|The technology of power plant piping is constantly evolving
The reliability of pipe joints cannot be compromised and the selection of pipe-joining methods can have a significant impact on both the initial installation and running costs and the efficient operation of a plant, writes Bill Lowar
Piping systems are an important part of power plant construction because they have a major influence on how efficiently and cost-effectively a plant operates.
When critical systems are down, the entire plant could cease to operate – and a plant that is not generating power is not making money.
Traditionally, pipes required for power plant services have been installed using welded and flanged joints in the medium- to large-diameter range, and threading for small-diameter pipe connections. Yet these methods are not ideal. Each presents risks and drawbacks for owners, engineering consultants, contractors and installers, including concerns about safety, cost, maintenance and long installation times. Alternative technologies such as grooved mechanical pipe joining can overcome many of these issues.
A grooved mechanical joint is comprised of four elements: grooved pipe, gasket, coupling housings, and nuts and bolts. The pipe groove is made by cold forming or machining a groove into the end of a pipe. A resilient, pressure-responsive elastomeric gasket enclosed in coupling housings is wrapped around the two ends of the pipe, and the key section of the coupling housing engages the groove. The bolts and nuts are tightened with a socket wrench or impact wrench, which holds the housings together. 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.
Once assembled, the mechanical coupling provides a permanent connection. Yet it can be disassembled if required to give quick and easy access for maintenance, reducing costly plant downtime.
Welding is a time-consuming process and must be performed to stringent procedures. Welders must cut, bevel and prepare the pipe lengths, align and clamp the joint, then undertake two to three passes using the selected welding method at each joint. On a large-diameter system, this process can take hours for each joint. If a pipe joint needs to be welded at a height, the erection and dismantling of essential scaffolding will significantly add to the time needed.
Welding is also an expensive pipe-joining method. Although material costs are lower, total installed costs will be higher than mechanical joints due to the installation time and the need for highly skilled workers. Non-availability of the necessary skills can cause project delays and potentially lead to heavy financial penalties.
Safety is a major concern during welding. The potential for fire or explosion necessitates a fire watch during and following the work, which slows the construction schedule and adds cost. Welding indoors also requires fume and smoke extraction equipment.
Joining pipe with a mechanical coupling is up to five times faster than welding because the gasket and housings simply need to be positioned on the grooved pipe ends, and the two bolts and nuts tightened with standard hand tools or impact wrenches. Following installation, the joint can be inspected visually: metal-to-metal bolt-pad contact confirms that the coupling has been properly secured into place without the need for x-rays.
No flame is required to install a mechanical coupling, eliminating the safety concerns associated with welding. Fume or smoke extraction equipment is not needed, nor are precautions such as a fire watch.
Grooved pipework can be prefabricated off-site in manageable sections, greatly simplifying transport, handling and erection. A man lift will normally allow quick and safe access for coupling installation at height.
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Flanges are difficult to work with and are time-consuming to connect, with multiple bolts and nuts that require star-pattern tightening numerous times to complete the joint. Maintaining reliable, leak-free performance of a flanged joint can be the biggest issue.
Flanges are bolted together, compressing a gasket to create a seal. The bolts and nuts of a flanged union and gasket absorb and compensate for system forces and, over time, the bolts and nuts can stretch or yield due to surges, system working pressure, vibration, and expansion and contraction. When these bolts yield, the gasket can ‘slip’, which can result in a leak. Flange gaskets can take on compression over time, resulting in leakage.
In addition to potential safety issues, flanged unions increase maintenance requirements. To prevent or stop leakage of the flange, routine bolt and nut tightening is required. If this maintenance is not performed on a regular basis, the system is at greater risk of leaks. Gasket replacement may also be required, particularly when the flange is taken apart. Over time, the gasket can bond to the flanged pipe ends. When the joint is disassembled, the gasket will need to be scraped off the flanged pipe end and replaced, again increasing downtime due to maintenance.
A solution to these common safety and maintenance problems is to use couplings in place of flanges. A gasket contained within the coupling housings is stretched over the two ends of pipe which have been grooved, creating an initial seal, and the key sections of the coupling housings engage the groove on the pipe ends. When tightened, the bolts and nuts pull the housings together, metal to metal, compressing the gasket a precisely controlled amount to form a reliable, secure joint.
Couplings can be used on balance-of-plant piping applications including water and air services and can be installed in a third of the time needed to form a flanged joint. They eliminate the regular maintenance associated with flanges, because they do not require regular retightening throughout the life of the system. Unlike a flange that puts variable stress on the gasket, nuts and bolts, a coupling holds the gasket in precise compression from the outside of the pipe joint and holds the pipe together.
|Overhauling piping can bring long-term cost savings
Threaded joint leakage
One of the most notorious issues in a plant is leaks in instrument air lines. Leaks are a problem because the cost of lost air is huge. Leaks cause pressure drops and machinery runs less efficiently by using more energy to make up for these losses.
Leaks result in a variety of additional problems, including inconsistent equipment performance due to fluctuating system pressure, increased maintenance costs, reduced service life of compressors due to excess load, and even corrosion of the steel piping system caused by moisture in the system.
A widely accepted joining method for small-diameter instrument air systems is threading and many of these air lines with threaded joints experience leakage. Two of the main causes of leaks are improper initial installation and ongoing plant operations that weaken the threaded seal. System vibration, for example, can compromise the thread tape or sealant, resulting in a leak. Poor thread cuts can also cause leaks.
One solution to this problem is replacing threaded instrument air systems with a press-to-connect system. These systems allow plain-end ANSI schedule 10 stainless steel pipes to be connected thread- and weld-free. A hand-held pressing tool compresses a fitting, containing O-ring seals on two pipe ends, resulting in a permanent, leak-free, precisely compressed seal. When installed correctly, the elastomeric seal of a press joint dramatically reduces the likelihood of leaks compared to threaded systems.
Although initial material costs are higher, many plants that have replaced galvanized carbon steel threaded systems with stainless steel press-to-connect systems have realized long-term cost savings due to reduced energy costs.
Bill Lowar is vice-president of Victaulic Power Division
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