10/19/2010
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Home > News > Engineering Art Articl icles es > On-Stream On-Stream Lea k R epair Techniques - Part 2
On-Stream Leak Repair Repair Techniques Techniques - Part 2 By Doug Stelling This is is the second a rtic rticle le in a series of articles articles de scribi scribing ng On-Stream Leak Repa ir Techniques Techniques (Part 1-Aug. 1997). In the previous article, two types of on-stream leak repair techniques were introduced. The first type was a welded-on leak containment device or leak box. The second type introduced was the bolted-on leak containment device or leak box. This article discusses the various design aspe cts of bolted-on bolted-on leak boxes. Typically Typic ally,, bolted-on bolted-on leak boxes are use d w hen the leak cannot be completely completely stopped using an e poxy wrap and a w elded-on leak box cannot be used. Welding on the line may be prohibited when the fluid contained in the line may combust or react if heated, the line is very thin and burn through is a concern, or welding on the line would require stress relieving. While these factors may be a concern in many instances, the bolted-on leak box is not a panacea. The bolted-on leak box is usually more costly than a welded-on box and may not work in some cases. Bolted-on leak boxes can be made in many Bolted-on many shape s and sizes to cov cover er straight pipe, elbows, reducers, tees, flanges flanges and e ven valves. Various types of standard leak boxes can be purchased from commercial vendors such as IPSCO or PLIDCO. However, in many cases, the piping is of nonstandard configuration and a tailor made box is required. While a bolted-on leak box can be made by a refinery/petrochemical refinery/petrochemical plant machine machine s hop, it is probably most most o ften des igned, fabricated, and insta lled by a contract leak sealing company such as Leak Repairs or Sealtek. These companies have much experience in design, fabrication, and installation of leak boxe s. Whe ther the refinery or a leak repa ir company company is contracted to build the b ox, it it is still im important portant to understand the bas ic ics s of the design.
Figure 1 shows the typical design features of a bolt-on leak box. This type of leak box typically uses bolts to: Restrain the internal pressure Restrain pressure forc force e acti acting ng on the component parts o f the leak box, Transmi Tr ansmitt forces forces across the w eakened area of the piping, and Compress a gasket or packing material to make a leak tight enclosure. The bolts and the bolting flanges must sometimes be quite large to restrain the pressure force in the piping. Basically, the pressure acting on the component component pa rts of the leak box must must be de signed to restrain the pressure forces that w ould tend to blow the box apart. The bolting bolting flange must be de signed to transmit these loads and a lso limit limit distortion distortion of the gas ket or packing packing and make the leak box leak tight. In many cases, the leak box has to be precision machined so that it fits snugly to the pipe and so tha t there is just the ri right ght amount of compression compression on the gasketing or packing. packing. While the bolt-on type of leak box can be us ed as a permanen t repa ir, it is typically typically use d as o nly a temporary repair until a conventional repa ir can can be made to the piping system. This This is becau se th e bolting may relax with time and tempera ture, and the packing may may leak w ith time. time.
Basic Design Considerations The design pressure, design temperature, pipe material, pipe thickness, corrosion allowance, fluid service, and any restrictions associated with the service should be determined. This usually entails checking the piping drawings, piping line list, and piping materials specifications specifications for the existing line(s). In addition to the des ign press ure and temperature , it is is importan importan t to det ermi ermine ne any piping forces that must be transmitted through or across the leak box. The piping forces may be due to pressure, weight, or thermal expansion. In addition to the weight of the line and its contents, the weight of the leak box itself and the filling of the voids voi ds w ithi ithin n the leak box with the process fluid fluid or sealant must be consi considered. dered. The next step is to dete rm rmine ine the s ize and type of box that must be used to completely enclose the affected area. Accurate measurements must be taken of the piping, including the exact diameters where the leak box will close on the pipe. The location of all weld seams, small connections, pipe supports, or
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10/19/2010
On-Stream Leak Repair Techniques - P…
other obstructions must be determined. A field check should also be made to determine if there are any installation problems or safety concerns.
Bolted-on Leak Box Design Aspects The leak bo x enclosure is us ually formed from cylindrical pipe se ctions or rolled plate ; howe ver, sometimes p ipe caps or conical reducers are used. Each piece of the leak box is then designed in accordance with ASME B31.3 (or other applicable Code). The box is typically split along a symmetry plane and bolting bars or flanges are installed along the cut edges and ends. Bolts are then use d to clamp one ha lf of the bo x to the other. This bolting must be designed to restrain the pressure a cting on the symmetry plane that w ould tend to blow the box apa rt. For tee-shaped leak bo xes, such as w hen a tee, branch connection, or a valve is e nclosed, the leak box is typically designed a s a branch connection in accordance w ith ASME B31.3 (or othe r applicable Code ). The ope ning in the main leak box shou ld be reinforced, if necessary, in accordance with the applicable Code. The bolting for a tee-shaped box must be appropriately proportioned be twee n the run and the stem of the tee . The layout of the bolting must also be properly proportioned to contain the pressure forces that must be carried around the opening. If the corrosion of the underlying pipe is very severe, such that separation of the piping is possible, it is usually necessary to design a leak box to clamp the pipe outside of the wea kened a rea to restrain the longitudinal pressure forces in the line. This feature can sometimes be incorporated into the des ign of the box, but is often des igned sepa rate from the box s imilar to the way tie rods span the flexible elements in a universal expansion joint. The bolting must also be designed with sufficient reserve to maintain a sufficient pressure on the packing to maintain a leak tight seal. The selection of the packing material, the size and the design of the packing grooves require experience. In some cases, packing alone is used to se al the e nclosure. In other cases, packing is use d as a temporary sealing device and a pumpable sealant is injected into sealant grooves to make the box leak tight. In a few case s, the leak box may even be pumped full with a sealant. Howe ver, this is usua lly less des irable due to the high cost of the sea lant involved.
Packing Selection The packing must also be suitable for the contained fluid, as well as the long time pressure and temperature conditions. While the type o f packing tha t is used for valve ste ms in the given service may be s uitable for the lea k box, packing compatibility with the fluid to be sealed should be checked with a packing manufacturer. Some types of packing are extruded square or rectangular in shape while other types are braided. Typical molded or extruded packing mate rials are Buna-N, Ethylene-Propylene Ne oprene , Silicone , Viton or Teflon. Molded or extruded p acking usually has better leak sealing capabilities. Braided packing materials such as Grafoil and Kevlar can be used at higher temperatures; however, because they are braided, they require higher compression forces to maintain a leak tight joint.
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