Does a multistage centrifugal pump have a lower necessary NPSH than a single stage pump?

This digression can only be misleading!

Centrifugal pump series and material selection considerations.

The series of centrifugal pumps refers to the pumps of the same structure and purpose produced by the pump factory, such as IS-type clean water pump, S-type double-suction centrifugal pump, IH type chemical pump, ZGB slurry pump, pipeline centrifugal pump, etc. The pump series and material are selected based on the process parameters and media characteristics.

1) According to the characteristics of the medium, decide which characteristic pump to choose, such as clean water pump, corrosion-resistant pump and impurity pump. When the medium is highly toxic, valuable or radioactive, and other substances that are not allowed to leak, a leakage-free pump (such as a shielded pump, a magnetic pump) or a double-acting mechanical seal with a leakage liquid collection and leakage alarm device should be considered. If the medium is liquefied and other easy-to-use liquids, a low NPSH pump, such as a cartridge pump, should be selected.

2) According to the selection and installation conditions, choose horizontal pumps and vertical pumps (including submerged pumps and pipeline pumps).

3) According to the flow rate, choose a single suction pump, a double suction pump, or a small flow centrifugal pump.

4) According to the head level, single-stage pump, multi-stage pump, or high-speed centrifugal pump, etc.

After the above items are determined, you can choose the appropriate pump series and production plant according to the characteristics of different series of pumps and the conditions of the production plant. Finally, according to the characteristics of the device and the process parameters of the pump, decide which type of manufacturing and inspection standard to choose. If the requirements are high, the API610 standard can be selected, and when the requirements are general, the GB5656 (ISO5199) or ANSI M standard can be selected.

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It is better for the centrifugal pump to have a large NPSH.

Because the disadvantage of throttling adjustment is that when the pump flow rate is small, the impeller is prone to cavitation. This is because the impeller of the centrifugal pump rotates at high speed under the drive of the prime mover, and when the valve opening is reduced and the flow rate is too small, the heat generated by the friction between the impeller and the liquid cannot be completely taken away, so that the temperature of the liquid in the pump increases.

As a result, the liquid vaporizes and cavitation is formed. This is especially true in power plants, where boiler feed pumps are used. Therefore, when throttling adjustment is adopted, a recirculation system should be set up to increase the output flow of the pump to prevent cavitation.

Use Maintenance:

Centrifugal pump downtime is mainly caused by the failure of mechanical seals. Most of the manifestations of failure are leaks, and the reasons for leaks are as follows:

The main reasons for the leakage of the sealing surface of the dynamic and static ring are: the flatness and roughness of the end face do not meet the requirements, or the surface is scratched; There is particulate matter between the end faces, which causes the two end faces to not operate in the same way; The installation is not in place, and the way is not correct.

The main reasons for the leakage of the compensation ring seal are: deformation of the gland and uneven preload; Incorrect installation; The quality of the sealing ring does not meet the standard; The seal is not selected correctly.

The actual use effect shows that the most failed parts of the sealing element are the end face of the dynamic and static rings, and the end face of the static ring is a common failure phenomenon when the centrifugal pump is sealed, and the cracks on the end face of the static rings are common.

Different centrifugal pumps have different NPSH requirements, which can be used under the premise that the inherent NPSH requirements of centrifugal pumps are met in a system device, and there is no question of whether it is better or smaller. In addition, the centrifugal pump with good cavitation performance has relatively better performance parameters in other aspects, and the selection area of suction lift is also wider.

Effect of cavitation on pumps

When cavitation develops to a certain level, it will affect the performance of the pump and hinder its normal operation. It is mainly manifested in the following aspects:

1. When the performance of the pump changes and cavitation is born, there is no obvious effect on the external characteristics of the pump. After cavitation develops to a certain extent, the bubbles will also clog the impeller channel, causing the pump to suddenly drop parameters such as power, efficiency, flow and head. When cavitation is fully developed, the effective flow area of the water flow will be reduced a lot, resulting in the interruption of the water flow and the failure to work.

2. When the vibration and noise bubbles burst, the liquid particles impact each other, resulting in noise and unit vibration, and the two excite each other to produce strong vibration of the pump, which is called cavitation resonance phenomenon.

3. Due to the continuous local impact on the surface of the overflow parts, the surface of the material will be gradually fatigued and damaged, causing the erosion of the metal surface, and then the appearance of large and small honeycomb cavities; In addition to damage to metal parts caused by impact, chemical corrosion phenomena can also be generated, oxidizing equipment.

In addition, the cavitation process is unstable and causes the pump to vibrate and generate noise.

1) Effective NPSH: After the liquid flow self-suction tank reaches the suction inlet of the pump through the suction pipeline, the excess energy head that is higher than the saturated vapor pressure of the liquid. It is denoted by δha.

2) The necessary NPSH of the pump: the total energy loss of the liquid flow from the pump inlet to the lowest pressure point k in the impeller, expressed by ΔHR.

3) Difference and connection between ΔHR and ΔHA:

The effective NPSH of the pump is greater than the necessary NPSH of the pump: the effective NPSH of the pump is equal to the necessary NPSH of the pump: the effective NPSH of the pump is less than the necessary NPSH of the pump: the pump is severely cavitated.

If the NPSH of the centrifugal pump must refer to the necessary NPSH of the pump, of course, the smaller the better!

Compare the required NPSH of a centrifugal pump with the critical NPSH ()aNecessary NPSH Allowance "The NPSH of the Rock Boundary is large.

b.Required NPSH = Critical NPSH.

c.Required NPSH Margin: Critical NPSH.

d.It's hard to say.

Correct Answer: Required NPSH Critical NPSH.

The greater the NPSH of a centrifugal pump, the more blind it is ()aCavitation is less likely to occur.

b.The more likely it is to have hollow cavitation.

c.Whether cavitation is susceptible or not is independent of the value of the critical NPSH.

d.It's not boring to say.

Correct answer: The more prone it is to cavitation.

The relationship between the necessary NPSH and the flow rate of the lead heart pump is ()aIt does not matter.

b.The higher the liquid flow, the smaller the required NPSH.

c.The higher the liquid flow, the greater the required NPSH.

d.It's hard to say.

Correct Answer: The higher the liquid flow, the greater the required NPSH.

When the actual NPSH of the centrifugal pump is equal to the critical NPSH, then () Cong Sakura.

a.The pressure at the inlet of the pump is equal to the saturated vapor pressure of the liquid.

b.The pressure at the center of the impeller is equal to the saturated vapor pressure of the liquid.

c.The pressure above the liquid level of the closed storage tank where the liquid to be conveyed is equal to the saturated vapor pressure of the liquid.

d.None of those three statements are true.

Correct Answer: The pressure at the center of the impeller is equal to the saturated vapor pressure of the liquid.

With the increase of flow rate q, the cavitation allowance δh of centrifugal pump increases (correctly buried in the bucket liquid).

The hollow mountain is small. No change.

It increases first, then decreases.

a.It's just a general idea, and you can't get a specific friend sales value.

b.The values are determined by the user himself by calibrating the pump.

c.The values are calculated by the pump user.

d.Values are provided by the pump manufacturer.

Correct answer: The value is provided by the pump manufacturer.