One, what is cavitation?
cavitation (cavitation) refers to a phenomenon in which the metal surface is cavitation under large pressure and high cavitation frequency impact and electrochemical corrosion of a small amount of oxygen and other active gases in the bubble on the metal surface, so that the surface of the impeller appears like sea surface and fish scale damage.
Second, the harm of centrifugal pump cavitation
Cavitation of centrifugal pump is one of the common faults of centrifugal pump. Once the pump cavitation occurs, its flow and head performance will not only decline, but also show significantly high noise and vibration, and even make the liquid flow in the pump interrupted and can not work normally. Cavitation will also damage the flow parts of the pump, and even affect the pipeline system.
There are many reasons for cavitation, such as centrifugal pump product quality problems, improper use of operators and so on. Products will go through multiple procedures of quality testing before leaving the factory, so the proportion of human factors is greater. In the working state, the influence of the working environment and operating factors of the centrifugal pump accounts for the majority of the proportion of cavitation in the centrifugal pump.
Third, the occurrence process and reasons of cavitation?
1. The process of cavitation.
When the centrifugal pump is working, the liquid pressure delivered by the centrifugal pump will decrease as the liquid in the pump drops from the inlet to the inlet of the impeller. When the liquid pressure near the inlet of the blade reaches the lowest point, the impeller begins to do work on the liquid, and the liquid pressure begins to rise. When the minimum pressure near the inlet of the impeller blade is less than the saturated steam pressure at the liquid transport temperature, the liquid will vaporize. At the same time, the gases dissolved in the liquid also escape, and they form bubbles. When the bubble flows with the liquid to the higher pressure in the duct, the outside liquid pressure is higher than the vaporization pressure in the bubble, then the bubble re-condenses and collapses to form a hole, and the surrounding liquid rushes to the hole at a very high speed, causing the liquid to collide with each other, and the local pressure suddenly increases. In this way, the normal flow of the liquid transported by the centrifugal pump is not only hindered. And when these bubbles break near the impeller wall, the liquid will continuously impact the inner surface of the centrifugal pump. Long-term impact will cause structural damage and spalling of the inner wall of the centrifugal pump. If the bubble is doped with some chemical gases such as oxygen, these gases will use the heat released when the bubble condenses (the local temperature can reach 200 ~ 300 ° C), it will also form a thermocouple, produce electrolysis, form electrochemical corrosion, and accelerate the destruction rate of metal denudation. Like this liquid vaporization, condensation, impact, the formation of high pressure, high temperature, high frequency impact load, resulting in the mechanical stripping of metal materials and electrochemical corrosion damage of the comprehensive phenomenon called centrifugal pump cavitation phenomenon. When cavitation occurs, the combined action of mechanical denudation and chemical corrosion causes the material to be damaged, and there will be noise and vibration. When cavitation develops seriously, the presence of a large number of bubbles will block the cross section of the flow channel, reduce the energy obtained by the fluid from the impeller, resulting in the liquid interruption in the pump and cannot work normally.
2. What causes cavitation?
In a word: Cavitation occurs when the pump wheel inlet is later located, or generally speaking, the lowest pressure in the pump is less than the saturated steam pressure of the conveyed medium.
In technical language: Cavitation occurs when the NPSHr of the pump is greater than the NPSHa of the unit.
Specific to the actual operation are:
The liquid gas pressure at the pump inlet suddenly drops, reaching or below the pressure at saturation temperature, and the liquid vaporizes.
Pump inlet into the air, or pump inlet flow drop.
Improper adjustment operation resulted in a sharp decrease in outlet flow.
The installation height of the pump is insufficient
The recirculation door does not open in time when the flow rate is low.
Deaerator, condenser and tank level is too low.
Fourth, cavitation treatment measures.
Preventive measures:
(1) Appropriately increase the diameter of the pump inlet and the inlet diameter of the impeller, reduce the liquid flow rate at the pump inlet, and reduce NPSHr. Or directly use double suction impeller, because the double suction impeller is equivalent to the inlet area of two single suction impeller, the inlet flow rate can be reduced by twice under the same flow condition.
(2) Thin the back of the blade head to improve the inlet crowding and reduce NPSHr. Or the induction wheel is installed to increase the pressure energy before the liquid enters the impeller.
(3) When selecting the pump, when the device cavitation allowance is low or the medium is easy to vaporize, the pump should use low speed as much as possible.
(4) When designing the pipeline system, the suction height of the pump is as low as possible, and the reverse irrigation is used if conditions permit. When piping, shorten the length of the suction pipe appropriately, increase the diameter of the suction pipe, and minimize the number of unnecessary valves and elbows in the suction road to reduce the pipeline loss of the suction pipe.
(5) The pump works in a state close to cavitation, such as the use of dense anti-cavitation materials (copper alloy, stainless steel, etc.) to manufacture the pump impeller can extend the life of the impeller. For example, the impeller welded with rolled steel plate has stronger cavitation resistance than the cast impeller. The impeller can also be coated with non-metallic coatings using epoxy resin, nylon, polyamine, etc.
(6) For the easy vaporizing medium, do a good job of heat preservation and cooling of the pipeline to avoid the temperature increase of the transported liquid.
(7) When cavitation occurs in the pump and cannot change its process conditions, a nozzle can be installed at the pump inlet to use the pump outlet pressure to make the high-pressure liquid feedback to increase the pump inlet pressure and reduce the possibility of cavitation.
(8) During the operation of the pump, the pump outlet valve should be used to control the flow rate in a reasonable range. Cavitation is most likely to occur when the pump is running at a high flow rate. Suction line valves are not allowed to regulate flow during operation.
(9) When the condensate pump and feed pump have low flow, check the recirculation door open in time.
(10) Keep the water level of the deaerator, condenser and water tank high, and set the low water level to automatically stop the pump protection.