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Reliability / Piping rations that can reconcile these conflicting Pressure counts criteria for more sustainable piping systems. Industry has made great strides in reducing the number of leaks from flanged joints, and some plants have even eliminated them. Because reliable piping system performance, including pumps, valves and other equipment, is critical to plant sustainability both operationally and environmentally, it’s vital to pay careful attention to these flanged joints and the gaskets that keep them from leaking. Before replacing a leaking gasket, make sure you’ve identified the root cause of the leak. A good place to start is to verify that the proper gasket was installed in the first place based on the operating parameters of the system, notably temperature, media and pressure. The first consideration should be temperature because it can eliminate many unsuitable gasket materials from consideration. Next, look at the media’s chemical compatibility with the gasket, including any secondary media to which the gasket might be exposed, such as fluids that are intermittently present during chemical or steam/hot-water flushing. Then consider pressure. Most systems operate at relatively consistent pressure, but it’s important to take into account severe spikes or surges that might occur. However, selecting the right gasket requires an understanding of the application that goes beyond these criteria. Flanged joints rarely leak because chemicals attacked the gasket. Most people understand that gasket materials must be compatible with system media. Likewise, it’s rare to find a piping system that exceeds a gasket’s maximum pressure or temperature ratings. Figure 2. This compressed sheet gasket installed in a flat-faced flange failed because of insufficient compression. flanges for vacuum service (Figure 2). The available stress for these types of flanges varies widely. For example, an 8-in., 150# raised-face flange and a ring gasket having a contact area of 32.2 sq. in., a bolt stress of 60,000 psi and a bolt torque of 200 ft.-lbs. produces 4,800 psi of gasket compression. The same size flange with a flat face, fitted with an 80-sq.-in., full-face gasket under the same bolt stress and torque, results in gasket stress of only 1,813 psi, about 38% of that from the ring gasket with its smaller contact area. As a general rule, therefore, the best choice for raised-face flanges are ring gaskets made of compressed sheet, PTFE, graphite or metal. Use full-face rubber gaskets or gaskets with molded-in sealing rings with greatly reduced contact area for flat-face flanges. Valves and pumps with flat flanges and standard ASME drilling can produce a stress between 1,000 psi and 2,000 psi; glass-lined flanges produce between 2,000 psi and 3,500 psi; flat-faced, non-metallic flanges from 150 psi to 500 psi; and angle-iron flanges around 100 psi to 300 psi. The bolting on some flanges used in vacuum service produces almost no compressive load. Even within these low-load flanges, there’s considerable variation in available load. Not all nonmetallic flanges are flat faced; many have raised ridges to concentrate the load. Some are stub-end designs in which the contact surface is confined to within the bolt circle and the bolted flange slips on behind Technological rescue Compressive load key to performance More often than not, flanged joint gasket failures result from the mechanics of the application, usually from incorrect compressive load. Eliminating such a simple cause requires knowing the type of flange in which the gasket will be installed, including the material and bolting information, to determine the compressive force. This is extremely important because nearly 70% of gasket failures are attributable to insufficient load (Figure 1). This root cause might be uneven bolt loads, but it’s far more common that achieving proper loading was impossible given the flange design and available bolting. Under the same bolting scheme, a flat-faced flange with a full-face gasket, for example, won’t apply the same compressive force to the gasket as a raised-face flange with its smaller compressed area. Flange types that produce low compressive gasket loads include flat-faced cast iron flanges commonly found in valves and pumps, flanges on glass-lined equipment, nonmetallic flanges, rolled angle-iron flanges for ductwork, and Figure 3. This gasket’s raised, molded-in rings allow it to seal with 75% less surface contact. The lower seating stress makes it suitable for nonmetallic flanges. 36 APRIL 2009 www.PLANTSERVICES.com
