Did diesel fuel for WTC 7’s emergency generators feed the fires?

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Short answer: we don’t know. The final NIST WTC 7 report should shed more light on this issue, which is an important one.
"Across the country, diesel-powered generators are used in buildings like hospitals and trading houses, where avoiding power outages is crucial. Partly for that reason, a definitive understanding of what happened in 7 World Trade Center is vital to investigators, said Jonathan Barnett, a professor of fire protection engineering at the Worcester Polytechnic Institute."
– “Diesel suspected in WTC 7 collapse” James Glanz, New York Times, November 29, 2001.

WTC 7 contained up to 43,000 gallons (162,273 liters) of diesel fuel for its emergency generators. It is believed that at least some fuel pumps did kick on after the Con Edison power plant went down at 9:59 (see FEMA, NIST reports), perhaps fueling the fires, although this remains speculation.

7,000 gallon tankers

Of particular interest in the investigation are the two 6,000-gallon diesel tanks installed below grade for Salomon Brothers in 1990. Through pressurized pipes, these fed nine emergency generators on the fifth floor. When the tanks were found beneath the debris, they were empty, but diesel fuel was not detected in the ground below, which seems to preclude leakage directly from the tanks:
"NIST reviewed the report of an environmental contractor hired in the months after the collapse of WTC 7 to recover remaining fuel and mitigate any environmental damage from the second system’s two 6,000 gal tanks. The tanks were damaged and appeared to be empty and the report stated that neither the underground storage tanks nor their associated piping contained any residual petroleum product. No residual free product or sludge was observed in either underground storage tank. Evidence suggests that this fuel did not leak into the underground soil and contaminate it, and, therefore, could have been consumed in the building."(Source: pdf, p. 4)

From NIST NCSTAR 1-1J: Design, Installation, and Operation of Fuel Systems for Emergency Power in World Trade Center 7, p. 46-47:
"Based on the available documentation, the pressurized loop could have endlessly discharged fuel out of a broken FOS pipe. A break in the FOS pipe, between the pump set and FOS valve rig, could have activated the pump set. In the event of a FOS pipe break, the fuel could have emptied out of the valve rig, sending a low level signal to the pump controllers from the liquid level switch and starting the pump set. Fuel would have flowed out the break until the storage tank(s) were emptied. However, if the double walled construction of the FOS piping was designed similar to that of the Base Building system, it could have been possible that a leak detector switch would have shut off the pump set. This cannot be determined from the available documentation. The electrical characteristics of the system are unknown and need to be analyzed to determine if an electrical malfunction would have started or stopped the fuel pump set."

Debunk911myths.com makes the interesting point that the south face damage seen in the photo below aligns with WTC 7's emergency generator fuel distribution system.