* Unit to remain shut for unknown amount of time

* Outage to have negligible impact on plant output

* US Power Gen looking for a buyer for the plant

NEW YORK, Aug 10 (Reuters) - A steam tank ruptured Tuesday morning at the Astoria oil/natural-gas fired power plant in Queens, New York, taking out of service the 175-megawatt Unit 2 for an unknown amount of time, said the plant’s owner US Power Generating Co.

Electricity traders guessed the unit, which is over 50 years old, would never return to service.

The incident had no impact on the other three units, and, since Unit 2 is the plant’s smallest unit, was expected to have negligible impact on the plant’s available power supplies, the company said.

The company called the New York City Fire Department to the plant but there were no injuries and no fire as a result of the rupture.

The tank was insulated and some of the insulation was disturbed by the rupture, the company said, adding, emergency response technicians were on site and testing to determine if asbestos is present in the disturbed insulation and what if any remediation is required.

Last month, US Power Generating hired Goldman Sachs Group Inc ( GS.N

Earlier this year, financial services company Standard & Poor’s said the Astoria plants require significant environmental capital spending to comply with state and federal environmental regulations, including the installation of variable speed circulating water pumps to address water intake issues.

US Power Generating has been selling off its assets to pay down debt, and on Monday sold its 2,950-MW generating fleet in New England to Constellation Energy Group Inc CEG.N for $1.1 billion. The assets are capable of meeting about half the demand in the Boston metropolitan area. See [ID:nSGE6780EY] —————————————————————————————- PLANT BACKGROUND/TIMELINE STATE: New York COUNTY: Queens TOWN: Queens OPERATOR: US Power Generating OWNER(S): US Power Generating CAPACITY: 1,296 MW UNIT(S): Astoria

2 - 175-MW steam turbine

3 - 369-MW steam turbine

4 - 376-MW steam turbine

5 - 376-MW steam turbine FUEL: Oil/Natural Gas DISPATCH: Peaking TIMELINE: 1954 - Con Edison completes Unit 2 1958 - Con Ed completes Unit 3 1961 - Con Ed completes Unit 4 1962 - Con Ed completes Unit 5 Erly 2000s - Con Edison sells generation as part of the

deregulation of the New York State power market.

Orion Energy buys Astoria and gas turbines

Reliant Energy buys Orion. 2006 - Reliant sells Astoria and the gas turbines to

Astoria Generating, which becomes part of US

Power Generating (Reporting by Scott DiSavino; Editing by Lisa Shumaker)