Leakages in Check Valve Flanges of Oxygen Pipelines

The emergency failure of the stainless steel check valve flange of the oxygen pipeline of two sets of air separation plants is briefly described, and emergency measures are taken from the perspective of production and safety, and a detailed emergency repair plan is formulated. Through the stages of pressure relief, nitrogen replacement, welding, penetration testing, purging and pressure testing, the failure is eliminated, and production is resumed. At the same time, analyze the cause of the failure and prevent the recurrence of such accidents from the source.
 
I. Introduction
The oxygen plant of Anyang Iron and Steel Co., Ltd. (hereinafter referred to as An Steel) currently has 6 sets of air separation equipment. They are 6000m3/h, 1#14 000m3/h, 2#14000m3/h, 1#23500m3/h, 2#23500m3/h and 3#23500m3/h. The 6 sets of air separation equipment are all external compression processes. The oxygen produced is compressed to a pressure of about 2.50MPa by an oxygen compressor, and enters the spherical tank area of the oxygen plant in three sets of pipelines. Oxygen produced by two sets of units with 6000m3/h and l#23500m3/h converges, l#14 000m3/h and 2#14 000m3/h, 2#23500m3/h and 3#23500m3/h. There are seven oxygen spherical tanks in the spherical tank area: five oxygen spherical tanks with 400m3 and two oxygen spherical tanks with 1000m3. Seven spherical oxygen tanks are connected in series to store gas and ease oxygen pressure for users. Among them, two sets of 2#23 500m3/h and 3#23 500m3/h units and two 1 000m3 oxygen spherical tanks were put into operation in 2007, which belong to the new system supporting the new production line of An Steel. The oxygen from the spherical tank is regulated by the pressure regulating valve group and sent to each oxygen user unit. The main oxygen users of An Steel include the first rolling mill, the second rolling mill, the second steelmaking plant, the iron mill and the sintering plant.
 
2. Occurrence and response of failure
1. Failure
On the morning of September 21, 2015, the operator of the spherical tank area heard abnormal noises during the inspection and immediately reported to the workshop personnel. After the workshop personnel arrived at the scene, they organized a detailed investigation and found that there was a crack in the weld seam between the flange of the check valve and the pipeline, and a large amount of oxygen leaked. It has been confirmed that this pipeline is the pipeline of the tank area after the compressed oxygen of the 6 000m3/h unit and the compressor of the l#23 500m3/h unit is compressed. Immediately report to the leader of the oxygen plant. Through on-site inspection and measurement, the diameter of the flange is DN300, and there are 3 cracks in the welding seam along the circumference of the flange. The total length is close to 1/2 of the circumference of the flange pipe. Cracks are in danger of deteriorating or even bursting at any time, causing major production safety accidents, and measures must be taken immediately.
 
2. Interim measures
All oxygen pipelines in the spherical tank area are connected. At present, steel is in short supply of oxygen. There were 7 sets of air separation equipment in the oxygen plant, and 1 set of 6000m3/h equipment was dismantled in 2014. The newly built 4#23500m3/h air separation equipment was still being installed. A slight move in one part may affect the situation as a whole. Stop supplying oxygen for maintenance, and all users of the company must take measures in advance to limit production and limit oxygen. Once the gas is suddenly stopped, it will cause great economic losses to the company. In the increasingly severe situation of the steel situation, which is one disaster after another. Through preliminary discussions, the following emergency safety measures are temporarily taken when no emergency repair plan has been formulated:
1) A warning belt is set near the leakage point, and a warning sign is hung; strictly prohibits people from approaching.
2) Notify the personnel who were working on the site at that time to stop the construction until the treatment is completed.
3) Oxygen users will be notified by the dispatching and coordination of the oxygen plant that the oxygen pressure in the control system should not exceed 1.8MPa. The oxygen pressure for normal operation is about 2.3MPa.
4) Strengthen on-site monitoring; use binoculars to observe the leakage of the flange at a distance without entering the warning area, and take immediate measures if there is any abnormal deterioration.