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Piston diaphragm pump - Optimal solution for filter press feeding

Filter presses are an effective method of separating liquid from solids, but the choice of pump plays a crucial role in performance. Piston diaphragm pumps have proven to be the most efficient solution for filter press feeding, thanks to their ability to handle high pressure and maintain a constant flow. In this article, we compare the advantages of piston diaphragm pumps over centrifugal pumps.

A filter press is used to dry sludge, either for landfill or to produce products. There are several different technologies for pumping sludge but piston diaphragm pumps have proven to be the most optimal solution. These pumps offer a filter cycle that provides low residual moisture, is energy efficient and allows for faster runs. Their robust construction, combined with the unique duty cycle, means that piston diaphragm pumps have a superior service life compared to other filter press solutions.

The filter press cycle

Figure: Filter press cycle with piston diaphragm pump. The solid line is the flow (Q) and the dashed line is the pressure (P). Note that the flow rate decreases only when the pressure has built up.

Phase 1: Pre-filling

When the filter press is empty, the system is under low pressure. The goal in this phase is to quickly fill the press with sludge. The piston diaphragm pump does this efficiently thanks to its high initial flow.

Phase 2: Increased back pressure

As the sludge starts to build up on the filter cloths, the back pressure increases. Continued high flow enters the filter press, despite the rapidly increasing back pressure. Conventional pumps would experience a decrease in flow rate at this stage.

Phase 3: Final phase

When the press is almost full, only small amounts of new sludge can be pressed in. The pump then continues to feed the same amount of sludge that flows out as filtered. Only now, when a preset back pressure (about two thirds of the maximum pressure) is reached, the flow rate will be automatically reduced by the pump's internal pilot system.

Are centrifugal pumps an option?

Centrifugal pumps are hydrodynamically driven and work by the electric motor driving the impeller to rotate at a speed of up to 2950 rpm. The centrifugal forces accelerate the liquid and the volute directs the sludge to the outlet. The resulting operating pressure is a state of equilibrium between flow rate and pressure. This makes this type of pump not ideal for filter press feeding.

What is problematic about filter press feeding with centrifugal pumps?

Figure: Emmerich SP piston diaphragm pump for abrasive and aggressive media. The pump is available in a variety of steel grades, also rubberised and plastic materials.

The strength of centrifugal pumps is that they deliver high and constant flows, which is unsuitable for filter presses that require a relatively low flow and pressure of up to 16 bar. In addition, the pressure in a centrifugal pump is not constant, but increases gradually during the filtration process.

When a centrifugal pump is used, the flow rate is affected every time the pressure resistance changes, leading to an increased filtration time. Another limitation is that the maximum pressure of centrifugal pumps is limited to their characteristic pumping curve, which means that they cannot provide the high pressure needed for the filter press. With the lower pressure, the best filtration result (dry cake) cannot be achieved.

How does centrifugal pump operation affect service life?

If centrifugal pumps are operated at their optimum operating point, internal circulation is reduced to a minimum, and axial and radial forces are so low that they can be absorbed by existing bearings.

Centrifugal pump manufacturers specify a recommended operating range of +10% to -20% at the best efficiency point. Running the pump outside this range reduces efficiency, while increasing bearing stress and cavitation, leading to shorter life and a greater need for regular repairs.

High flow rates also cause wear on both the impeller and the pump housing. Shaft seals often require external flushing and are affected by wear from particles, crystallisation, insufficient lubrication and part-load operation, making leakage a matter of time.

Advantages of piston diaphragm pumps

Piston diaphragm pumps are positive displacement pumps that deliver the same flow rate regardless of back pressure, resulting in fast filling of the filter press. Through speed control, the flow can be easily adjusted. The pump continues to build up pressure until the maximum pressure, specified by the filter press manufacturer, is reached. The result is a dry filtrate cake with minimal energy use.

Buy your piston diaphragm pump at Lyma

Piston diaphragm pumps, with their reliability and energy efficiency, are the optimal solution for filter press feeding and thus outperform centrifugal pumps in both performance and lifetime.

At Lyma Kemiteknik, we have more than 40 years of experience working with chemically aggressive liquids, and can help you make the right application and material choices. In addition to all types of pumps, you will also find valves, pipes and tanks in our large range. Welcome to contact us at Lyma!