By Doug Stelling
This is the second article in a series of articles describing On-Stream Leak Repair 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 aspects of bolted-on leak boxes.
Typically, bolted-on leak boxes are used when the leak cannot be completely stopped using an epoxy wrap and a welded-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 shapes and sizes to cover straight pipe, elbows, reducers, tees, flanges and even 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 plant machine shop, it is probably most often designed, fabricated, and installed by a contract leak sealing company such as Leak Repairs or Sealtek. These companies have much experience in design, fabrication, and installation of leak boxes. Whether the refinery or a leak repair company is contracted to build the box, it is still important to understand the basics 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:
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 parts of the leak box must be designed to restrain the pressure forces that would tend to blow the box apart. The bolting flange must be designed to transmit these loads and also limit distortion of the gasket or 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 that there is just the right amount of compression on the gasketing or packing. While the bolt-on type of leak box can be used as a permanent repair, it is typically used as only a temporary repair until a conventional repair can be made to the piping system. This is because the bolting may relax with time and temperature, and the packing may leak with time.
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 for the existing line(s). In addition to the design pressure and temperature, it is important to determine 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 within the leak box with the process fluid or sealant must be considered. The next step is to determine the size 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 other obstructions must be determined. A field check should also be made to determine if there are any installation problems or safety concerns.
The leak box enclosure is usually formed from cylindrical pipe sections or rolled plate; however, sometimes pipe 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 used to clamp one half of the box to the other. This bolting must be designed to restrain the pressure acting on the symmetry plane that would tend to blow the box apart.
For tee-shaped leak boxes, such as when a tee, branch connection, or a valve is enclosed, the leak box is typically designed as a branch connection in accordance with ASME B31.3 (or other applicable Code). The opening in the main leak box should be reinforced, if necessary, in accordance with the applicable Code. The bolting for a tee-shaped box must be appropriately proportioned between 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 weakened area to restrain the longitudinal pressure forces in the line. This feature can sometimes be incorporated into the design of the box, but is often designed separate from the box similar 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 seal the enclosure. In other cases, packing is used as a temporary sealing device and a pumpable sealant is injected into sealant grooves to make the box leak tight. In a few cases, the leak box may even be pumped full with a sealant. However, this is usually less desirable due to the high cost of the sealant involved.
The packing must also be suitable for the contained fluid, as well as the long time pressure and temperature conditions. While the type of packing that is used for valve stems in the given service may be suitable for the leak 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 materials are Buna-N, Ethylene-Propylene Neoprene, Silicone, Viton or Teflon. Molded or extruded packing 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.