Liquid at a certain temperature, reduce the pressure to the temperature of the vaporization pressure, the liquid will produce bubbles. The phenomenon of bubbles is called cavitation. Cavitation bubble generated when flowing to high pressure Department, the volume decreases so as to burst. This phenomenon of bubble disappearing in liquid due to pressure increase is called cavitation collapse.
Centrifugal pump in operation, if the over-flow part of the local area (usually impeller blade inlet somewhere later) for some reason, the absolute pressure of the liquid pumped down to the current temperature of liquid vaporization pressure, the liquid Where it begins to vaporize, generating a large amount of steam, forming bubbles that, when the liquid containing a large amount of air bubbles move forward through the high pressure region within the impeller, cause the bubbles to collapse sharply as a result of the high pressure liquid around the bubbles. At the same time as the bubble is broken and condensed, the liquid particles fill the cavity at a very high speed, causing a very strong water hammer moment at this moment, and striking the metal surface with a high impact frequency with an impact stress of several hundreds to several thousands Atmospheric pressure, the impact frequency of up to tens of thousands of times per second, will be severe wall breakdown.
Cavitation in a pump is the process by which bubbles in the pump and rupture of the bubble damage the overcurrent component. Pump cavitation in addition to the over-current components will have a damaging effect, but also produce noise and vibration, and lead to the decline in the performance of the pump, serious pump liquid will be interrupted, can not work properly.
Second, the basic relationship between centrifugal pump cavitation Centrifugal pump occurs cavitation conditions are determined by the pump itself and the inhalation device. Therefore, to study the conditions of cavitation should be considered from both the pump itself and the inhalation device, the basic relationship of pump cavitation NPSHc≤NPSHr≤ [NPSH] ≤NPSHa
NPSHa = NPSHrNPSHc - pump start cavitation NPSHaNPSHa> NPSHrNPSHc - pump no cavitation NPSHa - device NPSH, also known as the effective NPSH, the bigger the more difficult to cavitation;
NPSHr - pump NPSH, also known as the necessary NPSH or the dynamic pressure drop of the pump inlet, the smaller the better anti-cavitation performance;
NPSHc - Critical NPSH, which refers to the pump performance corresponding to a certain value of NPSH;
[NPSH] - Permissible NPSH, is the NPSHc used to determine pump operating conditions, usually NPSH = (1.1 ~ 1.5) NPSHc.
Third, the calculation of the device NPSH Fourth, to prevent the occurrence of cavitation measures To prevent the occurrence of cavitation NPSHa must be increased so that NPSHa> NPSHr can prevent cavitation as follows:
1. Reduce the geometric suction height hg (or increase geometric inversion height);
2. Reduce inhalation loss hc, for which you can try to increase the diameter, minimize pipe length, elbows and accessories;
3. Prevent long run at high flow rate;
4. In the same speed and flow, the use of double suction pump, due to reduce the inlet flow rate, the pump is not prone to cavitation;
5. Centrifugal pump cavitation, the flow should be reduced or reduced speed operation;
6. Centrifugal pump suction pump cavitation has a significant impact;
7. For pumps operating under harsh conditions, cavitation-resistant materials may be used to avoid cavitation damage.
Centrifugal pump in operation, if the over-flow part of the local area (usually impeller blade inlet somewhere later) for some reason, the absolute pressure of the liquid pumped down to the current temperature of liquid vaporization pressure, the liquid Where it begins to vaporize, generating a large amount of steam, forming bubbles that, when the liquid containing a large amount of air bubbles move forward through the high pressure region within the impeller, cause the bubbles to collapse sharply as a result of the high pressure liquid around the bubbles. At the same time as the bubble is broken and condensed, the liquid particles fill the cavity at a very high speed, causing a very strong water hammer moment at this moment, and striking the metal surface with a high impact frequency with an impact stress of several hundreds to several thousands Atmospheric pressure, the impact frequency of up to tens of thousands of times per second, will be severe wall breakdown.
Cavitation in a pump is the process by which bubbles in the pump and rupture of the bubble damage the overcurrent component. Pump cavitation in addition to the over-current components will have a damaging effect, but also produce noise and vibration, and lead to the decline in the performance of the pump, serious pump liquid will be interrupted, can not work properly.
Second, the basic relationship between centrifugal pump cavitation Centrifugal pump occurs cavitation conditions are determined by the pump itself and the inhalation device. Therefore, to study the conditions of cavitation should be considered from both the pump itself and the inhalation device, the basic relationship of pump cavitation NPSHc≤NPSHr≤ [NPSH] ≤NPSHa
NPSHa = NPSHrNPSHc - pump start cavitation NPSHaNPSHa> NPSHrNPSHc - pump no cavitation NPSHa - device NPSH, also known as the effective NPSH, the bigger the more difficult to cavitation;
NPSHr - pump NPSH, also known as the necessary NPSH or the dynamic pressure drop of the pump inlet, the smaller the better anti-cavitation performance;
NPSHc - Critical NPSH, which refers to the pump performance corresponding to a certain value of NPSH;
[NPSH] - Permissible NPSH, is the NPSHc used to determine pump operating conditions, usually NPSH = (1.1 ~ 1.5) NPSHc.
Third, the calculation of the device NPSH Fourth, to prevent the occurrence of cavitation measures To prevent the occurrence of cavitation NPSHa must be increased so that NPSHa> NPSHr can prevent cavitation as follows:
1. Reduce the geometric suction height hg (or increase geometric inversion height);
2. Reduce inhalation loss hc, for which you can try to increase the diameter, minimize pipe length, elbows and accessories;
3. Prevent long run at high flow rate;
4. In the same speed and flow, the use of double suction pump, due to reduce the inlet flow rate, the pump is not prone to cavitation;
5. Centrifugal pump cavitation, the flow should be reduced or reduced speed operation;
6. Centrifugal pump suction pump cavitation has a significant impact;
7. For pumps operating under harsh conditions, cavitation-resistant materials may be used to avoid cavitation damage.