The decision to use materials with low upfront costs rather than ones with low lifecycle costs has come back to haunt many a design engineer. Some in the oil and gas drilling industry have recently started to resist this pennywise-pound foolish approach when selecting a material to line the mud pumps that circulate a rig’s drilling fluid. Capable of circulating up to 200 gpm of water and abrasive slurry at pump pressures as high as 7,500 psi, these big triplex reciprocating pumps have traditionally been lined with chrome iron or alumina ceramics for wear resistance. Now an alternative made from a magnesia partially stabilized zirconia ceramic has started to shine as a liner material. Supplied by Carpenter Advanced Ceramics (CAC), these Z-Max zirconia ceramics do cost substantially more on a price basis than the two previous liner materials. Yet zirconia also has some compelling property advantages that more than offset the initial price penalty.
Performance and cost
Zirconia has three important property advantages compared to alumina, which became the state-of-the-art material not long after its introduction to the oil industry more than 15 years ago.
For one, zirconia exhibits better impact strength, according to Steve Thompson, who manages CAC’s oil and gas business. He doesn’t have test results to quantify the difference. Carpenter, however, does supply both kinds of ceramics, and he’s also seen the improvement in person. “Alumina breaks about as easily as a coffee mug,” he says. “But zirconia will withstand a heavy hammer blow.” For another, zirconia is also harder than alumina. In ASTM lab tests, zirconia wears anywhere from two to four times longer than alumina (see chart). Finally, zirconia can be honed to finer surface finishes than alumina. Finished to 4 RMS, the zirconia liners provide a surface finish that’s three to four times finer than alumina. These last two advantages are even more pronounced when zirconia is stack up against chrome iron.
All three of these property advantages translate to lower ownership costs. Thompson says the improved wear directly extends the service life of the sleeve, while the improved impact strength cuts down on the significant costs of replacing broken liners in the field. The surface finish improvements, meanwhile, have an indirect effect. As Thompson explains, the finer surface finish means less friction with the elastomer-and-metal pump pistons, which in turn extends piston life and reduces pump-cooling requirements.
This cost scenario isn’t just wishful thinking. Recently C&C Equipment Inc., a supplier of mud pump liners and other drilling expendables, conducted a field study that examined the ownership costs of zirconia, alumina, and chrome liners. Corey Cole, the company’s president, reports that zirconia came out persuasively on top.
Nice Touch: A typical zirconia sleeve for a mud pump has a 0.25-inch wall thickness, a length of 12-17.5 inches, and a honed inside diameter ranging from 4.5-7.5 inches. These zirconia sleeves fit inside a carbon steel hull to make up a completed mud pump liner.
Using its customers’ drilling rigs as a real-world lab, he found that the total average cost of owning and using zirconia liners continuously for three years was $4,000 per cylinder. The corresponding average three-year cost for alumina liners was $10,000. Most expensive were chrome iron liners, which cost an average of $27,000 to own and use for three years (see table). Cole attributes most of the cost advantage to wear performance. “The mud simply tears up the other liner materials faster,” he says. The study also confirmed the secondary cost benefits, including a 75 percent improvement in piston wear due to the surface finish improvements.
And the savings may be even more dramatic than the study suggests, since it ignores the substantial costs of labor and downtime associated with liner failure and changeovers. The oil industry hates downtime. “They say, ‘You have to make hole to make money’,” Thompson notes. Cole agrees and estimates that downtime costs related to mud pump liners account for as much as 10-20 percent of operating costs.
C&C’s evaluation of zirconia ceramics has implications beyond the oil industry. A number of large pumps, valves, and piping components share a similar problem that might be called “excessive cost of wear.”
Thompson says zirconia could be a good fit in many industrial pump applications where wear produces unacceptable replacement and labor costs—or results in a safety issue by placing maintenance workers in harm’s way.
As for the mud pump application, it seems to be on the upswing. Cole estimates that of roughly 2,500 rigs worldwide, more than 500 have either switched to zirconia or installed them for field trials over the past three years. C&C has even stopped selling alumina liners altogether. “We haven’t had anyone who tried zirconia liners not buy them,” Cole says.