A sootblower isolation wall box provided for sealing the wall box opening of a large scale boiler to provide sootblower lance tube access to the boiler. The wall box assembly includes a clamping seal assembly having radial sealing segments which are individually actuated. Applying fluid pressure to actuating cylinders causes the seal segments to be clamped against the outer circumference of the sootblower lance tube. During cleaning cycles, the segments are actuated to provide a radial clearance with the lance tube. Preferably, the clamping seal assembly is used in connection with another seal assembly which is relied upon to provide sealing during the cleaning steps. In a preferred embodiment, the additional seal assembly is in the form of a labyrinth-type seal assembly.