About cavitation
When the local pressure of the liquid in the pump drops to the critical pressure, bubbles will be generated in the liquid. Cavitation is the entire process of bubble aggregation, movement, splitting, and elimination. The critical pressure is generally close to the vaporization pressure.
01. What are the hazards of cavitation?
Pump performance degradation
During pump cavitation, the energy exchange inside the impeller is disrupted and damaged, resulting in a decrease in the Q-H curve, Q-P, and Q-η curves. In severe cases, this can interrupt the flow of liquid in the pump and render it inoperable.
For low specific speeds, due to the narrow and long flow channels between the blades, once cavitation occurs, bubbles fill the entire flow channel and the performance curve will suddenly decrease.
For medium to high specific speeds, the flow channel is short and wide, so there is a transitional process for bubbles to develop and fill the entire flow channel. The corresponding performance curve starts with a slow decline and then sharply decreases when it increases to a certain flow rate.
02. The most prone area for cavitation in centrifugal pumps
At the front cover plate with the maximum curvature of the impeller, near the low-pressure side of the blade inlet edge
The low-pressure side near the inlet edge of the snail shell baffle and guide vanes in the extrusion chamber
The sealing gap between the outer circle of the blade tip and the shell of a high specific speed impeller without a front cover plate, as well as the low-pressure side of the blade tip
The first stage impeller in a multi-stage pump。
03. Improve anti cavitation measures
Measures to improve the anti cavitation performance of centrifugal pumps themselves
By using a double suction impeller, the liquid flow enters the impeller from both sides simultaneously, doubling the inlet cross-section and reducing the inlet flow velocity.
The design condition adopts a slightly larger positive angle of attack to increase the blade inlet angle, reduce the bending at the blade inlet, reduce blade blockage, and increase the inlet area; Improve working conditions under high flow conditions to reduce flow losses. But the positive angle of attack should not be too large, otherwise it will affect efficiency.
Use materials that are resistant to cavitation. Practice has shown that the higher the strength, hardness, and toughness of a material, the better its chemical stability, and the stronger its resistance to cavitation.
Measures to increase the effective cavitation margin of the liquid inlet device
Increase the pressure of the liquid level in the storage tank before the pump to increase the effective cavitation margin.
Reduce the installation height of the suction device pump. Replace the suction device with a backflow device.
Reduce flow loss on the pipeline in front of the pump. Try to shorten the pipeline as much as possible within the required range, reduce the flow rate in the pipeline, minimize bends and valves, and increase the valve opening as much as possible.
Reduce the temperature of the working medium at the pump inlet (when the conveyed working medium approaches saturation temperature).
The above measures can be comprehensively analyzed and appropriately applied based on the selection, material selection, and on-site conditions of the pump.
04. Cavitation allowance and suction stroke
- When the pump is working, the liquid at the inlet of the impeller will generate vapor under a certain vacuum pressure. The vaporized bubbles will erode the metal surface of the impeller and other materials under the impact of the liquid particles, thereby damaging the impeller and other metals. At this time, the vacuum pressure is called vaporization pressure, and cavitation allowance refers to the excess energy per unit weight of liquid at the pump suction inlet that exceeds the vaporization pressure. The unit is marked in meters and (NPSH) r.
- Suction stroke is the necessary cavitation allowance Δ h: the vacuum degree that the pump is allowed to suction liquid, which is the installation height allowed by the pump, measured in meters. Suction stroke=standard atmospheric pressure (10.33 meters) – cavitation allowance – safety margin (0.5 meters) Standard atmospheric pressure can compress the vacuum height of the pipeline to 10.33 meters.
- For example, if the cavitation allowance of a certain pump is 4.0 meters, calculate the suction stroke Δ h
Solution: Δ h=10.33-4.0-0.5=5.83 meters
05. Each unit of measurement and letter of representation
Cavitation allowance refers to the difference between the total head of the liquid at the inlet of the pump and the pressure head when the liquid vaporizes, measured in meters (water column) and expressed in NPSH. It is divided into the following categories:
NPSHa – device cavitation allowance, also known as effective cavitation allowance, the larger the amount, the less likely it is to cavitation;
NPSHr – Pump cavitation allowance, also known as necessary cavitation allowance or pump inlet dynamic pressure drop. The smaller the NPSHr, the better the anti cavitation performance;
NPSHc – critical cavitation allowance, refers to the cavitation allowance corresponding to a certain decrease in pump performance;
[NPSH] – Allowable cavitation allowance, is the cavitation allowance used to determine the operating conditions of the pump, usually taken as [NPSH]=(1.1-1.5) NPSHc.
06. The difference between required cavitation allowance and effective cavitation allowance
The cavitation allowance is divided into effective cavitation allowance NPSHa and necessary cavitation allowance NPSHr. The required cavitation allowance of a pump is a characteristic of the pump, determined by the design, and the effective cavitation allowance of the pump is determined by the process pipeline.
The required cavitation allowance for a given pump at a given speed and flow rate is called the required cavitation allowance, commonly represented by NPSHr. Also known as the cavitation allowance of a pump, it is the cavitation performance parameter that the pump is required to achieve.
NPSHr is related to the internal flow of the pump and is determined by the pump head itself. Its physical meaning is to indicate the degree of pressure drop of the liquid at the inlet of the pump. In order to prevent cavitation of the pump, it is required that the unit weight of liquid at the pump inlet has excess energy beyond the vaporization pressure head.
The cavitation allowance must be independent of the device parameters and only related to the motion parameters of the pump inlet (vo, wo, wk, etc.), which are determined by geometric parameters at a certain speed and flow rate. This means that NPSHr is determined by the geometric parameters of the pump itself (suction chamber and impeller inlet).
For a given pump, regardless of the medium (excluding viscous media that affect velocity distribution), when flowing through the pump inlet at a certain speed and flow rate, there is the same pressure drop due to the same velocity, that is, NPSHr is the same. So NPSHr is independent of the properties of the liquid (without considering thermodynamic factors).
The smaller the NPSHr, the lower the pressure drop, requiring the device to provide a smaller NPSHa, thus improving the pump’s anti cavitation performance. Therefore, r represents required, determined by the pump body, and specifically related to the speed, impeller form, etc;
Effective cavitation allowance refers to the cavitation allowance determined by the installation conditions of the pump, commonly expressed as NPSHa. Also known as device cavitation allowance, it is the excess energy per unit weight of liquid provided by the suction device at the pump inlet that exceeds the vaporization pressure head.
The larger the NPSHa, the less likely the pump is to experience cavitation. The size of effective cavitation allowance is related to device parameters and liquid properties (p, pv, etc.). Because the hydraulic loss of the suction device is proportional to the square of the flow rate, NPSHa decreases with increasing flow rate.
Therefore, A represents available, effective, and available, which is determined by the system and pipeline and must be rigorously calculated;
To ensure that the pump does not suffer from cavitation, NPSHa must be greater than NPSHr. The specific size varies depending on the type of pump, and there are empirical values for different forms of pumps. Generally, the allowable cavitation allowance is determined by adding 0.5-1m of excess energy head to the required cavitation allowance of the pump.
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