-- Can cavitation still occur in centrifugal pump impeller when NPSHa is greater than NPSHr?
Question:
Can cavitation still occur in centrifugal pump impeller when NPSHa is greater than NPSHr? If so, why does the pump supplier display the NPSHr value while knowing that cavitation may occur?
Answer:
Yes. When NPSHa is greater than NPSHr, cavitation may still occur in the centrifugal pump impeller. However, this does not mean that cavitation will cause damage to the centrifugal pump impeller.
The standard procedure for determining NPSHr has been used for many years and was officially approved as an American national standard in 1994, named ANSI / hi 1.6 centrifugal pump impeller test.
In order to determine the value of NPSHr, the pump operates at constant flow and constant speed, and cavitation is generated by changing suction conditions. Figure 1 is a schematic diagram of NPSHr values measured under different constant flow and constant speed.
Figure 1: schematic diagram of NPSHr test with constant flow and speed for centrifugal pump impeller
With the decrease of NPSHa, the point where the curve deviates from the straight line trend is reached, indicating that the performance of the pump may be damaged. The 3% drop in head is the standard for determining NPSHr, which is the basis for judging cavitation (Note: this standard is generally used for ordinary centrifugal pumps. In fact, cavitation begins when the first bubble is generated at the inlet of the impeller). For more details on this test, see ANSI / hi 1.6.
Although cavitation may occur when the pump operates when the NPSHa value is greater than NPSHr, the impeller is not necessarily damaged. The actual damage of the impeller also depends on many other factors:
1) Thermodynamic properties of liquids. Under operating conditions, with the increase of the number of steam bubbles, they will cause greater damage in collapse. This is also related to the liquid vapor pressure. For example, cold water causes more damage than hot water, while hydrocarbons (hydrocarbons) cause less damage than hot or cold water.
2) Corrosive liquid. Inter granular corrosion and crevice corrosion will not only cause direct damage, but also weaken the structure, making it more vulnerable to the damage caused by the impact of collapsing steam bubbles.
3) Impeller material. Steel, cast iron and brass are more susceptible to cavitation damage, while stainless steel, titanium and aluminum bronze are more resistant to cavitation damage.
4) Impeller speed. For high-speed impellers, more energy is concentrated in a smaller volume, which will lead to the removal of more metal from the high-speed rotating impeller.
5) Run away from BEP. The mismatch between the actual flow angle and the impeller geometry will also aggravate cavitation damage.
6) High suction specific speed design. The special impeller design with low NPSHr value will be more affected because the impeller inlet diameter is larger. When it deviates from BEP (small flow), inlet backflow and cavitation will occur.
7) Duty cycle. Obviously, the longer the pump runs, the greater the possibility of cavitation damage.
8) High energy pump. Experience shows that for high-energy pumps, larger NPSH margin is usually required than ordinary centrifugal pumps.
