In seawater cooling systems a leak of the tubing or piping costs money and loses production. However, it has been demonstrated that the use of a “soft” start-up is of great benefit to many alloys and reduces the risk of a failure.

Copper alloys are widely used for heat exchangers and seawater piping, but in the presence of pollutants, such as sulphide and ammonia, they can suffer from accelerated localised corrosion, leading to failure. Once protective films form in clean, natural seawater, they are much more resistant to corrosion by pollutants. As pollution is often intermittent, a long life can be achieved by a start-up when no pollution is present. Simple monitoring of the seawater at the inlet can determine the presence of both sulphide and ammonia.

High alloy stainless steels, such as the 6% Mo austenitic and 25% Cr superduplex alloys, are limited by the maximum temperature of seawater that they can tolerate without suffering crevice corrosion or pitting at welds. This is typically 30 to 40°C, depending on the alloy. However, where discharge temperatures from heat exchangers are higher, it has been shown that running the system with cooler water first, strengthens the passive film and enables operation at 10 to 15°C higher than normal. One recommended start up regime is:

2 days minimum with cold, untreated seawater
5 days minimum with cold chlorinated seawater
Thereafter, turn on heat exchangers.

A superduplex seawater piping system was given a soft start-up and the piping after the heat exchangers ran at 55°C without any problems.

Posted on: 7th Dec 2016

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Image (top left) by Agnieszka