Ashish Nair 550 deg cent is not possible for ammonia converter operation at 250 bar. Could this be mistake in typing - 450 deg cent typed as 550 deg cent. Wire mesh of at the bottom of the converter sees higher temp than entry at the top? Is the failure related to bottom wire mesh. From your own experience in the same ammonia converter, S.S 321 internals - cartridge shell and gratings of higher thickness did not suffer a similar damage. This means the material had behaved well for the same service duties. If the wire mesh chosen is very thin - possibly it could have been attacked and quite likely it could become brittle. One question arises whether a thicker wire mesh would be better ? S.S 347 or S.S 321 will behave the same for converter service. The fact that your cartridge and other gratings did not suffer any damage in the same service is enough proof that the matrial had withstood the converter service duties well. May be you have to try a thickner wire mesh for the bottom of the same material or S.S 347. Both are stabilized grades suitable for application at this service condition. You should be prepared for a change of wire mesh during every replacement of converter catalysts or during every internal inspection of the converter even with a higher thickness wire mesh also. A 5 to 10 year service for a thin wire mesh is good enough service record at this temperature and for syn gas use. Trust this is of help to you. C.V.Srinivasan Nishi Engineers Pvt Ltd Chennai 20 India June 13 E-mail: nishi@vsnl.com >Request Urgent help >The wire mesh (of AISI 321) in our ammonia converter >suffered extensive nitriding damage in service (Synthesis >gas at 250 Kg/cm2 and 550 deg. C) and failed completely. >Similar damage was not noticed on the other SS 321 internals >(SA 240 Tp 321) such as the gratings and the cartridge >shell. Is it possible that Nitriding occurs rapidly in cold >worked / work hardened SS and that Solution annealed SS is >generally imune to the effects of nitriding. >How does SS 347 H or SS 330 compare with SS 321 in this >service. >Please suggest a suitable material and its final heat >treatment for the wire mesh. > >Thanking you all in advance > >desperately yours > >ashish

Ashish Nair Since your converter D.P is quite low and also the inlet and exit pressure are 207 ATA / 198 ATA, the design calls for operation at 50-80 deg cent higher i.e, 500-550 deg cent for you converter- i guess. Yes, normally wire meshes are not solution annealed. Nitriding depth for this service temp could be even more for this service temperature. It is true that solution annealed material has more resistantance due to large grain size. Cold worked S.S 321 will be susceptible to a higher depth of nitriding than other areas in the same converter. I guess that it is load / bending stress and cold work - all combined acts for this accelerated nitriding attack on a thin wire mesh of 1 mm thickness. Nitriding effect is more pronounced with fine grain material. This is well known. You can specify solution annealed S.S 321 and specify also a higher thickness. After all, in 1. 00 mm thick if it is nitrided by > 50% of the thickness, the SS. 321 material wire mesh will not have any strength and will be quite brittle. Solution annealing small quantum is a problem. You may have to use a smaller furnace to do or do a field wrap and solution anneal yourself to improve on the performance of the wire mesh. All the same, change of wrie mesh during catalyst change or during internal inspection is the best you could think of - even with solution annealed S.S 321 since the wire mesh sees both higher temp - 550 deg cent and also the load and hoop / bending stress at the bottom of the converter. Load and bending stress for the wire mesh- if you calculate- will be surprisingly quite high which might be contributing to higher nitriding rate in cold worked S.S 321 wire mesh of 1 mm thickness. The same wire mesh of 2 mm thickness will have double resistance in the same service duties. That is shy the larger gratings and cartridge of same S.S 321 is less nitrided compared to this 1 mm thick S.S 321 wire mesh. Trust this is of help to you C.V.Srinivasan Nishi Engineers Pvt Ltd Chennai India June 13 E-mal: nishi@vsnl.com. >Mr. Srinivasan > >The design conditions of the catalyst beds are 550 - 580 >deg. C and 207 ATA inlet and 199 ATA outlet (7.0 ATA being >the design pressure drop). > >The wire mesh is in the bottom of the catalyst bed which as >you rightly pointed out is the hotest portion. The wire dia >of the meshes are 1.0 mm and 2.0 mm respectively. Both the >meshes were completely damaged by nitriding. Metallography >ascertained the depth of nitriding to be varying from 0.5 to >0.7 mm. > >Nitriding effects were totally absent on the gratings and >other internals. > >The construction drawing of the converter does not mention >anything about the heat treatment of the wire mesh (AISI >321) while the other internals are specified as SA 240 TP >321 which by default is solution annealed. Hence the >question of whether cold working / work hardening enhances >the effect of nitriding of stainless steels in similar >srevice conditions. > >ashish