EXAMINATION PROCEDURE

Each sample was examined visually, and then the bore was measured. After that, the ends were cut off squarely, the length was measured accurately, and the weight per unit length was calculated. Photographs were taken of the bore after the tube was split lengthwise to reveal the bore.

DISCUSSION

The experiment carried out on new tube is likely to represent the most serious conditions applied to any tube because the oxide thickness develops on the tube bore in service whereas new tube has only a very thin surface oxide. Any damage to the bore will therefore be exaggerated when the cleaning operation is performed on a new tube.

The phosphor bronze flight on the plug scours the surface and appears to make point contact in a few areas as it passes down the tube. These marks were found to be remarkably consistent indicating that the plug centralises itself as it passes down the tube and in doing so will effectively scour any debris or deposits on the tube bore. It is therefore likely to perform an excellent cleaning operation.

The surface marks on the tube appeared to be due to localised scuffing of the oxide layer and the phosphor bronze flight material is a distinctive copper colour but none was found on any of the tubes used in the experiment. We are therefore confident that the flight material is not deposited on the tube as a different metal, which might under some circumstances be considered to be assisting corrosion, as it will be a more noble material.

The tubes cleaned with a new plug every time were quite similar to the tubes cleaned with a plug used for ten operations, so although it would be good practice to maintain the plugs in good condition, there appears to be no necessity for a new plug every time an operation is carried out. Indeed, the tube where one hundred operations were carried out with the same plug was very little different to the other two examples. On none of these tube samples, even after one hundred cleanings, was there any sign of metal pick up leaving a roughened surface. Also, there were no circumferential marks on the tube where the plug had chattered down the tube and dug into the tube wall.

In contrast therefore the bore from the tube cleaned with the steel flights was very different. This had actually picked up metal in many areas leaving a roughened texture to the tube bore, and the plug appeared to have chatted all the way down the tube leaving a series of circumferential indentations in the tube as shown in the attached photographs. For this type of tube, we consider that steel flighted plugs are unsuitable for a number of reasons. A roughened surface is likely to cause turbulence, and also a higher friction in the boundary zone of the water passing down the tube, which will increase the rate of fouling compared with tubes cleaned using phosphor bronze flights.

These roughened textures will have work hardening of the metal associated with them and will therefore produce a galvanic cell in which the work hardened areas are anodic with respect to the rest of the material.

For these reasons we would not be happy with multiple cleaning using steel flights.

CONCLUSIONS

Condenser tubes and other types of heat exchanger tubes will not be affected by the Rico process even after one hundred cleaning cycles.

J G Campbell BSc CEng MIM MIBF

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