The most complete summary of pump cavitation.

Table of Contents

1.0: What is cavitation?

2.0: Why does pump cavitation occur?

3.0:What are the hazards of cavitation?

4.0:How do you know if a pump is cavitating?

5.0:What are the measures to improve cavitation resistance?

6.0:The respective units of measurement and the letters indicating them.

7.0:What is the difference between the required cavitation allowance and the effective cavitation allowance?


Chapter 1.0: What is cavitation?

Cavitation is a general term for the process of accumulation, flow, splitting, and collapse of vapor bubbles when the fluid in the flow channel, which can be pumps, turbines, rivers, valves, propellers or even human and animal blood vessels, etc., such as water, oil, etc., local pressure drops to the critical pressure, the gas nucleus in the liquid grows into a vapor bubble. Pump in the suction vacuum degree is greater than the allowable suction vacuum degree, cavitation phenomenon occurs. It mainly occurs at the outer edge of the impeller blade and covers, volute, or guide wheel, not at the blade inlet. For example, when the flow rate is greater than the design flow rate, it occurs at the front of the blade inlet near the front cover plate. When the impeller inlet pressure down to be sent to the liquid in the working temperature of the saturated vapor pressure, the liquid will occur part of the vaporization, the generation of bubbles with the liquid from the low-pressure area into the high-pressure area, the bubble in the high-pressure area will shrink sharply, condensation, the liquid around it to a very high speed to the original bubble space, resulting in a high-intensity shock wave, impact impeller and pump casing, noise caused by vibration. Due to the repeated action of long-term impact and the chemical corrosion of trace dissolved oxygen in the liquid, the local surface of the impeller is speckled and cracked or even spongy damaged.

Chapter 2.0: Why does pump cavitation occur?

Pumps are designed to supply water at full flow, but in some cases, the water-filled inlet is not sufficient to maintain the pressure required to prevent cavitation. The inlet or suction side of the pump is the lowest pressure point in a given pump. For positive displacement pumps, the lowest pressure occurs before the rotor engages; for centrifugal pumps, the lowest pressure is near the impeller eye.

Cavitation can occur in all pump types, and because the principles are essentially the same, we will focus on centrifugal pumps. The eye is where the fluid is drawn into the impeller, and where the rotation of the impeller begins to act on the fluid. When the pressure acting on the fluid (available net positive suction head) is too low, bubbles are formed, and as the impeller rotates, the fluid accelerates, the pressure increases, and the bubbles burst.

Under normal atmospheric pressure conditions, the fluid has a predictable vapor pressure. When the pressure inside the pump is lower than the vapor pressure of the fluid, bubbles are formed. When the bubble reaches a region of the fluid where the pressure is higher than the vapor pressure, the bubble will burst. In the case of cavitation, this formation and collapse are both rapid and violent. Interrupted or poorly executed processing lines can result in a drop in suction or discharge pressure, which can lead to cavitation.

1.Poor pump inlet condition:

Flow interruptions can have a variety of causes, from system design to component degradation. Common causes of flow interruptions that lead to cavitation phenomena are:

  • Excessively long inlet piping
  • Higher than expected fluid viscosity
  • Blocked inlet
  • Clogged strainers and filters
  • Restricted or collapsed inlet hoses
  • Unspecified pump

2.Discharge cavitation:

At very high discharge pressures, some fluid circulates within the pump rather than being discharged. Fluid trapped between the impeller and housing at very high velocities causes a pressure drop that creates the same conditions as suction cavitation.

Chapter 3.0:What are the hazards of cavitation?

1.Corrosion of overflow components

There are two reasons for corrosion:

1.1.One is due to the high frequency (600~25000HZ) impact when the bubble breaks, the pressure is as high as 49Mpa, resulting in mechanical stripping of the metal surface.

1.2.The second is due to the heat released during vaporization, and there is a temperature difference between the role of the battery to produce hydrolysis, the resulting oxygen to oxidize the metal, chemical corrosion occurs.

2.Pump performance degradation

Pump cavitation when the impeller energy exchange is disturbed and destroyed, the external characteristics of the Q-H curve, Q-P, Q-η curve decline, serious will make the pump in the liquid flow interrupted, can not 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.

PS: The part of centrifugal pump most prone to cavitation:

1.0.The low-pressure side near the blade inlet edge at the front cover where the impeller curvature is greatest.

2.0.The low-pressure side of the worm casing spacer and guide vane near the inlet edge in the press-out chamber.

