Velocity Effects

With increasing seawater flow rate, corrosion rates remain low due to the resilience of the protective surface film. Once, however, the velocity for a given geometry exceeds a critical value, at which shear stress are sufficiently high to strip off the protective corrosion film, damage in the form of impingement attack may occur. General experience has shown that 90-10 and 70-30 copper-nickel alloys can successfully be used in condensers and heat exchanger with velocities up to 2.5m/s and 3m/s respectively. For pipeline systems, higher maximum design seawater velocities of 3.5m/s for 90-10 copper-nickel and 4m/s for 70-30 alloy can safely be used for pipes 100mm in diameter and larger, as described in British Standard BS MA18.

Although these values are now considered to be conservative, such guidelines have worked well because they take into account normal velocity raisers within piping systems such as bends, which can cause areas of high local flow rates. Nevertheless, extreme turbulence should be avoided. Instances where this may occur include tight radius bends, partial blockages and areas downstream of partially throttled valves. Minimum flow rates of more than 1m/s are usually preferred to avoid sediment build up.

In multistage flash desalination units, the 30%Ni alloy containing 2%Mn and 2% Fe which has higher resistance to impingement than the standard 70-30 alloy is commonly preferred for heat exchanger tubing in the heat rejection section. The normal design velocity in this section is about 2m/s but local turbulence and high velocities can occur if debris passes through screens or there is unsatisfactory flow conditions in water boxes or unsatisfactory entry conditions into the tubes. This alloy has been found to give very good performance.

The seawater velocities for copper-nickel alloys discussed until now have been for continuous flow situations. Fire-mains can encounter intermittent high velocities of 12-15m/s during test practices as well as during actual fires. Experience has shown that these high flow rates are acceptable for the short term practices used in such applications.

The hydrodynamics of ship hulls are somewhat different than piping systems. Experience to date has shown minimal corrosion after 14 months at 24 knots (12m/s) for the 90-10 alloy whereas the highest recorded velocity is 38 knots (19m/s) for a patrol boat which showed no measurable thickness loss after 200 hours at maximum operating speed. The upper service velocity for hulls is still to be established. The velocity capabilities increase upon going from condenser to piping systems and on to ship hulls because of fluid dynamic boundary layer growth.

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