Gas blows have been used in the power industry to clean fuel gas lines during the plant commissioning process, but this practice is being seriously evaluated and reconsidered by regulatory agencies and public safety organizations due to fire and explosion hazards, as well as recent accidents and loss of life. This article looks at the recent practices at Pacific Gas & Electric Company in power plant construction. It is not intended to reflect industry practices.
Air blows were selected for the Colusa Generating Station, PG&E’s second combined-cycle power plant, and the process can be summarized as follows:
- Compressed air at 125 psig was used. The process took 9 days total. Actual blowing time was about 35 hours. Remaining time was used for other activities in the process.
- Each blow lasted several seconds, and was repeated about every 15 minutes.
- Labor cost to install the contractor’s equipment and temporary piping for air blows is considered to be about the same as that for gas blows,
- U.S. Chemical Safety Board on the Kleen Energy Natural Gas Explosion, Final Report and Urgent Recommendations, 6/28/10.
At the Gateway Generating Station, the first combined-cycle power plant for PG&E, natural gas was used to clean the fuel gas piping system. To avoid risk of injury to any personnel working at the facility, the activity took place on a Sunday.
The gas blows lasted about 30 minutes, and the entire process took about 75 minutes.
About 0.5 million standard cubic feet of natural gas, which is primarily methane, was released to the atmosphere. In some other plants, much of the pipe was pre-cleaned by other mechanical means to reduce the time for the gas blows, and therefore the amount of natural gas used.
On Feb. 7, 2010, the Kleen Energy Facility in Middletown, Conn., experienced a natural gas explosion that killed 6 workers and injured approximately 50 others. As a result of this explosion and several others of a similar nature, the U.S. Chemical Safety Board (CSB) investigated this accident and provided a Final Report 1 on June 28, 2010.
Among the Urgent Recommendations made by the CSB in the Final Report, a significant recommendation to OSHA, NFPA, EPRI, the State of Connecticut and others was to make changes to codes/standards or enact legislation to prohibit the use of a flammable gas that is released to the atmosphere to clean fuel gas piping. The alternate methods include the use of air blows or pigging with air.
PG&E has recently constructed two combined-cycle power plants in California where gas blows and air blows for cleaning the fuel gas piping were used. At the Gateway Generating Station gas blows were used, a previously common method used at power plants prior to the Kleen Energy accident and the most recent recommendations by the CSB. At the Colusa Generating Station air blows were used, a method recommended by the CSB.
This paper is written to discuss the use of both methods of fuel gas pipe cleaning.
Colusa Generating Station – Air blow
Colusa Generating Station is a 640 MW, “two-on-one” combined-cycle power plant with two General Electric (GE) 7FA combustion turbine generators (CTG), two heavily duct-fired heat recovery steam generators (HRSG) and one GE 330 MW steam turbine generator (STG). The air blows for the Colusa plant were performed in the first week of August 2010, and the plant entered commercial operation in December 2010.
Boundaries and requirements
The air blows were designed to remove loose material, water and construction debris from the underground piping. Above ground piping and piping that might have been significantly dirty were hydrolazed and vacuumed out before air blowing.
The fuel gas piping to be cleaned was from the utility metering terminal point (TP) to the individual TP at each CTG’s fuel conditioning skid, and both HRSG duct burner pressure reducing stations.