Patent ID: 7849805

Claim:
A primary air system for a combustion plant providing a flow of primary air to at least one coal pulverizer emitting pulverized coal in a downstream direction to a furnace, the primary air system comprising: a first primary air line for feeding hot primary air to the coal pulverizer; a second primary air line for admixing of cold primary air into the hot primary air line at a mixing location, the mixing location being disposed at a position upstream of the coal pulverizer; and a control system for shutting off the flow of primary air to the coal pulverizer, when the coal pulverizer is being repaired or serviced while the furnace is in operation, the control system including a first flow control butterfly valve disposed in the first primary air line at a position upstream of the mixing location, the first flow control butterfly valve including sealing strips, a second flow control butterfly valve disposed in the second primary air line at a position upstream of the mixing location, a shut-off butterfly valve, disposed in the primary air line intermediate the mixing location and the coal pulverizer, the shut-off butterfly valve including a tandem wing having substantially parallel, first and second wings defining a space therebetween, the tandem wing being supported at a cental location and being movable between an open position and a closed position, and sealing strips, a pressure relief line providing fluid communication with the space between the first and second wings when the tandem wing is in the closed position, and a pressure relief butterfly valve disposed in the pressure relief line.