3.0.The sealing gap between the outer circle of the impeller tip and the casing and the low-pressure side of the impeller tip of a high-ratio impeller without a front cover plate.

4.0.The first stage impeller in a multistage pump.

Chapter 4.0:How do you know if a pump is cavitating?

The obvious symptoms of cavitation are noise and vibration. When steam bubbles implode, they make a series of bubbling, crackling sounds that sound as if gravel is crunching around the pump casing or piping. In addition to the noise, abnormal vibrations that would not normally occur when operating the pump and its associated equipment may occur.

With centrifugal pumps, the discharge pressure will be lower than that normally observed or predicted by the pump manufacturer. In positive displacement pumps, cavitation causes a reduction in flow, rather than a reduction in head or pressure, as vapor bubbles expel fluid from the pumping chamber, thereby reducing its capacity.

Power consumption may also be affected under the unstable conditions associated with cavitation. It may fluctuate and be higher to achieve the same throughput. In addition, in extreme cases, when cavitation damages the pump assembly, you may observe debris in the fluid expelled from the pump assembly, including seals and bearings.

Chapter 5.0:What are the measures to improve cavitation resistance?

1.Measures to improve the anti-cavitation performance of the centrifugal pump itself:

  • 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 inlet section of the impeller cover, reduce the rapid acceleration and decompression of the liquid flow; appropriately reduce the thickness of the blade inlet and round the blade inlet, so that it is close to streamline, can also reduce the acceleration and decompression of the head of the winding blade.
  • improve the surface finish of the impeller and blade inlet section to reduce drag losses; extend the blade inlet side to the impeller inlet, so that the liquid flow to accept work in advance, and 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.
  • Adopt a double-suction impeller, let the liquid flow enter the impeller from both sides of the impeller at the same time, then the inlet section is doubled, and the inlet flow rate can be reduced twice.
  • Design conditions using a slightly larger positive impulse angle to increase the blade inlet angle, reduce the bending of the blade inlet, reduce blade blockage to increase the inlet area; improve the working conditions under the large flow rate 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.

2.Measures to improve the effective cavitation margin of the liquid inlet device:

  • Increase the pressure of the liquid level in the storage tank before the pump to improve the effective cavitation margin.
  • Reduce the installation height of the pump on the suction device.
  • Replace the upper suction device with 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 elbow 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).

Chapter 6.0:The respective units of measurement and the letters indicating them.

Cavitation margin refers to the difference between the total head of the liquid at the pump inlet and the pressure head of the liquid when it vaporizes, the unit is marked in meters (water column) and expressed in (NPSH), which is divided into the following categories.

  • NPSHa – device cavitation margin also called effective cavitation margin, the larger the less likely to cavitate.
  • NPSHr – pump cavitation margin, also known as the necessary cavitation margin or pump inlet dynamic pressure drop, the smaller the better the cavitation resistance.
  • NPSHc – critical cavitation margin, is the cavitation margin corresponding to a certain value of pump performance decline.
  • NPSH] – allowable cavitation margin, is to determine the cavitation margin for pump use conditions, usually take [NPSH] = (1.1 ~ 1.5) NPSHc.

Chapter 7.0:What is the difference between the required cavitation allowance and the effective cavitation allowance?

The cavitation margin is divided into effective cavitation margin NPSHa and necessary cavitation margin NPSHr. The necessary cavitation margin of the pump is a characteristic of the pump, determined by the design, and the effective cavitation margin of the pump is determined by the processing pipeline.

For a given pump, the necessary cavitation margin at a given speed and flow rate is called the necessary cavitation margin, often expressed as NPSHr. Also known as the pump cavitation margin, is the cavitation performance parameters to be achieved by the pump.

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 a surplus of energy over the vaporization pressure head.

Must cavitation margin and device parameters, only with the pump inlet port of the motion parameters (vo, wo, wk, etc.), these motion parameters in a certain speed and flow rate are 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. So NPSHr and the nature of the liquid have nothing to do with (without taking into account the thermodynamic factors).

The smaller the NPSHr, the smaller the pressure drop, the smaller the NPSHa required to be provided by the device, and therefore 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.

The effective cavitation margin is the cavitation margin determined by the installation conditions of the pump, often expressed as NPSHa. Also known as the device cavitation margin, 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 less likely the pump is to cavitate. The size of the effective cavitation margin is related to the device parameters and the nature of the liquid (p, PV, etc.). Because the hydraulic loss of the suction unit 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 strictly calculated.

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 cavitation margin.

Chapter 8.0:Conclusion

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