Back to Information Document HSE 690/13
1 Un-cracked Boulton-type mill end plates have a large margin of safety if worked at pressures that are less than 0.5 bar.
2 Any cracks that develop in the end plates have to reach very large sizes before becoming unstable, when mills are worked at pressures below the 0.5 bar limit.
NOTE: Both these conclusions only stand if the mill is operated below 0.5 bar.
3 A mill running at 23 rpm with two adjacent cracked ribs (less than 900included angle) should have nearly three years of safe continuous working life remaining. Once the crack is through seven ribs the continuous working life is less than a day.
4 Operating procedures to ensure pressures do not exceed 0.5 bar are essential.
5 The findings of the report allow a realistic specification of safety procedures for the operation of potentially cracked mills. They identify those flaws which are:
1) not immediately significant;
2) significant but are sufficiently safe to allow the mill to continue running whilst repairs are effected;
3) dangerous and require the mill to be immediately withdrawn from service and repaired or scrapped.
6 The safe working of the mills depends upon inspection being carried out for potential flaws at agreed and regular intervals.