A primary air system for a combustion plant provides a flow of primary air to at least one coal pulverizer, which emits pulverized coal in a downstream direction to a furnace. The primary air system includes a first primary air line for feeding hot primary air to the coal pulverizer and a second primary air line for admixing of cold primary air into the hot primary air line at a mixing location positioned upstream of the coal pulverizer. A control system shuts 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 includes a first flow control butterfly valve disposed in the first primary air line at a position upstream of the mixing location and 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, includes a tandem wing supported at a cental location and movable between an open position and a closed position. The tandem wing has parallel, first and second wings forming a space therebetween. A pressure relief line provides fluid communication with the space between the first and second wings when the tandem wing is in the closed position. A pressure relief butterfly valve is disposed in the pressure relief line.