Recently a client who uses anhydrous ammonia (NH3) had a significant event involving their NH3. This event is the perfect scenario to demonstrate that “inherently safe design” is by far the best design. The incident involved unloading a tank truck of NH3 into a bulk tank. A single valve was in its CLOSED position when it was supposed to be OPEN. This was called out very clearly and specifically in the unloading SOP. Two (2) trained peopled missed this deviation from the SOP. The normal transfer pump was out of service so the delivery company had agreed to use their own pumps on their trucks. NO MOC was done on this change BTW.
As soon as the transfer began, there was a significant release of NH3 through the hydrostatic relief valves on the transfer piping. This LOPC event began with the failure to “follow the detailed SOP”, but that simple error and active failure is NOT the moral to this story.