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NPSH of water pump

Date: November 8, 2015 popularity: 144
NPSH refers to the difference between the total head of the liquid at the pump inlet and the pressure head when the liquid vaporizes. The unit is marked in meters (water column) and expressed in (NPSH). It is divided into the following categories:
Npsha -- NPSH of the device, also known as effective NPSH, the larger the NPSH, the more difficult it is for NPSH;
NPSHr - pump NPSH, also known as the necessary NPSH or pump inlet dynamic pressure drop, the smaller the anti cavitation performance is better;
Npshc critical NPSH, which refers to the NPSH corresponding to a certain value of pump performance reduction;
[NPSH] - NPSH is the NPSH used to determine the service conditions of the pump, usually taken as [NPSH] = (1.1-1.5) npshc.
When the centrifugal pump is running, the liquid pressure decreases along the pump inlet to the impeller inlet, and the liquid pressure PK is the lowest at the K point near the blade inlet. Since then, due to the impeller work on the liquid, the liquid pressure rises quickly. When the pressure PK near the impeller blade inlet is less than the saturated vapor pressure PV at the liquid delivery temperature, the liquid vaporizes. At the same time, the gas dissolved in the liquid can escape. They form many bubbles. When the vapor bubbles flow with the liquid to the higher pressure in the blade passage, the external liquid pressure is higher than the vaporization pressure in the steam bubble, then the vapor bubble condenses again and collapses to form a cavity. In an instant, the surrounding liquid rushes to the cavity at a very high speed, causing the liquid to collide with each other, resulting in the sudden increase of local pressure (some up to hundreds of atmospheres). In this way, it not only hinders the normal flow of liquid, but also, if these bubbles collapse near the impeller wall, the liquid will continuously hit the metal surface like countless small warheads. The impact frequency is very high (some can reach 2000-3000 Hz), so the metal surface is cracked due to impact fatigue. If some kind of active gas (such as oxygen) is mixed in the bubble, they will form a thermocouple by virtue of the heat released during condensation (local temperature can reach 200-300 ℃), which will produce electrolysis and electrochemical corrosion, and accelerate the damage speed of metal erosion. The above-mentioned phenomena of high-temperature corrosion and cavitation are called as high-temperature corrosion, high-frequency cavitation and impact.
The most vulnerable parts of centrifugal pump are as follows:
a. At the front cover plate with the largest impeller curvature, the low pressure side close to the blade inlet edge;
b. The low pressure side near the inlet edge of the volute tongue and guide vane in the press out chamber;
c. The sealing clearance between the outer circle of the tip and the shell of the high specific speed impeller without front cover plate and the low pressure side of the blade tip;
d. The first stage impeller in multistage pump.
Calculation formula
What is NPSH? What is suction? Unit of measurement and indication letters?
At this time, the pressure of vaporized liquid at the inlet of the pump is higher than the pressure of the liquid at the inlet of the pump. The unit is a meter liquid column, expressed as (NPSH) r.
The suction head is the necessary NPSH Δ / h, that is, the vacuum degree of liquid suction allowed by the pump, that is, the allowable geometric installation height of the pump. The unit is meter. Suction head = standard atmospheric pressure (10.33 m) - cavitation allowance - Pipeline loss - Safety amount (0.5) standard atmospheric pressure can press the vacuum height of pipeline by 10.33 M
For example: the cavitation allowance of a pump is 4.0 m, and the suction head Δ h is calculated
Solution: Δ H = 10.33-4.0-0.5 = 5.83m
Related knowledge
Cavitation phenomenon
At a certain temperature, when the pressure is reduced to the vaporization pressure at this temperature, the liquid will produce vapor bubbles. This phenomenon of bubble formation is called cavitation. The bubble generated during cavitation will decrease in volume and burst when it flows to high pressure. This phenomenon of bubble disappearance in liquid due to pressure rise is called cavitation collapse.
When the pump is in operation, if the local area of its flow passage part (usually a place later than the impeller blade inlet) for some reason, when the absolute pressure of the pumped liquid is reduced to the liquid vaporization pressure under the temperature at that time, the liquid will begin to vaporize at that place, and a large amount of steam will be generated to form bubbles. When the liquid containing a large number of bubbles passes through the high-pressure area in the impeller, when the liquid containing a large number of bubbles goes forward through the high-pressure area in the impeller, the surrounding air bubbles will be formed The high pressure liquid causes the bubble to shrink sharply and even to burst. When the bubble condenses and breaks, the liquid particle fills the hole at a very high speed. At this moment, it produces a strong water hammer effect and strikes the metal surface with a high impact frequency. The impact stress can reach hundreds to thousands of atmospheres, and the impact frequency can reach tens of thousands of times per second. In serious cases, the wall thickness will be broken down.
The cavitation process is the process of cavitation in the water pump that produces bubbles and breaks the flow passage parts. In addition to the normal operation of the pump, the pump will be seriously damaged by cavitation.
Improving cavitation resistance measures
a. Measures to improve the cavitation resistance of centrifugal pump
(1) The structure design of the pump from the suction to the impeller is improved. In order to reduce the speed-up and pressure drop of the flow around the blade head, it is necessary to increase the flow area, increase the curvature radius of the inlet section of the impeller cover plate, and reduce the sharp acceleration and pressure drop of the liquid flow; appropriately reduce the thickness of the blade inlet and round the blade inlet to make it close to the streamline shape, and reduce the acceleration and pressure drop at the head of the blade; improve the surface finish of the impeller and the blade inlet to reduce the resistance loss; turn the blade inlet side to the impeller inlet The extension makes the liquid flow accept the work ahead of time and increase the pressure.
(2) The front inducer is used to make the liquid flow work ahead of time in the front inducer, so as to improve the liquid flow pressure.
(3) If the double suction impeller is used to let the liquid flow into the impeller from both sides of the impeller at the same time, the inlet cross-section will be doubled and the inlet velocity can be reduced by one time.
(4) In order to increase the blade inlet angle, reduce the bending at the blade inlet, reduce the blade blockage, and increase the inlet area, improve the working conditions under large flow rate to reduce the flow loss. But the positive attack angle should not be too large, otherwise it will affect the efficiency.
(5) The cavitation resistant materials are used. The practice shows that the higher the strength, hardness and toughness of the material, the better the chemical stability and the stronger the cavitation resistance.
b. Measures to improve effective cavitation allowance of liquid inlet device
(1) Increase the pressure of liquid level in the tank before the pump to increase the effective cavitation allowance.
(2) Reduce the installation height of suction device pump.
(3) The upper suction device was changed to the upper one.
(4) Reduce the flow loss in the pipeline before the pump. For example, shorten the pipeline as far as possible, reduce the flow rate in the pipeline, reduce the elbow and valve, and increase the valve opening as much as possible.
(5) Reduce the temperature of working medium at the pump inlet (when the conveying working fluid is close to the saturation temperature).
The above measures can be comprehensively analyzed and applied according to the pump selection, material selection and pump use site conditions.

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