The last warning was issued two days before the plant blew up and burned on Feb. 7, killing 14 people and injuring many more.
But company spokesman Steve Behm said Monday that Imperial didn't receive that final report until after the disaster.
All the admonitions came independently either from companies or employees of companies hired by Imperial Sugar.
One consultant described the dust collection systems as "state of the art" - for the early 1960s.
Two separate federal investigations have said buildups of highly volatile powdered sugar were a key factor in the explosions and fire.
Dust collection problems are described in reports the company received in 2006, 2007 and 2008 and have been obtained by the Savannah Morning News.
2006: This report by local consultant and former Imperial employee Joe Tillman told the company that the dust collection was inefficient, outdated and "plugged up."
2007: A service technician for the company that sold bag packing machines to Imperial cited a major problem with dust control.
2008: MacAljon Engineering said one of six collection systems had multiple defects and major difficulties collecting explosive sugar dust.
They appear to support contentions by the U.S. Occupational Safety and Health Administration, which investigated the disaster.
OSHA contends that senior management at Imperial Sugar was aware of dust hazards at the plant and didn't eliminate those dangers.
"Imperial did take action in terms of repairs and maintenance to its dust collection systems prior to the Feb. 7, 2008, explosion," Behm said, "along with many other efforts towards improving safety at the facility."
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OSHA is seeking to impose $5.1 million in fines on Imperial for alleged safety violations at the Port Wentworth plant. It also wants $3.7 million for alleged violations at its refinery in Gramercy, La.
The company is appealing.
Whether violations related to worker deaths are willful is a consideration not only in fines but also in deciding whether criminal charges are filed, OSHA officials have said. They have not ruled out the possibility of criminal prosecution.
OSHA spokesman Michael Wald had no comment Monday.
In the past, Imperial President and CEO John Sheptor has denied there were any willful violations at the plant.
On Monday, he referred inquiries to Brian Harrison, vice president for sugar technology.
A recording said Harrison's voice mail box was full. There was no answer at an extension to which calls were referred.
Tim Hale, listed as a company contact in some of the documents, declined to comment. Hale no longer works for Imperial.
Another listed contact, Imperial project manager Derek Kight, did not return a phone call made to seek comment Monday.
"It's heartbreaking that they knew and that people had to die," said Mark Tate, a lawyer for victims of the catastrophe who are suing the company.
Brent Savage, another victims' lawyer, agreed.
"They simply waited until it was too late," Savage said.
He added that he is amending his clients' filings today to contend that Imperial's "willful conduct" led to the inferno.
Air flow issues
The Feb. 5, 2008, report by MacAljon Engineering of Savannah reported the findings of tests made some time after MacAljon proposed them in a letter dated Jan. 7. MacAljon found that two of the plant's collector systems "are incapable of providing enough energy ... ."
Both had air flow of only about half their designed capacity, the report said.
On one collector, it indicated, three of eight connections to the third floor were blocked, with no flow.
Two other collectors, the report said, "are not performing to original design." The operation of the other two were described as sufficient.
A purchase order, also obtained by the Morning News, said Imperial paid MacAljon $17,640 for the report.
"This testing was not required, but instead was performed voluntarily by Imperial," Behm said.
Steven Hunt, who wrote the report, did not return four phone calls made to seek comment.
In August 2006, Custom Technical Solutions of Savannah also found that some systems were running at less than designed capacity.
"There was a lot of buildup in the duct work in each system," the report said. "Most of this is related to the inefficiency of the collector. ... The system is plugged up.
"The units that you have were state of the art ... in the early 60's, but there are much better options available today."
Behm acknowledged the Custom Technical Solutions study covered "certain parts" of the dust collector systems.
Citing ongoing litigation, Joe Tillman, who wrote the report and is president of Custom Technical Solutions, declined to comment. Tillman is a former employee at the refinery.
Other documents - which dealt mostly with sugar bag packing machines - also cited dust control issues.
They were written by Derek Myers, a service technician for the Haver Filling Systems Inc., which sold the machines to Imperial.
Haver sent Myers to troubleshoot difficulties Imperial reported concerning the machines.
"One major problem is still the strength of the dust extraction," Myers wrote. "This is so weak that the dust is not transported away from the machine ... ."
In the same report, dated Feb. 4, 2007, Myers cited pressure buildup that he said caused dust to leak out of the bags when they were filling.
Myers also said he found repeated lapses in maintenance.
They included irregular lubrication and cleaning of machines, failure to make needed repairs, and disconnection or dismantling of some parts.
He said he found one part "covered with a black-brown grime," which damaged the seal on another part.
"This has been pointed out previously," he wrote.
Holger Ferst, an official at Haver Filling Systems, said Myers has left the company and lives in Germany.
Citing ongoing litigation, Thomas Reckersdrees, vice president of Haver, declined to comment.
Next report soon
The outside reports dovetail with some of OSHA's findings following its investigation.
It said it found that dust collector fans at the refinery's south packing house were running at 50 percent "or less" of design capacity.
The resulting dust buildup "contributed to fuel load and supported secondary explosions," OSHA said in one of its citations for an alleged safety violation.
At least seven fatal injuries and 15 nonfatal ones occurred in the packing house, said John Bresland, head of the U.S. Chemical Safety Board.
The safety board is due to release a report on its findings in June.