Hi
Normal life of hood arches is about 2 to 3 years. I don't expect a design problem.It is very difficult to come to conclusion without seeing the nature of failure. If the roof bricks are not laid tight enough to hold the bricks, it will fall down due to the thermal expansion and shrinking during any small stoppage.If the bricks became yellowing\sh or greenish it may be due to the alkali or sulfur attack. it will lead to spalling of the bricks and hence the cracks will form.
Regarding the side walls, if sufficeint expansion joints are not given , it will also lead to bulging of lining and hence the crak/failure.
You may also ask your refractory supplier to have a look at the bricks/castable during shutdown.they will be able to guide better. if you send some photograph along with lining data and past history, better solution can be given
Regards
Vinayak
vinayak_icl@rediffmail.com