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Causes of gas binding and cavitation in downhole wear-resistant self-balancing centrifugal water pumps
If the downhole wear-resistant self-balancing centrifugal water pump is improperly operated or the liquid is vaporized in the low-pressure area during the start-up and working process, it will cause cavitation. Cavitation and gas-binding phenomena can cause serious damage to the downhole wear-resistant self-balancing centrifugal water pump. Therefore, today I will take you to learn more about the causes of the two phenomena and the corresponding preventive measures, so as to avoid the phenomenon of cavitation and gas-binding during work. Occurs, ensuring the normal and efficient operation of the centrifugal pump.
A. Air binding phenomenon of downhole wear-resistant self-balancing centrifugal water pump
"Air binding": The phenomenon that the pump can't suck liquid after starting the pump because of the internal gas in the pump is called the "gas binding" phenomenon. After the phenomenon of "air binding" occurs, the pump has no liquid discharge, no noise or vibration. In order to prevent the occurrence of "air binding", the liquid should be filled before starting.
1. Reasons for air binding:
The centrifugal pump is not filled with the liquid to be transported before starting, or air has penetrated into the pump during operation. Because the density of the gas is less than the density of the liquid, the centrifugal force generated is small, and the air cannot be thrown out. The fluid in the pump casing When the negative pressure generated by the centrifugal movement of the motor is not enough to suck the liquid into the pump casing, the pump loses its self-priming ability and cannot transport the liquid as if it is trapped by "gas", which is called the air-binding phenomenon of the centrifugal pump.
2. Harmful situation:
The pump cannot pump liquid, the unit produces violent vibration, and at the same time, it is accompanied by strong harsh noise. The motor is idling and it is easy to burn the motor. Affect the efficiency of conveying liquid and the normal operation of centrifugal pumps.
3. Collection of preventive measures:
Before starting, fill the pump and fill the pump casing with the liquid to be delivered, and close the outlet valve when starting. In order to prevent the liquid poured into the pump casing from flowing into the low tank due to gravity, a check valve (bottom valve) is installed at the inlet of the pump suction pipe; if the pump position is lower than the liquid level in the tank, no filling is required when starting. Pump. Do a good job of sealing the shell, and the valve for filling water should not leak, and the sealing should be good.
B. The cavitation phenomenon of wear-resistant self-balancing centrifugal water pump downhole
"Cavitation": Because the suction height of the pump is too high and the pressure in the pump is equal to or lower than the saturated vapor pressure at the temperature of the conveying liquid, the erosion phenomenon caused by the process of liquid vaporization, bubble formation, rupture, etc., is called " "Cavitation" phenomenon. When the "cavitation" occurs, the liquid produces noise and vibration due to impact, which reduces the flow rate, and even no liquid is discharged. In order to prevent "cavitation" phenomenon, the actual installation height of the pump should not be higher than the allowable suction height.
1. Causes of cavitation:
When the liquid sucked in the pump casing just vaporizes at the suction port of the pump due to the decrease in pressure, it brings a huge hydraulic shock to the inner wall of the pump casing, causing the casing wall to be corroded by "gas". This phenomenon is called cavitation. phenomenon.
2. The main causes of cavitation are:
1). The resistance of the inlet pipeline is too large or the pipeline is too thin;
2). The temperature of the conveying medium is too high;
3). The flow is too large, that is, the outlet valve is opened too much;
4). The installation height is too high, which will affect the pump's suction capacity;
5). Selection issues, including pump selection, pump material selection, etc.
Liquid with bubbles condenses or ruptures sharply after being squeezed into the high-pressure zone. Due to the disappearance of the bubble, a partial vacuum is generated, and the surrounding liquid flows to the center of the bubble at a very high speed, and a huge high-speed impact force of up to tens of thousands of kpa is instantly generated, causing an impact on the impeller and pump casing, causing the material to be corroded and damaged. damage.
From the perspective of the different causes of cavitation and air binding: air binding is the presence of air in the pump body, which generally occurs when the pump is started, mainly because the air in the pump body is not exhausted; and cavitation is caused by the liquid reaching a certain temperature. At its vaporization pressure.
