By Doug Stelling
In the May 1999 issue of the Carmagen Report, pipe hot tapping was discussed. This article discusses hot tapping pressure vessels. Hot taps can also be made into atmospheric storage tanks, and some aspects of this was covered in the October 1996 Carmagen Report.
While is preferred to install nozzles during a turnaround, installing a nozzle with equipment in operation is sometimes advantageous, especially if it averts a costly shut down. Hot tapping is a process by which a nozzle can be added to equipment that is in service. In addition to just installing a nozzle to allow the flow of a fluid into or out of a vessel, hot taps can also be used to install pipe distributors, instruments (e.g., pressure, temperature, flow), corrosion probes, or sampling probes. Even TV cameras can be inserted into a vessel using hot tapped nozzles to determine the condition of vessel internals.
All hot taps should be designed, fabricated, inspected and tested in accordance with the applicable Codes and Standards. The applicable standard for pressure vessels is usually the National Board Inspection Code (NBIC) or API Standard 510, “Inspection, Repair, Alteration and Re-rating of Pressure Vessels.” These Codes or Standards require that the design of the hot tap should also be in accordance with the original code of construction. In most cases in the US, this is typically the ASME Code Section VIII Division 1. In addition, API Recommended Practice RP 2201, “Procedures for Welding or Hot Tapping on Equipment in Service,” should also be followed.
The design of a vessel hot tap nozzle is similar to that of a pipe hot tap. In most cases, the hot tap nozzle will be considered as an Alteration in accordance with the NBIC or API Standard 510. This requires that the design, all materials, welding procedures, and inspection be in accordance with the original Code of construction. Typically, all aspects of the hot tap should be reviewed with the Authorized Inspector before any physical welding work is done on the vessel.
The nozzle design should meet the ASME Code Par.UG-37 for pressure reinforcement, as well as the requirements of Par.UG-45 for nozzle neck thickness. In addition to designing the nozzle for pressure, the design should be checked for weight and thermal loads from the attached piping. If the hot tap is into the side of the vessel, support of the hot tap machine from the nozzle should also be considered. If a pipe distributor is to be installed in the hot tap nozzle, then the nozzle should be designed for the loads from the pipe distributor.
In many cases, special considerations must be addressed when a vessel is hot tapped (e.g., when the vessel is internally clad, weld overlaid, or refractory lined). However, in some cases, these situations may preclude hot tapping. However, successful hot taps have been made in such cases if the repair of the cladding, weld overlay, or refractory lining can wait until the next turnaround. Special welding procedures may be required if the vessel is very thick and was post weld heat-treated, if the vessel was impact tested, or if the vessel is of unknown toughness. Depending on the service (e.g., wet H2S), special materials, inspection methods, and/or welding procedures must be used.
Since welding on the hot tap nozzle results in localized heating of the shell, consideration must be given to dissipating the heat to avoid combustion of the fluid inside the vessel. If the vessel is thin, burn through of the shell may have to be considered.
The detail shown in the Figure is for a pipe distributor that was installed in a hot tapped nozzle in a distillation tower. The pipe distributor was installed while the vessel was in service to alleviate a plugging problem and avoid a unit shutdown. First, the nozzle was carefully located to meet the process objectives and avoid interference with any internals. The shell of the vessel was visually inspected to verify the location of all weld seams and any attachments that might interfere with the design. Weld seams in the vicinity of the hot tap were MT inspected to check for cracks. The shell was checked ultrasonically to determine actual thickness and the presence of any laminations or subsurface defects that could affect the strength of the hot tap nozzle.
The nozzle was then welded to the shell and pneumatically pressure tested. A pneumatic test was used in this case because the vessel temperature was over the boiling point of water and most fluids that could be used for a hydrostatic test. The reinforcing pad was then installed, MT inspected, and pneumatically pressure tested. The hot tap valve was then installed and the complete assembly pressure tested. The shell opening was cut using a hot tap machine, the hot tap cutter retracted, and the valve closed. The pipe distributor was installed using a larger hot tap machine in a fashion similar to inserting a stopple into a pipe. A hot tap cutter holder was modified to provide a flow passage and welded to the distributor pipe. The pipe distributor was mounted to the hot tap machine inside of an extra long stopple housing, which was fastened to the hot tap valve. The pipe distributor was then pushed in and locked into place using extra long Thread-O-Ring plugs. The plugs were installed using a hot tap machine. The hot tap machine was then backed out and the hot tap valve closed. The piping to the new distributor was then connected and flows through the distributor established.
Some of the concerns that were addressed were the design of the pipe distributor nozzle for the weight of very large hot tap machines, as well as static deflection of the assembly while the pipe distributor was being inserted. To ensure that the design would work, a mock-up of the whole set-up was built and tested in the shop before installing it on the actual vessel. The job went well and two pipe distributors were successfully installed.