A plant was restarting after a shutdown. The pulley of a double-lobe blower on a mist separator vent line (Figure 1) suddenly broke while rotating, explosively expelling fragments. Fortunately, the pulley guard (Figure 2) captured the fragments. If the guard had been intended only to prevent contact, or removed for inspection, anybody nearby could have been seriously injured or killed.
A significant amount of water was found inside the blower. The water came from the separator, which was designed to retain mist and small amounts of fibrous dust from the process. The water stopped the rotating lobes suddenly, shearing them off; their fragments were retained by the blower casing. Rotation of the pulley shaft was also stopped and this, combined with the continued rotation at the pulley rim, caused the pulley to break.
The separator drain line contained a water seal (Figure 1). This and a check valve were there to prevent reverse flow of air, enabling the separator to operate at slightly negative pressure.
“Think about your start-up procedures – and follow them!”