Selection of screw pumps for wastewater treatment and precautions for their use

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Selection and application considerations of screw pumps in wastewater treatment. Due to their capabilities in variable flow rate delivery, strong self-priming ability, reversibility, and the ability to transport liquids containing solid particles, screw pumps are widely used in wastewater treatment plants for transporting water, wet sludge, and flocculant solutions. The selection of screw pumps should follow the principles of economy, rationality, and reliability. If inadequate consideration is given to the design and selection, it will cause problems in future use, management, and maintenance. Therefore, choosing a screw pump that is suitable and reliable for the actual production needs not only ensures smooth operations but also reduces repair costs.  I. Selection of the rotational speed for screw pumps: The flow rate of a screw pump is directly proportional to its rotational speed. Compared to screw pumps operating at lower speeds, those operating at higher speeds do indeed produce a higher flow rate and head, but their power consumption increases significantly. Higher speeds accelerate wear between the rotor and the stator, which inevitably leads to premature failure of the screw pump. Additionally, the rotor and stator in high-speed screw pumps are relatively short in length, making them prone to wear and thus reducing the service life of the screw pump. The speed is reduced through a reduction mechanism or a variable-speed mechanism, keeping it within a reasonable range of below 300 revolutions per minute; compared to high-speed screw pumps, this extends their service life by several times.  II. Quality of screw pumps There are many types of screw pumps available on the market today. Relatively speaking, imported screw pumps feature sound design and high-quality materials, but they are more expensive. The after-sales service may not be satisfactory, the prices of spare parts are high, and the lead time for ordering is long, which can affect the normal operation of production processes. Most products manufactured domestically are imitations of imported ones, and their quality varies greatly. When choosing domestically produced products, it is necessary to consider cost-effectiveness by opting for those with a low rotation speed, a long lead length, high-quality materials for the transmission components, and a long service life. III. Prevent debris from entering the pump chamber. Solid debris mixed in with the wet sludge can damage the rubber-styled stator of the screw pump; therefore, it is important to ensure that no debris enters the pump’s chamber. Many sewage treatment plants install crushers before the pumps, or they use grid systems or filters to prevent debris from reaching the screw pump. The grids should be cleaned regularly to avoid blockages. IV. Avoiding material interruption – A screw pump must never operate when there is a break in the material supply. In such cases, the rubber stator generates high temperatures due to dry friction, which can lead to its destruction. Therefore, it is essential for the screw pump to function properly that the crusher is in good condition and the grids are unobstructed. To this end, some screw pumps are equipped with a shutdown mechanism that activates when there is a break in the material supply; thanks to the self-priming capability of the screw pump, a vacuum is created within the chamber, and this vacuum mechanism causes the pump to stop operating. V. Maintain a constant outlet pressure: A screw pump is a positive-displacement rotary pump; when there is obstruction at the outlet, the pressure gradually increases until it exceeds the predetermined value. At this point, the motor load increases sharply. The load on the components related to the transmission machinery can also exceed the design values; in severe cases, this can lead to motor burnout or breakage of the transmission components. To prevent damage to the screw pump, a bypass relief valve is generally installed at its outlet to stabilize the outlet pressure and ensure the proper operation of the pump. This post was last edited by johncom on 2009-3-3 08:12]