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The acceptance criteria for target blows on the fuel gas line are similar to what is used for target blows of steam lines, although they are not as stringent. These full pressure air blows were continued until two sequential good (clean) targets are achieved. A good target is one with no raised edge impacts and 3 particles no larger than 1/64? (0.016? or 0.4 mm) on each target.
Results and observations
The air compressor used for the blows was sized at 900 cfm.
The air receiver was sized at 1,000 gallons.
Due to noise concerns, air blows were done for only 11 hours each day, between 7 a.m. and 6 p.m.
Air blows were done without an air dryer for the first three days with little success. An air dryer sized for 900 cfm was added between the air compressor and air receiver. The total time to complete the air blows was 9 days; however, with an air dryer it is believed that the time could be reduced to 4-5 days. If air blows are performed around the clock, this duration could be shortened even more.
Each blow lasted several seconds
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Gateway Generating Station – Gas blow
The Gateway Generating Station is a 580 MW, two-on-one combined cycle power plant with two GE 7FA CTGs, two duct-fired HRSGs and one GE 250 MW STG. The gas blows for the Gateway plant were performed in October 2008 and the plant entered commercial operation in January 2009.
Boundaries and requirements
Similar to air blows, gas blows were used to remove loose material, water and construction debris inside the pipe. High pressure natural gas from the gas pipeline is readily available, so that no additional compression was needed for the cleaning process.
High pressure gas from the pipeline is used to blow through the two piping sections leading to the two CTGs. Bypass loops around equipment are installed to avoid damaging the equipment. Vent piping (6? in diameter) to the atmosphere at each CTG is provided to exhaust the gas to a safe, elevated location for proper dispersion. Gas is available at a much higher pressure than needed (in the range of 600-900 psig), such that the main block valve at the gas meter station was only opened slightly in order to provide a pressure of around 100 psig for cleaning the pipes. During the first blow, incremental steps were taken to increase the pressures from 10-100 psig, after proper observation of the overall system to ensure that there were no safety issues prior to going up to a higher pressure level.
Safety
Similar safety measures were provided, as in the air blow case, consisting of hearing protection, area barricades (especially at the gas exhaust point), etc. The actual blowing process was performed on Sunday, with close communication between both ends of the pipes and the main control room at all times. The block valve at the gas yard was operated by the gas line technician.
Pre-requisites to gas blow
Some minor modifications of the piping at the CTG filter skids were installed to add the vent line to the atmosphere. Bypasses around equipment were used during the blow. Similar to the air blow process, the complete system was walked down prior to the blow.
Gas blow sequence/procedure/equipment
As mentioned above, no additional compression was needed, due to availability of high pressure natural gas.
At the gas yard, the main block valve was cracked open slowly to flow gas into the line to one CTG, for several minutes. Initial gas pressure of up to 100 psig was used as the starting point. The first blow was done slowly to ensure that no problems were encountered, with the individual blow duration lasting up to about 10 minutes.
