Basic Principles Of Centrifugal Pumps: Cavitation
Types of cavitation in centrifugal pumps
To reduce or prevent cavitation in centrifugal pumps, it is important to understand the different types of cavitation that can occur. These types include:
1. Vaporization cavitation. Also known as "typical cavitation" or "Net positive suction head Deficiency (NPSHa) cavitation", this is the most common type of cavitation. The centrifugal pump increases the speed of the fluid as it is drawn through the impeller holes. An increase in velocity corresponds to a decrease in fluid pressure. The pressure reduction may cause part of the fluid to boil (vaporize) and form vapor bubbles, which collapse violently when they reach the high-pressure area and create a tiny shock wave.
2. Turbulent cavitation. Parts such as elbows, valves, filters, etc. in the piping system may not be suitable for the volume or nature of the liquid being pumped, which can create eddy currents, turbulence, and pressure differences throughout the liquid. When these phenomena appear at the entrance of the pump, they will directly erode the inside of the pump or cause the liquid to vaporize.
3. Leaf syndrome cavitation. Also known as "blade through syndrome", this type of cavitation occurs when the impeller diameter is too large or the inner coating of the pump housing is too thick/the inner diameter of the pump housing is too small. Either or both of these conditions will reduce the space (clearance) in the pump housing below an acceptable level. A reduction in the clearance in the pump housing results in an increase in the fluid flow rate, resulting in a reduction in pressure. Reduced pressure may cause the fluid to vaporize, producing cavitation bubbles.
4. Internal recirculation cavitation. When the pump is unable to discharge the fluid at the desired flow rate, it causes some or all of the fluid to recirculate around the impeller. The recirculated fluid passes through regions of low pressure and high pressure, resulting in heat, high speed, and the formation of vaporizing bubbles. A common cause of internal recirculation is running the pump when the pump outlet valve is closed (or with a low flow rate -).
5. Air entrainment with cavitation. Air may be drawn into the pump through a failed valve or loose connector. Once inside the pump, the air will flow with the fluid. The movement of fluid and air may form bubbles that "explode" when exposed to the increased pressure of the pump impeller.
Factors that cause cavitation
NPSH, NPSHa, and NPSHr
NPSH is a key factor to prevent cavitation in centrifugal pumps. NPSH is the difference between the actual suction pressure and the fluid vapor pressure, measured at the pump inlet. The NPSH value must be high to prevent the fluid from vaporizing in the pump. NPSHa is the actual NPSH under pump operating conditions. The required net positive suction head (NPSHr) is the minimum NPSH specified by the pump manufacturer to avoid cavitation. NPSHa is a function of the installation and operation details of the suction pipe and pump. NPSHr is a function of pump design and its value is determined by pump testing. NPSHr indicates the available head under test conditions, and usually takes a 3% drop in pump head (for multistage pumps, head impeller head) as the basis for cavitation identification. NPSHa should always be greater than NPSHr to avoid cavitation.
Air entrainment and its role in cavitation
When air enters the suction line of the pump, air entrainment occurs, resulting in an increased risk of cavitation. This can occur due to improper pump filling, leakage in the suction line, and eddy or turbulent flow in the suction line. The air in the liquid forms small bubbles that, under pressure conditions in the pump, can contribute to or exacerbate the cavitation process. Reducing air entrainment is key to minimizing the risk of cavitation. This can be achieved by complying with minimum immersion requirements, ensuring proper sealing of pipe connections, maintaining adequate NPSHa, and avoiding turbulence at the pump inlet.
Analysis of pump and system curves related to cavitation risk
Pump and system curve analysis is an important tool for understanding and reducing the risk of cavitation. The intersections between the pump and system curves illustrate the performance of the pump's flow, head, and efficiency under different system conditions. By analyzing pump and system curves, the operator can determine the optimal operating range of the pump and avoid areas known to cause cavitation. These areas include situations where the flow rate is very high or the suction head is very low. Special attention should be paid to the minimum flow point, as operating below this rate greatly increases the risk of cavitation. Proper use of pump curves helps to make decisions about pump selection, operating speed and necessary precautions to minimize cavitation in centrifugal pumps.
