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Using Hot Taps For In Service Pipeline Connections
Natural gas transmission and distribution companies need to make new connections to pipelines many times a year toexpand or modify their existing system. Historically, thishas necessitated shutting down a portion of the systemand purging the gas to the atmosphere to ensure a safe connection. This procedure, referred to as a shutdowninterconnect, results in methane emissions, loss of productand sales, occasionally customer inconvenience, and costsassociated 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 procedureinvolves 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 (greaterthan 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 isoften immediate.
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 (greaterthan 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 isoften immediate.
Technology Background
In natural gas transmission and distribution systems, it is frequently necessary to relocate or expand existingpipelines, install new valves or repair old ones, install newlaterals, perform maintenance, or access lines during emergencies. Historically, it has been common practice toshut down the portion of the system during the alteration,vent the gas within the isolated segment, and purge thepipeline with inert gas to ensure a safe connection.
The procedure for performing the shutdown interconnectdiffers slightly depending on system pressure. In high-pressure systems, the surrounding valves are closed toisolate the pipeline segment and additional stoppels (inserted plugs) are placed next to the valves to preventnatural gas leakage and improve the safety conditions atthe 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 pipelinedirectly around the area of the tap. In both cases, the gasin 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 canoccasionally cause gas service interruptions to customers.For example, a shutdown connection on a steel line canrequire 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 alsoreferred to as line tapping, pressure tapping, pressurecutting, and side cutting. The process involves attaching branch connections and cutting holes into the operatingpipeline without interruption of gas flow, and with norelease or loss of product. Hot taps permit new tie-ins toexisting 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 pipelinesize, pipe material, and pressure rating found in transmission and distribution systems. The primaryequipment 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.
The procedure for performing the shutdown interconnectdiffers slightly depending on system pressure. In high-pressure systems, the surrounding valves are closed toisolate the pipeline segment and additional stoppels (inserted plugs) are placed next to the valves to preventnatural gas leakage and improve the safety conditions atthe 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 pipelinedirectly around the area of the tap. In both cases, the gasin 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 canoccasionally cause gas service interruptions to customers.For example, a shutdown connection on a steel line canrequire 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 alsoreferred to as line tapping, pressure tapping, pressurecutting, and side cutting. The process involves attaching branch connections and cutting holes into the operatingpipeline without interruption of gas flow, and with norelease or loss of product. Hot taps permit new tie-ins toexisting 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 pipelinesize, pipe material, and pressure rating found in transmission and distribution systems. The primaryequipment 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 generallyconsists of a mechanically driven telescoping boringbar that controls a cutting tool. The cutting tool is used to bore a pilot hole into the pipeline wall inorder to center a hole saw that cuts out the “coupon,”or curved section of pipeline wall.
Fitting
Connection to the existing pipe is madewithin 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 completelyaround the pipeline, and when welded, provides mechanical reinforcement of the branch and carrier pipe.
Valve
The valve on a hot tap connection can beeither 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 permanentvalve.
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 valveand close the valve.
4. Remove the tapping machine and add the branchpipeline. Purge oxygen, open the valve, and the new connection is put into service.
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 permanentvalve.
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 valveand close the valve.
4. Remove the tapping machine and add the branchpipeline. 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 technologyallows for taps to be made on all types of pipelines, at all pressures, diameters, and compositions, even older pipesmerging with new. New, lightweight tapping machines are also available that allow a hot tap to be performed by asingle operator, without additional blocking or bracing.
Safety manuals and procedural outlines are available fromthe American Petroleum Institute (API), American Societyof Mechanical Engineers (ASME), and other organizationsfor welding on in-service pipelines for all sizes, flow rates, and locations. These manuals provide information on what to consider during welding, including burn-throughprevention, flow in lines, metal thickness, fittings, post weld heat treatment, metal temperature, hot tap connection and welding design, and piping and equipment contents.
Vendor manuals and equipment catalogues are also goodsources for determining which size and type of equipmentis most appropriate. Several vendors have publishedcomprehensive outlines and guides for performing hot tap procedures, including information on tapping on variousmaterials, 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 beevaluated on a case-by-case basis and a detailed, writtenprocedure should be prepared or reviewed before startingeach job to ensure that all steps are taken properly and safely.
Safety manuals and procedural outlines are available fromthe American Petroleum Institute (API), American Societyof Mechanical Engineers (ASME), and other organizationsfor welding on in-service pipelines for all sizes, flow rates, and locations. These manuals provide information on what to consider during welding, including burn-throughprevention, flow in lines, metal thickness, fittings, post weld heat treatment, metal temperature, hot tap connection and welding design, and piping and equipment contents.
Vendor manuals and equipment catalogues are also goodsources for determining which size and type of equipmentis most appropriate. Several vendors have publishedcomprehensive outlines and guides for performing hot tap procedures, including information on tapping on variousmaterials, 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 beevaluated on a case-by-case basis and a detailed, writtenprocedure should be prepared or reviewed before startingeach job to ensure that all steps are taken properly and safely.
Source:
Lessons Learned from Natural Gas STAR Partners
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