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Solutions to technical problems of centrifuge surge

2021-05-28 11:22:03

Centrifuge surge is the killer of centrifuges. High speed refrigerated centrifuges and ultra-high speed refrigerated centrifuges are more likely to surge. In serious cases, centrifuge rotors and other accessories will be damaged. Let's analyze the causes and solutions of centrifuge surge.

Causes and solutions of centrifuge surge

1. Condenser fouling

The water quality on the inner surface of the condenser heat exchange tube is CRUD (the open circulation cooling water system is easy to crud), which leads to the increase of heat transfer resistance and the reduction of heat exchange effect, which increases the condensation temperature or reduces the evaporation temperature. In addition, due to the untreated water quality and poor maintenance, it also causes the deposition of sand, impurities, algae and other substances on the inner surface of the heat exchange tube, resulting in the increase of condensation pressure and the surge of the centrifuge.

Solution: remove the dirt on the heat transfer surface and clean the cooling tower.

2. There is air in the refrigeration system

When the centrifugal unit is running, because the evaporator and low-pressure pipeline are in a vacuum state, it is very easy to infiltrate air at the connection. In addition, the air is a non condensable gas, and the insulation index is very high, which is 1.4. When the air condenses on the upper part of the condenser, the condensation pressure and condensation temperature rise, resulting in the surge of the centrifuge.

Solution: when the temperature of refrigerant in the air exceeds K11 ℃, it indicates that there is liquid in the system. Elimination method: start the extraction recovery device to discharge the non condensable gas. Generally, the pressure of refrigerant R11 is slightly lower than the saturation pressure corresponding to the temperature of refrigeration liquid, that is, the corresponding pressure below 28 ℃: 117.68kmp.

3. The cooling water circulation of the cooling tower is insufficient and the inlet water temperature is too high

Due to the poor cooling effect of the cooling tower, the condensation pressure is too high, resulting in surge.

Solution: carry out anti surge regulation. When the energy regulation is greatly reduced, the suction capacity is insufficient, that is, the steam cannot flow evenly into the impeller, resulting in a sudden drop in the exhaust pressure, and the compressor is in an unstable working area, resulting in surge. In order to prevent surge, part of the compressed steam can be bypassed to the evaporator through the exhaust pipe, which can not only prevent surge. It is also beneficial to the start-up of the centrifuge: reducing the steam density and the pressure during the start-up can reduce the start-up power.

4. Evaporator evaporation temperature too low

Due to the insufficient refrigerant in the system, the cooling capacity load is reduced, and the opening of the ball valve is too small, resulting in too low evaporation pressure and surge.

Solution: check the reason why the evaporation pressure is too low, the refrigerant is insufficient, add refrigerant, the refrigeration load is small, and close the energy regulating blade.

5. During shutdown, the guide vane angle is not turned down and the pressure at the exhaust port of the centrifuge is not reduced

When the centrifuge is shut down, due to the sudden disappearance of pressurization, the high-pressure refrigerant vapor in the volute and condenser is poured back, which is easy to surge.

Solution: when stopping the centrifuge, pay attention to whether the main motor is reversed, and turn down the guide vane angle as much as possible to reduce the pressure at the exhaust port of the centrifuge. During the operation of the centrifuge, the condensation pressure and evaporation pressure shall be kept stable to make the cooling capacity of the centrifugal mechanism higher than the corresponding cooling capacity at the surge point to prevent surge.

Causes of surge of centrifugal compressor

When the inlet pressure or flow decreases suddenly and is lower than the allowable operating point, the gas in the compressor will be seriously rotated and separated due to the change of flow, resulting in sudden stall (referring to the great deviation between the flow direction of gas at the inlet of blade channel and the inlet angle of blade). At this time, the impeller can not effectively increase the gas pressure, resulting in the decrease of outlet pressure. However, the pressure of the system pipe network does not drop instantaneously, so that the gas flows from the system pipe network to the compressor. When the pressure of the system pipe network drops below the outlet pressure of the turbine, the gas flows to the system pipe network again. Such repetition will cause periodic axial low-frequency and large amplitude air flow oscillation between the unit and the pipe network. This will greatly affect the service life of the impeller and blade. The existing manufacturers can partly solve this problem by adding an anti surge device, but it also causes the increase of vulnerable parts and energy consumption.

Generally speaking, the energy regulation range of the centrifugal unit is 40% - 100%. When the load is lower than 40%, the centrifugal unit is prone to "surge". When the surge is serious, the whole core component of the unit - impeller can be damaged and the centrifugal compressor can be scrapped. At present, many centrifugal unit manufacturers use "air replenishment" means that the critical point of "surge" of the unit reaches "20%" or "10%", and "air replenishment" needs to consume a lot of energy, so that the energy efficiency ratio of the unit is relatively low below 50%.

The above is an introduction to the causes of centrifuge surge. How to prevent centrifuge surge? Keep the condensation pressure and evaporation pressure stable so that the cooling capacity of the centrifugal mechanism is higher than the corresponding cooling capacity at the surge point.