Automation Beats Handmade Hands Down
High-throughput, low-cost honing technology helps meet demand for critical gas-metering components.
By Tony Tharp
Business Development Manager
Sunnen Products Co.
The shale gas boom in the US is creating its own boom for producers of metering equipment. Meter tubes, also called “meter runs,” are critical parts of the orifice gas meter, a device that’s been used for more than a century to measure gas flows. Millions of meter tubes are used worldwide, usually at custody transfer points to determine billing amounts. Thus, there’s a requirement for high accuracy, and the meter tube plays a key role in accurately measuring gas flow.
ID finishing of these fabricated tubes is a critical process, but it has been one of those jobs that falls through the cracks of being just low-tech enough that it’s often still tackled with a die grinder or some form of homemade hone. And when you see a worker with a die grinder or homemade hone doing this work, you know you’re watching the guy who drew the short straw that day.
How it Works
The beauty of an orifice meter is that it has no moving parts. It consists of specified lengths of “meter quality” pipe on either side of an orifice plate, which may be several inches thick, with a precisely sized round hole in it. Gas flow through the pipeline is restricted by the orifice plate. The pressure difference on either side of the orifice, measured by sensors in the meter tube on both sides of the orifice, is used to compute fluid flow.
The length of meter tubes varies with the pipe diameter, beta ratio and whether the tubes are upstream or downstream from the orifice plate. The beta ratio is the ratio of the orifice diameter to the pipe diameter, and the maximum is 0.75:1. Meter tubes can be less than an inch (25 mm) in diameter, and up to 24" (610 mm) or more, but the majority fall between those two extremes.
Accuracy of the orifice meter requires a uniform and consistent gas flow, which the meter tube helps produce. The meter run provides a length of straight pipe, precisely sized and finished, which allows swirl or turbulence in the gas stream to smooth out, producing an optimum flow profile before and after the orifice. A flow conditioner is often used inside the meter run upstream from the orifice to help smooth out the flow within a shorter length of pipe. The maximum and minimum surface roughness on meter run ID’s are functions of the beta ratio and nominal diameter of the tube, but typically fall in a range of 100–300 µin. Ra. It seems contradictory, but an excessively smooth surface can cause irregularities in the gas velocity.
Standards setting agencies, such as the American Gas Association (AGA) and the American Petroleum Institute (API), spell out detailed specifications for meter assemblies and components, and manufacturers use meter-quality tubing or standard pipe for meter runs, as long as the end product meets specification.
How it’s Made
The meter run itself is actually a fabrication with welded flanges or fittings on both ends, so manufacturers must eliminate evidence of weld beads and restore a correct ID surface finish. This is a multistep process. The pipe may first be chucked into a lathe to machine out most of the weld bead. More typically, it is ground down with a die grinder using an aluminum oxide “rock” or carbide burr on a long rod. The final step is honing to remove the last trace of the bead and create the required surface finish. Traditionally, this has involved a manually stroked, homemade hone of some kind.
The manual homemade hone usually consists of a drill motor with a shaft several meters long that mounts standard honing tools. The lack of power stroking, a clutch system, automatic tool feed or load meter/control makes this an unpredictable and physically demanding job. It requires good tactile sense to find tight spots in the bore, while avoiding binding the tool in the pipe—a condition that industry wags find darkly humorous.
Manual hones have dominated in this application because it simply does not demand the power, precision and higher cost of an industrial tube hone, but that’s changed with the introduction of low-cost, automated tube hones that can use legacy tooling.
The budget-priced HTA series tube hone is the first in this new class—an all-electric, lean design, engineered specifically for lighter-duty applications where initial machine cost is critical. Able to mount ANR275-type tooling, the most widely used tools on manual hones, it’s aimed at “surfacing” applications, where stock removal is in the range of 0.030" (0.76 mm). The base price of the machine is less than $52,000 for a model with a 13.4' (4.1-m) stroke, and it is ideal for meter runs because it is automatically stroked, safer (clutched), more accurate and pays for itself by helping quickly reduce cycle times, operator fatigue and workman’s comp claims. It includes high-end features not available with a homemade machine, such as Siemens drives and PLC control with touch-screen HMI for setting all machine parameters, including stroke reversal point, spindle/stroking speeds, and crosshatch angle calculation.
Notes from the Field
Early users of the HTA report cycle-time reductions of 40% compared to manual honing because of the automation on this machine. The control features a load meter to determine areas of bore tightness, with capability for dwelling the tool in tight spots using a joystick. Other advanced capabilities include automatic tool tensioning, set through the PLC. The tool-feed system can be programmed to expand the honing tool to a predetermined spindle load that is adjustable for any application, protecting the machine, tooling and workers, and allowing inexperienced operators to produce consistent results. This also optimizes the performance of the honing abrasive.
To give some idea of the impact this has, the beta-site company that assisted in development of this machine subsequently purchased five of them, and has completely eliminated manual honing of meter tubes at all its plants.
The meter-tube hone is available in five standard sizes with stroke lengths of 2.1, 4.1, 6.1, 8.1 and 10.1 m. Its standard fixturing allows it to handle a bore ID range of 2.5–21" (63.5– 533 mm) and maximum part OD of 24" (610 mm). All models also handle part weights up to 8000 lb (3629 kg) with appropriate fixturing. The powered section of the HTA is available by itself for machining large, difficult-to-fixture parts, such as turbine rotors.
An electronically controlled, 3-hp (2.24-kW), fan-cooled, AC gear motor powers the spindle at 20-300 rpm, while the servo-driven stroking system has a speed range of 5–90 ft/min (1.52–27.4 m/min). The all-electric design eliminates the noise, leaks, valve adjustments and performance variability inherent to hydraulic systems.
The HTA includes a standard 55-gal (208-L) coolant reservoir and paper-media coolant filtration unit. Set on casters with a level gage for easy maintenance, the coolant tank tucks under the machine frame to conserve floor space.
At Home in the Oil Patch
Since the machine’s introduction better than 10% of those sold have gone into meter-tube applications. This machine is also widely used for light honing in other energy-industry applications, such as reconditioning/refurbishment of hydraulic cylinders, drilling jars and other downhole tools.
Initial grinding on a meter tube’s weld bead may still be needed before honing, but the speed, consistency and safety of meter-tube production are greatly enhanced with this affordable honing solution. In addition, this specific application is evolving as manufacturers seek to hone greater lengths of tubing and better manage the uneven centerline that results from welding lengths of pipe together. This is driving the development of improved, heavier-duty tooling, guided with input from key flow-meter manufacturers.
This article was first published in the 2013 edition of the Energy Manufacturing Yearbook.
Published Date : 12/6/2013