#Industry News
Some Standard Requirements For Psa Oxygen Plant Installations In Central Oxygen Supply Systems
Requirements for Psa oxygen plant installations
Oxygen generator or pressure swing adsorber (PSA) systems may be an alternative to the more traditional supply systems (the terms oxygen generator and PSA plant are interchangeable). Typical installations where PSA oxygen plant systems should be considered are those sites having no access to reliable liquid supplies, such as remote or off-shore locations, or where the safety criteria for a bulk liquid vessel cannot be met (for example, very restricted sites). Or, PSA systems should be installed when an investment appraisal shows them to be economical. When installed, a PSA system will deliver product gas via the "oxygen" pipeline system.
Oxygen generator plant operate by adsorbing, under pressure, other gases in the atmosphere onto materials which have specific physico-chemical properties, thus freeing the oxygen which is stored and transmitting it for use. The adsorbents are known as artificial zeolites, more commonly referred to as molecular sieves. The sieve units are arranged in pairs, one adsorbing whilst the other regenerates. The waste product, essentially nitrogen, is discharged to atmosphere during regeneration of the adsorbents. In some systems, the use of vacuum to remove the nitrogen increases the efficiency of the regeneration/adsorption process. Regeneration requires the use of a small proportion of the product gas.
The PSA process has reached a high level of technical sophistication and is capable of producing oxygen with a concentration of about 95%. (For the UK the minimum level, below which the emergency/reserve manifold will come into operation, is 94%.) The remainder is mainly argon with some nitrogen. The highest concentration is not likely to exceed 97/98%, except when the emergency/reserve manifold is in use, when it will be 100% if these are from a gas supplier.
The major components of a PSA oxygen plant system and their layout are shown in Figure 22.
The typical major components of the system are the compressors, receiver(s), dryers, molecular sieves, vacuum pumps, filters and regulators. Other components are identical to those used for medical air and vacuum plant, which are described fully in the appropriate sections. A suitable operating and indicating system is also required, as specified below. Package supply systems, which should be specified to meet the requirements given in this memorandum, are available from manufacturers.
Plant configuration
The plant should comprise:
a. a duplex compressor – if more than two compressors are installed, the plant should provide the design flow with one compressor out of service;
b. duplexed air treatment/molecular sieve devices, that is, two sets of filters and a pair of molecular sieves (one adsorbing whilst the other regenerates) and one vacuum pump (if required by the manufacturer).
Compressors and vacuum pumps
The compressors for the PSA oxygen plant systems may be any of the type recommended for compressed air systems. It is also possible to provide a combined medical air PSA plant. Generally, the compressed air requirement per litre of product gas is of the order 4:1; as a result the compressor plant will be on longer than that typically seen in hospitals.
A vacuum pump may be required as part of the system. The vacuum pump, if provided, is utilised during the adsorption/regeneration process. Vacuum pumps may be of any type as for the piped medical vacuum system. It will not generally be practicable to use water-sealed pumps or the medical vacuum plant.
Molecular sieves
Duplex molecular sieves should be provided in pairs to permit continuous generation of oxygen. One of the pairs of duplex sieves will be in the adsorbing stage, whilst the other regenerates.
Dryers
Air dryers of the desiccant type are usually integrated within the molecular sieves and therefore do not regenerate independently. Refrigerant dryers may also be included.
Oxygen monitoring system
The PSA oxygen plant should include a calibrated paramagnetic oxygen monitoring system comprising oxygen analyser, oxygen concentration indicator, oxygen flow monitor and oxygen concentration/flow recorder. Connections for calibration cylinders should also be provided. In the event of the concentration falling below 94%, the monitoring system should isolate the PSA system from the pipeline distribution system so that the emergency/reserve manifold operates. Additionally, an independent monitoring system should be provided to isolate the plant when the concentration falls below 94%. The second system need not be provided with a flow indicator or recorder.
