Accidents sometimes occur when draining oil in an ammonia system. What can happen is that the operator cracks the shutoff valve, shown in Fig. 13.6, but nothing happens because the oil is cold and stiff. The operator then slowly continues to open the shutoff valve, which may require about eight or ten turns to open completely. At some point during the opening process, a tunnel breaks through the oil, allowing ammonia liquid to rush out the drain. The operator may at first try to close the shutoff valve, but the required time is so long that the operator is forced to try to escape. Drain lines should be equipped with a quarter-turn ball or butterfly valve that could quickly interrupt the rush of ammonia and oil. Instead of the quarter-turn valve, some plants are equipped at this position with a spring-closing valve that automatically closes if the manual opening force is withdrawn.