Strategies to reduce cavitation
Raise NPSHa to prevent cavitation
Ensuring that the NPSHa is greater than the NPSHr is essential to avoid cavitation. This can be done by:
1. Reduce the height of the pump relative to the suction reservoir/tank. Can increase the level in the suction reservoir/pool, or reduce the installation height of the pump. This increases the NPSHa at the pump inlet.
2. Increase the diameter of the suction pipe. This reduces fluid velocity at a constant flow rate, thereby reducing suction head losses for pipes and fittings.
3. Reduce the head loss of accessories. Reduce the number of connections on the pump suction line. Use accessories such as long radius bends, full diameter valves and taper reducing pipes to help reduce the loss of suction head caused by pipe fittings.
4. As far as possible, avoid installing filters and filters on the pump suction line, because they usually cause cavitation in the centrifugal pump. If this cannot be avoided, ensure that the filter and filter on the pump suction line are regularly checked and cleaned.
5. Cool the pumped fluid to reduce its steam pressure.
Learn about the NPSH margin for preventing cavitation
The NPSH margin is the difference between NPSHa and NPSHr. A large NPSH margin reduces the risk of cavitation because it provides a safety factor that prevents NPSHa from falling below normal operating levels due to fluctuations in operating conditions. Factors that affect the NPSH margin include fluid characteristics, pump speed, and suction conditions. Engineers must calculate and maximize this margin during the design and operational planning stages to ensure reliable pump performance and minimize the risk of cavitation. Regular monitoring and adjustment based on real-time operational data helps maintain an effective NPSH margin.
Maintain minimum pump flow
Ensuring that the centrifugal pump is operating above the specified minimum flow rate is critical to reducing cavitation. Centrifugal pumps operating below their optimal flow range (allowable working area) increase the likelihood of low pressure zones, which can induce cavitation. Each centrifugal pump has a pump characteristic curve that shows the minimum flow rate required to prevent operating problems such as cavitation. This minimum flow can be maintained using flow control methods such as bypass lines, control valves, or variable speed pumps. This is especially important during the start-up or shutdown phase, when the demand for the pump changes.
Impeller design considerations to reduce cavitation
The design of impeller plays an important role in whether the centrifugal pump is prone to cavitation. An impeller with fewer and larger blades tends to accelerate the fluid less, reducing the risk of cavitation. In addition, impellers with large inlet diameters or tapered blades help manage the flow of fluids more smoothly, minimizing turbulence and bubble formation. The service life of impellers and pumps can be extended by using materials that are resistant to cavitation damage.
Use anti-cavitation devices
Cavitation prevention devices, such as flow control accessories or cavitation suppression linings, can effectively reduce cavitation. The role of these devices is to control the fluid dynamics around the impeller, providing a more stable flow and reducing turbulence and low pressure areas that cause cavitation. Flow rectifiers can be used to reduce the vortices in the fluid and improve pump inlet conditions. The cavitation suppression lining breaks the bubble before it implodes, protecting the impeller and pump housing from damage.
The importance of correctly sizing the pump to prevent cavitation
Selecting the right pump type and specifying the right size for a particular application is critical to preventing cavitation. Over-sized pumps may operate less efficiently at lower flow rates, leading to an increased risk of cavitation, while under-sized pumps may need to work harder to meet flow requirements, which also increases the likelihood of cavitation. Proper pump selection includes detailed analysis of maximum, normal and minimum flow requirements, fluid characteristics and system layout to ensure that the pump is operating within the specified operating range. Accurate selection prevents cavitation and improves the efficiency and reliability of the pump throughout its life cycle. Cavitation in centrifugal pumps can affect efficiency and shorten service life by damaging important components. Implementing the strategies discussed, such as optimizing pump design and selection, maintaining appropriate flow rates, and ensuring adequate NPSH margins, will significantly reduce the risk of cavitation. Regular monitoring and maintenance ensures that the pump is operating under optimal conditions, thereby increasing the service life and reliability of various applications. By taking proactive measures, equipment can improve performance and avoid costly damage and hazards caused by cavitation.
