Duplex tubing heat exchanger

The duplex grade stainless steels are also used in shell and tube-heat exchangers because of their high resistance to SCC and lower thermal expansion than austenitic steels. These steels are also used in high-pressure piping and also in systems requiring high-erosion resistance. 

A duplex heat exchanger tube containing a single small perforation was examined. Perforation occurred due to internal surface wastage beneath a deposit layer containing large concentrations of sulfur compounds (Fig. 4.28). 

The use of stainless steel in oil production has increased, and is of particular significance for production in deep waters. This applies both to production and process equipment carrying oil and gas and to seawater systems. For seawater systems the experience from nuclear power plants has been useful. In these plants, some high-alloy steels have performed well in equipments such as seawater pumps and heat exchangers. But the performance depends on service conditions, design and combination of materials. On the oil/gas side, good experience has been obtained with 13% Cr steel in production tubing, in components at the wellhead and in well valves. The same can be said about the use of other stainless steels in process systems offshore, but there have been some exceptions. For instance, SCC has occurred on duplex steel (22-5-3) under external heat insulation (compare Section 7.12) [10.11]. 

The quality of cooling water and steam must he considered when selecting materials of construction. The chosen materials may be resistant on the process side, but fail on tbe utility side. For example, 316 stainless steel may be applicable for vertical heat exchanger tubing that handles a weak organic acid on the process side, but it may fail from chloride stress cracking on the waterside. A duplex stainless steel may be a better choice. 

Heat transfer. A number of options exist for the heat exchanger/chemical reactor. Examples of alternative interfaces include traditional heat exchangers, radiation heat transfer (thermal infrared between tube banks), duplex tubes (tubes constructed of two metals), and intermediate heat exchanger loops. It is not clear what the preferred option is. The very high temperatures does create new options such as the use of a heat exchanger that operates on radiation heat transfer (Fig. 5). Such options provide very high degrees of separation between the nuclear and chemical facilities. 

The compatibility of INOR-8 and sodium is adequate in the temperature range presently contemplated for molten-salt reactor heat-exchanger operation. At higher temperatures, mass transfer could become a problem, and therefore the fabrication of duplex tubing has been investigated. Satisfactory duplex tubing has been made that consists of Inconel clad with type-316 stainless steel, and components for a duplex heat exchanger have been fabricated, as shown in Fig. 13-20. 

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