Coal, O&M

Piping Prefab: Boosting Efficiency with Grooved Systems

Issue 4 and Volume 119.

According to the Government Accountability Office, just over 42 GW or about 13 percent of coal-fired capacity has been retired since 2012 or is planned for retirement by 2025. The coming years will bring a boom in gas-fired plant construction as power producers rush to replace the retiring units. Indeed, there are currently more projects in the planning and design stage than we’ve seen in the last 15 years. With so much capacity to replace in a small window, strategies that reduce construction time will be vital, and the availability of skilled labor will be critical.

Prefabrication of piping is a common strategy to shorten construction schedules and reduce field labor, particularly for combined cycle plants. High-pressure, high-temperature critical process systems must be welded, but there are economic, schedule and constructability advantages to using grooved mechanical piping for balance-of-plant (BOP) water, air and utility systems. Prefabricating pipe spools with grooved-end components increases fabrication shop productivity and eases field installation, resulting in a much shorter fabrication and installation schedule.

Mechanics of Grooved

A grooved pipe joint comprises four elements: grooved-end pipe, a gasket, coupling housing, and nuts and bolts. The pipe groove is made by cold forming or machining a groove into the end of a pipe. A gasket enclosed in the coupling housing is wrapped around the pipe ends, and the key sections of the housing engage the grooves. The bolts and nuts are tightened with a socket wrench or impact wrench. In the installed state, the coupling housing encases the gasket and engages the grooves around the circumference of the pipe to create a leak-tight seal in a self-restrained pipe joint.

Major enhancements have been made to the original grooved coupling in recent years, which enables this technology to be successfully used in pipe fabrication for major power plants. The heart of the system is the elastomer gasket, which is permanent and never needs replacement, unlike flanged seals that require replacement over time. The system utilizes state-of-the-art roll grooving shop and field tools to groove 2- to 72-inch standard wall steel pipe without removing any wall thickness. The operation is clean and fast. Two types of grooved mechanical couplings are available: those that provide flexible pipe joints (the coupling allows limited axial and angular deflection) and those that provide rigid pipe joints (the coupling does not permit any axial or angular deflection). The vast majority of systems in the power market are rigid, with flexible couplings used only around rotating equipment to isolate equipment noise or vibration. This enables the grooved system to be supported and guided in accordance with the industry standard ASME B31.1.

In recent years, the introduction of ready-to-install couplings has greatly enhanced the speed-of-installation and performance of grooved systems. These couplings are not disassembled during the installation process, which greatly reduces field man-hours required for pipe spool installation. Proper coupling assembly is verified through a simple visual inspection to ensure contact between the two housings at the bolt pad junctions. No specific or measurable bolt torque or special installation tools (torque wrench) are required. Once properly installed, the coupling does not require retightening of bolts/nuts, unlike a flange.

Advantages of Grooved Prefabrication

Prefabricating with grooved piping is much faster than welding and enables quicker, easier field joints and long-term maintenance advantages. Total installed costs can be reduced by as much as 30 percent.

Grooved systems generate time savings and other benefits in the fabrication shop as well as the field. In the ideal shop environment, fabrication and assembly of grooved pipe spools is approximately three times faster than welding the spools. Many steps associated with preparation, alignment, finishing and testing are eliminated. For example, stainless steel systems do not require pickling and passivation, and non-destructive testing is unnecessary as a grooved joint can be visually inspected.

Several national companies recommend and implement grooved fabrication for major projects.

Using grooved piping where applicable greatly reduces the amount of welding on a project, reducing costs and allowing certified welders to focus on high-temperature, high-pressure systems. With the imminent skilled labor shortage already being felt in some parts of the country, allocating highly skilled welders to critical systems will increasingly become a necessity.

In terms of transportation, grooved pipe spools can be kept straight-what’s known as two-dimensional fabrication. Defined as spools with no change in direction, 2D spools include a linear length of pipe, sometimes with a valve in the middle and an elbow on the end. This makes the spools easy to load and enables an increase in the linear footage of pipe on a truck. In fact, it’s possible to load up to three times more grooved spools than welded spools on a single truck. This makes transportation more efficient, minimizing trips to and congestion at the construction site and reducing shipping costs.

Unlike flanged valves, grooved-end valves can be assembled onto pipe spools at the fabrication shop, reducing labor and time in the shop and field. The manner in which a flanged joint holds a seal is sensitive, particularly in transport. On a grooved joint, the seal is on the outside of the pipe and the coupling carries the load, mitigating concerns about weakening the seal during transportation. The ability to include valves in prefabricated spools enables better constructability in piping system design, as a valve doesn’t need to be a break point. Grooved-end valves also eliminate handling, two-holing and special tightening sequences in the field.

On site, grooved pipe spools are easier to handle and install. The shorter two-dimensional spools are easier to lift, move into position and rig. Grooved spools ease alignment because elbows and other fittings can be adjusted in the field, which eliminates rework in many cases. Proper alignment can be achieved by loosening the nuts on a coupling, adjusting the orientation of a fitting and retightening the coupling. Field assembly effort is dramatically reduced as the spools can be quickly joined with a two-bolt ready-to-install coupling rather than field welding. Grooved couplings provide a permanent joint, which, unlike flanges, never need retightening or gasket replacement. The coupling does, however, allow easy access for maintenance, repair and future expansions or field modifications.