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One of the practices frequently found in sludge dewatering plants is to use two sludge suction pumps in parallel to feed the filter press.
The description ‘in parallel’ is actually not even entirely correct since the pumps do not run together, but simply feed the filter press in sequence.
What does this mean? Simply that there is – usually – a centrifugal pump for filling and a pump (usually in this case a positive displacement pump) for the final filtration phase.
This is because, as we have seen, one follows the demand of the filter during the filtration phase and also because centrifugal pumps are suitable in the initial phase where a high flow rate is needed, but at low pressure values.
When the centrifugal pumps are then unable to reach the pressure required to finish filtration, the positive displacement pump is started, which typically delivers a higher pressure and a lower flow rate.
In this way, it is then possible to conduct the filtration phase in both the filling and final stages.
The problem is, however, that two pumps must be run, which adds to the cost of what is involved in running two machines instead of one.
With Autemi’s variable displacement piston pumps, it is possible to remedy this choice and use only one pump that takes care of both the filling phase and the final pressure increase phase.
In this way I have energy saving and am not forced to install and manage two pumps when I can comfortably install only one.
One of the other mistakes often made by some plant engineers or those who install a plate filter press in their plant is to make the wrong choice of pump or, even worse, to opt for a cheap or unsuitable sludge pump because the budget available has been eaten up by the installation of the filter press.
It is in fact common to think that the goodness of the filtration process depends only on the filter press, and that the pump does not contribute much at this stage.
Nothing could be more wrong: the feed pump plays a crucial role in filtration, sometimes even more so than the filter press.
It is easy to see how then, the pump plays a pivotal role in the filtration process: in fact, if a web breaks or if by chance some automatic function is no longer responsive, I still manage to complete the filtration or do some other emergency cycle. If, on the other hand, the pump is out of order, there is no way to get the filtration done.
This also explains why in a Series A system, there are always two sludge suction pumps in parallel (one spare to the other) precisely to remedy this problem or for preventive maintenance.
Ideally, a filter press is nothing more than a frame with a hydraulic cylinder that holds a series of filter elements (plates and cloths) under pressure. The end.
Then you can have more or less options depending on the degree of automation required.
Actually, no one considers that it is the pump that conveys the liquid to be pressed inside the filter press, so it is responsible for how the filling phase takes place.
So at least two factors come into play here, namely correct sizing and pump management.
Firstly, the pump must have the capacity to obtain a sufficiently compact and dehydrated solid cake, and this may mean that the sludge feed pressure must reach certain pressure values, so there are cases where 6 bar may be sufficient, but there are also cases where 12-14 bar is reached.
And the difference in terms of the compactness of the cake – and thus in relation to the degree of dryness – you pay for it at the end of the year when you do the disposal bills.
Another issue is that of flow rate: I cannot send as much flow as I want to the filter press, because the cloths would get clogged, I would have an inlet resistance which would result in an incorrect reading of the pump supply pressure and it would be very difficult to manage the filtration cycle, as the pump flow rate
must be adequate for what the filter press allows me to receive.
Then we come to the subject of management.
We have already explained how it is correct to manage the feed pump, especially the volumetric one, during the filling of the filter press in order to give it the most suitable pressure/flow rate pair.
In fact, it is important to avoid discontinuous or pulsating filtration cycles, because it does not help to obtain compactness in the discharge cake.