Air Blow Startup
Air blows are a relatively new technology in the power plant construction industry. The use of air blows must be worked into the schedule early in the preparation process, to ensure proper cleanliness specifications are issued to all vendors supplying portions of the steam generation system. Mill scale must be completely removed and proper welding techniques must be employed during the erection of the piping in preparation of air blows. Millscale removal utilizing a chemical cleaning is often recommended to ensure that the surfaces are adequately prepared for the air blows. A solid passivation layer on the piping will prevent flash rusting during the air blows.
It should be noted that air blows are the only method available to achieve a cleaning force ratio (CFR) or disturbance factor above 1.2 (industry standard) in the superheaters of the boiler. Manufacturers of boilers and HRSGs discourage steam flow rates during steam blows that would allow for carryover of moisture from the drum to the superheaters. As a result of this, steam flows through the superheaters cannot be greater than maximum normal operation. Air blows do not have this limitation and thus provide much more effective cleaning of the superheaters.
A comprehensive engineering analysis of the plant design is needed to ensure the feasibility of air blowing. The engineering study must reveal the pressure required to achieve proper disturbance factors, pump up times, blow down times, required exit piping sizes, disturbance factors in all portions of the flow path and the order in which the piping systems should be blown. The use and selection of the proper equipment is mandatory. Oil free air compressors, boosters capable of easily achieving the required blow pressures, air drying and cooling provisions should also be included. Heavy wall piping with full thickness and pressure rated flanges, heavy-duty blow-down silencers with debris containment systems and fast acting valves are also part and parcel of air blows.