Piping for power generation

Piping systems are an important part of power plant construction. They have a major influence on how efficiently and cost effectively a plant operates
Whatever the method of power generation – nuclear, natural gas, hydroelectric, or coal fired – multiple types of piping systems will need to be installed for essential services. From the small-diameter stainless steel pipes needed for instrument air lines, through flue-gas desulphurisation and coal-handling systems, to the large-diameter penstock lines required for hydroelectric plants has a vital role to play. Many of these systems are critical: when they are down, the entire plant could cease to operate, and a plant not generating power is not making money.With so much at stake, 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.
Traditionally pipes required for power plant services have been installed using welded and flanged joints in the medium- to large-diameter range, with threading used for small-diameter pipe connections. Yet, these methods are not ideal. Each presents risks and drawbacks for engineering consultants, contractors and installers, including health and safety issues and concerns about cost, susceptibility to adverse weather conditions, space constraints, maintenance requirements and the lengthy installation process. Alternative technologies such as grooved mechanical pipe joining can overcome many of these issues.
Mechanical joiningA 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 union at every joint, allowing for ease in future system access and maintenance, and reducing costly plant downtime.
Couplings fall into two categories: flexible and rigid. Both provide the security of full circumferential engagement of the coupling into the groove for high pressure and end load performance.
Rigidity is achieved with rigid couplings. The unique angled-pad design provides positive clamping of the pipe to resist torsional and flexural loads. Flexible couplings allow controlled angular, linear and rotational movement at each joint to accommodate thermal expansion, contraction and deflection, offering a range of advantages when designing piping systems.
Welding concernsWelding is a time consuming. 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. There are other aspects of welding that can lengthen the construction schedule. For instance, welding on the side of a mountain or on rough terrain as may be the case in some hydroelectric projects is tricky. The weather can also present challenges. If it is dry and the risk of forest fires is high, welding activities may be prohibited or limited. If it is raining or cold, welding is difficult, because the pipes need to be covered and/or preheated first. On completion of the weld, X-rays may be required to ensure a sound joint. An average of 5-6 per cent of welded joints requires rework according to industry standards, adding time and material costs to construction.
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, and the physical impact on the individual welder is significant. Welding exposes the worker to hazardous fumes and particulate matter, as well as potential burns, eye damage and the risk of fire or explosion. 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 or underground also requires fume and smoke extraction equipment.
Non-welded benefitsJoining 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 bolts and nuts tightened with standard hand tools or impact wrenches. The coupling requires only two bolts to secure the joint and following installation the joint can be inspected visually. Metal-to-metal bolt-pad contact confirms that the coupling has been properly installed and secured into place; no X-ray is needed.
No flame is required to install a mechanical coupling, eliminating the safety concerns associated with welding. Mechanical couplings can be installed in any weather condition, from downpours to extreme cold and dry climates, preventing weather delays and keeping the construction on schedule.
Until recently, joining methods for larger size piping systems required multiple housings. Now mechanical couplings are available with a two-piece housing in sizes up to 1,525 mm, making them ideal for the fast, reliable installation of penstock lines in hydroelectric plants for example. A typical large-diameter joint that requires several hours to weld can be installed in less than an hour with this method. These couplings have a wedge-shaped groove that delivers pressure ratings of up to 2,500 kPa and are also offered in a rigid or flexible style. The rigid style forms a completely rigid pipe joint, whereas the flexible style allows for some expansion, contraction and deflection within the piping system.
Flanging shortcomingsFlanges 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. They require enough clearance around the pipe to be able to secure each bolt, making use of flanges in confined spaces difficult. Flanges can also result in maintenance challenges: after a valve or other piece of equipment has been removed, it is difficult to squeeze back in between the flanges. But maintaining reliable, leak-free performance of a flanged joint can be the biggest issue.
Non-flanged benefitsA 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, air, slurry and bearing lube oil feed 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, decreasing maintenance downtime, because they do not require regular retightening. 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. While the bolts and nuts of the coupling hold the housings together, the coupling itself is what holds the pipe together. Over the life of the system, the nuts and bolts of a coupling do not require regular maintenance and can last the life of the system.
Threaded joint leakageOne of the most notorious issues in a plant of any kind is leaks in compressed air/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. Sometimes additional compressors are needed to compensate, further increasing energy costs. 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 number of factors can cause leaks, and they can occur at any point in the compressed air/instrument air system.
A widely accepted joining method for small-diameter compressed air/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.
Unlike leaks in water lines, leaks in compressed air/instrument air lines can go undetected because they are not visible to the naked eye. The first step in solving the problem of leaks is to find them. This is usually done through sound, feel, soapy water or by using specialised leak detection equipment. In a threaded system, the leak is usually “fixed” by tightening the joint. The problem with this is that tightening one end of the threaded joint ultimately loosens an adjacent joint, so fixing one leak may lead to a new one.
Press-to-connect solutionsOne solution to this problem is replacing threaded compressed air/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 galvanised carbon steel threaded systems with stainless steel press-to-connect systems have realised long-term cost savings due to reduced energy costs. The other benefits of the system are also ideal for power plants: installation that is up to five times faster than other joining systems and safer than welding, simple installation with hand-held pressing tools that do not require highly trained labour and reduced total installed costs.
When selecting a press-to-connect system for compressed air/instrument air lines, a stainless steel system with nitrile or HNBR O-rings, which are designed to resist oil vapours that may be present in compressor fluids, is recommended. Another factor of press-to-connect systems that can simplify integration with larger systems is to ensure the system is available in IPS pipe sizes. For systems smaller than 50 mm, press-to-connect may offer advantages over traditional systems in terms of installation time, costs and future maintenance needs.
ConclusionOwners, engineers and contractors could do well to consider alternatives to welding, flanging and threading for joining power plant piping systems. Grooved mechanical pipe-joining and press-to-connect methods bring a host of practical and economic advantages. They make for safety, ease and speed of installation and maintenance but more importantly deliver long-term reliability and efficiency.
Authored by—Bill Lowar, Vice President – Power Division, Victaulic
 

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