Natural gas transmission and distribution companies need to make new connections to pipelines many times a year to expand or modify their existing system. Historically, this has necessitated shutting down a portion of the system and purging the gas to the atmosphere to ensure a safe connection. This procedure, referred to as a shutdown interconnect, results in methane emissions, loss of product and sales, occasionally customer inconvenience, and costs associated with evacuating the existing piping system. Hot tapping is an alternative procedure that makes a new pipeline connection while the pipeline remains in service, flowing natural gas under pressure.
Hot tapping is an alternative technique that allows the connection to be made without shutting down the system and venting gas to the atmosphere. Hot tapping is also referred to as line tapping, pressure tapping, pressure cutting, and side cutting. The process involves attaching branch connections and cutting holes into the operating pipeline without interruption of gas flow, and with no release or loss of product. Hot taps permit new tie-ins to existing systems, the insertion of devices into the flow stream, permanent or temporary bypasses, and is the preparatory stage for line plugging with inflatable, temporary balloon plugs (stoppels).The hot tap procedure involves attaching a branch connection and valve on the outside of an operating pipeline, and then cutting out the pipe-line wall within the branch and removing the wall. Section through the valve. Hot tapping or pressure tapping, is the method of making a connection to existing piping or pressure vessels without the interruption of emptying that section of pipe or vessel. This means that a pipe or tank can continue to be in operation whilst maintenance or modifications are being done to it. The process is also used to drain off pressurised casing fluids. Hot tapping is also the first procedure in line stopping, where a hole saw is used to make an opening in the pipe, so a line plugging head can be inserted.
Situations in which welding operations are prohibited on equipment which contains:
• Mixtures of gases or vapours within their flammable range or which may become flammable as a result of heat input in welding operations.
• Substances which may undergo reaction or decomposition leading to a dangerous increase in pressure, explosion or attack on metal. In this context, attention is drawn to the possibility that under certain combinations of concentration, temperature and pressure, acetylene, ethylene and other unsaturated hydrocarbons may decompose explosively, initiated by a welding hot spot.
• Oxygen-enriched atmospheres in the presence of hydrocarbons which may be present either in the atmosphere or deposited on the inside surface of the equipment or pipe.
• Compressed air in the presence of hydrocarbons which may be present either in the air or deposited on the inside surfaces of the equipment or pipe.
• Gaseous mixtures in which the partial pressure of hydrogen exceeds 700 kPa gauge, except where evidence from tests has demonstrated that hot-tapping can be done safely.
Hot tapping equipment is described below:
• Drilling machine. The drilling machine generally consists of a mechanically driven telescoping boring bar that controls a cutting tool. The cutting tool is used to bore a pilot hole into the pipeline wall in order to center a hole saw that cuts out the “coupon,” or curved section of pipeline wall.
• Fitting. Connection to the existing pipe is made within a fitting, which can be a simple welded nipple for small (e.g., one inch) connection to a larger pipeline, or a full-encirclement split-sleeve tee for extra support when the branch is the same size as the parent pipeline. The tee wraps completely around the pipeline, and when welded, provides mechanical reinforcement of the branch and carrier pipe.
• Valve.The valve on a hot tap connection can be either a block valve or a control valve for the new connection, and must allow the coupon (section of pipeline wall cut out by the drilling machine) to be removed after the cutting operation. Suitable valves include a ball or gate valve, but not a plug or butterfly valve.
Schematic of Hot Tapping Machine with Profile
The basic steps to perform a hot tap are:
1. Connect the fitting on the existing pipeline by welding (steel), bolting (cast iron), or bonding (plastic) and install the valve.
2. Install the hot tap machine through the permanent valve.
3. Perform the hot tap by cutting the coupon from the pipeline through the open valve. A special device retains the “coupon” for removal after the hot tap operation. Withdraw the coupon through the valve and close the valve.
4. Remove the tapping machine and add the branch pipeline. Purge oxygen, open the valve, and the new connection is put into service.
Operators can assess the economics of performing a hot tapas an alternative to a shutdown connection by following the five steps below:
1. Determine physical conditions of the existing line.
2. Calculate cost of performing a shutdown interconnect.
3. Calculate the cost of a hot tap procedure.
4. Evaluate the gas savings benefits of hot tapping.
5. Compare the options and determine the economics of hot tapping.
Bibliography:
http://www.koppl.com/pdfs/Hottap.pdf
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