Instrument air for air separation unit
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In air separation units, instrument air is generally designed to be air purified using molecular sieves. I would like those with experience in air separation units to discuss the following: what effects would it have on such units if the instrument air supplied by them were replaced with air provided by a dedicated air compressor station, that is, if external instrument air were used?
Originally, the instrument air used when the air separation unit is in operation comes from external supply; in some installations, the low-pressure nitrogen pipeline is connected in parallel with the instrument air pipeline, with a valve installed between them, which is opened when the air separation unit is shut down. Air separation plant manufacturers or designers consider using air treated with molecular sieves as the instrument air for such plants, in order to avoid a sudden interruption of this instrument air from affecting the normal operation of the plant. It’s a bit like the effect of a shaft seal pump on a compressor. These are just my personal opinions; I welcome any corrections
After the plant is shut down, the instrument air we use is supplied from an external source. Regarding the problems in our actual production process: first of all, it is necessary to ensure the quality of the gas supplied from outside – the dew point, pressure, and the levels of various substances such as oil are all very important. Our argon pumps have even stopped working due to the poor quality of the instrument gas supplied from outside. Another point is to ensure that switching is convenient and fast. Otherwise, it will definitely have an impact during an emergency stop
It is preferable that the instrument air supply for air separation units be provided in multiple ways: 1. Air supply from air pumps； 2. Low-pressure nitrogen (0.5 Mpa) ； 3. Low and medium pressure nitrogen/dry air storage tanks.
In our workshop, there are two Atlas air compressors with a capacity of 59 cubic meters per hour. From the very beginning, air is supplied to the air compressor, the pressure boost expander, and other equipment for sealing purposes as well as for use in instruments. The gas from the molecular sieve enters the booster; the gas obtained in the second stage is used as instrument air for the entire facility.
To the experts in air separation: my idea is as follows. The liquid throttle valves in air separation towers (such as those for liquid nitrogen, liquid oxygen, or contaminated liquid nitrogen) do not come equipped with solenoid valves, nor are they designed with interlocks. In order to protect the tower, they still operate on a gas-open, gas-close principle; in other words, they rely on air supplied by the air separation system itself. When the air compressor stops working, the liquid oxygen, liquid nitrogen, or contaminated liquid nitrogen throttle valves will open or close completely due to the loss of instrument air. However, if instrument air is not used, after the air compressor shuts down, these valves will lose the protective function of being operated by air for opening and closing purposes, as instrument air continues to flow to them. It’s just my personal opinion; I’m not sure if it’s correct Please, experts, correct me!
Reply to 6# wei*aobao: It is not impossible to use externally supplied instrument air, but air separation units are quite special and have high requirements for instrument air; oil contamination is strictly prohibited. Ordinary instrument air compressors find it difficult to meet these requirements, and moreover they consume a lot of energy, so it is not cost-effective from an economic perspective. I don’t understand why you want to change the instrument air used for the air shut-off and isolation valves, but one thing I need to clarify is that even if the air compressor fails, the supply of instrument air will not be interrupted, as these valves will close as part of the interlock protection system
When the air separation unit is shut down, instrument air is supplied by using backup liquid nitrogen vaporization; once gas is available after the molecular sieve starts operating, a switch can be made to use that gas instead. Keep the reserve in a supply-ready state to switch at any time. Regarding those liquid throttle valves mentioned on the 6th floor, it is incorrect to rely on the supply and cutoff of instrument air to control their opening and closing when the system stops operating, if there is no interlock in place. The air separation unit has been shut down, and there are many valves that need to be closed; however, in an emergency situation some valves must remain open so that the instrument air supply is not interrupted.
Reply to 6# wei*aobao: It’s not a problem either; after parking, the operator can go and close or open those valves to ensure a safe shutdown of the equipment. Air-on and air-off valves are designed only for emergency situations (that is, when the control valve loses access to instrument air and the control room can no longer operate it; in such cases, the valve can rely on its own protection mechanism). Nowadays, instrument air from the air compressor station is used as the power source, so there is no need for such self-protection mechanisms – it is humans who are required to provide protection. I’m not sure if this is what you wanted to know! :)
Reply to 6# wei*aobao: The choice between air-open and air-close modes is determined based on the conditions during an emergency. In the event that the air separation system comes to a stop, it is best to have access to qualified instrument air from an external source; this ensures that the instrument valves of the air separation system, as well as the sealing air for the units and the cryogenic liquid pumps, can continue to function properly in emergency situations, thereby ensuring a safe shutdown of the facility!