Section: CRUNS Plant Manager+

Every urea plant continuously fights again corrosion. The intermediate product ammonium carbamate is extremely corrosive under synthesis conditions. The applied materials of construction require oxygen to form a protective passive layer of chromium oxides. The ammonium carbamate solution will continuously dissolve the passive layer, therefore it is vital to continuously supply oxygen, typically in the form of air, to maintain the passive layer. During blocking-in conditions of the synthesis section it is not possible to add air and the oxygen present will be consumed as a result of the passive corrosion reactions, while at the same time the passive layer dissolves in the ammonium-carbamate solution. At a certain point, the oxygen content in the solution becomes too low to assure a passive layer. At that moment active corrosion will start with much higher corrosion rates than passive corrosion. The picture on the left side shows the passive layer (blue, brown, grey surface) and the picture on the right side shows active corrosion (a shiny silver surface). It is important to realise that once active corrosion starts it cannot be stopped, adding more oxygen at this stage, for example, will not work. Active corrosion will continue, leading to the risk that the protective layer will be severely damaged. The only way to solve this situation is to drain the synthesis section and re-passivate the surfaces.

The fact that biuret is toxic to plants has been known for a long time – since the middle of the last century. Very sensitive (pineapple, citrus), moderately sensitive (cereals, legumes) and resistant (conifers) plant species were identified. The maximum concentration of biuret in urea for each species has been established for soil application and foliar application. Most field crops easily tolerate foliar fertilization with urea, which contains 1 wt-% biuret. Potatoes and tomatoes are more sensitive – for foliar feeding of these crops, it is advisable to use urea with an admixture of biuret of no more than 0.5 wt-%. Citrus fruits and pineapples are so sensitive that biuret in urea for foliar feeding should be no more than 0.35 wt-%.

The heart of any urea plant is the high-pressure urea synthesis section. Properly functioning safety valves are vital to protect the high-pressure section against ruptures due to pressures that are too high in case of upset conditions. However, the very corrosive intermediate ammonium carbamate makes the reliable and proper functioning of safety valves more challenging.

In the March-April issue of Nitrogen+Syngas, Plant Manager+ reported on how to prevent safety risks with a proper leak detection system. In this issue we continue the discussion by further exploring the benefits of vacuum based leak detection systems, which provide several benefits including: less clogging, no build up of pressure, only one ammonia analyser needed for the high pressure equipment, works when there is only one leak detection hole, as well as when there are clogged or no grooves.

Several safety risks threaten urea high pressure equipment such as high pressures, high temperatures, various kind of corrosion phenomena, crystallisation risks, and the release of large volumes of toxic ammonia in case of a leak. A significant number of serious incidents with high pressure urea equipment still occur in the industry and, in 50% of cases, a failing leak detection system was one of the main causes. UreaKnowHow’s Risk Register for a 316L urea grade reactor identifies 50+ safety risks of which 75% can be prevented by operating a proper leak detection system. In this article, UreaKnowHow answers some key questions about the importance of an effective active leak detection system.

The following case studies describe serious incidents associated with leak detection system failures. Incident 4 relates to the failure of the active pressurised leak detection system for the loose liner of a urea reactor and Incident 5 relates to a passive leak detection system which failed to work leading to serious corrosion of the carbon steel wall of the HP scrubber.

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.