The calculation of centrifugal pump cavitation for coal mine centrifugal pump plays a vital role in the selection and normal use of pump equipment. Once the calculation is wrong, the centrifugal pump may work in the cavitation state, which will reduce the life of the pump. Three types of NPSH commonly used in centrifugal pumps used in coal mines:
① Effective NPSH (NPSHa), the characteristic parameters of the inlet side of the system after the pump is installed, are provided by the suction device;
②Necessary NPSH (NPSHr) , Is determined by the performance parameters of the pump itself;
③NPSH3, the necessary NPSH when the first stage head of the pump drops by 3%.
The cavitation measurement of centrifugal pumps used in coal mines is actually the measurement of the pressure at the inlet and outlet of the centrifugal pump. Currently, pressure gauges or pressure transmitters are often used. The NPSH is the residual energy that the unit mass of the liquid at the inlet of the centrifugal pump exceeds the liquid vaporization pressure, and it is also an important indicator of the performance of the centrifugal pump. It is determined by the calculation of related parameters and the test.
Whether the cavitation allowance of the centrifugal pump meets the performance requirements of the centrifugal pump. The working schematic diagram of the centrifugal pump for coal mine is shown in the figure below:
The cavitation margin of the pump at the test site is equal to the total suction pressure of the pump suction port minus the vaporization pressure of the liquid at this time. The calculation formula is:
Where: P1 is the negative value Pa when the pressure of the pump inlet pressure gauge is less than atmospheric pressure; P is the liquid surface atmospheric pressure Pa during the test; P is the vaporization pressure Pa of the liquid at that temperature; V is the pump suction port flow rate, m/s; p is the liquid density kg/m3; Z is the height difference between the pump inlet datum (the horizontal plane passing the centerline of the pump inlet interface) and the NPSH datum, m inlet velocity V can pass through the flow Q and the inlet pipe diameter D The obtained relationship is:
Where: Q is the flow rate of the pump m3/h; D is the diameter of the inlet pipe, m effective NPSH (NPSH) is determined by the parameters in the suction pipe, the installation height of the centrifugal pump and the flow rate of the pump, and the design of the centrifugal pump The structure of NPSHa is irrelevant, therefore, the larger the NPSHa, the less likely to be cavitation. The cavitation test of the centrifugal pump is aimed at the centrifugal pump used in coal mines using an upward suction device. The effective cavitation margin at this time is determined by the formula ( 3) Calculation:
Where: P is the pressure of the suction liquid surface, Pa; h is the suction geometric height, m; h is the suction section pipe loss height m
