HomeCoal FiredEquipmentAlternatives to conventional welded pipe systems

Alternatives to conventional welded pipe systems

Figure 1. A complete non-welded piping system with a Parflange F37 connection

Piping solutions utilizing non-welded piping technologies and cold bent piping offer reductions in fabrication and commissioning time while improving workplace safety, writes Brian Smith

Since the invention of the first commercial oxyacetylene welding torch and the coated metal electrode in the early 20th century, welding has become the predominant method for joining metals and fabricating pipe systems. Welded pipe systems can be found in nearly every industry that relies on the movement of process streams at elevated pressure or temperature.

For certain applications, however, welded systems may no longer represent the most effective or economic choice. Piping solutions using non-welded connections and cold bent piping offer significant value through reductions in fabrication and commissioning time, while improving workplace safety.

The most common joint employed in fabricating welded pipe systems is the circumferential butt weld. Butt welding is typically used for weld neck and lap joint flange connections because attaching the weld neck flange or lap joint stub end requires full penetration welds. For butt welded connections, the ends of pipes, fittings, and flanges must be chamfered (beveled) to approximately 37.5à‚° in order to accommodate the weld bead and assure a strong joint. Butt welds are commonly used in severe service applications where the weld joint must be able to withstand higher pressures.

Less severe applications enable the use of more economical slip-on flanges. Slip-on flanges require less accuracy in cutting/beveling the pipe or fittings, but two circumferential fillet welds are required to attach the flange – one at the pipe outside diameter and one on the flange face. This necessitates refacing after welding. Slip-on flanges are standard in most 150-pound and 300-pound flange classes.

For piping systems containing small bore (two-inch NPS or smaller) piping and fittings, socket weld flanges and fittings are often used. These typically require one fillet weld per joint (two joints per fitting). Construction costs are lower than for butt welding because precise fit-ups and beveling are not required. Socket welding, however, does not come without challenges. Compared to butt welding, socket welds have lower resistance to vibration and shock. Further, proper weld gapping to obtain proper weld penetration is needed to ensure welds do not fail prematurely due to shear stresses induced into the fillet during the welding/cooling process.

To different extents, both butt welding and fillet welding impose procedures and costs that cannot be avoided. For example, welded joints between straight tubes and elbows often require the prevention or removal of heat ‘tint’ around the weld. Tint removal is typically accomplished using mechanical or chemical methods or, for best corrosion resistance, acid pickling, introducing an extra processing step and additional handling concerns.

The welding process involves much more than the physical act of striking an arc and applying the weld bead. There are many pre- and post-weld tasks that often have to be performed to produce a structurally sound joint (see Table 1). Each of these can add time and labour.

Two butt weld SAE hydraulic flanges being aligned prior to final installation

For example, consider the double-90à‚° piping system, which contains two socket-welded elbow connections and one welded straight coupling. Six welds are required for this configuration (two per fitting). Total installed costs would need to account for surface preparation, welding time, nondestructive examination, cleaning, and post-weld pickling and passivation. For a high-pressure installation requiring Schedule 80/160 piping, welding time per joint can exceed two hours. Notably, the double-90à‚° configuration also presents operational concerns because the 90à‚° elbows impart abrupt flow changes and associated pressure losses.

Viable alternative

There is a viable alternative to weld fittings that is routinely in power and industrial applications: cold bending. Cold bends can provide a cleaner solution to addressing directional flow changes in piping systems without limiting the design to the use of ‘off the shelf’ 90à‚°, 45à‚°, and 22à‚½à‚° degree fittings.

Powered benders are commonly used by piping/mechanical contractors and fabrication shops. Modern CNC controlled pipe and tube benders are now being used to provide more accurate and repeatable pipe fabrication in the shop environment for pre-fabrication. Material-specific bending parameters and pipe spool co-ordinates are programmed into the bending software, allowing the CNC bender to provide a complete and repeatable pipe spool after loading in a cut-to-length pipe. CNC benders also can be equipped with mandrel bending capabilities wherein an inside diameter supporting ball or plug mandrel is attached to a rigid steel rod, supporting the tube or pipe as it is bent providing extra support and resistance to wrinkling, collapsing and ovality at the bend.

