First, cavitation phenomenon causes

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.

Impurities In operation, if the local area of ​​the overcurrent portion (usually somewhere later in the impeller vane inlet) is, for some reason, the absolute pressure of the pumped liquid drops to the liquid vaporization pressure at the current temperature, The vaporizer begins to vaporize and generate a large amount of steam, forming bubbles. When the liquid containing a large amount of bubbles moves forward through the high-pressure region inside the impeller, the high-pressure liquid around the bubbles causes the bubbles to shrink sharply to rupture. 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.

The process of creating bubbles and bubble ruptures in water-contaminated pumps that cause damage to overcurrent components is the cavitation process in water-contaminated pumps. In addition to the damage caused by overcurrent components, cavitation pumps produce noise and vibration and lead to the performance degradation of impurity pumps. In severe cases, the impurities in the pump will be interrupted and can not work normally.

Second, impurities pump cavitation basic relationship

Impurity pump cavitation conditions are impurities pump itself and inhalation device determined in two ways. Therefore, to study the conditions of cavitation should be considered from both the impurity pump itself and the inhalation device, the basic relationship of impurity pump cavitation

NPSHc≤NPSHr≤ [NPSH] ≤NPSHa

NPSHa = NPSHr (NPSHc) - The impurity pump starts cavitation

NPSHaNPSHa> NPSHr (NPSHc) - Impure pump cavitation free

NPSHa - NPSHa - also known as the effective NPSH, the greater the more difficult to cavitation;

NPSHr - impurity pump NPSH, also known as the necessary NPSH or import pump dynamic pressure drop, the smaller the better anti-cavitation performance;

NPSHc - Critical NPSH, which means that the NPSHc corresponds to a certain value of NPSH.

[NPSH] - Permissible NPSH, is the NPSHc used to determine the conditions for the use of impurity pumps. Usually, [NPSH] = (1.1 ~ 1.5) NPSHc.

Third, the calculation of the device NPSH

NPSHa = Ps / ρg + Vs / 2g-Pc / ρg = Pc / ρg ± hg-hc-Ps / ρg

Fourth, to prevent cavitation of the measures

To prevent cavitation must be increased NPSHa, NPSHa> NPSHr prevent cavitation measures are 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 impurity pump, due to reduce the inlet flow rate, impurity pump is not prone to cavitation;

5. Impurities pump cavitation occurs, the flow should be reduced or reduced speed operation;

6. Impurity pump suction pump cavitation has a significant impact on impurities;

7. For impurity pumps that operate under harsh conditions, cavitation-resistant materials may be used to avoid cavitation damage.

This article Keywords: impurity pump

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