There are four main categories of causes:
1) Insufficient discharge capacity of the air compressor, resulting in inadequate feed air to the fractionation column, causing tray weeping and impairing distillation efficiency. The reasons for insufficient compressor discharge include:
① Leakage at the intake and exhaust valves. This may be caused by uneven or insufficient valve spring force, broken springs, valve wear, worn lift limiters, fractured valve plates, or carbon/coke deposits on the valves;
② Cylinder or piston ring wear and leakage, or poorly assembled or worn packing glands causing air leakage;
③ Significant leakage from the blow-off valves and other valves. The blow-off valve in particular is prone to damage and leakage due to frequent operation;
④ Insufficient motor speed or excessively low voltage, resulting in reduced discharge capacity;
⑤ Severe clogging of the air filter, leakage in the cooler, or leakage at flange connections in external piping, etc.
A severe shortage of feed air will prolong the time required for the fractionation column to produce oxygen. This is particularly critical for units equipped with an argon column or high-purity nitrogen production, where a reduction in air volume has a major impact on product quality.
2) Inadequate adsorption of moisture and carbon dioxide by the purifier, allowing large quantities of moisture and CO₂ to enter the fractionation column. This blocks the heat exchangers and sieve holes of the column trays, increasing internal resistance, causing unstable operation, and even leading to liquid flooding that disrupts the distillation process. The main causes of reduced purifier adsorption performance are:
① Excessive operating time of the purifier, incomplete regeneration of the adsorbent, or premature switching to service before the adsorbent has been adequately regenerated;
② Contamination of the adsorbent by oil and dust, resulting in increased flow resistance and reduced adsorption capacity.
3) Severely insufficient refrigeration production by the expander, causing inadequate cooling capacity in the fractionation column. The main causes of reduced refrigeration output from the expander (referring to piston-type expanders) are:
① Wear and leakage of the intake and exhaust valve cores. Fatigued springs with insufficient force prevent the valves from closing tightly;
② Over-tightening of the seal ring nut, causing sluggish movement of the valve stem;
③ Severe wear of the cylinder and piston rings, or scoring of the cylinder surface, resulting in gas leakage;
④ Loose expander drive belt, preventing the gas flow into the expander from reaching the design value. This reduces refrigeration output and may also cause overspeeding;
⑤ Damage to the expander inlet regulating valve (e.g., the bypass valve, safety shut-off valve, or cam regulator), resulting in loss of control over the gas flow rate and temperature entering the expander.
4) Decline in the distillation efficiency of the fractionation column. In addition to the above causes, the main reasons include:
① Internal gas and liquid leakage in the fractionation column, resulting in significant gas loss and increased cold loss;
② End-of-run condition of the fractionation column, with blockage caused by moisture, carbon dioxide, or dust;
③ Moisture absorption by the insulation material inside the cold box, followed by settlement after vibration, exposing the top of the column and increasing cold loss;
④ Non-vertical alignment of the column body, causing uneven liquid levels on the trays;
⑤ Less common faults include: inter-stream gas bypassing between the medium- and low-pressure sides of the condenser-reboiler or the liquid air/liquid nitrogen subcooler, and gas bypassing between the oxygen and nitrogen passages in the heat exchangers.
Of course, many other factors can affect the decline in oxygen and nitrogen purity and yield in the fractionation column, such as liquid flooding caused by excessively rapid operation. Therefore, careful operation and close observation are essential. Faults must be promptly eliminated, and routine maintenance of machinery and equipment must be given due attention.
