The carbon steel casings of Heat Recovery Steam Generators (HRSGs) in a co-generation plant were found to have developed cracks that originated on the inside surfaces. The HRSGs receive hot exhaust gases from the primary Gas Turbines (GTs), which are fuelled with pipeline natural gas. The GTs was initially operating without any suppression of nitrogen oxides (NOx).

After being in service for about over three years, some of the cracks were found to have penetrated right through the ¼ in. thick casings. The casings are fitted with insulation on the inside (hot gas side).

Examination of steel samples cut from the casing walls revealed (i) fine cracks propagating through the steel from the inside surfaces, and (ii) the presence of relatively large amounts of nitrate ion on the inside surfaces.

It was concluded that the nitrate ions originated from nitric acid that was condensing on the relatively cool walls of the casings behind the insulation. Nitric acid was formed in the GT exhaust gases by the combination of water vapour (formed from combustion of the hydrocarbon fuel and from moisture in the combustion air) and nitrogen oxides (formed from nitrogen and oxygen in the combustion air).

Subject
Steel surfaces, Water, Materials, Insulation, Intergranular stress corrosion cracking, Liquids, Nitrates, Internal surfaces, Nitrogen, Steel, Carbon steel, Gases, Stress corrosion cracking
Keywords:
carbon steel, nitrate stress corrosion cracking, co-generation, heat recovery steam generators, oxides of nitrogen
© 2007 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
2007
Association for Materials Protection and Performance (AMPP)
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