Cavitation in centrifugal pumps

You can refer to the causes of cavitation: at a certain temperature, when the pressure is reduced to the vaporization pressure at that temperature, bubbles form in the liquid. This phenomenon of bubble formation is called cavitation. The bubbles formed during cavitation, when they move to areas of higher pressure, shrink in size and eventually burst. This phenomenon of bubbles disappearing in the liquid due to rising pressure is called cavitation collapse. While the pump is in operation, if the absolute pressure of the liquid being pumped drops to the vaporization pressure of that liquid at the current temperature in a certain local area of its flow path – usually somewhere slightly downstream of the inlet to the impeller blades – the liquid begins to vaporize there, generating large amounts of steam and forming bubbles. When this liquid containing many bubbles moves forward through the high-pressure areas inside the impeller, the high-pressure liquid surrounding the bubbles causes them to shrink rapidly until they burst. As the bubbles condense and burst, liquid particles fill the voids at high speeds; this creates a very strong water hammer effect, which strikes the metal surface at a high frequency. The impact stress can reach several hundred to several thousand atmospheres, while the impact frequency can be in the tens of thousands of times per second. In severe cases, this can cause the wall thickness to be breached. The process in which bubbles are formed and burst in a water pump, causing damage to the flowing components, is known as cavitation in water pumps. After cavitation occurs in a water pump, in addition to causing damage to the flow-through components, it also generates noise and vibration, leading to a decline in the pump’s performance; in severe cases, it can even interrupt the flow of liquid within the pump, preventing it from functioning properly. Measures to prevent cavitation: To prevent cavitation, it is necessary to increase NPSHa so that NPSHa > NPSHr. The measures to prevent cavitation are as follows: 1. Reduce the geometric suction head hg (or increase the geometric backflow head) ； 2. Reduce the suction loss hc; this can be achieved by increasing the pipe diameter, minimizing the length of the pipelines, as well as reducing the use of elbows and other fittings ； 3. Prevent operation at high flow rates for extended periods ； 4. At the same speed and flow rate, a double-suction pump is used, as it reduces the inlet flow velocity and thus makes cavitation less likely to occur ； 5. When cavitation occurs in the pump, the flow rate should be reduced or the pump should operate at a lower speed ； 6. The condition of the pump suction tank has a significant impact on pump cavitation ； 7. For pumps operating under harsh conditions, to avoid cavitation damage, materials resistant to cavitation can be used. As the poster mentioned, having an elbow close to the pump inlet can lead to cavitation; I think this simply means that the inlet resistance increases, as elbows indeed create some resistance, and such a situation is possible. However, this is not the main factor, as the resistance at the elbow is relatively low. I don’t understand why gas keeps being drawn in. Is it the gases dissolved in the medium? There must be another reason.

Having the elbow too close to the pump can have an impact; it creates vortices that cause some energy loss in the pipeline, thereby reducing the effective net positive suction head. If gas is continuously introduced, it might help reduce cavitation, as this gas will increase the pressure in the low-pressure areas, preventing the liquid from vaporizing. But make sure to control the amount. Hehe, just my personal opinion; for reference only

It has been explained in great detail upstairs; if gas is continuously carried into the pump along with the liquid, cavitation will occur. I don’t understand either why gas keeps getting carried into the pump along with the liquid Is the operating temperature not appropriate?

Placing the inlet elbow too close to the pump primarily causes turbulence in the liquid, and this flow condition is equivalent to reducing the effective net positive suction head. Furthermore, if gas continuously enters the pump along with the liquid, similar to cavitation, this can also cause phenomena such as noise and vibration.

Ask on the second floor: what is the pressure and temperature at which cavitation occurs in centrifugal pumps?

The pressure and temperature required for cavitation in centrifugal pumps are related to the saturated vapor pressure and temperature of the fluid being transported. The required net positive suction head given on the performance curve refers to that for clean water at normal temperatures! :)

Provide a detailed description of the usage conditions so that everyone can analyze whether cavitation is the cause; some of the manufacturer’s service staff are not professionals – they might just be in sales!

