The introduction of advanced materials, such as hyper duplex stainless steel, offers chemical plants a wider operational window that can accommodate fluctuations in service conditions providing problem free operation and increased plant uptime. Correct material selection for a given application is paramount in order to maximise service life and operational performance of a heat exchanger. Costly maintenance programmes and re-tubing as a direct result of incorrect material selection can add considerably to the operational and whole life costs of a heat exchanger. However, premature failure of a heat exchanger can lead to significant lost production, which is often far more costly than re-tubing with the correct material.
Hyper duplex materials
Now, as a result of the introduction of advanced hyper duplex materials this selection, particularly where aggressive environments are concerned, can be made considerably easier with the increased operating capabilities of the material.
Traditionally, chemical plants are situated alongside estuaries, lakes or on the coast to ensure a plentiful supply of cooling water for the various processes. This can, however, have an impact on the equipment within the plant leading to premature failure usually as a direct result of corrosion.
In heat exchanger applications involving saltwater or chloride containing waters under oxidising conditions, pitting or crevice corrosion is a common failure mode for stainless steels.
In order to satisfy these requirements Sandvik developed the latest hyper duplex grade Sandvik SAF 2707 HD designed to provide a higher corrosion resistance at increased temperatures.
Seamless heat exchanger tubes supplied in the hyper duplex material are not only able to withstand increases in the corrosive nature of the cooling water they can operate in more aggressive environments and at increased temperatures.
Another problem encountered when using estuarine/seawater is a build-up of silt deposits within the heat exchanger tubes. This, in turn, can encourage the development of under deposit corrosion which in itself is extremely difficult to detect until the tube breaks down causing system leakage significantly reducing the heat exchanger's operational capability. Eventually, as its efficiency falls, the heat exchanger has to be taken off-line for re-tubing resulting in lost operational time and additional expenditure.
Essentially developed for use in acidic chloride containing environments, the material's greater resistance to pitting and crevice corrosion is due to the addition of chromium, nitrogen and molybdenum to its chemical composition. This gives it a Pitting Resistant Equivalent (PRE) value >48.
Super duplex and super austenitic material grades have a PRE value >40, insufficient to replace expensive nickel based alloys. This means there is a definite requirement for a cost effective hyper duplex material with a PRE >48. As a result the material can be used to replace nickel-based alloys used for chloride solutions that up until now have been the only suitable option for heat exchanger applications requiring a high PRE value.
Already installed in a number of heat exchanger applications within chemical plants around the world, Sandvik SAF2707HD has resolved issues such as operational reliability, corrosion attack, elevated seawater temperatures and condensing hydrocarbons to provide a reliable solution previously unavailable from other materials.
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Knut Tersmeden is with Sandvik Materials Technology, Sandviken, Sweden. www.sandvik.com