Thread Milling in Advanced Energy-Recovery Operations
By Jeff Dei
Initially developed over 60 years ago, hydraulic fracturing of shale for the purpose of releasing hydrocarbons (or “fracking,” as it is generally known) has become a major factor in the domestic recovery of oil and natural gas and promises to play a significant part in managing energy costs, as well as securing independence from foreign sources. The process involves the pumping of water that has been chemically treated into deep shale formations at pressures up to 15,000 psi. The pressurized fluid cracks the shale or widens existing gaps and liberates the energy resources to flow toward the well for recovery.
Geologists estimate that the gas and petroleum reserves contained in one of the major shale sources—the Marcellus Shale—could meet domestic energy needs for at least two decades. A recent article in Popular Mechanics magazine quoted the Energy Information Administration as predicting that “shale gas will account for nearly half of the natural gas produced in the US by 2035.”
At the heart of the recovery system are the pumping units capable of generating the extraordinary pressures and, at the same time, withstanding the flow of the abrasive-laden pumping fluid. In one specific case, pump construction begins with the forging of a 3 × 6' (0.9 × 1.8-m) billet of an alloy derived from 4340 steel, chrome and nickel. Initially weighing 12,000 lb (5400 kg), the billet will be reduced to 3000–5000 lb (1350–2250 kg) after machining. Under field conditions, the pumps, which operate at up to 2500 hp (8949 kW), can have a working life as short as nine hours before requiring rebuilding.
No element of the pump is more critical than the threaded ports that connect the unit to the high-pressure lines. Connections are typically machined from the same alloy as the pump and, given the pressures involved, the fit must be perfect, both to ensure efficient operation and to facilitate connect and disconnect operation.
Depending on the design, typical pumps are produced with three to five threaded holes per block. Bores are generally 4–8" (101–203 mm), with finished threading ranging from 5.25 to 9.25" (133–235 mm). The most common threads are 4 pitch American Buttress, 4 pitch Acme and 8 UN. Depth of threads ranges from 2 to 4" (51–102 mm).
Carmex has worked with major pump builders to meet customer demands for connections that literally “hold up under pressure” and provide efficient connect/disconnect performance as well. In analyzing various tapping and thread milling operations, it was determined that a key component in delivering a desired performance is surface finish. The quality of the finish is essential to proper fit, as well as ease of connection and removal.
The Carmex helical indexable thread mill not only delivered a better end product but resulted in a reduced cycle time of up to 80%. Improvement in tool life over conventional straight-flute thread mills ranged from 50 to 200%. Given the extremely high value of the part at the time the threading process is performed, thread milling is the required procedure.
The company's design utilizes sequential cutter engagement that eases the tool into the cut, thereby minimizing the harmonic chatter and vibration attendant on straight-flute cutters that engage the entire surface area immediately. As a result, the tool works more easily, resulting in smoother cutting, better surface finish, improved thread quality, and longer tool life. (Ironically, helical design has long been a factor in the manufacture of drills and milling cutters.)
Further advantages inherent in the design include solid carbide construction, heavy TiAlN coating and fast, easy insert change capability. Unlike designs that utilize a cumbersome ring, locator screws and wedges, this design permits fast and easy changing through the use of a single retention screw. ME