The generation of cavitation and impeller cutting in centrifugal pumps
Centrifugal pumps are prone to cavitation, which is related to many factors. What is the purpose of cutting the pump impeller and what role does it play?
1、 Cutting of Centrifugal Pump Impeller
1. Purpose of cutting:
A centrifugal pump has only one performance curve at a certain speed. To expand the working range of the pump, the method of cutting the outer diameter of the impeller is often used to change its working range from a line to a surface. When the cutting amount is small, it can be considered that the outlet angle and flow area of the blades before and after cutting remain basically unchanged, and the pump efficiency is approximately equal.
2. The expression for the cutting law:
H ‘/H=(D2’/D2) 2
N ‘/N=(D2’/D2) 3
In the formula, Q, H, N represent the rated flow rate, head, and shaft power of the pump
Corner marks’ represent the corresponding parameters of the impeller after cutting
D2 represents the outer diameter of the impeller
2、 Cavitation and suction characteristics of centrifugal pumps
1. Cavitation phenomenon
According to the working principle of a centrifugal pump, the liquid flow enters the impeller under the pressure difference (Pa Pk) formed between the suction tank pressure Pa and the low pressure Pk at the impeller inlet. The lower the pressure Pk at the impeller inlet, the greater the suction capacity. But if Pk decreases to a certain limit value (currently, the saturation steam pressure Pt of the liquid at the conveying temperature is often used as the critical value for liquid vaporization pressure), cavitation phenomenon will occur.
2. Serious consequences caused by cavitation:
(1) Generate vibration and noise.
(2) It has an impact on the working performance of the pump: when cavitation develops to a certain extent, a large number of bubbles are generated, which will block the flow channel, causing a significant decrease in the pump’s flow rate, head, efficiency, etc.
(3) The material of the flow channel will be damaged, mainly due to fatigue erosion of the metal near the blade inlet.
3. Suction characteristics of centrifugal pumps:
1) The basic condition for pump cavitation is that the low liquid flow pressure Pk at the blade inlet is ≤ the saturated vapor pressure Pt of the liquid at that temperature.
2) Effective cavitation margin: The excess energy head that is higher than the vaporization pressure after the liquid flows from the suction tank and reaches the pump suction port through the suction pipeline. use Δ Ha represents.
3) The necessary cavitation margin of the pump: the total energy loss of the liquid flow from the pump inlet to the low pressure point K in the impeller, calculated using Δ Hr represents.
4)· Δ HR and Δ Differences and connections between ha:
Δ Ha> Δ No cavitation of the hr pump
Δ Ha= Δ Hr pump begins to cavitate
Δ Ha< Δ Severe cavitation of the hr pump
5. For a pump, in order to ensure its safe operation without cavitation, a safety margin should be added to the necessary cavitation margin of the pump, usually 0.5 meters of liquid column. Therefore, the allowable NPSH of the pump is:[ Δ Hr]= Δ Hr+0.5.
The expression for the allowable geometric installation height of the pump is: [Hg1]=(Pa Pt)/r-hA-S-[ Δ Hr].
Pa — suction tank pressure
Pt — Saturated vapor pressure of liquid at delivery temperature
R — Liquid gravity
HA-S — Flow loss in the suction pipe
[ Δ Hr] — allowable cavitation margin
7. The methods to improve the anti cavitation performance of centrifugal pumps include:
A. Improve the pump structure and reduce Δ HR is a design issue with the pump.
B. The main commonly used method to increase the effective NPSH inside the device is to use a filling head suction device
In addition, efforts should be made to minimize the resistance loss of the suction pipeline and reduce the saturated vapor pressure of the liquid. When designing the suction pipeline, measures such as selecting larger pipe diameters, shorter lengths, fewer elbows and valves, and minimizing the temperature of the liquid being transported can all improve the effective cavitation margin of the device.
8. Balance device for axial force
① Causes of axial force generation
a. The axial force A1 caused by the different distribution of fluid pressure on the front and rear sides of the impeller (low pressure on the wheel cover side and high pressure on the wheel disc) is directed from the back side of the impeller towards the inlet of the impeller.
b. The dynamic reaction force A2 generated by the different directions and velocities of fluid entering and exiting the impeller is opposite to A1, so the total axial force A=A1-A2, and the direction is generally the same as A1 (generally A2 is smaller).
② Balance of axial force
a. Adopting a double suction impeller: the impeller is symmetrical on both sides, and the fluid is sucked in from both ends. The axial force is automatically counteracted to achieve balance.
b. Opening balance holes or installing balance pipes: A: Open several balance holes on the impeller disc relative to the suction port. B: To avoid increasing hydraulic loss due to disturbance of the main flow after opening the balance hole, a balance pipe can be installed instead of the balance hole, which uses a small pipe to introduce pressure to the back side of the wheel disc.
c: Using balanced blades: Several radial ribs are cast on the back of the impeller disc, which drive the fluid in the clearance on the back of the impeller to accelerate rotation, increase centrifugal force, and significantly reduce the pressure on the back of the impeller.
d: Use thrust bearings to withstand axial force. Generally, small single suction pumps can withstand all axial force and prevent pump shaft movement by stopping the thrust bearing.
③ Balance of axial force in multi-stage centrifugal pumps:
a. Same method as single stage centrifugal pump
b. Symmetrically arranged impeller
c. Using a balance drum to partially balance axial force
d. Adopting an automatic balancing disc, all axial forces are automatically balanced.