Corrosion allowance, which is generally 1mm to 3mm, in various equipment deplete over a period of time and prolonged exposure to air results in degradation of material strength and appearance. During the normal operation of power plant when all the equipment (turbine,Generator, boiler, piping and other BOP equipment) are performing their usual functions, they are most likely to be working in their own environment protected from the atmospheric oxygen. But during power plant idling and shutdowns, metallic parts of a machine are liable to corrode in the presence of oxygen and moisture of atmospheric air. There are many ways by which we can safeguard equipment from corrosion such as surface treatment and blanketing it with inert-gases like Nitrogen, Carbon dioxide, etc. The method used to prevent corrosion has their own limitations, for example internal coating of process equipment is not acceptable and many inert gases are very expensive to procure or produce. In this scenario blanketing the process equipment with Nitrogen gas is in practice widely.
To safeguard equipment, Nitrogen purging is a viable solution. Firstly, Nitrogen when filled in large volume and slightly positive pressure, displaces air well. Secondly, Nitrogen is typically dry with a low dew point. These two characteristics make Nitrogen very desirable as an inert gas. In power plants, Nitrogen is used for purging equipment and pipelines as a safety mechanism.
One major cause of concern for many of the power plants is the supply crunch of fuel materials such as coal and gas. In cases where fuel availability challenges are experienced, many power plants are forced to shut down for several days at a stretch. Hence, during these periods of prolonged idling or shutdowns, equipment have to be "preserved" to avoid major corrosion due to contact with air.
Availability of Nitrogen
Nitrogen can be procured either by a merchant gas supplier or by installing onsite gas plant at the point of use. It has been observed that procurement of Nitrogen is costlier as well as have the users face other challenges such as storage and constant supply problem. Thus, installing a onsite gas plant is much viable solution for ensuring instant supply of gas when required.
Given that various challenges faced by the industry such as rising price and supply constraints, many power plants are seriously evaluating options of setting up on-site gas generators.
Nitrogen onsite generator generally comes with the following two technologies: Membrane Nitrogen technology: It is simple process to separate nitrogen from the atmospheric air with the help of membrane modules. It's a compact size facility. PSA Technology: Pressure Swing Adsorption technology employs method of separating nitrogen form atmospheric oxygen with the help of carbon molecular sieves. This unit is a little complex in construction and relatively bigger in size.
Both the technology require compressed air around at around 7 barg pressure. In case, plant air is not provisioned, then a small air compressor, dedicated for Nitrogen generation, can be provided as an integral part.
For power plants, membrane technology is the most preferred option as it can be set up with a low cost investment. It also has other advantages such as compact size, minimal maintenance, easy to operate and life time of 12-15 years.