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Calculation and calculation formula of cavitation allowance for self-balancing horizontal multistage centrifugal pump
Cavitation phenomenon of self-balancing horizontal multistage centrifugal pump
When the liquid is at a certain temperature, when the pressure is reduced to the vaporization pressure at that temperature, the liquid produces vapor bubbles. This phenomenon of generating bubbles is called cavitation. The air bubbles produced during cavitation, when flowing to a high pressure, their volume decreases and collapses. This phenomenon in which the bubbles disappear in the liquid due to the increase in pressure is called cavitation collapse.
When the self-balancing horizontal multistage centrifugal pump is in operation, if the partial area of the overflow part (usually somewhere behind the inlet of the impeller blade) for some reason, the absolute pressure of the pumped liquid is reduced to the liquid vaporization at the current temperature Under pressure, the liquid begins to vaporize at this place, generating a large amount of steam and forming bubbles. When the liquid containing a large number of bubbles passes through the high-pressure zone in the impeller, the high-pressure liquid around the bubbles causes the bubbles to shrink sharply and even burst. While the bubbles are condensed and ruptured, the liquid particles fill the cavities at a very high speed. At this moment, they produce a very strong water hammer and hit the metal surface with a high impact frequency. The impact stress can reach several hundred to several thousand. Atmospheric pressure, the impact frequency can reach tens of thousands of times per second, and the wall thickness will be broken down in severe cases.
The process of generating bubbles and bubble bursting in the water pump to destroy the flow parts is the cavitation process in the water pump. After the cavitation of the pump occurs, in addition to damaging the flow components, it will also produce noise and vibration, and cause the performance of the pump to decline. In severe cases, the liquid in the pump will be interrupted and the pump will not work properly.
Self-balancing horizontal multistage centrifugal pump cavitation basic relationship
Self-balancing horizontal multi-stage centrifugal pump NPSH refers to the excess energy that exceeds the vaporization pressure per unit weight of liquid at the pump suction port. The unit is marked in meters and (NPSH)r. The suction stroke is the necessary cavitation allowance Δh: the vacuum degree of the multi-stage pump allowed to suck liquid, that is, the allowable installation height of the pump, in meters.
Suction range = standard atmospheric pressure (10.33 meters)-cavitation margin-safety amount (0.5 meters) The vacuum height of the standard atmospheric pressure energy pressure pipeline is 10.33 meters.
The NPSH refers to the difference between the total head of the liquid at the inlet of the pump and the pressure head when the liquid is vaporized. The unit is marked by meters (water column) and expressed by (NPSH). It is divided into the following categories:
(1) NPSHa ——The device cavitation allowance is also called effective cavitation allowance, which is provided by the suction device. The larger the NPSHa, the less likely the pump is to cause cavitation;
(2) NPSHr-pump cavitation margin is also called necessary cavitation margin, which is the cavitation performance parameter that the pump must achieve. The smaller the NPSHr, the better the anti-cavitation performance of the pump;
(3) NPSHt-test cavitation margin, which is the value calculated during the cavitation test. There are any number of test cavitation margins, but there is only one test cavitation margin corresponding to a certain drop in pump performance, which is called critical The NPSH is expressed by NPSHc.
(4) [NPSH] ——Permissible cavitation allowance, which is the cavitation allowance used to determine the use conditions of the pump (such as installation height). It should be greater than the critical cavitation allowance to ensure that no steam occurs during pump operation. eclipse.
These NPSH have the following relationship: NPSHc ≤ NPSHr ≤ [NPSH] ≤ NPSHa
(1) After the liquid flows into the self-balancing multi-stage pump, the part of the value that the pressure energy head decreases before the energy is increased by the impeller is caused by the flow rate change and hydraulic loss. The main factor affecting it is the geometry of the pump suction chamber The shape and flow rate have nothing to do with the suction pipeline, the nature of the liquid and other parameters, it is only related to the pump structure, to be precise, it is only related to the impeller shape and the inlet shape;
(2) NPSHr and NPSHa are two parameters with different properties. NPSHr is determined by the characteristics of the pump itself, and is a parameter indicating the anti-cavitation performance of the pump itself. The larger the NPSHr, the worse the cavitation performance of the pump, and vice versa. NPSHa is determined by the characteristics of the external inhalation device.
(3) The limit of pump cavitation NPSHa = NPSHr
NPSHa=NPSHr corresponds to (pk=pv) the pump starts cavitation;
NPSHa>NPSHr corresponds to (pk>pv) pump without cavitation.
Calculation method of cavitation allowance for self-balancing horizontal multi-stage centrifugal pump device
(1) Calculation of cavitation allowance for self-balancing horizontal multistage centrifugal pump
In the formula, ps/ρg —— the inlet pressure head (m) converted to the datum level;
v2s/2g —— The average liquid velocity head (m) of the measuring pressure ps section;
pv/ρg —— vaporization pressure head at the temperature of the pumped liquid (m);
NPSH —— NPSH (m), its value is expressed as the value converted to the reference surface (that is, the pressure head ps/ρg converted to the reference surface is used to calculate NPSH).
(2) The standard of the pump reference plane: that is, the horizontal plane passing through the center of the circle drawn by the outer end of the inlet side of the impeller blade, as shown in the figure below.
The datum level of self-balancing horizontal multistage centrifugal pump
Suction up-the suction liquid level is below the reference level;
Backflow-the suction liquid level is above the reference level.
For horizontal self-balancing multi-stage centrifugal pumps, the datum plane is the horizontal plane passing through the axis of the pump;
For self-balancing multi-stage pumps, take its first stage impeller as the benchmark;
The opposite two-stage pump is based on the upper impeller suction port.
(3) Self-balancing horizontal multi-stage centrifugal pump suction and backflow calculation formula
Suction calculation of self-balancing horizontal multistage centrifugal pump
Backflow calculation of self-balancing horizontal multistage centrifugal pump