2.7. Comparison of recommended capacities
We have now studied three different separators with respect to the specific load recommended by the manufacturer.
During the sales process it is the recommended capacity expressed as m3/h that is discussed rather than the specific load per KQ or Ae. The specific load is of no meaning to a customer as long as the separation efficiency curve is not discussed.
Let us look at the performance graphs with separation efficiency versus flow rate. Figure 6 shows the graphs for the same three separators as in figure 5 plus a fourth separator of the same make as number 1. The big dots on the curves show the recommended capacity chosen by respective manufacturer.
The tests are run with 5 micron particles at a viscosity corresponding to that of HFO 380 cSt at separation temperature of 98 ℃.
The difference between the separation curves depends in this type of graph both on size related parameters (Ae or KQ) and non size related parameters influencing the separation efficiency (e.g. detailed disc stack design).
The position of the curves in the diagram is the technical expression of the size of the actual separators compared to each other. In this way size represents the capability to separate sludge from the oil.
The commercial "size" of the respective separator is determined by the recommended capacity chosen by the manufacturer.
From a sound engineering point of view a correlation is expected between the commercial and the technical size of a separator. But the fight for market shares and orders is so hard that it must be tempting for the separator manufacturers to increase the separator "size" by moving the point of recommended flow rate to the right in the diagram. But this is done at the cost of lower efficiency if no technical improvement moves the whole curve to the right. That would lead to a race towards lower capital cost per unit treated oil but also a shift towards lower performance level. If this will be the case it is time to think about the main reason why the cleaning systems are installed - TO PROTECT THE ENGlNES.
If the recommended capacity of the four separators in figure 6 were chosen at an equal separation efficiency to be technically comparable the situation would be as shown in figure 7.
The sizes with respect to capacity at equal separation efficiency will be completely changed. The order of size will then be changed from land 3, 4, 2 to the new series 1, 2, 3, 4.
Notice that the level of efficiency chosen is not important for the comparison. The order of size will be the same despite on which efficiency level the comparison is made.
Fig. 7 The same separation curves as in fig.6 compared with the recommended capacities chosen to give the same separation efficiency
3. CONCLUSION
Alfa Laval has developed a laboratory method which accurately, and with very good repeatability, can be used to measure the separation efficiency versus the flow rate for centrifugal separators. Comparisons between the laboratory method and other tests using actual heavy fuel oil have shown that the correlation between separators is maintained but on different efficiency levels.
By choosing a standard efficiency level, in figure 7 set to 85 %, the recommended capacity can be standardized to the flow rate corresponding to this standard efficiency level. Such a method would be of great advantage to the market because the separator development will be more focused on efficient separation, which is the basic reason why the separators are installed.
If this new Alfa Laval method will be accepted by classification societies, engine builders, oil companies etc., the market would have an excellent tool to classify separation equipment and to compare different brands of equipment on equal conditions.
