Three Measures to Improve Cavitation Performance of Centrifugal Pump

Category: Water pump knowledge

2024-06-07

Centrifugal pump cavitation refers to the design condition of the pump, when the inlet pressure of the pump is reduced, the liquid in the pump cavity will produce steam bubbles, which will affect the normal operation of the pump. At this time, the flow of the pump will be limited, adjust the control valve or change the speed can not restore the normal flow, only by increasing the inlet pressure to make the pump work again. Centrifugal pump NPSH is generally used to describe the minimum allowable inlet pressure when the pump is working. If the inlet pressure is less than this value, cavitation is likely to occur.

During the operation of the pump, when the pumped liquid flows through some local areas of the flow-passing component for some reasons, the absolute pressure of the liquid decreases to the saturated vapor pressure (vaporization pressure) of the liquid at the pumping temperature, the liquid vaporizes here, generating steam and forming bubbles; At the same time, the gas dissolved in the liquid will also precipitate in the form of bubbles, forming two-phase flow in local areas.

When the bubble moves to the high-pressure area, the high-pressure liquid around the bubble will quickly condense and shrink the bubble and burst. At the moment when the bubble condenses and shrinks and ruptures, the liquid around the bubble will fill the hole (formed by the bubble condensation and rupture) at a high speed, resulting in a strong shock wave. When bubble rupture occurs on the solid wall surface of the flow-through component, high-speed micro-jets will be formed on the solid wall surface, resulting in local corrosion damage. The process of generating bubbles and bursting bubbles to destroy the flow components is the cavitation process of the pump.

Cavitation is a common problem of centrifugal pump, which can cause the increase of pump vibration and noise, the decrease of performance, and the serious damage of pump parts. Only a comprehensive and correct understanding of the cavitation performance of centrifugal pumps can scientifically and reasonably design or select products that meet the requirements of user devices.

There are four main reasons for 1. centrifugal pump cavitation:

1.1 vaporization cavitation（Vaporization cavitation）

When the pressure of the liquid is lower than its vaporization pressure or the temperature is too high, the liquid will vaporize, resulting in cavitation. This cavitation occurs on the suction side of the pump.

1.2 internal backflow cavitation（Internal recirculation cavitation - Suction specific speed）

Backflow occurs when the pumping flow is too small or the inlet pressure is too high. When the pumping flow is too small, internal backflow occurs at the inlet of the impeller; when the pump inlet pressure is too high, internal backflow occurs at the outlet of the impeller. The internal reflux causes an increase in the liquid flow rate until vaporization produces a vapor bubble, which then bursts at the ambient higher pressure. When internal backflow occurs at the suction port, irregular crackling noises will be emitted around the suction port of the pump, accompanied by a high-intensity knocking sound.

1.3 leaf runner syndrome cavitation（Vane passing syndrome cavitation）

This cavitation damage is caused by the small clearance between the guide vane (for a guide vane pump) or the volute (for a volute pump) and the outer diameter of the impeller blade. As the liquid flows through the small channel, the increase in the flow rate of the liquid causes a drop in the pressure of the liquid, local vaporization, generation of vapor bubbles, and then rupture at a higher pressure, resulting in cavitation. This cavitation usually occurs on the inside of the guide vane or tongue and in the center of the impeller blade.

1.4 turbulent cavitation（Flow turbulence cavitation）

When there is something that interferes with or hinders (such as corrosion, blockage, diameter mutation, etc.) the normal flow of liquid in the pump, turbulence will occur to change the flow rate of the liquid, the change of the flow rate of the liquid will cause the change of the pressure, and the change of the pressure will eventually cause the vaporization and cavitation of the liquid. This cavitation usually occurs in the pipes connected to the pump.

2. parameters that affect the NPSH of centrifugal pump:

1. The structural parameters of the pump: such as the inlet and outlet diameter of the pump, the speed, the diameter of the impeller outlet, the number of impeller blades, etc., will affect the NPSH of the centrifugal pump.

2. Working conditions: such as inlet pressure, the opening of the outlet valve and the flow rate, etc., will also have an impact on the NPSH of the centrifugal pump.

3. Medium: the density, viscosity and gas content of the medium will also affect the NPSH of the centrifugal pump.

3. measures to improve the cavitation performance of centrifugal pumps mainly focus on the following three points:

(1) Improve the cavitation resistance of materials

Use anti-cavitation materials, such as copper, austenitic stainless steel, martensitic stainless steel.

Improving the density of the metal (such as the use of special die-casting process, etc.) and hardness can delay the speed of cavitation damage.

(2) Reduce the necessary NPSH

Reduce the pump speed.

Adopt double suction impeller (multi-stage pump first stage adopts double suction).

Install an inducer. However, in petrochemical and petrochemical engineering applications, many users do not want to use pumps with inducers. Because when the inducer is improperly designed or corroded, it is easy to cause it to work in the overload area, not only does not play a role in pressurization, but also more likely to cause cavitation of the pump. The UOP specification clearly stipulates that horizontal pumps are not allowed to have inducers.

Increase the impeller inlet diameter. However, if the impeller inlet diameter is too large, this method not only affects the operating performance of the centrifugal pump, but also causes a significant increase in pump vibration.

Use several smaller pumps. Three half-capacity pumps may be cheaper than one large pump plus one backup pump and more energy efficient at smaller loads.

The impeller blade extends to the pump inlet side properly, which is equivalent to adding a small inducer.

The blade is swept back to reduce any cavitation at its leading edge.

Optimizing the leading edge profile of the impeller blade (e. g. using parabolic leading edge profile, reducing the thickness of the suction side blade, etc.) can effectively limit the pressure peak at the leading edge of the blade and reduce the sensitivity to operation under partial load.

The use of modern computer calculation and analysis technology given the impeller inlet design can be optimized conditions, so as to better control and understand the impeller flow and pressure distribution. The pressure distribution on the blade is more uniform, which ensures that the blade does not undergo cavitation corrosion, and at the same time ensures that the impeller has the best performance.

(3) Improve device NPSH

Increase the level of the suction sump.

Increase the pressure of the water tank.

Install the pump into a pit, such as a VS6 pump.

Reduce hydraulic losses in suction lines.

Install a front (booster) pump for the main pump, such as the main feed pump of a million-unit nuclear power plant.

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