If the elbow is located very close to the pump’s inlet, especially with volatile media, cavitation is very likely to occur. This is because, as the liquefied gas passes through the elbow, its fluid state changes, resulting in the formation of numerous bubbles. When these bubbles enter the pump along with the fluid, they are crushed by the impeller and burst violently. If the pressure at the inlet is not high enough to meet the pump’s requirements or is below the saturation point for the liquid entering the pump, a large number of bubbles will be generated, leading to cavitation in the pump. If cavitation occurs, the owner can increase the inlet pressure of the pump, such as by raising the pressure level, increasing the saturation pressure, or reducing pipe resistance.

When gas enters the pump, it is called gas entrapment!

Too many inlet elbows on the pump can reduce the inlet pressure; once it drops below the saturated steam pressure, cavitation occurs. It is generally necessary to calculate the effective net positive suction head available for the device to determine whether it meets the requirements.

For this pump where cavitation occurs, the inlet is located in the solution circulation tank; the liquid level is about 1.8 meters above the pump’s centerline. The resistance in the pump’s inlet pipeline should not be high, as it was designed by a reputable design firm, with a maximum flow velocity of no more than 1 meter per second. The staff from the pump manufacturer said that the straight section after the elbow in the inlet pipeline should be made longer; it’s not clear whether this is necessary or not. Additionally, continuous gas is introduced because there is liquid flowing in from a higher level above the liquid surface at the outlet of the pump’s inlet circulation tank; there is a drop of about three to four meters, and as the liquid falls, it carries air into the solution in the tank. Below that is the pump’s inlet, so I suspect that this gas comes from that area...

If it is cavitation, it is definitely necessary to follow the methods suggested by the manufacturer, as this increases the effective net positive suction head. Check the pump’s net positive suction head and then calculate the effective one. This is because the conditions for cavitation in a pump are determined by both the pump itself and the suction setup. Therefore, to study the conditions under which cavitation occurs, it is necessary to consider both the pump itself and the suction system. The basic relationship for pump cavitation is NPSHc ≤ NPSHr ≤ NPSHa. NPSHa = NPSHr / (NPSHc) – this is the point at which cavitation begins in the pump; when NPSHa > NPSHr / (NPSHc), there is no cavitation in the pump. Here, NPSHa represents the net positive suction head available, also known as the effective net positive suction head, and the higher this value, the less likely cavitation will occur ； NPSHr —— pump net positive suction head, also known as the required net positive suction head or the dynamic pressure drop at the pump inlet; the lower this value, the better the pump’s resistance to cavitation ； NPSHc —— Critical net positive suction head, refers to the net positive suction head at which the pump’s performance declines by a certain amount ； ——The allowable NPSH is the NPSH used to determine the operating conditions of a pump, and it is usually taken as = (1.1～1.5) NPSHc. Calculation of the net positive suction head available for a device (refer to http://blog.sina.com.cn/s/blog_5aca3b150100ah3o.html). NPSHa = Ps/ρg + Vs/2g – Pc/ρg = Pc/ρg ± hg – hc – Ps/ρg. Measures to prevent cavitation: increase NPSHa so that it is greater than NPSHr. For example, adding a straight pipe at the pump inlet ； Pump height decreases (this is not suitable for you) ； If there is a surplus flow in the secondary pump, the simplest way is to lead a portion of the outlet pipe back to the pump inlet in order to increase its energy. As for introducing air into the liquid, whether it has an impact or not, it can be solved simply by connecting a hose directly below the liquid level, hehe.

If the inlet pipe diameter is several times that of the pump’s inlet diameter, could there still be an impact? In my opinion, the above statement is not rigorous enough; if it refers to the standard size of the piping, it indeed is related to the elbows

Continuous introduction of gas can also help reduce cavitation? ? ? Then you might as well try not to fill the pump while driving. In my opinion, bent pipes are a secondary cause of cavitation; the real reason is the continuous intake of gas.

It is recommended that the straight pipe section after the elbow should have a length of 5 times the diameter