In all process industries, valve failures due to improper, welded valve jacketing occur regularly. Their annual costs in lost production, spare parts and maintenance overtime can run into millions of dollars. How can such losses be avoided? It's simple: Avoid welding jackets for valves. Instead, use bolt-on jacketing to effectively provide thermal maintenance for valves. This will avoid jeopardizing valves, and save time and money.
At a Taiwan-based refinery recently, a butterfly valve had "torqued itself to death." Besides shutting down a sulfur recovery unit for days, the failure 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.
As specialists in thermal maintenance solutions for difficult heated processes since 1971, we hear about many incidents like this one every year. And 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 industry standards for governing 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
For many years, control-valve and flow-meter manufacturers have 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? That equipment can be just as critical: If the valve or the jacket fails, the line shuts down. The ability to restart the line is only as good as your ability to repair or replace the component or the jacket. Either option is much easier to accomplish 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, depending on service temperature requirements and customer preferences).
Failure-prevention aside, bolt-on jacketing typically cuts valve ownership cost by at least 25% versus a traditional weld-on jacket with oversize flanges. It is comparable with partial weld-on jackets. There are several reasons for this. First, standard jackets are available for most major valve manufacturers in most types, models, sizes and ratings. Weld-on jacketing, by contrast, is always a custom job requiring the receipt of the valves prior to the jacket being manufactured. Weld-on jackets also typically involve higher costs, longer lead times, and stocking of custom spares. Also important, the bolt-on jacket is removable and reusable.
Understanding valves
It's important to remember that a valve is a precisely made flow-control device. Performance depends on tight tolerances. It is not designed to withstand 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. Inevitably, castings have tiny areas of porosity, inclusions and tiny cracks that go undetected when tested under internal pressure. Those 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 designed to limit heat input and the resulting thermal expansion can result in problems. Most valves are not designed to be jacketed or modified prior to use.
Sometimes you will encounter a non-negotiable specification or existing equipment that precludes bolt-on jacketing. And there are those few applications where welded jacketing is imperative because of the process requirements. For example, 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 didn't. With bolt-on jacketing it's easy to jacket this area. This solution works regardless of whether the body is jacketed with a bolt-on or weld-on jacket.
Thirty years ago, when bolt-on jacketing was new, proven weld-on jacketing was the mainstay technology. But over those years, tens of thousands of bolt-on jackets have gone onto valves and process components of all types. They have proven their value, functionality and reliability for thermal maintenance in 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. While properly jacketed valves may represent only a small percentage of total plant cost, they exert enormous leverage where it matters: in throughput and uptime. Without codes or standards to govern 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?