WATER JET CONFERENCE - Waterjet Technology Association

SLURRY FEEDS.
In order to get around the problem of clogging and the need to feed dry abrasive, there
has been considerable development of wet slurry systems. However, these in themselves had
further problems in that there is the need for a wear resistant feed pump, the abrasive has a
tendency to settle out in feed lines and the mixture must be kept agitated. However, the main
disadvantage is that under similar operating parameters a slurry system can be 50% less efficient
than with a dry feed system. This is almost entirely due to the fact that it is very nearly
impossible to feed slurry with greater than 50% by volume of abrasive. Thus, nearly half the
power of the water jet is being used to accelerate water, which has next to no cleaning effect. The
following graph (Figure 3) illustrates the relationship between abrasive feed and cleaning rate for
typical wet and dry feed systems.
While most abrasive devices are of the type described previously, other devices have
been used. They all work on the principle of the velocity of the water jet creating a venturi to
entrain the abrasive and accomplish mixing.
ENGINEERING CONSIDERATIONS OF CLEANING HEADS:
Abrasive cleaning heads must have the following characteristics in order to be usefully
employed :
1. Maximum cleaning speed for minimum abrasive usage.
2. Maximum band width to translate into slow linear speed when held on the end of a gun.
3. Minimum wear of internals, in order to be used reliably for long periods of time.
4. Easy replacement of wearing parts, as these must Often be replaced on site under difficult
conditions.
5. Light weight when used manually to reduce operator fatigue.
Because early sand guns were made from normal steels it was found in practice, that spools and
discharge pipes were lasting less than 2 hours. Wear on these units occurred in three main areas
1. Abrasive passage in centre of spool due to high velocity within venturi system.
2. Water jets wear outwards so that they no longer intersect at a fixed point, so they quickly
wear through discharge tube.
3. Discharge tubes normally made from @" gas pipe wear out at best in approximately 2
hours.
In order to minimize wear, modern cleaning heads are of a different design (Figure 5).
RESISTANT MATERIALS: BARREL LIFE.
While normal steel barrel life of 1-2 hours is satisfactory in a contracting
hand held situation it is impractiacal in a continuous blasting operation or process machine. It
was necessary to design a wear resistant outlet tube (Figure 6).
Trials carried out early in the 1970's showed that the cleaning rate was proportional to the
amount of abrasive up to a certain point. [see Fig 4] As the amount of abrasive increased beyond
this point the performance began to tail off. This was because the abrasive head could entrain
more abrasive than it could accelerate to optional velocity. At the same time, it was found that
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