Maintenance on tap

In the process industries, where downtime is costly and unscheduled shutdown could be crippling, the ability to undertake necessary repairs and maintenance or plant modifications without disrupting operation inevitably has a high value. Pipeline intervention technologies, including hot tapping, line plugging and pipe freezing, enable this to be achieved.

Broadly, these services are applied where temporary isolation of a section of plant, piping or component is needed to allow the plant to remain operational during maintenance, modifications, retrofitting, emergencies or system expansions.

The principle of these services is not new, but on-going developments mean the technological boundaries and capability parameters are constantly being extended. Companies now offer hot tapping and line plugging at large diameters (to 72 inches, size on size or reduced branch taps), high pressures (over 100 bar), high temperatures (to 370°C), and on large scales requiring multiple simultaneous operations.

Typically, hot tapping into products from chemicals, hydrocarbons, natural gas and heavy oils to water and steam is used to provide additional branches or modifications, instrumentation and monitoring points, or entry points for isolation.

Line-stopping techniques, on the other hand, temporarily stop the flow, providing a safe, cost-effective and efficient isolation while the plant remains on-line and pressurised, without the cost of fitting permanent valves. Typical applications include a double stop and bypass for valve insertion or permanent relocation of a line for plant expansion.

Hot tapping uses a welded or mechanical fitting, compatible with the pipework specification, which is installed on the pipeline together with an appropriately-rated full-bore valve. A drilling machine is then used to remove a section of the existing pipe, using a pilot drill to break through the pipe wall.

Retaining pressure

The product content fills the void beneath the drill, and air is expelled through the drilling machine purge valve, which is then closed to retain the pressure. The drilling process continues to complete the cut, and the cutter and disc produced by the process are withdrawn into the void above the valve.

Choice of cutters and pilots, machine and swarf clearance measures depend on material and size, for instance, while pressure and connection configuration determine the adapter to be used. Temperature affects the machine's seals and whether cooling spools are needed, and product and chemical constituency will clearly impact in terms of corrosion, flammability, and toxicity.

A line-plugging operation also begins with a hot tap and temporary valve, through which a mechanical plugging head is inserted into the line, rotated and locked into position. The line-plugging head is fitted with a sealing element, which is activated in the line under pressure — achieving a temporary seal and stopping pipeline flow or re-directing it through a by-pass.

Once the work is complete, the plugging head and equipment is removed, a completion plug installed and locked into position, and a blind flange is installed on the hot tap connection to complete the process. A feature of this system is that it allows the process to be reversed and the hot tap connection can be re-used to repeat the line plugging process in future.

In one example, a hot tapping and line- plugging solution enabled a manifold and associated valves on a 5,000-tonne ethylene vessel to be replaced for Huntsman European Petrochemicals as part of modification work, while the vessel remained full.

Indeed, insertion or replacement of defective equipment such as pumps, valves and filters is a typical situation in which hot tapping or line-plugging services are called upon. This was the case at another process plant, where an isolation valve had failed and line-plugging was used to allow the valve to be changed without disrupting operation.

Although branch fittings are usually size on size, at large diameters (18 inches and over), this can introduce considerable added weight considerations and potential problems. In such cases, a folding head line stop, which is lighter than a conventional plugging head and allows access through a reduced branch fitting, offers significant advantages.

In the case of the above example, line-plugging of the 60-inch line was completed using folding head technology via a 36-inch hot tap connection that afforded substantial weight savings over the traditional full line diameter pivoting plugging head technology.

The folding head reduces load bearing considerations and minimises risk, and in some cases enables line-plugging to take place where the dimensions and weights involved could otherwise render it impossible, or extremely costly.

Where the challenges faced centre on high temperatures, on the other hand, a special HTP stopping head, using a copper-faced metal plug to provide a metal-to-metal seal at the pipe face, can provide a positive seal, typically for 4- to 16-inch lines at temperatures to 370°C with pressures to 48 bar.

This technology is commonly used by refineries, petrochemical plants, and steel and paper mills, and special HTP fittings can be designed and manufactured to suit specific applications. Because the sealing surface is metal-to-metal on the upstream side, the stopping head can be left on the line longer than elastomer seals, providing a safe method of isolation while downstream work is undertaken.

Mike Tucker is business development manager at technical service company Furmanite International