Bolt-on jackets can boost valve reliability.
At a Taiwan-based refinery recently, a butterfly valve had "torqued itself to death," according to plant personnel. The failure shut down a sulfur recovery unit for days, and cost thousands of dollars in labor and replacement costs. The root cause of the failure was a poorly designed weld-on jacket that did not provide adequate heat to the bearing housing area and the adjacent process flanges. Sulfur condensed and froze because of the heat loss through those critical areas. Weld-induced distortion in the bearing housing area exacerbated the problem. When the valve actuator received the signal to apply power to move the stem, the disc would not budge. The result was inevitable. The weldments were there because the valve was jacketed with a conventional welded-on steam jacket that had been designed and fabricated by a traditional pipe-fabrication shop.
Over-torquing a frozen valve is just one of many possible failure modes. Casting defects, weld failure and pancaking (concaving) are also common. These occur because fabricators of weld-on valve .jackets often fail to apply good design practices and because there are no standards for this fabrication. Further, most corporate engineering specifications for jacketed valves are old, very broad, and do not specify a design or fabrication standard. Left to their own devices, fabricators typically default to the lowest-cost design. Most leading valve manufacturers have encountered problems or failures after fabrication and modification of the original equipment in an attempt to provide a jacketed valve.
Consider bolt-on jacketing
Control-valve and flow-meter manufacturers have long used bolt-on jacketing because of the sensitivity of the equipment and its expense. For example, no fabricator wants to accept responsibility for welding a jacket on a Fisher Vee-Ball series valve or a Micro Motion coriolis flow meter. But why treat your ball, gate, check, butterfly or plug valve differently? This equipment can be just as critical. The ability to restart the line is only as good as your ability to repair or replace the component or the jacket. Either is easier to do if a bolt-on jacket is employed.
As the name implies, bolt-on jacketing bolts around the valve body and is readily removable. Usually a two-piece aluminum casting made specifically for each type, size and model of process component, it fits snugly for good heat transfer, usually with the help of a thin layer of heat-transfer mastic. An integral cast-in pressure chamber made from carbon steel or stainless steel contains the heating medium. The pressure chamber, designed and tested in accordance with ASME Code, is fabricated with the requested inlet and outlet connection to transfer the heating medium (water, steam, hot oil, hot oil vapor or glycol/water).
Failure-prevention aside, bolt-on jacketing typically cuts valve ownership cost by at least 25% versus a traditional weld-on jacket. It is comparable to partial weld-on jackets for several reasons. First, standard jackets are available for most major valve manufacturers in most types, models, sizes and ratings. Weldon jacketing, by contrast, is always a custom job. Weld-on jackets also cost more, take longer to install and require custom spares. Also important, the bolton jacket is removable and reusable.
A valve is a precision flow-control device. Performance depends on tight tolerances. It is not designed to with-stand the heat input from welding a jacket to its body and flanges.
Furthermore, most process valves are made of castings that are machined to final tolerance, but have areas of porosity or tiny cracks that go undetected when tested. These flaws are exacerbated and can be problematic when external pressure is applied. While original equipment is designed to perform for years, once you strike an arc on it, no matter how carefully, all bets are off. Even the most careful fabrication techniques can cause problems.
While most valves are not designed to be jacketed or modified prior to use, before bolt-on jackets, there was no easy way to bring the jacketing up and around the bonnet flange and the packing-gland area. So most fabricators did not. With bolt-on jacketing it's easy to jacket this area.
In the 30 years since bolt-on jacketing was introduced, the technology has proven its value for thermal maintenance in multiple areas. These include molten sulfur processing and transport, as well as production of phthallic anhydride, caprolactam, acrylic acid, dimethyl teraphthalate, asphalt, resins, hot melt adhesives, Bisphenol-A and many other heated processes.
Properly jacketed valves may represent only a small percentage of total plant cost, but they exert enormous leverage where it matters: throughput and uptime. Without codes or standards for this specialty fabrication, use of the proper bolt-on jacket eliminates the need to alter original equipment. Why risk the potentially damaging effects of welding if you don't have to?
Controls Southeast, Inc.
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|Title Annotation:||Tech Check|
|Publication:||Industrial Maintenance & Plant Operation|
|Date:||May 1, 2004|
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