2024 06 Najafian Rupture of a HP Stripper tube in a urea plant

In this Technical Paper, we discuss the major problem of leakage in the high-pressure stripper of urea production units. The main conclusions and recommendations are:
Thickness measurement should be done by Eddy Current method in the first upcoming overhaul and at regular time intervals depending on the findings;
All liquid dividers should be tested and checked and the diameter of liquid divider holes should be measured according to the Licensor procedures;
Plugging of tubes with a too low thickness to operate safely until the next turnaround should be done;
If the number of plugged tubes increases, the purchase order of stripper with Safurex material should be made for replacement. Although Safurex also experiences a passive corrosion rate and thus will reach an end of lifetime situation, its main benefit is that it does not suffer from active corrosion and thus allows for more operational flexibility;
Also a larger number of stripper tubes is recommended in order to assure that the realized plant capacity with the old stripper can be realized again with the new stripper;
Stamicarbon’s safety study known as the Y2K-paper to support the DHS 2004 showed that due to the fact that the shell side of the stripper is vapor/steam filled there is more than enough time for a pressure safety valve to respond on the pressure increase as result of a tube rupture. Therefore since Stamicarbon’s DHS 2004 the rupture discs on the HP – and the LP steam saturator are removed. The only rupture disc still present in Stamicarbon plants is on the (liquid filled) shell side of the HP scrubber heat exchanger. Also, as a result of gas cloud dispersion calculations carried out at that time, it was decided to point the discharge of the pressure safety valve „upward‟ (min. 75° to horizon) instead of horizontal.

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