Reliability / Piping the stub-end. These back-up flanges often are metallic and the stub-end design yields a contact area comparable to that of a raised-face flange. These assemblies can generate significantly higher compressive stresses than flat-faced flanges. Elastomer gaskets might appear to be the obvious solution to low-load flanges. They’re suitable for some flanges types, but compressive loads are typically too high when using glass-lined or stub-end flanges. Moreover, elastomer gaskets aren’t always compatible with system media. PTFE-based gaskets provide chemical compatibility, but require higher loads to seal effectively. The message is that you should calculate the available compressive stress for a given flange when selecting a gasket or troubleshooting a leak. If the available stress falls between More often than not, flanged joint gasket failures derive from the meCHanics of the aPPlication. 600 psi and 1,200 psi, a rubber gasket will work. If the calculated stress is below 600 psi, you might need special gaskets to maintain a consistently tight, leak-free seal. By contrast, a stress greater than 1,200 psi might crush and split a rubber gasket, but still might be too low to produce an effective seal with harder materials, such as rubber-bound fiber sheet gaskets or standard PTFE-type materials. These materials work best at stresses of 3,000 psi or more. Gasketing for low-load flanges For extremely low-load flanges, consider rubber-based gaskets with molded ridges that concentrate the compressive load (Figure 3). These gaskets often seal where standard rubber sheet gaskets leak. They achieve a tight seal more easily because the ridges contact the flanges first. As more compressive load is applied, larger areas of the flange come in contact with the gasket, preventing it from being crushed, even though the initial contact area is limited. These gaskets can be beefed up with metal backing rings that slip behind the flanges to provide more joint strength. The rings permit greater bolt torque that produces a significantly larger pressure capability. These bolted joints can survive pressures that nearly equal the burst pressures of the piping. There are many soft, highly compressible PTFE-based gaskets that collapse to 20% to 40% of their original thickness. This, technically, makes these materials softer than Water power TOPIC Gasket tutorial Figure 4. This gasket reacts with water or oil to generate its own load, eliminating leaks in low-load applications. more resources at www.plantservices.com Joint sealants searCH “Rethinking the purchase of valves and valve repairs” “Joint sealants to the rescue” For more, search www.<strong>Plant</strong><strong>Services</strong>.com using the keywords flange, gasket and load. some rubber gaskets, but they still require minimum compressive stress to seal the leak paths through the gasket body. Some flange types noted above can produce sufficient compression on these gaskets, but others, especially non-metallic flat-faced flanges, might not consistently seal using these gaskets. One possible alternative is an elastomer gasket with a PTFE envelope that wraps around the inside diameter and a portion of the gasket face. The envelope can be affixed to the rubber with an adhesive or bonded to the rubber during vulcanization. Another solution to a low-load situation is a gasket that swells in the contained liquid (Figure 4). These gaskets typically are of the fiber variety, and are less compressible than rubber gaskets. The advantage of a fiber gasket is that it handles low loads and withstands stresses that would crush a rubber gasket. A secondary advantage is the availability of sheet sizes to 150 in. square, making it possible to use a onepiece gasket in very large flanges. Another advantage of fiber-based gasket material with swelling characteristics is its ability to be used with flatfaced flanges when the operating pressure or, in some cases, the test pressure, is too high for a rubber gasket. The higher pressure rating of the fiber gasket allows the joint to function reliably at elevated pressures, while the swelling seals the flat-faced flanges. Compressive load has an enormous effect on the performance of gasketed joints in industrial piping systems. Available bolt loading is a critical factor in determining the type of gaskets to be used. Many types of gasket material and designs are available to meet the challenges posed by various flange configurations. Keeping your gasket supplier involved in the selection and replacement process will help your plant’s piping systems perform reliably, efficiently and sustainably. David Burgess is senior applications engineer for Garlock Sealing Technologies, Palmyra, N.Y. Contact him at dave.burgess@ garlock.com and (315) 597-4811. www.PLANTSERVICES.com APRIL 2009 37
Page 1 and 2: WWW.PLANTSERVICES.COM PERFORMANCE |
Page 3 and 4: COMPRESSEDAIR&GAS Ultra-Filter Sup
Page 5 and 6: TABLE OF CONTENTS APRIL 2009 / VOL.
Page 7 and 8: from the editor paul studebaker, cm
Page 9: PS0901_01_Cover.indd 1 WWW.PLANTSER
Page 12 and 13: up & Running Automation and Power J
Page 14 and 15: UP & RUNNING ABB technology can red
Page 16 and 17: what works Crack Troubleshot Over W
Page 19 and 20: HUMan CAPitaL Tom Moriarty, P.E., C
Page 21 and 22: asset manager david berger, P.Eng.
Page 23 and 24: Technology Toolbox Sheila Kennedy A
Page 25 and 26: Table 1. On tap at www.PlantService
Page 27 and 28: performance / software What organiz
Page 29 and 30: vendor selection committee, consens
Page 31 and 32: Compressor Power Is Not Measured by
Page 33 and 34: Efficiency / Compressors When amps
Page 35: Properly sealed piping systems are
Page 39 and 40: SKILLS / INSTRUMENTS AND CONTROLS A
Page 41 and 42: SkILLS / Instruments and Controls a
Page 43 and 44: For the Pacific Northwest Martin Ka
Page 45 and 46: our group of experts and provide an
Page 47 and 48: product picks Stainless solenoid va
Page 49 and 50: KwikBond_Ad.pdf 12/5/07 11:58:45 AM
Page 51 and 52: Short-term maintenance never sticks

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