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:
Q '/Q=D2'/D2
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.
