Hot Tapping – Advice on Successful Pipeline Tapping
In many cases for production lines and plants it is crucial to keep pipelines online and producing, however, it also becomes necessary to reroute, tie in additional lines or do maintenance on pipelines. This is where hot tapping can provide critical and much-needed support to keep the product flowing in the line while doing work.
In short, this process involves welding a stub to your pipe, installing a valve, opening the valve, drilling through the open valve, retrieving the cut-out piece and closing your valve for future extensions.
Start by identifying the part of the pipeline where the new tie-in will be placed. All coatings must be removed at that location to ensure a perfect weld to the pipeline. If a complete size to size tie-in is being done the whole circumference of the pipe needs to be cleaned.
When a complete size to size tie in is done, a split T-piece is welded over the pipeline. This is also true when doing hot tapping greater than half of the parent pipe diameter. If doing smaller taps, you can weld on a compensation plate and the stub. Always ensure that the stub length is as short as possible seeing that your tapping machine has a limited travel distance.
Once the T piece is welded on it is a good idea to pressure test your welding, since in most cases Xray NDT is not possible. Depending on the client specifications they might request to do dye penetrating tests on the welds. Install the valve on the stub now. For taps that are very long, it is advisable to install a sandwich valve to save some travel distance. Other full-size valves will also work for this application but are usually very large in size.
You can now move to the hot tapping machine. The tapping adapter on your hot tapping machine should match the flange size of your stub that is welded to the pipe. The cutter in the machine will always be one size smaller than the adapter. This will ensure that there is enough clearance to pass through the valve and get most of the shavings out. Install a pilot drill onto the machine that protrudes past the cutter. The pilot drill should have at least one catch wire installed that also protrudes past the cutter teeth by a margin greater than your pipe wall thickness. THIS IS CRITICAL! It will ensure that the catches drill through before the cutter is through and thus you will be able to retrieve the coupon.
Before placing the machine on the valve it is important to measure and record all internals of the operation. When the machine is installed, you will only rely on your measurements for a successful tap. Critical measurements include:
Distance from tip of pilot to tip of cutter
Distance from tip of cutter to base of cutter
Distance from top of flange to the pipe surface that is being cut.
Wall thickness of the pipe.
The tapping machine is now installed on the valve. While the valve is open, feed the machine down to the pipe until it stops on the pipe surface. In this position, you can record your starting measurement for cutting. By taking the pipe diameter and wall thickness into account you can now determine the depth you need to cut into the pipe. Do not under any circumstances cut deeper into the pipe than the base of the cutter. This might cause the cutter to jam into the pipe and the machine will not be able to complete the tap.
The machine can now be switched on and the cutting process started. The machine usually has an automatic feed and must be stopped when you reach your predetermined cutting distance. While the machine is operating you will also hear it moving freely once the cutter is through and the pressure will drop significantly.
Once through you can stop the machine and reverse feed it back past the valve. The main valve can now be closed. The tapping adapter has a relief valve on it. This valve must be opened before removing the machine to release the internal pressure. The machine can now be removed from the valve and the coupon retrieved.
With the valve successfully installed you can now add additional piping to your network without stopping the flow of your pipeline. To do pipeline maintenance, two hot taps and stopples are performed and a bypass line installed. This allows you to maintain or cut out a piece of your line while your processes are never interrupted.
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