Cavitation is a phenomenon that occurs when the pressure of a fluid drops below its vapor pressure, causing bubbles of vapor to form and collapse. This can damage the pump, reduce its efficiency, and create noise and vibration. As a chemical engineer, you need to know how to prevent cavitation in a pump and ensure optimal performance. Here are some of the best ways to do that.
The first step to prevent cavitation is to select a pump that matches the flow rate, pressure, and viscosity of the fluid you are pumping. Different types of pumps have different characteristics and limitations. For example, centrifugal pumps are suitable for low viscosity fluids with high flow rates, while positive displacement pumps are better for high viscosity fluids with low flow rates. You should also check the pump's NPSHr (net positive suction head required), which is the minimum pressure at the suction side of the pump to avoid cavitation.
Most importantly when it comes to avoiding pump cavitation is to calculate the NPSHa (Nett Positive Suction Head available) in the system that the pump is to be installed into. This is the most overlooked or ignored aspect in pump system design yet the number one cause of cavitation within pumps when the NPSHr (required) exceeds the available. NPSHr should always be at least 0.5m less than NPSHa. Factors which most affect NPSHa are high altitude sites with low atmospheric pressures on the fluid entering the pump, sealed suction tanks with no atmospheric pressure at all or vacuum conditions, high suction lifts eg: where pumps need to “lift” water out of a pit, and fluids at high temperatures that are counteracting atmospheric pressure.
Choosing the right pump for the job is crucial. A pump with a higher Net Positive Suction Head Required (NPSHr) than the available Net Positive Suction Head Available (NPSHa) in your system is setting itself up for cavitation. So, during pump selection, ensure a good margin between NPSHa and NPSHr (a rule of thumb is NPSHa 10% more than NPSHr).
The second step to prevent cavitation is to avoid any restrictions or obstructions in the suction line that can reduce the pressure of the fluid entering the pump. These include valves, elbows, tees, filters, strainers, and fittings. You should minimize the length and number of these components and use smooth and wide pipes. You should also avoid air entrainment, which is the presence of air bubbles in the fluid, as it can lower the pressure and increase the risk of cavitation.
System Design Matters: Even with the right pump, your system design can influence cavitation. Here's how to optimize it: Minimize friction losses: Reduce friction in the suction piping by using larger diameter pipes, smooth bends, and clean filters. This helps maintain pressure at the pump inlet. Gravity is your friend: If possible, position the pump below the liquid source so gravity feeds the pump inlet, reducing the pressure drop in the suction line. Submerged suction: In some cases, submerging the pump inlet in the liquid can ensure a constantly flooded inlet.
The third step to prevent cavitation is to increase the suction head, which is the height of the fluid above the pump's centerline. The higher the suction head, the higher the pressure of the fluid at the pump inlet. You can increase the suction head by placing the pump below the fluid source, such as a tank or a reservoir, or by increasing the level of the fluid in the source. You should also avoid negative suction head, which is when the pump is above the fluid source and has to lift the fluid.
Optimizing system design by minimizing suction line restrictions, such as sharp bends or undersized pipes, reduces the likelihood of cavitation. Regular maintenance, including inspection of pump components for wear or damage, and timely replacement of worn parts, such as impellers or wear rings, is crucial for sustained pump performance. Implementing effective operational practices, such as avoiding sudden changes in flow rates and maintaining proper alignment and installation of the pump, further mitigates cavitation risks. Continuous monitoring of pump performance parameters, including suction pressure and temperature, enables early detection of cavitation symptoms, allowing for timely intervention and corrective measures.
Subcooling liquid is also a method to increase the NPSHa of the fluid. In applications were the liquid being pumped is at equilibrium (such as the bottom of a distillation column or sour water stripper) it is possible to use a cooler upstream of the pump. However, it is very important to keep this cooler clean from fouling. I have seen this methodology used primarily on sour water strippers in petroleum refineries. Another way that subcooling can be achieved is by recycling some small amount of bottoms liquid after it has been cooled. This is common practice in vacuum distillation columns used in crude oil refineries.
The fourth step to prevent cavitation is to reduce the discharge head, which is the height of the fluid above the pump's centerline at the outlet. The higher the discharge head, the higher the resistance and back pressure that the pump has to overcome. This can reduce the flow rate and increase the chance of cavitation. You can reduce the discharge head by placing the pump closer to the destination of the fluid, such as a reactor or a separator, or by lowering the level of the fluid in the destination. You should also avoid excessive discharge line restrictions, such as valves, elbows, and fittings.
Installing a vacuum gauge on the suction line can help in monitoring the suction pressure and detecting potential cavitation issues.
The fifth step to prevent cavitation is to control the flow rate of the fluid through the pump. The flow rate affects the pressure and velocity of the fluid at different points of the pump. If the flow rate is too high, the pressure at the suction side can drop below the vapor pressure and cause cavitation. If the flow rate is too low, the pressure at the discharge side can increase and cause backflow and recirculation, which can also cause cavitation. You can control the flow rate by using a flow meter, a control valve, or a variable speed drive.
Operating the Pump: Once you have the right pump and a good system design, here are some operational practices to prevent cavitation: Reduce motor speed: Lowering the RPM reduces the pressure drop across the impeller, lessening the chance of cavitation. However, this also reduces flow rate and head pressure - so find a balance. Temperature control: Colder liquids have a lower vapor pressure, so if feasible, try to reduce the temperature of the liquid being pumped.
The sixth step to prevent cavitation is to monitor the pump performance and detect any signs of cavitation. You can use indicators such as pressure gauges, flow meters, vibration sensors, and noise meters to measure the pressure, flow, vibration, and noise of the pump. You can also inspect the pump regularly and look for any signs of wear, erosion, or corrosion on the impeller, casing, or seals. If you notice any abnormal or deteriorating conditions, you should take corrective actions, such as adjusting the pump settings, cleaning the pump, or replacing the damaged parts.
Pump Modifications: If all else fails, some modifications can be made to the pump itself: Impeller inducer: Adding an inducer to the impeller can improve NPSH characteristics. Larger impeller eye: An impeller with a larger eye diameter can reduce the pressure drop at the inlet, mitigating cavitation.
Cavitation in pumps is a complex issue that can lead to equipment failure and process inefficiency. Beyond the listed strategies, understanding the fluid's properties is crucial. Fluids with high vapor pressure at operating temperatures are more prone to cavitation. Therefore, thermodynamic analysis is essential. Additionally, maintaining proper pump maintenance schedules and using anti-cavitation valve trims can further mitigate risks. It's also worth considering the pump's material compatibility with the fluid to prevent degradation that could exacerbate cavitation problems.