Tube failures of waste heat boilers in the synthesis loop of ammonia plants are a frequent failure have been reported often from several plants throughout the world. There are various design alternatives however they are suffering from common and different types of failure mechanisms.
Type A
Hot Inlet Field – Cold Outlet Field symmetrically to one axis of the tube field (with or without cooling bores in the tube sheet)
Type B
Hot Inlet Centre – Cold Outlet Circular Area with fountain tube arrangement (with or without cooling bores in the tube sheet)
Type C
Hot / Cold – Alternate Tubing
For these types of boilers, the observed failures are as follows:
Type A
- Corrosion under deposits (without cooling bores) in hot zone
- Wall thinning at end of the ferrules (with cooling bores) in hot zone
- Corrosion under deposits in some cases on top of baffles
- Nitriding and Disbonding
Type B
- Tube failures from corrosion under deposits just above the tube sheet
- Corrosion under deposits in some cases on top of baffles
Type C
- Possible failures same as type A
All these effects and failures are limiting the lifetime of the equipment and are causing unplanned shut downs and loss of production.
To prevent unplanned shut downs and to extend the lifetime of the equipment, measures and procedures were developed and applied by the referenced authors, considering the individual boundary conditions.
The technical background, development, practical use and experience of the following measures are described below with typical examples and practical cases:
- Sleeving of damaged tubes (before they fail)
- Modification of classical U-tubing to
- Hot/Cold alternate tubing / Hot/Cold Flow Reversal (HCFR)
- Ferrule Design Change
- Sleeving of damaged tubes (before they fail): The method of sleeving by hydraulic expanding (after preparation of the inner tube surface with VACUBLAST–cleaning) has been applied with success in ammonia plants and even in Nuclear Power Plants. This repair procedure can, depending on the boundary conditions, be a permanent solution or, in this case, a method of “buying life time” for the equipment until a replacement is available.
- Modification of classical U-tubing to Hot/Cold alternate tubing: The main improvement for trouble free operation over a long period was achieved with modification from the classical U-tube design to Hot/Cold alternate tubing with optimization of feed water distribution to the tubes (grids for tube support and flow distribution plate) and the change of the water treatment from a phosphate system to an all volatile treatment (AVT). The modification of the boiler improved the uniformity of steam generation and reduced the tube sheet temperature to reduce the nitriding rate. More than 13 years of operation were achieved after these modifications and the equipment is still in operation.
- Hot/Cold Flow Reversal (HCFR): It was proposed to reverse the gas flow; the original hot inlet side would become the cold outlet side, and vice versa. Since all the corrosion was observed to be on the original hot side, it was felt that reversing the gas flow would buy time until a new boiler could be procured.
- Ferrule Design Change: The most critical area of the heat exchanger tube is the location of the highest heat flux (peak heat flux), Without ferrules this point is just behind the tubesheet, with ferrules this point moves further to the water/steam space with better cooling conditions.
Depending on the design of the ferrules, turbulence can increase the heat transfer and create unforeseen high peak heat fluxes. This can result in spontaneous boiling at the outer surface and does not allow for building and rebuilding of the protective magnetite layer, the tube material is just “eaten up”.
A smooth transition from the end of ferrule to the tube is essential to avoid this type of failure. In most of the cases with inbore welded tubes the weld seam does not allow for those optimized ferrules. If the beginning of wall thinning is detected, (IRIS test) during inspections in this area, a simple solution to prolong the life time of the boiler is a change in ferrule design to longer ferrules. This will protect the already affected area. The minimal loss in heat transfer surface will not be noticed in operation.
Reference:
- Life Extension of Waste Heat Boilers in Ammonia Plants, Authors: Wolfgang – Hegner Balcke-Dürr GmbH, William K. Taylor – Saskferco Products Inc. Petrus Bleijenbergh – YARA Global Support, Cato Johansen – Qatar Fertiliser Company AICHE Ammonia Safety Symposium 2006