Valve solves problems in space restricted applications
Universal Fiber Systems LLC, Bristol, Va., manufactures high-quality solution-dyed synthetic-filament-based fibers for the flooring, transportation, and performance textile industries. The company uses several high-pressure dense-phase conveying systems to transfer various materials from large outdoor silos into the facility’s production processes. In early 2004, the company wanted to install a new high pressure dense-phase conveying system beneath an existing 70-foot-tall silo to transfer nylon pellets. When designing this new conveying system, the company worked with a valve supplier to maximize the system’s capacity and improve its design by eliminating several components used in the existing conveying systems.
Determining existing system flaws
Each existing conveying system consists of a pressure vessel with a high level switch inside it, a rotary valve, an inflatable–seal butterfly valve, a system controller, and various lengths of 4–inch–diameter conveying line. During operation, a system‘s rotary valve feeds material from a silo into the pressure vessel. When the material inside the pressure vessel reaches the high–level switch, the system controller shuts off the rotary valve and closes the butterfly valve, stopping the material flow and sealing the pressure vessel. The pressure vessel is then pressurized, and the material is pushed out through the conveying line in dense phase to a receiving vessel that feeds a downstream production process. After a preset time, the system controller opens the butterfly valve and activates the rotary valve to meter more material into the pressure vessel, repeating the process. The company operates the conveying systems continuously, 24 hours a day, 7 days a week.
The company wasn’t entirely satisfied with the systems’ operation. “They operate inefficiently because we can only fill a pressure vessel to about eighty percent of its capacity,” says Tim Peyton, Universal Fiber Systems plant engineer. “We can‘t put in more pellets because we have to ensure that the material doesn‘t obstruct the butterfly valve. Material obstructing the valve will prevent the valve from sealing the vessel properly, and the conveying system won‘t function correctly.”
The company was also dissatisfied with the existing systems‘ design and installation requirements. “Each conveying system requires a lot of vertical space between a silo and pressure vessel because of the rotary valve and butterfly valve,” says Peyton. “And since the new conveying system was to be installed beneath an existing silo where one had never been before, we needed to minimize the vertical space requirements to fit it under the silo with minimal modifications.”
Looking for a new system component
When designing the new conveying system, Peyton called Robbins & Associates, a local manufacturers’ rep, and told Jim Robbins what the company was looking for. Robbins told Peyton about a flat rotating-disc valve made by Everlasting Valve Co., South Plainfield, N.J., that’s capable of cutting through a standing column of material and sealing a dense-phase conveying system’s pressure vessel. With this valve installed above the new conveying system‘s pressure vessel, Peyton realized, the pressure vessel would be able to be filled to 100 percent capacity, and less space would be required between the silo and pressure vessel for installation because the rotary valve and butterfly valve would no longer be needed.
Peyton contacted the valve supplier to learn more about the valve. “The valve’s capabilities almost sounded too good to be true,” says Peyton. “So I asked the supplier’s vice president of sales, Frank Hawley, to prove the valve’s capabilities by allowing me to install a valve in our new conveying system with an assurance that it would work. To my surprise, Frank agreed.”
After installing the 4-inch valve, the company experienced some minor problems with its operation.“At first, the valve cut the standing column of pellets and sealed the pressure vessel pretty well,” says Peyton. “But after about two thousand cycles, the valve would stop closing properly and we’d have to stop the conveying system to clean it out. So we asked the supplier to fix the problem. We told them that a valve would have to operate satisfactorily on a bench test for five thousand cycles without any problems before we would install it in our conveying system.”
The supplier tested the valve using the company’s nylon pellets and found that its single cylinder wasn’t powerful enough to continuously actuate the valve properly on the standing column of pellets and that the beveled edges of the discs weren’t sharp enough to effectively cut through the standing column of pellets. To solve this problem, the supplier added another cylinder to the valve and increased the angle of the discs’ edges.
“Over time, the air pressure to the initial valve’s cylinder naturally tended to drop, and when that happened, there wasn’t enough pressure to force the discs completely through the column of pellets every time,” says Hawley. “So we sharpened the discs’ edge angle and added a second, same–size air cylinder to the valve. We call it a tandem cylinder. Now, the two air cylinders provide more power to the discs during actuation, maximizing the air pressure so that the discs always cut through the standing column of pellets.“
After the supplier successfully modified and tested a replacement valve, Peyton installed it in the conveying line, and it‘s been operating continuously since.
The flat rotating–disc valve
The Bulk Material Valve (BMV) flat rotating–disc valve is installed in the short section of 4–inch–diameter vertical drop line between the company‘s silo and the new conveying system‘s pressure vessel. The valve consists of two discs and two seats, a disc drive, and a pneumatically actuated tandem air cylinder. The discs are connected to the disc drive, which is powered by the tandem air cylinder, and the full–ported seats are part of the valve body. The pneumatically actuated valve requires a compressed–air source capable of supplying at least 60 psi of air pressure to operate properly. The valve‘s body is constructed of cast iron, and its discs and seats are constructed of an investment–cast Type 440 stainless steel that has a hardness rating of 48 to 52 on the Rockwell C Scale.
In operation, the pellets move by gravity through the valve’s unobstructed open body down into the pressure vessel. The discs and seats are perpendicular to the material flow, which prevents the pellets from impacting and wearing the discs and seats and other valve components. After a predetermined time, the system controller actuates the valve, and the tandem air cylinder moves the disc drive to push the discs through the valve body and over their respective seats. When this occurs, the discs cut through the standing column of pellets to stop the material flow and seal the pressure vessel. The discs can easily slice through the column because each disc has a beveled edge to facilitate cutting.
A spring presses each disc against its seat to help maintain the pressure vessel‘s seal during pressurization. If the pressure inside the vessel lifts one disc off its seat, breaking the seal, then that same pressure is held by the back side of the other disc to maintain the seal. In this case, the other disc becomes the sealing disc. (Because both discs are capable of sealing the pressure vessel, this valve is also called a bi–directional valve.)
Some pellets may remain in the space between the two discs after the valve closes, but when the valve opens, these pellets are moved out of this area by the pellets moving from the silo to the pressure vessel. And since the valve‘s body is round and open, there are no small spaces where material or fines can accumulate, which can be a problem with valves that have pockets or ledges.
Increasing conveying capacity
The new valve has increased the amount of pellets that the company can load into the conveying system‘s pressure vessel.“The ability to fill the pressure vessel to capacity increases the amount of pellets we can transfer each conveying cycle, maximizing the transfer rate,” says Peyton. “In fact, I don‘t have to worry about installing another dense-phase conveying system to transfer nylon pellets because this conveying system has enough capacity to handle all of our needs.”
The supplier’s valve also made it possible for Peyton to install the new conveying system beneath an existing silo with minimal modifications. “The valve requires very little vertical space for installation,” says Peyton, “and it eliminated the need for a rotary valve and butterfly valve. This simplified the conveying system’s control system by decreasing the number of controls needed to operate the system. Removing the rotary valve, high-level switch, and butterfly valve also reduced the conveying system’s maintenance and power requirements.”
Since Peyton installed the supplier‘s valve in early 2005, it‘s been operating 24 hours a day, 7 days a week, transferring about 1,400 pounds of nylon pellets per hour flawlessly and completing more than 350,000 cycles. (If necessary, the system can handle up to 3,000 lb/h.) “In the two years that the valve has been operating, I’ve only had to replace one solenoid,” says Peyton. “Other than that, the valve has operated without incident.” PBE
Note: To find other articles on this topic, look under “Valves” in Powder and Bulk Engineering’s Article Index at www.powderbulk.com or in the December 2006 issue.