Abstract

A refinery has for decades been using tubes in CuNi 70/30 for several seawater cooled heat exchangers. Over the years the refinery has been facing a lot of corrosion issues with these heat exchangers, leading to frequent leakages, plugging of tubes and ultimately complete retubing of the tube bundles. Apart from high maintenance costs, this also leads to production losses with associated lost earnings. This paper will show the long-term cost of the CuNi 70/30 heat exchanger solution, and compare this to an initially more expensive stainless steel solution but which has a lower long-term maintenance cost. The cost calculations are based on data supplied from fabricators, material suppliers and end-users.

Introduction

Copper alloys such as copper nickel (CuNi) and Admiralty Brass (CuZn) are often successful material selections for seawater coolers. The copper alloys successes in these highly corrosive environments can be attributes to the ability of copper to form a protective scale, thus stopping corrosion of the material. On copper alloys in seawater, the protective scale formed comprises a mix of cuprous oxide (Cu2O), copper oxide (CuO) and copper hydroxy chlorides [1]. This protective scale is quite soft though and can be eroded by solid particles in the seawater or sheer stresses from high flow velocities. While the flow rates can be adequately calculated and kept below certain alloy dependent limits, turbulence in the tube entrance in heat exchangers can be very difficult to control and failures are common in these areas. Another drawback with copper alloys is that the protective scales is sensitive to sulphide species in the process water. Sulphide species will promote the formation of copper sulphide (CuS) which makes the scale soft and non-adherent to the tube surface. Therefore, formation of CuS will lead to very rapid corrosion of the tubes.

By contrast, stainless steel alloys are not sensible neither erosion-corrosion in seawater, nor are they affected by sulphide species in the seawater. This can be attributed to the nature of the passive layer of stainless steel, being constituted of relatively hard chromium oxyhydroxides which rapidly reforms when damaged. Therefore, when erosion-corrosion or sulphide species cause rapid failures of copper alloys, stainless steels can be attractive alternative solutions.

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