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From Same Source: Hot Tapping & Plugging or Pipe Freeze
Since TDW Services has its own pipe-freeze capability, customers have a choice when it comes to stopping flow in a line and taking it out of service. They can ask for either the traditional hot tapping and plugging service, or TDW Services' pipe-freeze capability, Freeze Service Inc.
In many instances, customers will have this choice. This is a real boon to nuclear power plant operators because many of them prefer the pipe-freeze technique to stop flow rather than welding on a fitting, then cutting holes in pipe to insert a plugging head. There was no choice in this particular job, however. The pipe was vertical, there was not enough room to apply the nitrogen freeze jackets, and there was flow in the pipe—all handicaps to freezing the line.
Not to worry, however, because TDW Services reverted to the traditional hot tapping and plugging method.
A 10-inch common header provided cooling water to a number of heat exchangers and other process operations in this Midwestern nuclear power plant. The carbon steel, standard wall pipe operated at 90 psi at 60-70°F. One of the heat exchangers had to be taken out of service for maintenance. The block valve between the header and heat exchanger did not work properly, and the choice was either to isolate the exchanger in some manner or shut down the entire header system. The latter, and its implications, was certainly not the preferred choice. Only 9.5 inches of clearance existed on the 10-inch vertical pipe between the header and the faulty valve—too short to install a freeze jacket. A freeze plug normally requires 2.5 pipe diameters in length to establish a freeze boundary. It was even too short to install a 10-inch tapping fitting for normal size-on-size tapping and plugging.
An 8 x 10-inch SHORTSTOPP® fitting would work, however. SHORTSTOPP® II Class 150 fittings meet ASME B31.1 power plant codes without any additional reinforcement. The 8-inch fitting would permit tapping an 8-inch hole in the pipe, then inserting a 10-inch folding plugging head, which would be opened to plug the line.
Once the SHORTSTOPP® Fitting was welded in place, the job went like clockwork. A TDW Model 660 hydraulic-powered tapping machine was installed on the line and the tap made. The recovered tapping coupon confirmed the wall thickness and the appropriate sealing element was selected. The SHORTSTOPP® II Folding Plugging Machine was then installed on the fitting. The plugging head was inserted into the line, opened, and a complete seal obtained, stopping all flow to the heat exchanger.
All of this was done in one day. The heat exchanger could then be taken out of service under controlled conditions for the necessary maintenance. The SHORTSTOPP® Plugging seal remained in place three days while work on the heat exchanger was performed. When the heat exchanger was ready for return to service, the plugging head was retracted, returning flow to the exchanger. A SHORTSTOPP® Completion plug was installed in the fitting, permitting removal of the completion machine, tapping valve, and the installation of a blind flange on the fitting, completing the job. Reentry into the line through this fitting at a future date, if necessary, may be possible by removing the completion plug.
Hot tapping and plugging, and the pipe-freeze technique are both viable options to be considered when performing plant maintenance. In some applications, such as this, one method must be chosen over the other. In others, either would be viable but one may be more suitable because of site conditions and price considerations.
Whether you want to plug a pipe system or freeze it, TDW Services, Inc., and Freeze Service, Inc. want to be your full service provider. Now that we have acquired Freeze Service, Inc., which operates as a division of TDW Services, we offer a full range of plugging and freeze services. Hot tapping and plugging goes hand-in-hand with the pipe-freeze technique. Where one may not work, or is not acceptable to management, the other will work.
For more information about TDW Services and our line stopping capabilities, contact us at 800-828-1988 or 1-918-447-5100.
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