How to understand the cavitation allowance of Dalian Chemical Pump?

The Net Positive Suction Head (NPSH) of Dalian Chemical Pump is a key performance parameter for measuring the avoidance of cavitation in pumps. It needs to be comprehensively understood from the aspects of principle, classification, influencing factors, and practical applications. The following is a specific analysis:

1、 The essence and harm of cavitation

Cavitation principle

When the absolute pressure of the liquid at the inlet of the pump is lower than the saturated vapor pressure of the liquid at the current temperature, the liquid will vaporize and form bubbles; When these bubbles flow into the high-pressure area of the pump with the liquid, they will quickly rupture and produce high-frequency impacts (similar to the "water hammer" effect), repeatedly impacting components such as impellers and pump casings, causing material fatigue damage (such as honeycomb pits on metal surfaces), accompanied by vibration and increased noise. In severe cases, it can cause a decrease in pump performance or even damage.

hazard

Performance degradation: Flow rate, head, and efficiency significantly decrease.

Equipment damage: The impeller and pump casing are corroded, the seals are worn, and the service life of the pump is shortened.

System failure: may cause pipeline vibration, valve damage, and even interrupt the process flow.

2、 Definition and classification of cavitation allowance

1. Effective cavitation head (NPSHa, Available NPSH)

Definition: The "excess energy" provided by a pump suction system that exceeds the saturated vapor pressure of the liquid, measured in meters (m) or pressure (Pa).

Physical meaning: Reflecting whether the suction system can provide sufficient energy for the pump to avoid liquid vaporization.

2. Required NPSH (NPSHr)

Definition: The minimum cavitation allowance required to avoid cavitation under specific operating conditions, determined by the pump's own design, is determined by the pump manufacturer through testing, with the same unit as above.

characteristic:

Related to the structure of the pump (such as impeller shape, suction chamber design), speed, flow rate, and other parameters.

The larger the flow rate, the higher the NPSHr (as high-speed flow will exacerbate the pressure drop at the inlet).

Function: Used to determine whether the NPSHa provided by the inhalation system is sufficient.

3、 Key judgment principle: Relationship between NPSHa and NPSHr

Safe operating conditions: It is necessary to ensure that NPSHa>NPSHr and leave a certain safety margin (usually 1.1-1.3 times).

Risk scenario:

If NPSHa ≈ NPSHr, the pump may be in a critical state of cavitation, and long-term operation will accelerate damage.

If NPSHa<NPSHr, the pump will inevitably experience cavitation and the system parameters need to be adjusted immediately.

4、 Core factors affecting cavitation allowance

Liquid properties

Saturated vapor pressure (Pv): The higher the temperature, the greater Pv, and NPSHa needs to be correspondingly increased (such as high-temperature media being more prone to vaporization).

Density (ρ): Density affects pressure calculation, but its impact on NPSH needs to be analyzed in conjunction with specific formulas (such as pressure being proportional to density, while the kinetic energy term is independent of density).

Inhalation system design

Installation height: When the pump installation position is higher than the liquid level, the higher the suction height, the smaller the Ps, and the NPSHa may decrease (to avoid "suction empty").

Pipeline resistance: If the diameter of the suction pipeline is too small, or if there are too many or too long valves/elbows, it will increase the flow resistance, reduce Ps, and lead to a decrease in NPSHa.

Operating parameters of the pump

Flow rate: As the flow rate increases, the pump inlet velocity vs increases, and the suction effect of the impeller on the liquid is enhanced, which may lead to an increase in NPSHr and insufficient NPSHa.

Speed: The higher the speed, the greater the centrifugal force of the impeller, and the inlet pressure drop may increase, requiring a higher NPSHa.

5、 Optimization measures in engineering applications

Improve NPSHa (improve inhalation conditions)

Reduce the installation height of the pump (such as placing the pump near the reservoir or installing it below the liquid level).

Increase the diameter of the suction pipeline, reduce accessories such as elbows and valves, and lower the resistance along the way.

For high temperature or easily vaporized liquids, use reverse suction (with the liquid level higher than the pump inlet) or increase pipeline insulation.

Reduce NPSHr (optimize pump selection and design)

Choose a pump type with good cavitation performance (such as low cavitation allowance centrifugal pump, positive displacement pump).

Require manufacturers to optimize impeller design (such as using double suction impellers, increasing impeller inlet diameter, and reducing blade inlet angle).

Avoid long-term operation of the pump under high flow conditions (flow can be controlled through variable frequency speed regulation or valve adjustment).

other measures

Regularly clean the impurities in the suction pipeline to prevent blockage and increase in pressure drop.

Set up anti cavitation protection devices in the system (such as pressure sensor linkage alarm, or automatic adjustment of reflux valve to maintain NPSHa).

The essence of cavitation allowance is an energy balance issue - the suction system needs to provide sufficient energy (NPSHa) to overcome the tendency of liquid vaporization and meet the design requirements of the pump (NPSHr). In practical applications, it is necessary to ensure the safety margin of both through theoretical calculations (combined with process parameters) and data from pump manufacturers, in order to avoid cavitation risks from the source and ensure the stable operation of chemical pumps.