Above: Clean natural gas flows through a completed pressure vessel system built by Mueller Environmental.

Manufacturer patents a technique for speeding assembly of vessels

October 2014 - When we switch on a light, turn up the heat or fill our gas tank, how often do we think about how the energy got there? 

One everyday source of energy is natural gas—used for many purposes, including cooking, heating, cooling, energy generation, transportation and more. But the natural gas that is extracted from wells is quite different than what we use in our homes, businesses and vehicles. 

Out of the ground, gas is dirty. It contains a variety of undesirable liquids and solids that must be removed before it is pumped to processing and distribution stations. Unclean gas can quickly ruin millions of dollars worth of equipment. The old method for cleaning the gas required large paper filters in special pressure vessels. These filters had to be changed regularly depending on to what degree the gas was unprocessed—sometimes as often as every eight hours. This can be a very time-consuming and environmentally costly process. 

One company outside Houston has transitioned to a new method for cleaning natural gas. Mueller Environmental Designs is manufacturing a patented helical separator system that eliminates solids, water, oil, black powder and a variety of contaminants from natural gas. Their separator delivers clean natural gas for processing and distribution.

Drilling challenges

The pressure vessels Mueller Environmental manufactures measure anywhere from 4 to 8 feet in diameter and are made from 1⁄2-inch to 3 1⁄2 -inch-thick grade 5617 rolled steel plate. The diameter and material thickness depends upon the pressure the vessel must withstand. Sometimes pressures can reach 3,000 psi. Inside the vessels is a proprietary gas-cleaning separator and an internal container to hold the accumulated solids and liquids that are removed. This material is drained automatically as it fills up the container. Liquid Level Indicators measure the amount of waste material inside the tank. A low-level indicator is stationed roughly 1 inch to 2 inches off the bottom of the tank and a high-level indicator is located at the tank’s midsection.

A vessel manufacturer must drill holes into each vessel for installing level indicators. Drilling a hole in the center of a large pressure vessel can present its own set of challenges, but nothing like the challenges of creating a radically angled hole near the bottom of the vessel. These holes range from 1 inch to 2 inches in diameter.

For years the method used to drill the low-level indicator holes was an old magnetic drill with a drill chuck and twist drills. Starting with a 3⁄8-inch twist drill, you would drill a pilot hole, then use a 5⁄8-inch twist drill to enlarge the hole to 3⁄4-inch. This process continues until the proper hole diameter is achieved. Depending on the hole angle and location, sometimes you need to arc gouge and grind to finish the hole. The entire process took eight to 12 hours per hole, eating up numerous skilled man-hours and equipment usage.

Less horsepower, better results

When Mueller Environmental opened a new manufacturing facility a couple years ago, one of Plant Manager Mark Allen’s jobs was to reduce the time spent on certain operations. His first instinct was to work on the drilling of holes in pressure vessels. “I knew there had to be a better method,” he recalls.

After some research, Allen called in  Hougen Manufacturing sales representative Terry Fuller to take a look at the application. “When Mark drew the hole on the end of the rolled steel to show the angle of the cut, I took a step back. A severely interrupted hole like this on the side of a curved vessel was not an everyday application,” Fuller says. After taking notes and thinking about Mueller’s drilling process, Fuller returned with a standard Hougen HMD915 magnetic drill and a couple of Rotabroach cutters. 

On a thinner walled vessel, Allen had his operators weld a platform that was perpendicular to the angle that needed to be drilled. The platform would serve two purposes: First, as something that attracts the magnetic drill, and second, the platform would act as a guide bushing for the cutter to drill through and into the edge of the vessel. The tests proved successful. The Hougen drilled through the wall of the vessel and at the proper angle without difficulty. But looking to future projects, the depth of cut on the larger vessels was going to be up to 8 inches. Standard Rotabroach cutters are available up to 6 inches.

Fuller took the test information back to Hougen’s engineers, who took a stock HMD915 magnetic drill and increased the stroke from a standard 3 inches to 6 inches to accommodate a longer Rotabroach Cutter. With this extra stroke on vessels requiring a 1-inch-diameter hole, Mueller Environmental could use a 6-inch-long Rotabroach to drill straight through. On 2-inch diameter holes and through the 3-inch-thick sidewalled vessels, Mueller could employ a step-up method of drilling. This means drilling down with one short cutting tool, removing it and then using a longer cutting tool to continue drilling through the remaining wall thickness.  

For deep holes this method lessens the horsepower needed for drilling and helps in chip extraction when the cutting tool is so deep into the hole. 

Extreme angles

Based in Swartz Creek, Michigan, Hougen suggested Mueller use its Industrial Series of Rotabroach Cutter, which offers a thicker cutter wall for increased durability, patented tooth geometry for the long, deep cuts and extended ability to resharpen the blade. Hougen custom-manufactured 2-inch-diameter by 8-inch-long Rotabroach Cutters so Mueller would be able to drill through the longest sections presenting the most extreme angles.

When it came time to build the next challenging pressure vessel, Mark Allen and his team at Mueller Environmental put this new method to the test. Using the modified HMD915 magnetic drill and Hougen’s Industrial Series of Rotabroach Cutters, they went to work.

The first holes needed were 1-inch-diameter holes through a 2-inch-thick side wall vessel. The guide platform was welded on and everything was aligned. The HMD915 drilled down through the plate, into and through the side wall of the vessel in 20 minutes, saving Mueller more than 7.5 hours compared with its previous method.

The next vessel required 2-inch-diameter holes drilled through a 3-inch-diameter sidewall with a total drilling depth of 8 inches. Once the guide plate was attached, Allen’s team drilled through the guide plate and down 3 inches. Next, they inserted a 6-inch-long cutter into the hole and continued drilling down until the cutter bottomed out. Finally, they used the custom 8-inch Rotabroach Cutter and completed the hole. Total drilling time was 45 minutes, saving Mueller 11 hours. 

“It wasn’t only the time savings we are experiencing but also the quality of the holes,” Allen says. “Each hole is round and true with a great finish. We get more accurate readings from our level indicators now. It’s much easier to stay within tolerances, and the tool will pay for itself.” 

With the success of its patented vessels to clean natural gas, and the time-saving methods being used by Allen and his team, Mueller Environmental expects to increase production by the end of the year. 

“Hougen has been a big problem solver for us. We’ve been able to find a better way and in doing that, it saved us money,” Allen says. The company also saved time—eight to 12 hours per drilled hole, depending on the hole size. “We hope that once we expand our manufacturing facility, we will be able to bring in more Hougen drills—not only for use on the pressure vessels but on other parts we fabricate as well.” MM

With more than 16 years of experience in the industrial and construction field, Greg Londrigan has been Hougen Manufacturing’s marketing manager for the last 12 years.