The elimination of weld fittings, however, only addresses part of the challenge in fabricating a viable piping scheme. Piping systems also must account for service breaks and equipment connectivity – typically accomplished through a weld flange. Mechanical flange alternatives to welded connections and flanges are available, such as through the use of an orbital flaring process. Figure 1 (page 24) shows a complete non-welded piping system comprising two 90à‚° cold bends combined with a Parflange F37 non-welded connection. This non-welded connection can be used for à‚½-inch to 10-inch pipe sizes at pressures up to 6000 psi/ 420 bar.

The Parflange F37 technology and the cold bending of pipe and tubing are widely used in hydraulic systems, providing interconnect piping between hydraulic power units, actuators, valve stands, accumulator banks, motors and grease/lubrication points. It can be field-fabricated, pre-fabricated, or some combination of both. For streamlined piping installations (minimizing field assembly), piping kits can be pre-fabricated and delivered to the job site ready for installation, reducing on-site installation time compared to field welded systems. In addition, the Parflange F37 system has been tested in compliance with the design requirements of ASME B31.1 and ASME B31.3.

The heat exchanger system in Figure 2 shows seven pre-fabricated pipe assemblies incorporating cold bent tubing and Parflange F37 connections. On this piping system, nearly 40 welds were eliminated.

Phastite PMAF non-welded fittings

Source: Parker Hannifin

The benefits of non-welded piping technology are summarized in Table 2, ranging from reduced preparation and inspection time to a safer work environment.

Piping systems in the oil and gas industry have historically relied on welded pipe and fittings for interconnecting hydraulic components such as pumps, valves, accumulators, motors, reservoirs, and filters. For severe applications such as subsea hydraulic systems, heavy duty (schedule 160) and extra-heavy duty (XXS) pipe and fittings are used to withstand both internal and external pressures at depth. Multi-pass socket and butt welds are typically used for these connections.

Non-welded connections provide an alternative in these applications as well. Axially-swaged permanent mechanical attached fittings (PMAFs) can reduce installation time, improve throughput, reduce weld-related costs, and minimize potential chromium exposure. PMAFs can be designed for use with three-quarter-inch to two-inch ASTM A312 type 300 Series stainless steel pipe in schedules 40/STD, 80/XH, 160, and XXS (double extra heavy duty).

For example, Phastite is a patent-pending axially-swaged PMAF pipe fitting technology that enables permanent connections to off-the-shelf schedule pipe. The Phastite system can withstand pressures up to 10,000 psi at subsea depths of 15,000 feet (4570 metres). These systems are tested in accordance with most ASTM F1387 requirements and meet the requirements of ASME B31.1 and B31.3.

Figure 2. Heat exchanger with cold bent tubing and Parflange F37 connections

Phastite fittings are axially swaged to the pipe using a bench-mounted attachment tool.

During pipe attachment, the Phastite collar is axially engaged to the fitting body as radial fitting teeth are locked to the pipe. A precise amount of pipe compression is achieved through the radial force and axial load, providing a permanent connection in a matter of minutes.

Phastite connections are highly resistant to pressure cycling, vibration, and external loading that occur in high-pressure fluid power/hydraulic systems.

The time savings associated with PMAFs enable manufacturers to use their scarce welding resources on critical welding requirements such as structural and large bore piping.

Whereas an acceptable weld on a two-inch 6000-pound class socket weld fitting can take up to two hours to weld per joint (not including inspection and flushing time), a similar PMAF can be fitted in about 10 minutes, achieving the same functional result and working pressure. Significant gains in productivity are possible through the use of Phastite PMAFs; for this example, the number of two-inch schedule 80 connections completed per day could increase from eight to 24.

Piping solutions utilizing non-welded piping technologies and cold bent piping offer significant value to clients through reductions in fabrication and commissioning time while improving workplace safety. Time-sensitive scheduled outages are well suited for non-welded piping systems as the planning, fabrication, installation, and commissioning of piping systems are significant cost and time drivers for capital construction projects.

Brian Smith is Business Development Manager at Parker Hannifin Corporation