By Bill Lowar and Doug Dole, Victaulic
Common piping sealing problems in power plants can lead to increased safety risk, heightened operating costs and general efficiency issues. These issues often arise as a result of ongoing plant operations that cause joints to leak and otherwise degrade over time. This article will identify one general issue that arises in seals throughout plant piping systems: gasket leakage in flanged pipe joints. In addition, two specific but common problems will be addressed: safety issues associated with traditional expansion joints used on pulverized coal boiler feed lines and cost issues associated with threaded compressed air/instrument air systems. A general class of fixes will be detailed for each of these problems.
General Sealing Inefficiencies
Depending on the pipe joining system, the connective device used to maintain the seal can increase installation and maintenance time. A flanged pipe joint is a good example of this. 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, can stretch 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. Depending on the location and service of the piping system, subsequent leaks can be hazardous. For example, when flanges are used on pulverized coal boiler feed lines, a leak can cause a fire hazard due to the highly combustible nature of pulverized coal.
In addition to potential safety issues, flanged unions also increase maintenance requirements. To prevent or stop flange leakage, routine bolt and nut tightening is required. If this maintenance is not performed on a regular basis, the system is at greater risk for 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 for these common problems is to use couplings in place of flanges. The mechanical grooved joint is comprised of four elements: grooved or shouldered 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. On systems where maximum abrasion resistance is required, a shoulder that contains a groove may be welded to the end of the pipe. A gasket is then stretched over the two pipe ends/shoulders creating an initial seal. The key sections of the coupling housings engage the groove on the pipe ends. The bolts and nuts are tightened with a socket wrench or impact wrench, pulling the housings together—metal to metal—compressing the gasket by a controlled amount. In the installed state, the coupling housings encase the gasket and engage the groove around the pipe’s circumference to create a leak-tight seal in a self-restrained pipe joint.
|Self-restrained expansion joint couplings used in combination with shouldered couplings. Designed for pulverized coal feed lines, the expansion joint coupling provides expansion/contraction in boiler piping . All photos courtesy Victaulic.|
Couplings—which can be installed on balance-of-plant piping applications including water, air and slurry services—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 compression from the outside of the pipe joint. While the coupling’s bolts and nuts hold the housings together, the coupling itself is what holds the pipe together. Nuts and bolts of a coupling do not require regular maintenance and have been known to last the life of the system.
|Coupling designed especially for flue gas desulfurization units.|
Flanged joints have also proved challenging in another coal-fired plant application: flue gas desulfurization (FGD). Within a wet FGD absorber, slurry is dispensed through ceramic nozzles connected to header pipes. Traditionally, these nozzles are connected to the pipe using flanged joints; however, the ceramic material is fragile and over-tightening the bolts point loads the brittle ceramic nozzle flange, which can lead to cracks, increasing installation and maintenance time as well as costs. Installing flanged-end nozzles requires certain torque values to be met, as well as a star-pattern tightening sequence. When these instructions are not followed and a bolt is overloaded, it can crack the nozzle. If left in place, a cracked nozzle can result in leaks, uneven spray patterns and unstable joints when the system is pressurized. Not surprisingly, this can cause sub-prime emission absorption. As much as 10 percent flanged ceramic nozzle breakage is an accepted jobsite norm. At a cost of several hundred dollars per nozzle and numbering, potentially, thousands of nozzles per installation, this is a costly and time-consuming problem.
To combat the difficulties associated with flanged nozzle connections, an alternative joining technology is couplings designed specifically for FGD units. These feature one-bolt installation compared to four bolts of a typical flanged connection. Manufactured from material similar to the internal absorber piping, the coupling creates a cushioned joint, which reduces the likelihood of nozzle fractures or cracks. Flanged connections permit only 90-degree indexing of the nozzle based on bolt-hole alignment. Couplings, on the other hand, allow for 360-degree indexing, permitting nozzle position adjustments based on actual spray patterns to eliminate slurry impingement on the absorber wall. During system maintenance and cleaning, the coupling’s single bolt can be removed, even if encrusted in limestone. These features can make FGD unit installation and maintenance faster, easier and less costly.
Leaks caused by deficient seals can carry far worse problems than increased maintenance downtime; they can create safety issues as well. On a pulverized coal feed line, for example, a leak can create a fire hazard. However, it’s not always a leak that creates the problem. Sometimes the seal itself is cause for concern.
