Understanding Cavitation and NPSH (net positive suction head)

Posted by angroup on October 3, 2021

Table of content

What is cavitation
How does cavitation occur
What is the suction range?
What is NPSH(net positive suction head)
NPSH(net positive suction head) Related Terms
What is the difference between NPSHr and NPSHa
The result of cavitation in the centrifugal pump
Measures to prevent cavitation

What is cavitation

Cavitation is a phenomenon in which vapor bubbles are generated in a flowing fluid due to local pressure reduction. When cavitation occurs in a pump, it causes erosion of the machine parts at the cavitation site, and further development will cause a drop in head and vibration noise.

How does cavitation occur

The pressure inside the pump at a certain place drops below the saturation vapor pressure at that temperature, then there are bubbles generated in the liquid, while the gas dissolved in water will also precipitate out, at this time the separated vapor and gas bubbles, sent by the flow of liquid to the high-pressure area, where the rapid breakage (at this time the continuation of time T ≤ 0.001s), accompanied by excessive pressure rise so that this process has all the characteristics of the breakage This produces the impact phenomenon quickly spread in turn, so that the vapor bubbles immediately adjacent to the eroded surface, as well as in the pores (micro-pores, fractures, cracks, etc.) are broken so that the lower strength of the place will produce cracks. The bombardment of the surface caused by bubble breakage causes vibration and even damage to the material layer immediately adjacent to the cavitation zone

What is the suction range?

The suction range is the necessary cavitation margin Δh: that is, the vacuum degree of the pump allowed to suck the liquid, which is also the installation height allowed by the pump, in meters. Suction range = standard atmospheric pressure (10.33m) - NPSH margin - safety amount (0.5m) The standard atmospheric pressure can press the vacuum height of the pipeline 10.33m.

What is NPSH(net positive suction head)

When the pump is working, the liquid at the inlet of the impeller will produce vapor under certain vacuum pressure, the vaporized bubbles will be under the impact movement of the liquid masses, which will exfoliate the impeller and other metal surfaces, thus destroying the impeller and other metals, the vacuum pressure is called vaporization pressure, the vapor corrosion margin is the surplus energy per unit weight of liquid at the suction inlet of the pump over the vaporization pressure, the unit is marked with meters, with (NPSH)r.

NPSHa: available net positive suction head, the larger the less likely to cavitate
NPSHc: critical net positive suction head, which is the NPSH corresponding to a certain value of pump performance reduction.
NPSHr: pump NPSH, also known as the required NPSH or pump inlet dynamic pressure drop, the smaller the better anti-cavitation performance
[NPSH]: allowable NPSH, is to determine the NPSH for pump use conditions, usually take [NPSH] = (1.1 ~ 1.5) NPSHc.

What is the difference between NPSHr and NPSHa

NPSHr is the characteristics of the pump, determined by the design, and NPSHa is determined by the process of piping

NPSHr and the internal flow of the pump are determined by the head of the pump itself, its physical meaning is to indicate the degree of pressure drop in the inlet port of the pump, that is, in order to ensure that the pump does not occur cavitation, the requirements of the pump inlet unit weight of liquid has more than the surplus energy of the vaporization pressure head. and flow rate is determined by the geometric parameters. This means that the NPSHr is determined by the pump itself (geometric parameters of the suction chamber and impeller inlet section). For the established pump, no matter what kind of media (viscous media due to the impact of the velocity distribution, except), in a certain speed and flow rate through the pump inlet, because the velocity size is the same, so there is the same pressure drop, that is, the same NPSHr. The smaller the NPSHr, the smaller the pressure drop, the smaller the NPSHa required to be provided by the device, and thus the better the pump's resistance to cavitation. Therefore:r represents the required necessary, determined by the pump body, specifically related to the speed, impeller form, etc.

v0, absolute velocity before blade inlet
w0, relative velocity before blade inlet
σ, pressure drop coefficient of blade inlet
g, acceleration of gravity

NPSHa is the NPSH determined by the pump installation conditions, is provided by the suction device at the pump inlet unit weight of liquid has more than the surplus energy of the vaporization pressure head. the larger the NPSHa, the pump is less likely to cavitation. the size of NPSHa and device parameters and liquid properties related. Because the hydraulic loss of the suction device is proportional to the square of the flow rate, NPSHa decreases with the increase of the flow rate. Therefore: A stands for available effective and available, this is determined by the system and piping and must be rigorously calculated.

