONS OF GROOVED PIPING
Unlike other industries that have readily accepted grooved piping, HPI facilities have been hampered by a perception that the joining method wont work and reluctance to use a gasketed joint. Fears exist that the coupling will leak or even fail, and that the grooving process weakens the pipe. These ideas have arisen due to limited exposure to grooved piping systems. The concerns can be easily rectified by reviewing strength and pressure performance capabilities.
With regard to pipe end preparation, roll grooving does not compromise the integrity of the pipe joint. The inward radial displacement that occurs at the groove during the roll-grooving process causes pipe material property changes comparable to similar cold-forming manufacturing operations. Any potential increase in pipe hardness, reduction in tensile strength, or reduction in elongation due to the roll-grooving process has no effect on the pressure capability of the joint.
The pressure rating of a grooved systemestablished after extensive performance barometers including ultimate pressure, bending moment and cyclic loading testsis based on the components of the joints. Grooved pipe has no rating without the corresponding coupling, and coupling pressure ratings vary based on the pipe material and wall thickness. The published maximum rated pressures for couplings are based on test data and field experience. Any effect that roll grooving has on the pipe material has been accounted for in coupling pressure ratings.
Component performance requirements for many piping applications are dictated by standard codes relevant to the service. To comply with the code requirements, the piping materials must be able to maintain published performance capabilities while in service. Based on their proven performance capabilities, use of couplings on grooved pipe meets the requirements of ASME B31.1, B31.3 and B31.9, as well as NFPA 13.
The suitability of grooved pipe for use in piping systems is recognized in such standards as ASTM F1476, Performance of Gasketed Mechanical Couplings for Use in Piping Applications, and ANSI/AWWA C606, Grooved and Shouldered Joints. These pipe standards have been established in recognition of the widespread use of grooved piping in air- and water-conveying systems, and the subsequent need for sufficient clarity in the performance and dimensional requirements of grooved joints.
Grooved pipe joining has been proven through research, testing and extensive evaluation. Provided the coupling is correctly installeda process that is substantially easier than most other pipe-joining methodsthe joint will not leak or fail as long as the working pressure of the system is within the couplings pressure rating for the type and thickness of the pipe. With couplings currently rated up to 4,000 psi, grooved pipe joining can be used on almost all utility services.
BENEFITS FOR UTILITY SERVICES
There are multiple reasons why grooved mechanical piping is an ideal choice for plant utility services. The three key benefits are also factors that drive equipment and material decisions in refinery expansions and turnarounds:
Ease of installation and maintenance is one of the most appealing aspects of grooved piping. To assemble a grooved joint, two grooved pipe ends are abutted, the gasket is positioned over the joint, the housings are placed over the gasket, and finally, the bolts and nuts are tightened to secure the housings together. Welding and special tools are not required. Grooved systems offer 360° of rotational allowance for field flexibility, meaning alignment of the pipe by the bolt-hole index, as would be required with flanging, is unnecessary.
Unlike other pipe-joining methods, visual inspection can confirm correct installation of most grooved systems. Metal-to-metal bolt-pad contact confirms that the assembled joint is properly and securely installed, and no re-work is necessary.
Couplings decrease maintenance time because, unlike flanges, they do not require regular retightening. A coupling holds the gasket in precise compression from the outside of the pipe joint. While the bolts and nuts of the coupling hold the housings together, the coupling itself is what holds the pipe together. Over the service life of the system, the nuts and bolts do not require regular maintenance and will not relax.
Should access to the piping system be required for maintenance, expansion, alteration or equipment/component replacement, the coupling can be removed quickly, and with no special tools. Following completion of the work, the coupling can be reassembled just as quickly on the joint. Maintenance of grooved systems is far simpler than maintenance of welded, threaded and flanged systems. The ease of access allows piping systems to quickly adapt to changes in plant operations.
Grooved is also beneficial for specialty applications such as lined pipe and galvanized pipe. Typical specifications do not allow torch cutting or welding lined pipe because it can compromise the integrity of the internal linings. As grooved systems are cold-formed, they meet the requirements of most piping specifications. Grooving the pipe does not have an effect on the internal coating. Furthermore, gaskets with a central leg that acts as a pipe stop protect the pipe ends from installation damage that can cause a holiday in the coating. As a result, grooved piping maintains the integrity of internal pipe coatings. In fact, grooved is the only pipe-joining method that can ensure a holiday-free system.
Grooved piping eliminates the disadvantages associated with welding and flanging galvanized pipe. Because welding is not required, no toxic fumes are created. Furthermore, there is no increased risk of leaks, as there would be with flanged due to leak paths created by zinc buildup. The fabrication and assembly of a grooved galvanized system are much quicker than other joining methods.
Another key factor during plant expansions, retrofits and turnarounds is the schedule. It is quite obvious that the shorter the downtime, the sooner the plant is online and producing revenue-generating products.
Installation of grooved piping is up to 10 times faster than welding and up to 6 times faster than flanging. Although installation time will vary by installer, conservative estimates require approximately 15 minutes to assemble a 4-in. grooved joint and 45 minutes to assemble a 12-in. joint, a vast difference compared to the 2.25 hours and 4 hours required to weld joints of the same size. As shown in Fig. 3, the ease and speed of installation can reduce onsite manhours by up to 45% compared to welding.
In the volatile environment of a refinery, any procedure that can reduce risk is worth exploring. In terms of pipe joining and maintenance, grooved is among the safest methods due to the elimination of hot work. Most injuries on job sites occur via material handling, but the most significant risks are caused by fire and fume hazards.
Because the assembly of a grooved pipe joint does not require welding, flame or heat of any kind, it can be installed by almost anyone. It does not require time-consuming X-rays of joints, purge gases, fire watches, hot-work permits, cutting/grinding of weld bevels, tacking, slag cleaning or dealing with weld fumes, weld splatter and sparks, and welding cable trip hazards. No-flame grooved systems pose no fire or respiratory risk, do not necessitate increased ventilation, and often reduce or eliminate system cleaning and flushing.
The primary obstacles in the use of grooved piping are lack of knowledge and fear. As demonstrated, concerns regarding the strength of the system are unfounded, and awareness of the grooved systems array of benefits can undoubtedly outweigh the reliance on traditional, inefficient joining methods. Grooved mechanical piping can offer improved constructability, speedy expansion, retrofit and turnaround completion, and also reduce safety risks. It is a quality pipe-joining method for utility services in any HPI facility. HP
Grady Wilkerson is vice president of oil, gas and chemical sales with Victaulic, a producer of mechanical pipe joining systems. He began his career with Victaulic in 1980 as a member of the West Texas Oil Metro Group in Odessa, Texas. Mr. Wilkerson holds a BBA degree from Texas A&M University.