A three person Hydro crew (pipefitters) were tasked to lower a 20-pound propane cylinder from a deck approximately 60 feet above grade using a rope and a propane cylinder sling. A quick release knot was used to secure the rope to the propane cylinder sling. While lowering the cylinder, it is believed that the frayed end of the knot caught in the grating and the weight of the load caused the knot to untie; as a result the propane cylinder dropped approximately 55 feet to the ground. The cylinder was damaged upon impact, but was not breached. The Emergency Response Team was notified and they assessed the cylinder for leakage (no leak detected). The propane tank was removed from the area and was taken to a safe location to be vented (emptied). No one was injured; however, a person was only 5 feet from where the cylinder impacted the ground.