Increasing the flow rate by 20 times for stock preparation at a paper mill had several benefits, including saving energy at the rate of $30,000 to $40,000 per year. Achieving that savings took some clever automation by Givens Control Engineering.
The paper mill had purchased a pulp screen sized for 1200-1400 gpm, assuming that future paper grades would require it. But there were few grades that required even 350 gpm, and most used 70-100 gpm. "It occurred to people in the mill that a lot of energy could be saved by running the system at the max flow of 1400 gpm and having it fill a very large tank they were already using, and then have it stop for many hours," explains Glenn Givens.
Basically, the system had a fixed-speed pump followed by a pressure control valve, a rotating basket screen and an Accepts flow control implemented by a flowmeter and valve. It also had a Rejects flow control, a Rejects dilution flow control and an Accepts recirculation flow—each of which had their own flowmeter and valve. In total there were four flow control loops and one pressure control loop, all of which had to be well-tuned.
Two alarms had to be changed from low- to high-priority because operators were ignoring them when the screen was started automatically. The big problem, though, was that the existing shutdown sequence for the screen flushed the pipes, purging some pulp to the floor. The cost of the pulp was significant, so Givens had to rewrite the sequence to flush the pipes back to the source.
"When we first started the program, operators were certain that something catastrophic would happen because we were operating the screen at a far higher throughput than they had ever seen," says Givens. "But at that point, we were just over half the minimum design flow. During the implementation phase, we found that the screen's cleaning efficiency was improved at high flow rates. Needless to say, the project paid for itself quickly."