Tag: Ureaknowhow

A proper leak detection system for loose liners of high-pressure urea equipment is the number one safeguard for any urea plant, as has been proven by detailed safety studies and incidents investigations. However, leak detection systems easily choke especially when urea is present in the leaking solution. When no urea is present, the ammonium carbamate will dissociate above 60°C when flashing to atmospheric pressure in the leak detection system. But, when urea is present, it can solidify and at higher temperatures polymerise into biuret and triuret, which have even higher crystallisation temperatures. An early and reliable leak detection system is therefore very important. UreaKnowHow.com has developed such a system: the state-of-the-art AMMO LASER Leak Detection System.

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 year the CRU Nitrogen + Syngas conference is going virtual. From 1 to 3 March 2021, the CRU virtual event will offer a live and on-demand agenda, interactive exhibition and enhanced networking capabilities on a platform tailored to make remote access as easy as possible.

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 NH 3 /CO 2 ratio on the corrosion rate is questionable.