3. The location where cavitation occurs
According to the different parts of water pump cavitation, cavitation can be divided into the following four categories:
(1) Leaf cavitation:
Leaf cavitation is the cavitation that occurs on the surface of the blade, mainly because the pump is installed too high or the flow deviates from the design flow is too large. The formation and collapse of the cavitation mostly occur on the front and back of the blade or the inner surface of the front disc and the root of the blade.
(2) Interstitial cavitation: When the water flow in the interstitial cavitation pump passes through the suddenly narrowed gap, the speed increases and the local pressure drops, which will also cause cavitation. For example, in the gap between the outer edge of the axial flow pump blade and the pump casing, the gap between the centrifugal pump seal ring and the outer edge of the impeller, due to the large glands on the inlet and outlet sides of the impeller, resulting in high-speed backflow and local pressure drop , Causing interstitial cavitation.
(3) Vortex cavitation: vortex cavitation may also produce a top-down ribbon vortex (referred to as vortex belt) under the impeller due to poor design of the water collection tank and inlet flow path or the pump working under undesigned conditions. When the center pressure of the vortex is lower than the vaporization pressure, the vortex becomes a cavitation zone.
(4) Rough cavitation:
Rough cavitation is when water flows through the uneven inner wall and flow parts of the pump, it is easy to generate partial negative pressure downstream of the protrusion to cause cavitation. This cavitation is called rough cavitation.
4. The resulting hazards
(1) The performance of the water pump is deteriorated. When cavitation occurs, a large number of cavitations will be produced. When the water contains a large number of cavitations, the normal law of water flow will be destroyed, and the effective flow surface area of the blade groove will be reduced, and the flow direction will change accordingly. The loss increases, which causes a rapid drop in the flow, head and efficiency of the pump, and even cut-off occurs when the cavitation is severe.
(2) If the flow parts are damaged, the wall surface of the water pump will be locally deformed and hardened and become brittle under the repeated action of the high-strength impact force, resulting in metal fatigue, causing the metal to crack and peel off. In addition to the mechanical action, it is also mixed with the chemical corrosion of the metal by the deep active gas (such as oxygen) that escapes from the water body and the electrochemical corrosion of the metal by the water body. Under the combined effect, the wall surface of the water pump appears pitted at first, and then becomes a honeycomb shape. In severe cases, the wall surface will be knocked out in a short period of time.
(3) Vibration and noise are generated. When the bubble collapses, the liquid particles collide with each other and also hit the metal surface, producing noises of various frequencies. In severe cases, a "crack" explosion can be heard in the pump, and the unit will vibrate at the same time. Under the repeated action of the huge impact, the impeller part has spots and cracks on the surface, and even gradually falls off in a spongy shape, which reduces the service life of the pump. Therefore, noise and vibration are also one of the main basis for judging whether cavitation occurs and disappears.
5. Collection of preventive measures
An effective measure to reduce cavitation is to prevent the generation of bubbles.
First of all, the surface that moves in the liquid should be streamlined to avoid eddy currents in local areas, because the pressure in the vortex area is low and bubbles are easy to generate. In addition, the gas content in the liquid and the disturbance in the liquid flow should be reduced, which will also limit the formation of bubbles.
Choosing the right material can improve the cavitation resistance. Generally, metal materials with high strength and toughness have better cavitation resistance, and improving the corrosion resistance of the material will also reduce cavitation damage.
The pressure at the inlet of the centrifugal pump should not be too low, but there should be a minimum allowable value. At this time, the corresponding cavitation margin is called the necessary cavitation margin, which is generally determined by the pump manufacturer through the cavitation test and used as the performance of the centrifugal pump Listed in the pump product catalog. When the pump is in normal operation, the actual cavitation margin must be greater than the necessary cavitation margin, and it is specified in our country's standard that it should be greater than 0.5m.
At the same time, clean up the foreign matter in the inlet pipeline to make the inlet unblocked, or increase the size of the pipe diameter.
In addition, for pump manufacturers, it is necessary to improve the anti-cavitation ability of the centrifugal pump itself, such as improving the structural design from the suction port to the vicinity of the impeller; using a front inducer to increase the liquid flow pressure; increasing the blade inlet angle to reduce The bend at the inlet of the small blade to increase the inlet area.