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 productand 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. The hot tap procedure involves attaching a branch connection and valve on the outside of an operating pipeline, and then cutting out thepipe-line wall within the branch and removing the wall section through the valve. Hot tapping avoids product loss, methane emissions, and disruption of service to customers.
While hot tapping is not a new practice, recent design improvements have reduced the complications and uncertainty operators might have experienced in the past. Several Natural Gas STAR transmission and distribution Partners report using hot tap procedures routinely—small jobs are performed almost daily while larger taps (greater than 12 inches) are made two or three times per year.
By performing hot taps, Natural Gas Star Partners have achieved methane emissions reductions and increased revenues. Gas savings are generally sufficient to justify making all new connections to operating lines by hot tapping. The payback period for utilizing hot tapping is often immediate.
In natural gas transmission and distribution systems, it is frequently necessary to relocate or expand existing pipelines, install new valves or repair old ones, install newlaterals, perform maintenance, or access lines during emergencies. Historically, it has been common practice to shut down the portion of the system during the alteration, vent the gas within the isolated segment, and purge the pipeline with inert gas to ensure a safe connection.
The procedure for performing the shutdown interconnect differs slightly depending on system pressure. In high-pressure systems, the surrounding valves are closed to isolate the pipeline segment and additional stoppels (inserted plugs) are placed next to the valves to prevent natural gas leakage and improve the safety conditions at the interconnection site. In a low-pressure system, the length of pipeline that is shutdown is typically much shorter. Rather than shutting the surrounding valves, stoppels are used to isolate the portion of the pipeline directly around the area of the tap. In both cases, the gas in the isolated pipeline segment is vented and the line is purged.
The impacts associated with performing a shutdown interconnect are both economic and environmental. Gas vented from the pipeline segment represents a loss of product and an increase in methane emissions. In addition, removing a pipeline segment from service can occasionally cause gas service interruptions to customers. For example, a shutdown connection on a steel line can require one to three or more days of pipeline outage and possible interruption of natural gas shipments in addition
to the release of methane to the atmosphere.
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).
Hot tapping equipment is available for almost any pipeline size, pipe material, and pr essure rating found in transmission and distributi on systems. The primary equipment for a typical hot tap application includes a drilling machine, a branch fitting, and a valve. Hot tapping equipment is described below and shown in Exhibit 1.
- 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 or 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.
Exhibit 2 provides a general schematic of a hot tapping procedure. 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.
Hot taps can be vertical, horizontal, or at any angle around the pipe as long as there is sufficient room to install the valve, fitting, and tapping machine. Current technology allows for taps to be made on all types of pipelines, at all pressures, diameters, and compositions, even older pipes merging with new. New, lightweight tapping machines are also available that allow a hot tap to be performed by a
single operator, without additional blocking or bracing.
Safety manuals and procedural outlines are available fromthe American Petroleum Institute (API), American Society of Mechanical Engineers (ASME), and other organizations for welding on in-service pipelines for all sizes, flow rates, and locations. These manuals provide information on what to consider during welding, including burn-through prevention, flow in lines, metal thickness, fittings, post weld heat treatment, metal temperature, hot tap connection and welding design, and piping and equipment
Vendor manuals and equipment catalogues are also good sources for determining which size and type of equipment is most appropriate. Several vendors have published comprehensive outlines and guides for performing hot tap procedures, including information on tapping on various materials, job-site evaluation and preparation, selection and installation of fittings and other equipment, and safety precautions. Most importantly, because this is a hazardous procedure, each potential hot tap must be
evaluated on a case-by-case basis and a detailed, written procedure should be prepared or reviewed before starting each job to ensure that all steps are taken properly and safely.
Economic and Environmental Benefits
Key economic and environmental benefits of employing hot tapping procedures instead of shutdown connections include:
- Continuous system operation—shutdown and service interruptions are avoided.
- No gas released to the atmosphere.
- Avoided cutting, realignment and re-welding of pipeline sections.
- Reduction of costs associated with planning and coordination—meetings, schedules, paperwork, lost production, and direct manpower.
- Increased worker safety.
- Elimination of obligations to notify customers of gas outages.
By ensuring that best practices are followed when performing a hot tap, the time required for the procedure, as well as the potential for failure, is reduced.
Taken from: http://www.epa.gov/gasstar/documents/ll_hottaps.pdf (page 1-3)