Common Fluctuations in Cold Boxes

Main causes

Solutions

Frequent fluctuations in pressure 

1. The vent valve is stuck or the circuit parameters are not adjusted properly

2. Unstable raw material air pressure

3. Changes in resistance of product conveying pipelines

4. The gas removal speed after the air compressor is too fast

1. Disassemble, clean/replace stuck valves, optimize pressure regulation PID parameters, and optimize control valve tracking parameters;

2. Coordinate upstream stable raw material air pressure;

3. Check if the product pipeline is blocked and promptly clear it

4. Slowly open or close the air intake valve after the compressor

Uneven temperature distribution (local supercooling/overheating)

1. Blockage of heat exchanger channels and imbalance of fluid distribution

2. Abnormal fluctuations in the flow rate of some gases involved in heat exchange

3. Internal components (tray/packing) of the distillation tower are offset. 4. Abnormal opening of the throttle valve

1. After shutdown, blow the heat exchanger with dry nitrogen and adjust the valve opening to balance the flow rate;

2. Check if there are any blockages or abnormal instruments in the heat exchanger related pipelines

3. Check the tower internals and shut down for calibration if necessary;

4. Verify the throttle valve positioner to ensure accurate opening

Excessive liquid level fluctuation (main cooling/condensing evaporator)

1. Improper adjustment of liquid nitrogen reflux rate;

2. Poor gas-liquid contact in the distillation tower of air separation plant;

3. Measurement error of liquid level gauge (instrument tube blockage or transmitter failure)

1. Fine tune the liquid nitrogen reflux valve to stabilize gas-liquid equilibrium;

2. Check if there is frost/blockage on the tower internals, and if necessary, heat them up and blow them off;

3. Calibrate the level gauge/transmitter and check if the measuring pipeline is blocked

Product purity fluctuation (oxygen/nitrogen purity not meeting standards)

1. The adsorption efficiency of the purification system decreases (water/CO ₂ penetration);

2. Imbalance of reflux ratio in distillation tower;

3. Leakage inside the tower leads to gas-liquid mixing

1. Replace the adsorbent of the purifier to shorten the adsorption cycle;

2. Adjust the opening of the reflux valve to optimize the distillation process;

3. Use a leak detector to locate the leak point, repair welding or replace the seal

Localized frosting/sweating fluctuations in the cold box of ASU

1. Local damage and inadequate sealing of the cold insulation layer;

2. Localized moisture absorption of pearl sand;

3. Cold insulation failure caused by changes in environmental humidity

1. Local repair of the cold insulation layer and tightening of the sealing surface bolts;

2. Replace the pearl sand in the damp area and strengthen the sealing and moisture resistance of the cold box;

3. If necessary, add a moisture-proof layer and optimize the ventilation around the cold box

Pipeline vibration accompanied by pressure fluctuations

1. The fluid flow rate is too fast (improper valve opening);

2. Loose pipeline fixing brackets;

3. Cavitation is formed after the throttle valve

1. Adjust the valve opening to reduce the flow rate and avoid critical flow;

2. Check and reinforce loose brackets;

3. Optimize the operating conditions of the throttle valve to avoid cavitation