In the fluid transportation applications of twin screw pumps, Net Positive Suction Head Available (NPSHa) and Net Positive Suction Head Required (NPSHr) are crucial, having a significant impact on the stable operation and service life of the pump. Understanding these two parameters can help you better operate the twin screw pump and avoid potential problems.
I. What are NPSHa and NPSHr?
- Net Positive Suction Head Available (NPSHa)（Net Positive Suction Head Available）When liquid enters a twin screw pump, it’s similar to an athlete needing an energy reserve for a competition. NPSHa is the surplus energy of the liquid per unit weight at the pump inlet exceeding the vaporization pressure. It is the energy margin provided by the system to prevent the liquid from vaporizing. For example, in a water transportation system, if there is sufficient NPSHa, water can smoothly enter the pump without vaporizing.
- Net Positive Suction Head Required (NPSHr)（Net Positive Suction Head Required）This is a characteristic parameter of the twin screw pump itself. Just as an athlete has a minimum energy requirement for a competition, for the pump to work properly, the liquid at the inlet also needs a specific minimum energy, which is NPSHr. It reflects the minimum energy required to prevent the liquid from vaporizing when it reaches the lowest pressure point inside the impeller. This value varies for different pump models.
II. Their Importance in the Transportation of Twin Screw Pumps
- Avoid Cavitation Phenomenon Cavitation can be regarded as the “health killer” of twin screw pumps. When NPSHa is less than NPSHr, the pressure of the liquid at the pump inlet is so low that it reaches the vaporization pressure, causing the liquid to vaporize and form bubbles. When these bubbles enter the high – pressure zone, they burst, impacting components such as the screws and bushings, resulting in pitting and honeycomb – like damage, affecting the pump’s performance and service life.
- Ensure Stable Operation Adequate NPSHa ensures that the liquid can smoothly enter the pump, maintaining stable flow and pressure. If NPSHa is insufficient, the pump will experience flow fluctuations, a decrease in head, and an increase in vibration and noise, making the transportation system unable to operate stably. In chemical production, cavitation may also lead to safety hazards such as leakage.
III. Ensuring the Coordination of the Two to Guarantee the Normal Operation of the Pump
- System Design Stage
- Pay attention to NPSHa during the design of the transportation system. Reasonably plan the pipeline, shorten the suction pipeline, increase the pipe diameter, and reduce the number of pipe fittings to reduce pressure loss and increase NPSHa. Accurately calculate its value according to the actual working conditions to ensure that it is greater than the NPSHr of the pump.
- Selection Stage “Selection Stage”
- Choosing the right twin screw pump is crucial. When selecting, check the performance parameter table to understand NPSHr. According to the properties of the liquid (density, viscosity, etc.) and system conditions (suction height, etc.), select a pump model whose NPSHr matches the system’s NPSHa. For example, for high – viscosity liquids, choose a pump with a low NPSHr.
- Operation Monitoring Stage
- Regularly monitor parameters during the pump’s operation. Install a pressure sensor to measure the pressure at the pump inlet and calculate NPSHa for comparison with NPSHr. When NPSHa approaches or is lower than NPSHr, adjust the operating parameters in a timely manner and check whether the suction pipeline is blocked or leaking.
Net Positive Suction Head Available and Net Positive Suction Head Required are the guarantees for the stable operation of twin – screw pumps. Mastering and coordinating these two parameters can give full play to the pump’s performance, extend its service life, and ensure efficient and safe transportation. It is hoped that this knowledge will enable you to use twin – screw pumps more proficiently.
