Wear of straight pipeline
Straight pipeline is rarely a problem with regard to erosive wear, although there are specific circumstances where it should be taken into account. Reference has already been made above to the deflection of particles issuing from a well reinforced eroded bend. Similar deflections can be promoted from poorly aligned pipe sections, and large abrasive particles present a particular problem.
Following bends
It will be seen from Figure 23.23 that the straight section of pipeline, following a well reinforced bend, is liable to erosive wear. Although the angle of impact of the particles
is generally low, for a ductile material low angle impact is likely to result in significant wear, because of the remarkably steep increase in erosion with very small increase in impact angle, as illustrated in Figure 23.1. To extend the life of the pipeline following a bend, therefore, it is suggested that a short section of thick walled pipe should be placed between the bend and the main pipeline, as illustrated in Figure 23.25.
It is also recommended that a short section of thick walled pipe should be used after a blind tee bend, such as that shown in Figure 23.16b. The turbulence generated in such a bend is quite significant and ultimate failure generally occurs a short distance downstream of the bend. The section of thick walled pipe following a bend does not have to be very long for the deflecting flow is soon dampened out. A 2 m section is generally long enough for small bore pipelines, and something of the order of 20 pipe diameters should be allowed for larger bore pipelines.
Since the flow of deflected particles issuing from a bend will generally impinge constantly on the same area of the thick walled pipe it is also recommended that this short section of pipe should be connected by flanges to the bend and the following sec- tion of regular pipeline so that it can be rotated on a regular basis. This will both help to extend the life of this section of pipe and prevent a large wear pocket forming which could result in a further site for particle deflection to occur.
Hot dust laden gases from boilers and reactors are often passed through heat exchangers for generating steam. The tubes through which the gases flow often wear, and are generally very expensive to repair. The wear is usually only at the start of the tube. This is because the dusty gases on entry to the tube are in a very turbulent state and numerous particle impacts occur. After a short distance the flow is effectively straightened out and little further pipeline wear occurs. An effective solution to the problem is to provide a sacrificial extension to the pipe prior to the tube plate and the heat exchange section for flow straightening purposes.
Pipe section joints
Misaligned flange joints, and welded joints with weld metal protruding inside the pipeline, as illustrated in Figure 23.26, can often lead to straight pipeline failure, par- ticularly in small bore pipelines. It is a similar situation to the wear pockets formed in bends, since the step produced can result in particle streaming. This is particularly a
problem if rubber hose is attached to steel pipe by means of pushing the hose over the steel pipe. A small step will be formed and this can cause severe streaming of particles.
Large particles
Small particles will generally be conveyed through a pipeline with little contact with the pipeline wall in dilute phase suspension flow, in the absence of flow streaming and turbulence promoting sites. With large particles, however, gravitational force has a much greater effect. Large particles can be conveyed quite successfully, but in hori- zontal flow they will tend to skip along the pipeline. They will convey in suspension, but gravity will give them a low trajectory in their flow, and hence they will impact fairly frequently with the pipeline wall. The impact angle will be very low but, as has been discussed before, wear of ductile pipeline materials can be significant as a result of glancing impact from abrasive particles.
Erosive wear, as a result, will be concentrated along the bottom of the pipeline. Since it is not generally very convenient to reinforce a pipeline along its entire length, in order to overcome this problem, it is recommended that the pipeline should be rotated period- ically. By this means the pipeline will last for a very much longer period of time. It is important to recognize this problem when the pipeline routing is being planned, how- ever, for the horizontal sections of pipeline need to be located where convenient access can be gained to carry out the rotating process.