Recently a question was circulated about how to safely install a feedwater heater on line. This consultant simply stated that it is necessary to have double block valves to protect pesonnel. However, Mr. Steven Kidwell,Project manager of Burns and McDonnal made a more informative comment. It is repeated here. This consultant is in complete agreement with Mr. Kidwell.
Here is Mr. Kidwell's comment
Double block and bleed is the only safe way of performing an on-line replacement. And it needs to be on all the pressurized (or vacuum) pipes, not just the mahor ones. The second block can be a spectacle flange to save money, but those spectacle flanges need to be installed ahead of time at an outage.
The other consideration is that normally the heater serves as an anchor for the pipe when the thermal stress analysis is done. If the heater is removed while the pipe is hog, the pipe will spring when the cut is made. So large temporary restraints need to be designed to hold the pipes in place so the new heater nozzles will mate up. Vof feedwater pipes and extraction pipes, these restraints will be massive. Commonwealth Edison pulled this off years ago, but because of the expense involved,they never considered it again.
Here is Mr. Kidwell's comment
Double block and bleed is the only safe way of performing an on-line replacement. And it needs to be on all the pressurized (or vacuum) pipes, not just the mahor ones. The second block can be a spectacle flange to save money, but those spectacle flanges need to be installed ahead of time at an outage.
The other consideration is that normally the heater serves as an anchor for the pipe when the thermal stress analysis is done. If the heater is removed while the pipe is hog, the pipe will spring when the cut is made. So large temporary restraints need to be designed to hold the pipes in place so the new heater nozzles will mate up. Vof feedwater pipes and extraction pipes, these restraints will be massive. Commonwealth Edison pulled this off years ago, but because of the expense involved,they never considered it again.
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