The cold box (condensating fractionating column) is the core part of the ASU, which is related to the safe and reliable operation of the whole system. How to check the construction quality of its installation is particularly critical.
Installation works include: Equipment assembling and installation; Pipeline configuration; Instrument pressure tube installation; Laying of pipe support. These four links all involve welding, which is also the main control point of installation quality control. In addition to nondestructive testing (NDT) under normal temperature, there are pressure test and cold commisioning (cold run) test to verify the quality of installation works.
Difficulties of pressure test include: (1) In the low pressure system which is the largest subsystem, it is easy to have leakage points. The demanded leakage rate is high, generally 24 hours over 98%. The inspeciton work requires carefulness and patience, better carried out in the evening when it is easier to find the leak points. At the same time, attention should be paid to the ambient temperature, which should not vary much during the test, since the volume expansion caused by temperature difference may cause miscalculation. If non-system gas is sued for the test, it is recommended to prolong the pressure holding period, that is, in the early stage, ensure that the pressure test temperature is close to the external atmospheric temperature before the start time of pressure holding, to reduce the calculation deviation.
(2) The instrument pressure guide tubes are relatively slim and have scattered welds, normally nondestructive testing is not arranged. Even if there is a leakage, it is not easy to find. But the hazard can be great, such as it can cause measurement inaccuracy, leakage of gas or liquid and system downtime, so it is recommended that the guide tubes should be assembled on the ground, and checked with pressure air to reduce risk of leakage.
(3) It is easy to neglect blind spots such as: the outsourced devices or components with manufacturers' own welds/solder joints and scattered instrument ports, and locations where it is not easy to climb to.
Matters needing attention in pressure test. (1) there must be a complete plan before pressure test. The classification of each pressure level should be clear, not only to prevent overpressure test, but also to prevent step-down inspection, otherwise the purpose of pressure test can not be achieved. (2) The pressure test between subsystems of different pressure levels must be done with good isolation, to prevent damage to the equipment. (3) Ensure that the air in the cold box is smooth. When other type of gas is used, prevent suffocation in case of any leakage. (4) During the pressure test, ensure isolation from the external pipe network, to prevent the invasion of harmful gases from the external sources, which may result in personal injury.