At an organic chemical plant, an ammonia (NH3) discharge occurred inside a urea unit operating in slowdown mode in preparation for a repair job. A regulation valve was activated to limit the quantity of NH3 fed to the reactor by re-injecting a portion of the flow to supply the pump; however, the valve was vibrating strongly while in this operating mode. Sensing difficulties in adjusting the quantity of NH3 intake, the technicians partially closed a manual valve positioned downstream of the regulation valve. A short time thereafter, 500 kg of NH3 escaped from the unit, which underwent an emergency shutdown. A water curtain prevented the NH3 vapours from becoming airborne. This accident was due to the partial opening of a safety valve discharging NH3 into the absorption device, causing an overload and evacuation into the open air via the washing column. In order to avoid repetition of these conditions, the operator installed orifices on the expansion pipes to evacuate liquid ammonia towards the absorption device. In short, this accident was due to human error.