Handling Pressure

As surface resources are gradually depleted, mines must reach increasingly greater depths. With depth comes the challenge of pressure. The deeper the shaft, the greater the head pressure, and conversely, the greater the pressure required to transport water back to the surface.

Deep-mine chilled water and dewatering piping systems must accommodate this pressure, as well as offer flexibility to accommodate the expansion and contraction inherent with temperature changes at different depths. Piping backfill solids also requires high pressures.

Engineers have typically specified heavy-duty flanges to join high-pressure mining piping systems such as these, but flanges are inefficient in installation and maintenance, do not offer flexibility and come at a high cost. Another option for joining high-pressure piping systems is actually very well known in the mining industry: grooved couplings.

Typically specified for low- to moderate-pressure air and water applications, grooved mechanical couplings offer quick and simple installation; create a union at every joint to simplify system maintenance, expansion and repositioning; accommodate expansion, contraction and deflection; and reduce safety risks.

Although not as widely known as their standard counterparts, grooved couplings are available to suit high-pressure applications. In fact, couplings currently on the market have the ability to withstand pressures of up to 4,000 psi (27,500 kPa) and end-loads in excess of 300,000 lbs (136 t). The design of these high-pressure double-groove couplings is slightly different than standard couplings.

As the name implies, double-groove couplings engage into two grooves on each pipe end, unlike standard couplings that engage into one groove on each pipe end. This double-groove engagement of mating coupling keys provides superior joint integrity at high pressures by distributing pipe end-loads. The housing design provides increased strength at points of high stress for high-pressure security. Instead of a single bolt on each side of the coupling, double-groove couplings feature two bolts on each side to reinforce the double-groove positioning for positive high-pressure reliability.

Double-groove couplings contain a standard C-shaped gasket. The groove profile is the same as that of a standard cut groove, with the exception that there are two grooves on each pipe end instead of one. Double-groove couplings must be used on cut-grooved schedule 80 or heavier pipe. These couplings do not require special weld-on nipples or collars, eliminating additional fabrication needs.

Double-groove couplings can withstand pressures of up to 4,000 psi (27,500 kPa), the highest pressure rating available for any grooved mechanical pipe joining system. They also have an extremely high end-load rating of up to 300,000 lbs (136 t). Although the flexibility of the double-groove joint allows the coupling to move within the groove to accommodate expansion, contraction and deflection, the joint will not pull apart if the end load is within specifications. Furthermore, the restrained joints create a system that does not require additional pipe supports.

The other type of high-pressure coupling is known as an EndSeal coupling. EndSeal couplings feature heavy-duty housings and a different groove profile than standard couplings. Compared to a standard cut groove, the EndSeal groove has a shorter A-dimension (length from the pipe end to the beginning of the groove) and is deeper and narrower.

EndSeal refers to the gasket that is used within the coupling. Rather than a standard C-shaped gasket, the gasket features a center lip that positions between the pipe ends when the coupling is engaged, creating a smoother flow path that reduces wear on the pipe ends.

EndSeal couplings, such as the Victaulic Style HP-70ES, can withstand pressures of up to 2,500 psi (17,250 kPa). EndSeal couplings must be used on schedule 40 or heavier pipe, and for high-pressure applications, the pipe should be cut grooved.

Comparing grooved couplings to heavy-duty flanges in high-pressure applications demonstrates the advantages of the grooved joining method.

In terms of flexibility, only couplings offer built-in expansion, contraction and deflection capabilities. Flanges would require expansion joints to provide this flexibility, which adds costs to an already expensive joining method. Comparing material costs alone, heavy-duty flanges are costlier than couplings. The labor costs involved in pipe preparation and field installation further increase the cost of flanged system installation.

Flanged joints require more time to fabricate and install than grooved joints due to the make-on and make-up processes. And unlike grooved systems, flanged systems do not provide a visual indicator when properly installed. Like standard couplings, both the double-groove and EndSeal couplings offer visual confirmation of proper assembly through metal-to-metal bolt pad contact.

As for maintenance, flanges can be difficult to work with and are not reusable when a section of pipe requires replacement, which increases material costs. Grooved couplings, on the other hand, can be reused and are extremely efficient in completing maintenance tasks. In fact, couplings can reduce maintenance time by up to two-thirds compared to flanges, reducing system downtime and costs. They can also reduce the labor needed to complete maintenance tasks.

Grooved couplings are not just for standard air and water lines. High-pressure couplings meet the pressure challenges of backfill and deep-mine chilled water and dewatering services, and they offer all the benefits of standard couplings, most notably, time and cost savings.

Information for the Special Report provided by Marc Carriere, Global Mining Manager for Victaulic, a major producer of mechanical pipe joining systems.