By Doug Stelling
Occasionally excessive corrosion, erosion, vibration or thermal stress can cause leaks to develop in a piping system. The leaks may occur in straight pipe, elbows, miter bends, tees, or at flanges or valves. In other instances, thinned areas of piping may be found during routine inspections required by API 570, Inspection, Repair, Alteration, and Re-rating of In-Service Piping Systems. In some cases, the leak can be stopped with a fiberglass reinforced epoxy wrap or gasket and pipe clamp until a permanent repair can be made to the piping. However, in many cases the temperature/pressure is too high, the time required for the epoxy wrap to be relied on to contain the leak is considered too long, or the weakening of the piping component is so severe that separation of the pipe is possible. In these cases, a longer term and more reliable repair is required. Depending on whether welding is permitted on the line, there are two basic types of leak containment devices:
This first article deals with the design and installation of welded-on leak containment devices. These are usually less expensive to fabricate and can be designed in many different shapes and sizes to enclose various piping configurations and components. A later article will discuss bolted-on leak containment devices. It should also be noted that in some cases, a combination of a bolted and welded-on leak containment device may be used.
Welded-on leak containment devices can be as simple as a lap patch or a pipe sleeve welded on the outside of the pipe. In other cases, a welded-on leak containment device can become more complex (e.g., if it must be fit over epoxy wraps or other bolted-on devices as added assurance against leakage). This type of welded-on leak containment device will be referred to as a welded-on leak box for the rest of this article. The welded-on box can contain leaks in some very complex piping configurations, such as at tees, reducers, elbows, miter bends, flanges and valves. Typically, the box is custom designed and then fabricated in the shop. The box is then cut along a convenient plane to facilitate field installation, fit-up, and field welding. Figure 1 shows the typical features of a welded-on leak box installed at a branch connection.
The first step is usually to consider whether the pipe is actively leaking and if there are any service restrictions on installing a welded-on leak box. There are several services where welding on the pipe is typically not permitted or may be restricted (e.g., air, oxygen, caustic, acid, amine, chlorine, and ethylene services). Typically, these are the same services in which a welding of a hot tap would not be permitted. The next step is to ensure that the weld to the pipe can be made without the possibility of burn-through during installation. Since many of these problems are similar to those dealing with hot tapping on lines, API RP 2201, Procedures for Welding or Hot Tapping on Equipment in Service, should be referred to in determining restricted services and precautions.
It is important that all design details of the box be in accordance with the applicable piping Code in so far as possible. One of the first design considerations is to determine whether the box must also contain the pressure thrust in the piping system if the pipe were to corrode/crack completely through. If the corrosion is localized, and a complete pipe separation is not a consideration, then the leak box can be designed much like a jacketed pressure vessel (e.g., ASME Code Section VIII, Div. 1, Appendix 9).
The leak box typically has a circular cross-section and as such can be designed as a straight piece of pipe using the straight pipe pressure thickness equations the applicable piping Code. If the leak box is covering a tee-shaped object, then reinforcement of the opening in the run of the leak box should be in accordance with the reinforcement requirements of in the applicable piping Code. Similarly, if the leak box is mitered, then the pressure-thickness design equations for miters in the applicable piping Code should be used.
If the jacket need not be designed for pressure thrust, the end closures on the box can be designed like a jacket vessel closure. Depending upon whether the jacket is fabricated from pipe or plate, a mill tolerance may be included. Although the welded-on box is often only temporary, a corrosion allowance is typically applied. It should be noted that the fluid in the box will be similar to a stagnant dead leg and could be subject to corrosion itself. Since it is difficult to radiograph the box after it is installed, a 0.6 to 0.65 weld joint efficiency is typically used depending on whether a backing strip will be used during welding of the long seam welds.
If separation of the pipe is possible, then the leak box is usually designed for the pressure thrust. In This case the jacket, and especially the end closures, must be designed to transmit the load across the weakened pipe section. In cases when the pressure is relatively low and the box size is small, the end closures can be flat plates and be designed as flanges (e.g., ASME Code Section VIII, Div. 1, Appendix 2). Figure 1 shows a thick end closure on the branch and a relatively thin end closure on the run of the tee. This is because the corrosion has severely affected the strength of the branch and the endplate must be designed to withstand the pressure thrust should the branch separate from the run pipe. If the pressure is high, either conical or formed reducers are typically preferred over flat end plates since flat plates can become very thick and the welds on the pipe can be very large. Pipe caps can sometimes be used as end closures, but special consideration must be given to reinforcing the opening in the pipe cap. In some designs, external tie rods can be used to carry the pressure thrust over the weakened piping section in a manner similar to that used in a universal expansion joint. In these cases, the end closures can again be designed as end closures of a jacketed vessel.
All fabrication details should be in accordance with the applicable piping Code. The orientation of the split in the box should be made to facilitate the field installation and welding. Discussion with maintenance staff who are installing and welding the leak box should be conducted before the design is finalized. Typically, the long seam welds are made with backing strips and welded first. Care must be used when installing the welds on each end of the box, as these are typically large fillet welds and burn-through is possible. API 570 Appendix D, API Publication 2009, Safe Welding, Cutting and other Hot Work Practices in Refineries, Gas Plants and Petrochemical Plants, and API RP 2201 contain many recommendations regarding welding on in-service piping.
After the box is tacked into position, fit-up and weld root gaps should be checked. Weld inspection usually consists of in-process MT/PT and RT of any major welds such as miter joint welds that can be done in the shop before the box is installed. In general all piping system components should be leak tested before being put into service. However, since a leak box is more akin to a hot tap nozzle, the leak box need only be tested to a maximum of 110% of the line operating pressure, unless calculations are made and show that buckling the inside pipe is not a concern. While a hydrotest is preferred, in many cases the internal pipe temperature is at or near the boiling point of water, and/or complete drainage of the water after the test is a problem. In these cases, a pneumatic leak test can be considered. Pneumatic tests are generally avoided due to their increased risk when compared to hydrotests. However, in the case of a leak box, the box volume is usually small, the test pressure is not as high as in a normal hydrotest, and therefore such a test is often done.
The above procedure outlines the typical considerations in the design, fabrication, inspection and testing of welded-on leak boxes. In addition, the following considerations may have to be addressed when installing a welded-on leak box: