Recommendations
The rate of impingement attack on copper also depends to some extent on the temperature of the water. The maximum velocities for fresh waters at different temperatures recommended in Sweden are given in the table below. These figures are for aerated waters of pH not less than about 7.
Recommended Maximum Water Velocities at Different Temperatures for Copper (m/s)
| 10°C | 50°C | 70°C | 90°C | |
|---|---|---|---|---|
| For pipes that can be replaced: | 4.0 | 3.0 | 2.5 | 2.0 |
| For pipes that cannot be replaced: | 2.0 | 1.5 | 1.3 | 1.0 |
| For short connections to taps, etc.§: | 16.0 | 12.0 | 10.0 | 8.0 |
§ These velocities give a risk of impingement attack and are acceptable only for small bore connections to taps, flushing cisterns etc., through which water flow is intermittent.
BS 6700 gives the following maximum water velocities although it does note that these are currently under investigation and the velocities specified will be amended if the results of this investigation so require.
| Water Temperature °C | Maximum Water Velocity (m/s) |
|---|---|
| 10 | 3.0 |
| 50 | 3.0 |
| 70 | 2.5 |
| 90 | 2.0 |
The minimum water speed at which copper pipes suffer impingement attack depends also to some extent on water composition. Aggressive waters that tend to be cupro-solvent are the most likely to give rise to impingement attack. Installations in large buildings where flow rates may be high and water is in continuous circulation are much more susceptible to attack than ordinary domestic installations. A high mineral content or a pH below 7 is likely to increase the possibility of corrosion-erosion occurring while a positive Langelier Index and consequent tendency to deposit a calcium carbonate scale is generally beneficial. The presence or absence of colloidal organic matter is also probably of some importance.
Remedial measures for impingement attack include modifications to the system to reduce the average water velocity, e.g. by using larger diameter tubes or, if appropriate, to lower the pump speed, and/or to redesign the part of the installation concerned to eliminate the cause of local turbulence, e.g. by using slow or swept bends and tee fittings rather than elbows and square tees. It is important to minimise the possibility of any local turbulence occurring by ensuring that the ends of tubes cut with a tube cutter are deburred and that the tubes are inserted fully to the stops in the fitting before the joints are made, as referred to earlier in this section. In some cases, where the above approaches are not possible, the length of copper tube affected can sometimes be replaced by materials more resistant to corrosion-erosion, e.g. 90/10 copper-nickel (BS Designation CN102) using appropriate fittings, or stainless steel to BS 4127:1994.
Read more about this topic: Erosion Corrosion Of Copper Water Tubes