The Trinity Packaging plant in Pueblo, Colo., converts blown film into retail plastic bags and wanted to increase the efficiency of doing so. The converting facility has a variety of specialized equipment to create bags with complex features, such as multi-colored printing or airtight closures. The project began with the goal of improving the efficiency of wicket-style bag-making machines.
Five wicket style bag making machines use aluminum arms with vacuum holes to grab the bag after it has been cut. The vacuum holes then stack the bag on a conveyor. Roots blowers develop the required vacuum pressure; they are multi-stage blowers with several fans stacked together.
The goal was to cut costs and increase efficiency by improving the vacuum system. When the project started, each bagging machine required two centrifugal blowers, each using a 10 hp motor running at full rated speed to create the required vacuum.
Two blowers per machine were used. The pumps sucked much of the petroleum waste released into the air as a result of the bag cutting and sealing process. As residue builds up on the blowers, the vacuum suction would weaken, making the blowers very inefficient. There was no way to accurately monitor the machines to assess the amount of residue buildup, so two blowers had to run all the time to ensure sufficient vacuum.
Moreover, blowers couldn’t be shut down when the machines weren’t running, because the amount of residue buildup would often cause the pumps to lock up and trip the control system at start-up. Furthermore, the motors always ran at full speed even when the process didn’t need that much vacuum.
Maintenance, total cost of ownership
A pilot project on one machine aimed to improve efficiency and overall operations. The first step was to install vacuum pressure transmitters at the inlet of the pumps.
Five transmitters were installed to measure the actual vacuum pressure at the pump and send an analog signal to the input card in the machine’s programmable logic controller (PLC).
Alarms provide notification when the pressure increases too high, a sign that filters need to be changed. A low-pressure alarm means the pump lost its belts. A temperature transmitter monitors the drive enclosure to turn the cooling fans off and on.
Upon switching to the new system, it seemed as if there was a problem with the controls because one line showed very high pressure. Examination of the machine showed that one vacuum hose had come off and sucked several bags in, a problem that would have gone undetected by the previous system.
Moreover, improved monitoring helped with creation of a precise cleaning schedule based on data collected by the transmitters. That information allowed use of the second blower as a backup, instead of having it run all the time, significantly reducing overall equipment and maintenance costs.
Installing drives provides savings
After being cleaned, the blowers run more efficiently, so motors require less power. However, without a variable frequency drive, blower motor speed could not be reduced. A blower was fitted with a drive, and results were monitored for approximately six months. Results showed that when everything is clean, the motor only needs to run at half speed.
Now, based on vacuum transmitter readings, the drive increases motor speed automatically as the arms get dirtier to maintain the required vacuum pressure, which increased energy efficiency significantly.
The pilot expanded to all wicket-style bag-making machines.
Products used
The control and monitoring system includes vacuum pressure transmitters, PLCs, and variable frequency drives. Better pressure and vacuum monitoring along with the variable speed motor drives supplied useful data, so an HMI touchscreen panel was added to the control system to display and help interpret data. An Ethernet card in the PLC communicates with the touchscreen HMI. Data can be monitored on the panel or remotely, which speeds response to any possible issues.
Drives communicate directly with the HMI using the Modbus protocol. Use of Modbus protocol eliminated a wiring and associated costs, avoiding the need to buy analog sensors to send current, rpm and other readings to the HMI, which transmits that information digitally from each drive to the HMI.
Valuable information
It was very easy to program the HMI using the Microsoft Windows-based graphical editor. Creating alarm parameters only required entering setpoints for low and high pressure. When an alarm is triggered, it automatically appears on the screen, and an e-mail is sent.
Hours of operation for each pump is monitored; an alarm tells when to change the oil to increase operating efficiency and machine life span. If any drive fault, belt failure, or pressure fault occurs, the PLC opens the backup gate and turns the backup pump on. The system then e-mails the event tag from the HMI. This happens instantly and automatically, allowing production to continue as if the event never occurred.
As the project progressed, images and specifications for every component used in the system were saved from vendor’s website. This information was converted into bitmaps, and then imported into the HMI. Now, when information is needed on any part, it can be pulled up immediately on the plant floor from the HMI touchscreen.
Dramatic savings
The new automation system greatly reduced the amount energy used and cut the number of blowers needed for regular operations in half. This has significantly decreased overall operating costs on these machines. As energy prices continue to rise, the increases have been offset by running the blowers only as needed to maintain sufficient vacuum pressure.
The old system needed two 10 hp motors running at full rated speed for the two centrifugal blowers to successfully operate each machine. Removing the need for a second full-time blower on each machine has eliminated approximately one week of maintenance for each pump on the five machines. The load on the remaining motor also has been reduced because the drive can adjust motor speed to deliver only the amount of vacuum needed to hold the bags in place.
An outside firm hired to perform an energy audit and calculate the overall savings used, as a basis, a total annual operating cost of $49,000 per year for the wicket bag making machines. The audit said operating costs will decrease by $13,000 a year, more than a 25% reduction.
A new project using the latest PLCs is underway to upgrade older equipment in other parts of the plant.
About the company
Trinity Packaging Corp. is a privately held family business. Founded in 1917 as a paper bag converter, Trinity Packaging installed its first plastics extruder in 1979. Since then it has expanded its focus to include the flexible films market, enabling the company to offer a wider variety of products. The company has three plant locations in the U.S. that manufacture plastic products for a broad base of American customers in a wide range of industries.
- Charles Landry is maintenance manager, Trinity Packaging Corp. Edited by Mark T. Hoske, content manager CFE Media, Control Engineering and Plant Engineering, mhoske(at)cfemedia.com.
