A new breed of quadraplex pumps aims to replace triplex units for heavy-duty use on shale play drilling sites
Duplex and triplex mud pumps have been used by drillersfor upwards of 50 years. But with increased demands being placed on drill rigs – from extended reach drilling in shale formations to deepwater drilling – contractors are pushing triplex pumps to their limits. Adding horsepower, however, means adding additional cylinder capacity to pump designs.
While some manufacturers have gone as far as a five-cylinder mud pump design, the White Star Pump Company of Houston believes that four cylinders provide the optimal combination of power, smooth operation and limited footprint. The company launched its first quadraplex pump, the Quatro, five years ago, on the cusp of a resource industry slowdown.
“Drillers still had a large number of triplex pumps in stock, so it took a while for us to begin to penetrate the market,” says White Star engineering and product development director Shaun White. “However, we have continued to offer triplexes simply because people are comfortable doing what they’ve always done. When I was an engineering student in 1976 on an American rig, the 70-year-old toolpusher said he thought we needed to go back to duplexes. We have to overcome that same inertia.”
WORKING HARDER
What’s wrong with triplexes? Nothing, says White – the company continues to make them. And triplex pumps remain the dominant unit in the field and will continue to be used in many applications. But White contends that drillers are more and more pushing them beyond design limits, which opens the door for four-cylinder pumps at drilling sites.
“Most of the triplexes manufactured during the last 50 years have been designed for a 5,000 psi pressure rating, but it was rare to see them used above 2,800 psi,” he says. “But with extreme deep-sea drilling and pumping through shale plays, we’re seeing them run consistently at as much as 4,500 psi. Your car may have a speedometer set at 120 mph, but the engine will suffer if you drive it consistently over 100.”
The company’s four-cylinder models are rated at 7,500 psi across the line, from the 750 hp model to the 2,450 hp model, to provide overcapacity for today’s drilling environments. “Pumps are brutally used in the field,” says White. “You not only need to deal with rugged handling, and use that stretches the limits of the design, but also anything up to gloves and coats going through the pumps with the fluid.”
Four-cylinder action can provide a smoother output than three cylinders, says White, just as a car engine with eight or 12 cylinders runs more smoothly than a four-cylinder engine, reducing vibration and increasing torque.
A typical triplex features a cast or full-penetration welded crankshaft with bearings located on either end, set in retainers in the pump frame or housing, leaving the middle cylinder unsupported at the middle cam.
“The distance from the center cam to the outside bearings can be as great as 36 inches, allowing for a large bending moment and significant flex in the crankshaft,” says White. “If the crankshaft is unsupported close to the middle cam, the bending moment is extreme and crankshaft cracking is inevitable. Even if the pump is rotating at 100 to 150 rpm, then, after a week of drilling, the crank will experience one million cycles of 300,000 ft-lbs of torque and continuous crankshaft bending.”
CRANKSHAFT REDESIGN
The four-cylinder design uses a redesigned modular crankshaft assembled from forged parts that are machined to very high tolerances. “It’s assembled simply like Legos,” says White. “You can assemble and disassemble it easily to get direct access to the bearings.”
A traditional crankshaft relies on the eccentric shape of its cams to create forward motion in the connecting rods. This four-cylinder design turns that principle on its head with a straight crankshaft and cams locked onto the shaft, allowing the shaft to be manufactured in isolation without the need for welds or the possibility of casting faults. The connecting rods that provide the forward motion look like lollipops, with the business end mounted eccentrically around the crankshaft cams.
“With four cylinders along a straight shaft, you can place the two main bearings between the cylinders, close to the cams, so the cam-to-bearing distances run no more than 10 inches,” says White. “The bending moment on the quadraplex shaft is about one quarter that of a triplex.”
THE HUMAN ELEMENT
White notes that the four-cylinder design also helps to limit the width of the pumps.
“You could keep on adding cylinders but you can’t ignore the human element of the design,” says White. “The cylinders are mounted a minimum of 20 inches apart, just wide enough for a technician to get into the liner bay space for service.” The company’s largest four-cylinder unit measures 82 inches across, and fits on a standard-width trailer.
The four-cylinder model’s symmetry is further reflected in its engineering design, which employs one suction desurger and pulsation dampener per pair of cylinders, to provide better fluid balance. That translates into a near-zero discharge pulse and smooth fluid flow.
“Traditional triplex pumps discharge all three cylinders into the discharge manifold in sequence and the fluid goes through a strainer where the dampener knocks out the vibration,” says White. “Our dampener is located upstream of the strainer, knocking out vibration close to the source, reducing pulse and other pump vibrations.”
With pump maintenance a critical issue, the company has installed system monitors to deliver operational information via data acquisition sensors. “This system can significantly reduce downtime by alerting the operator of problems and maintenance issues in real time, both locally and remotely,” says White.
QUADRAPLEX MARKET
Much of the recent interest in White Star’s four-cylinder pumps is now coming from shale drilling contractors.
“Fortunately for us, the drilling market is shifting to the shale plays where extreme drilling conditions require extra power,” says White. “It’s the shale plays where the resource market is looking at new ways of doing things.”
In shale plays, the units can also be used as frac pumps.
“It would be a constant-duty pump and can deliver a high volume of fluid rather than the intermittent-duty frac pumps that have to be over-run to achieve high volumes,” says White. For mining and other pumping applications the company’s quadraplexes, rated from 1,000 hp or greater, can produce a 2,400 gpm flow rate for low-pressure applications.
But one of the biggest problems faced by domestic drillers is the width and weight of fully unitized mud pumps being transported to the shale plays.
“With some pumps measuring 10 to 13 feet wide once unitized with power, we’re getting reports in Pennsylvania of drillers paying in excess of $100,000 per month in overweight and overwidth fines for transporting them on local highways,” says White. “The four-cylinder pumps weigh 25 percent to 35 percent less than conventional triplex mud pumps and can be transported as a standard-width load on a conventional trailer.”
White Star is currently designing a lightweight version of its 1,600 hp four-cylinder pump, targeted specifically to Pennsylvania, where pumps must travel across narrow roads and bridges to access the Marcellus Shale.
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