Pc, absolute inlet pool liquid level pressure
Pv, liquid saturation vapor pressure
hg, pump installation height, the maximum value is the suction range of the pump.
hc, inlet pipeline head loss
ρ, the capacitive weight of water
g, acceleration of gravity

To ensure that the pump does not cavitate, NPSHa must be greater than NPSHr. How much larger, a variety of different forms of pumps have empirical values, generally the pump must increase the cavitation margin of 0.5-1m surplus energy head as the allowable NPSHa.

The result of cavitation in the centrifugal pump

Generate noise and vibration

As the pump cavitation, the bubble in the high-pressure area continuously sudden ruptures, as well as the accompanying strong water shock, and the generation of noise and vibration, you can hear the crackling sound like popping beans. The initial nature of cavitation can be detected according to the noise, but it is difficult to distinguish this cavitation noise from the surrounding noise and machine noise in the surrounding environment, so it is very difficult to determine its length quantitatively.

Shorten the life of the pump

When cavitation occurs, the long-term effect of mechanical erosion and chemical corrosion will make the inlet and outlet of the first impeller, the inlet of the guide vane or the snail casing become rough and porous, producing microscopic cracks, and in serious cases, honeycomb erosion and even the formation of cavities, thus shortening the service life of the pump

Decreased pump performance

The energy exchange in the impeller is disturbed and destroyed when pump cavitation occurs, which is manifested by Q-H curve, Q-P, Q-η curve drop in external characteristics, which will seriously interrupt the liquid flow in the pump and make it unable to work.
For low speed, due to the narrow and long flow channel between the vanes, once cavitation occurs, bubbles fill the entire flow channel, the performance curve will suddenly drop. For medium to high speed, the flow channel is short and wide, so the bubble from the development to fill the entire flow channel needs a transition process, the corresponding performance curve is slowly declining at the beginning, and then increased to a certain flow rate before a sharp decline.

Measures to prevent cavitation

Centrifugal pump design to prevent cavitation

Improve the structural design of the pump from the suction port to the vicinity of the impeller. Increase the overflow area; increase the radius of curvature of the impeller cover inlet section to reduce the rapid acceleration and decompression of liquid flow; appropriately reduce the thickness of the blade inlet and round the blade inlet to make it close to streamline, which can also reduce the acceleration and decompression of the winding blade head; improve the surface finish of the impeller and blade inlet section to reduce drag loss; extend the blade inlet side to the impeller inlet to make the liquid flow accept work in advance. Increase the pressure

Adopt the front induction wheel to make the liquid flow work in advance in the front induction wheel to improve the liquid flow pressure

The use of double suction impeller, so that the liquid flow from both sides of the impeller into the impeller at the same time, the inlet cross-section doubled, the inlet flow rate can be reduced twice

Design conditions using a slightly larger positive impulse angle, in order to increase the blade inlet angle, reduce the bending of the blade inlet, reduce blade blockage, in order to increase the inlet area; improve the working conditions under the large flow rate, in order to reduce the flow loss. But the positive impulse angle should not be too large or affect the efficiency.

The use of cavitation-resistant materials. Practice shows that the higher the strength, hardness, and toughness of the material, the better the chemical stability, the stronger the performance of cavitation resistance

Increase NPSHa of the liquid feeder

Increase the pressure of the liquid level in the storage tank before the pump

Reduce the installation height of the pump of the suction-up device.

Change the upper suction device to a backflow device

Reduce the flow loss on the pipeline before the pump. Such as in the required range as short as possible to shorten the pipeline, reduce the flow rate in the pipeline, reduce the bend and valve, try to increase the valve opening, etc.

Reduce the pump inlet medium temperature (when the transported medium is close to the saturation temperature)

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