This case study describes a backflow scenario in a pressurised leak detection system. Backflow can occur in case of a relatively large leak (crack in liner), where the flowmeter installed downstream of the equipment acts as a flow restriction, allowing pressure to build up.
The following case study reports on a serious incident in a urea plant where a leak in the weld overlay in the urea reactor bottom resulted in a costly plant shutdown and near miss of rupture of the high pressure vessel. It is known that carbon steel can corrode with rates of 1000 mm/year not taking into account erosion due to flashing. The impact of the NH3/CO2 ratio on the corrosion rate is questionable.
High pressure urea equipment often has lined nozzles. A lined nozzle is a full strength carbon steel nozzle that is protected against carbamate corrosion by a 5 or 6 mm thick stainless steel liner plate, which is welded to the carbon steel nozzle on either end. This design is however very vulnerable to fatigue cracking due to the difference in thermal expansion between the austenite liner and the carbon steel nozzle. History shows that such a design will lead to cracking in the long term. The following case study reports on a serious incident in a urea plant where a leak in a urea reactor nozzle caused a plant shutdown but could have resulted in rupture of the high pressure vessel.
A Stamicarbon urea plant attempts to start up after a scheduled turnaround. Due to maintenance issues, it is necessary to shut down and block in the synthesis section several times. Although licensor’s procedures have been followed, several signs of active corrosion are noticed in the liner of the reactor. What could the cause be for this unexpected behaviour? Can sharing experiences from colleagues from other urea plants provide valuable support to find the root cause or even provide new insights into possible new causes? One observation is that the typical heating up rate of a liner in a reactor is much higher than recommended (refer to diagram). The condensation heat of steam heats up the liner much faster than the carbon steel pressure bearing wall. This creates stress on the liner and affects the lifetime of the liner. Another observation is a temperature rise in the reactor during a blocking in situation. This can be a cause for loss of oxygen required for passivation, resulting in higher corrosion rates.
The falling film high pressure carbamate condenser (HPCC) was the first type of high pressure carbamate condenser applied in urea stripping plants. Did you know that the first Snamprogetti stripping plants also had falling film carbamate condensers? In 1978 Mr Umberto Zardi, later founder of Casale, invented the horizontal kettle type condenser for Snamprogetti urea plants.
We look at safety, health and environmental (SHE) management and hazards at nitrogen fertilizer plants and the importance of the International Fertilizer Association’s ‘Protect & Sustain’ certification scheme.
Liquid ammonia is one of the feeds to the urea plant. It typically comes from the ammonia plant at a pressure of about 20 bar. In the urea plant this liquid ammonia is further pumped up to synthesis pressure conditions, sometimes after passing through a buffer tank, a heater and/or a filter. According to the ammonia phase diagram, when liquid ammonia flashes to atmospheric pressure temperatures can drop to as low as -33°C under worst case conditions. In case one or more of the above mentioned items of equipment needs to be drained for maintenance, a decision needs to be made where to drain this liquid ammonia. In a urea plant the options are the ammonia water tank or the vent stack, but which is best option?