Cavitation phenomenon, generally not for people to pay attention to and understanding. The actual production of cavitation problems are often the main cause of centrifugal pump impeller damage. The general understanding of impeller rot and aging. But how did this happen? The mechanism is unclear. So it is necessary to understand how cavitation is caused and how to avoid it. Here to talk about cavitation problems and preventive measures.
1 cavitation generated
We all know that water heated to a certain extent will become water vapor, and water vapor to a certain temperature will condense into water, and, under the action of an atmospheric pressure, the water heated to 100 ℃, began to boil, produce A lot of bubbles (called vaporization). While temperature and pressure are the conditions for their conversion to one another. When the pressure on the water surface is high, the temperature required to vaporize the water is high. When the pressure on the water surface is low, the temperature required for the water to vaporize When the pressure acting on the water surface is low to a certain extent, the water can vaporize to generate a large amount of bubbles even at normal temperature. The pressure at which the liquid begins to vaporize at a certain temperature is called vaporization pressure. Cavitation of the pump is caused by the vaporization of the liquid.
People on the principle of centrifugal pump water are more clear, in order to make the water pump sucked up the lower, you must make the import of the pump impeller vacuum, which created the conditions for the evaporation of water. At a certain temperature, when the impeller inlet pressure is low to a certain extent, the water entering the pump begins to vaporize, resulting in a large number of bubbles. These bubbles and a small amount of gas dissolved in water, with the water into the impeller. Since the pressure changes as the water flows in the pump impeller runner, the bubble re-starts at high pressure as the water moves the bubble from the low pressure zone (the inlet of the pump) to the high pressure zone (the outlet of the pump) Condensation into water and loss of the bubble disappears, the surrounding water at a high rate of impact to the rupture of bubbles, like numerous small warheads, continuously acting on the metal surface of the impeller. Impeller wall in a continuous blow at high-speed water, over time will appear honeycomb-shaped cavities, the shape of a random distribution. This phenomenon is called cavitation. People used to regard this phenomenon as impeller decay. Centrifugal pump impeller is very sensitive to cavitation. A long time will eclipse the impeller wall. Pump cavitation will produce a lot of noise and vibration. At this time the pump head was significantly reduced, the flow rate decreases, the more serious cavitation will make the pump off. Therefore, the pump is not allowed to operate in the case of cavitation.
A chemical centrifugal pump , when its speed and flow rate of a certain time, the production of cavitation limit the water pump suction height. General pumps on the nameplate are mentioned "to allow suction on the vacuum height" or "necessary NPSHr NPSHr" is based on the pump at rated speed and flow without cavitation principle, determined by the manufacturer's test The maximum water absorption height limits the pressure at the pump impeller inlet not to fall below a certain value in order not to cause cavitation. In other words, the pump impeller inlet vacuum can not be higher than a certain value. Therefore, the lowest allowable pressure at the inlet of the pump impeller, which is also called the "allowable suction of the vacuum" of the pump, is the same as "allowable suction of the vacuum".
2 cavitation prevention measures
The actual installation height of the shield pump must be lower than or equal to the actual water absorption height of the pump. Water pump inlet pipe to shorten the length of as much as possible, less elbows and valves, and some aquaculture production units make a straight elbow bend, rather than the way shrimp shells bent, it will increase the resistance of the suction pipe, Cavity capacity to produce cavitation. Strictly observe the rated parameters on the pump nameplate. Water inlet pipe joints, pads must be tight and reliable, should always check the fastening flange bolts. Bottom suction valve should be flexible opening and closing (especially for a long time without using the pump unit). Stop pumping in the process of continuous stop pump. Do not think the submerged suction pipe will increase the absorption of water depth, in fact, this is counterproductive. Because of the excessive flooding depth lengthened the suction pipe, thereby increasing the loss along the way, the result has reduced the water absorption height, but also easy to absorb air and produce cavitation. Tide pumping times pumping aquaculture production units, should pay more attention to changes in seawater levels.
Aluminum gravity casting Introduction:
The gravity casting means the part is injected by the earth's gravity. The generalized gravity casting includes sand casting, Metal mold casting (permanent mould casting), investment casting, Lost Foam Casting, Mud casting etc. The narrow meaning of gravity casting is metal mold casting (permanent mould casting).
The permanent mold casting is modern technology that the mould made by heat resistant alloy steel.
The metal liquid generally injected into the gate by manual. Full of cavity depend on weight of liquid metal, exhaust, cool, open mould and get products.
Process flow: smelting, filling cavity, exhaust, cool, open mould, heat treatment, downstream processing.
The characteristics of the aluminum gravity casting:
1. Less casting porosity, heat treatment can be performed. The strength of the product can be improved greatly after heat treatment, better than Die Casting Parts.
2. Product density is low, molding depend on the liquid weight, the density of the product is lower than die casting parts, its strength is slightly worse, but the elongation is higher.
3. The product surface finish is not high. Easy form similar blasting pit on the surface of casting products after cooling contraction.
4. Slow filling and low production efficiency, the production efficiency is only about a quarter compare the high pressure casting, caused the product cost is high.
5. Mould life is longer than die casting mould, and mould cost is low also.
6. Process is relatively simple.
7. Not suitable for production of thin wall parts.
The aluminum gravity casting conventional materials
Production and QC control of aluminum gravity casting
Aluminium Gravity Casting, Aluminum Gravity Casting Part, Aluminum Permanent Mould Casting, A356 Gravity Casting Part, Aluminum Low Pressure Casting Part, Aluminum Permanent Mold casting Part
NINGBO BEILUN LEMA MACHINERY TECHNOLOGY CO.,LTD , http://www.china-lema.com
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