Piping around coal boilers is installed at ambient temperatures. Once the boilers are ignited, the temperature increases, which causes the boiler and piping systems to expand. Pulverized coal feed lines accommodate this expansion through expansion joints such as modified sleeve couplings. Standard sleeve couplings are typically not self-restrained joints, meaning they don’t hold the pipe ends together. A seal is created, but the pipe can slip in and out. To use this type of coupling, brackets are often welded to the pipe to keep it together.
Furthermore, modified sleeve couplings that have been used for this application have bolt tips that project directly into the pneumatically conveyed coal stream and fit within windows cut directly into the pipe. These protrusions can result in heavy coal buildup within the pipe, erosion and other damage to the piping and burner nozzles. This buildup, or “roping,” can cause fires, not to mention reduced flow that prevents the boiler from operating at peak efficiency.
|A press-to-connect piping system can reduce the likelihood of leaks due to the elastomeric seal.|
To address fire concerns associated with traditional and modified sleeve couplings, plant operators should consider alternative joining methods such as self-restrained expansion joint couplings. This generation of expansion joint couplings offers space savings, longer service life and a connection that is often safer than modified sleeve couplings. Operators should seek couplings that do not reduce the pipe wall thickness and do not have parts that project into the coal stream. Couplings that accommodate deflection and rotation as well as expansion and contraction will further reduce the forces transmitted to the furnace connections and pipe supports. The space savings achieved with self-restrained expansion joint couplings allows for more compact pipe routing in toggle sections of coal feed lines because the pipelines can run closer to adjacent lines and equipment.
Not every joint on a pulverized coal feed line requires an expansion joint. Where expansion joint couplings are not installed, the pipe is typically joined using flanges or shouldered couplings. The latter method is usually preferred because flanges—a rigid pipe-joining method—are not ideal for environments with a lot of thermal movement. Expansion and contraction can stress the flange and piping, which can compromise the gasket over time. When this occurs, the joint is at risk for a leak. In a coal feed line this is a major fire hazard.
Shouldered couplings join pipe by connecting rings welded on to each pipe end. These rings maintain the full pipe wall thickness, while the coupling used on the rings provides additional flexibility for the piping system by accommodating deflection, expansion and contraction. When properly installed, the coupling creates a sealed joint that does not require routine tightening throughout the life of the piping system, preventing leaks.
One of the latest technologies to advance the shouldered joining method in this application is gaskets designed for pneumatically conveyed solids and abrasives. The gasket design creates a seal that is flush with the pipe ends, minimizing entrapment of pulverized coal in the coupling’s gasket well. Particles are prevented from entering the gasket wells under all deflection and expansion/contraction conditions, sustaining the coupling’s flexibility capabilities without damaging the gasket.
One notorious sealing issue in a plant of any kind is leaks in compressed air/instrument air lines. Leaks are a problem because the cost of lost air can be significant. 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 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 compressed air/instrument air systems is threading, but 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 specialized 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. As a result, fixing one leak may lead to a new one.
One solution is replacing threaded compressed air/instrument air systems with a press-to-connect system. These systems allow plain-end pipe 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 reduces the likelihood of leaks compared to threaded systems. In fact, in third-party accelerated aging and thermal cycling performance testing of the Victaulic Pressfit system conducted by Southwest Research Institute, the initial test plan had to be altered because virtually every threaded pipe joint in the flow loop developed leaks at one point in the testing. The researchers also reported that, as a rule, the larger the pipe diameter, the more difficult the task of obtaining a satisfactory thread seal.
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. Other benefits are also suitable for power plants: installation that is up to five times faster than other joining systems and safer than welding, installation with hand-held pressing tools that do not require trained labor, and reduced total installed costs.
When selecting a press-to-connect system for compressed air/instrument air lines, look for a stainless steel system with nitrile O-rings. These are designed to resist oil vapors that may be present in compressor fluids. 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 larger than two inches, it may be worthwhile to compare press-to-connect with grooved systems in terms of installation time, costs and future maintenance needs.
Sealing issues need not be a challenge in power plants. If regular maintenance is required or leaks are commonplace, consider fixing the problem with one of the solutions outlined above. Not only could this reduce expenses, it could increase safety and reduce maintenance downtime. Most importantly, fixing seal problems will help the plant operate at peak efficiency.
Authors: Bill Lowar is the industrial division manager and Doug Dole is the research manager at Victaulic, a manufacturer of mechanical pipe joining systems. Lowar has been with the company for 35 years and Dole has been with the company for 33 years and holds more than 90 U.S. patents. Based in Easton, Penn., Victaulic develops products for industrial, commercial and institutional piping system applications. For more information, visit www.victaulic.com.