Among the tangle of pipes and valves at Tonawanda Coke’s sprawling 188-acre facility, the small, narrow “bleeder” valve poking out from the coke oven gas line might seem innocuous at first glance.

But after six days of testimony in a downtown courtroom, that seemingly inconsequential pressure relief valve is emerging as a centerpiece in the federal government’s criminal prosecution of the Town of Tonawanda manufacturer.

The valve and what escaped from it – the allegation is that it’s coke oven gas with benzene – are at the heart of the charges of clean-air violations and obstruction of justice facing the company in one of the biggest environmental trials in decades.

Courtroom revelations about a bleeder valve that may have released coke oven gas with benzene into the air are almost certain to fuel the ongoing debate over the health of residents living in and around Tonawanda’s industrial corridor.

State Health Department officials recently released a health data review that found elevated levels of certain cancers and birth defects among those people but stopped well short of identifying a cause.

The review was prompted by an air quality study by the state that found concentrations of benzene and formaldehyde much higher in the Tonawanda area than other industrial and urban areas.

Witness after witness in the federal court trial have testified about the frequency of the valve’s openings and what they describe as Tonawanda Coke’s nervousness about inspectors learning of the emissions spewing from it.

“We can’t have that going off when they’re here,” Mark L. Kamholz, the company’s environmental controls manager and a defendant in the case, is alleged to have told another employee in the days leading up to an April 2009 inspection.

Patrick Cahill, the employee who said he heard Kamholz say that, also claims he overheard Kamholz tell state and federal inspectors that it was steam, not coke oven gas, coming out of the valve.

“I was mad,” Cahill said when asked about Kamholz’s comments. “Mark should have known it was coke oven gas.”

Cahill, who now is plant manager, said he felt put on the spot by Kamholz’s comments during the 2009 inspection but admitted he had already taken steps to prevent the valve from opening.

He acknowledged adjusting the back pressure on the coke oven gas line each day the inspectors were on site so that the bleeder valve would not open as often.

He never told anyone about the adjustments but later testified that he assumed that’s what Kamholz and the company wanted him to do while the six-day inspection was under way.

“I didn’t want anyone to find out why I was making the adjustments,” Cahill told the jury and Chief U.S. District Judge William M. Skretny.

The bleeder valve is at the heart of six of the 19 criminal charges against Tonawanda Coke and Kamholz, and yet it wasn’t until the 2009 inspection by state and federal officials – years after the valve was put in place – that it became an issue.

An official with the U.S. Environmental Protection Agency who was there the day the valve went off said Kamholz had previously denied knowing of any bleeder valves at the plant.

“Mr. Kamholz was asked whether there were any pressure relief valves, and he said, ‘No,’ ” Martha Hamre, one of several EPA officials at the 2009 inspection, told the court.

Under cross-examination, Hamre acknowledged that the bleeder valve had been disclosed in a 2003 report by Tonawanda Coke and that she and others missed that initial reference to the valve.

An official with the state Department of Environmental Conservation also conceded that the valve had been overlooked on previous inspections of the River Road plant.

“You’re supposed to look for them, aren’t you?” Rodney Personius, Kamholz’s defense attorney, asked the DEC official. “If you were doing a full-blown compliance evaluation, you should have found the PRV?”

Personius and Gregory Linsin, a lawyer for Tonawanda Coke, have argued since the first day of the jury trial that EPA and DEC officials should have been well aware of the valve.

They also have tried to suggest that Cahill acted alone and without Kamholz’s knowledge when he adjusted the pressure on the coke oven gas line during the inspection.

“Did you ever tell Mr. Kamholz you were changing the pressure on the valve?” Personius asked at one point.

“No,” Cahill said.

“Did Mr. Kamholz know what you were doing?” Personius asked.

“No,” Cahill said.

Cahill and others also testified that coke oven gas, not steam, was coming out of the valve and at a rate far greater than what the defendants have claimed in the past.

“What was the frequency that the valve would release coke oven gas into the atmosphere?” Assistant U.S. Attorney Aaron J. Mango asked at one point.

“Every 20 minutes,” said Anthony Brossack, a company employee.

Like Cahill, Brossack said the valve, which was shut down in 2010, released coke oven gas as a way of relieving pressure in the coke oven gas line.

In addition to the Clean Air Act violations, Tonawanda Coke and Kamholz are accused of violating a federal law known as the Resource Conservation and Recovery Act. The law covers the storage and treatment of coal tar sludge, one of the byproducts of making coke.