Foreword

A special phenomenon in the operation of the main fan of sulfuric acid plant is surge. Preventing surge is a very important problem in fan operation. The surge of the main fan can cause great harm. During the surge, the airflow produces strong reciprocating pulse, which impacts the rotor and other parts of the fan back and forth. The strong irregular vibration of the air stream causes the unit to vibrate strongly, resulting in a variety of serious consequences: it will cause the rotor shaft bending; Seal damage, resulting in serious air leakage, oil leakage; Surge increases the axial thrust and burns out the thrust bearing. Damage the alignment and installation quality, so that vibration intensifies; Strong vibration can cause instrument failure; Severe and persistent surge can cause the rotor to collide with the stationary part, the main shaft and partition to break, and even the entire main fan to be scrapped. Therefore, surge is a problem that must be always guarded against in operation.

When the sulfuric acid device is in operation, the signs of surge are generally the first significant decrease in flow rate, fluctuation of air outlet pressure, strong vibration of the fan and accompanied by intermittent low roar. The temperature of sulfur burning furnace increases rapidly, and the steam production of boiler decreases.

1. Conditions for surge generation

According to the surge principle, surge occurs under the following conditions:

1.1 When the flow rate is too small, the flow rate falls to the surge flow rate at this speed, which is determined by the characteristics of the fan. Under the condition of a certain speed, a certain flow rate corresponds to a certain outlet pressure or boost ratio, and there is a limit flow rate at a certain speed. — Surge flow. When the flow rate is lower than this surge flow rate, the fan cannot operate stably and surge occurs. The comprehensive relationship between the flow rate, outlet pressure, speed and surge flow rate constitutes the characteristic line of the fan, which is also called the performance curve. On the contrary, if the flow rate is greater than the surge flow rate at a certain speed, the surge will not occur.

2. Cause of surge during operation

2.1 High system resistance

This situation is mainly caused by the continuous operation of the sulfuric acid device, due to the converter, absorption tower and other flue gas flow path equipment caused by the system resistance continues to rise, resulting in the system resistance exceeds the fan operation boost pressure, so that the fan flow can not flow, resulting in fan surge and sublimation of sulfur.

2.2 Operation error

The main flue gas pipe of the sulfuric acid plant is equipped with a number of flue gas butterfly valves for controlling the flue gas, and the opening of these butterfly valves should be &gt during operation. 75% so as not to cause the system resistance to increase, because these butterfly valves are DCS remote control of the electric valve, if the DCS misoperation to close the opening below 75% is very easy to cause fan surge and sublimation of sulfur.

2.3 Insufficient suction flow

Due to external reasons, the suction volume is reduced to below the surge flow rate, and the reasons for the fan entering the surge area are: the fan inlet filter is blocked, the resistance is too large, and the compressor speed is not adjusted, resulting in surge; The filter element is too dirty, or ice may occur in winter; The inlet air source is reduced or cut off. If these conditions are not found and adjusted in time, the fan may surge.

2.4 Improper operation

Speed up the pressure is too fast, and the pressure is not reduced before speed down. The speed and pressure should be slow and uniform, and pressure relief measures should be taken before the speed reduction: such as empouting, backflow, etc., so as to avoid airflow backpouring after the speed is reduced. During normal operation, the anti-surge is not automatic, when external factors change (such as steam pressure drop or gas volume fluctuation); The turbine speed drops and it is too late to adjust manually. Or to gas interruption; Because the automatic anti-surge device is not used, it may cause surge.

3. Methods to prevent and eliminate surge

3.1 Measures to prevent and eliminate surge

A return circulation can be taken at the inlet of the fan. It can increase the gas flow through the fan and eliminate surge; But the pressure is reduced, power is wasted, and economy is reduced. If the system needs to maintain constant pressure, the speed should be increased after empouting or reflux to achieve the original level of discharge pressure. Before pressure increase, speed reduction, and shutdown, the vent valve or return valve should be opened in advance to reduce the back pressure, increase the flow rate, and prevent surge.

3.2 When boosting and shifting, emphasis “ Pressure increase must first increase speed, speed down must first reduce pressure ” The principle of

Fan pressure boost should be carried out after the turbine governor is put into operation; Check the performance curve before the pressure boost, determine the speed that should be reached, and then increase the pressure after rising to the speed; The fan speed reduction should be started after the anti-surge valve is arranged properly; Speed up, the pressure can not be too fast too fast; Speed reduction and pressure reduction should also be slow and uniform.

3.3 Controlling Raw material ash

It is necessary to control the content of ash in raw materials or sulfur to avoid ash entering the system, increasing the resistance of the system, resulting in the resistance of the fan booster low-pressure system. Sulfur acid making device should avoid bad atomization or bending of sulfur gun, insufficient combustion leads to corrosion of sulfur and iron, and iron sulfide is blown to the catalytic layer to increase system resistance, resulting in fan surge.