Preservation & Maintenance Techniques of Boilers |Boiler Preservation Practices | Boiler Preservation Procedures in Thermal Power Plant | Short/Long Term Preservation
The primary purpose of the cycle chemistry is to provide protective oxide surfaces on all components throughout the steam and water circuits; the primary purpose of the shutdown and layup periods should be to maintain those protective surfaces.
Severe corrosion damage to all power plant cycle components has been experienced because of insufficiently protected metal surfaces during inactive periods. Examples of such damage include the following:
Turbine blade and disk pitting;
Boiler drum and tube, feedwater heater, and condenser pitting and oxidation; and
Stress corrosion of condenser and feedwater heater tubing in stagnant oxygenated water.
Corrosion damage can occur on the water and steam-side surfaces as well as on other surfaces, including pipe surfaces under insulation.
The generation of excessive amounts of metal oxides and the contamination of layup water, if used, with oxygen and carbon dioxide adversely influence water and steam chemistry during subsequent startup and operation. The effect is prolonged startup period.
Other considerations of layup include its cost (chemicals, equipment, manpower) and the proper disposal of layup water when practicing wet layup and using chemicals such as ammonia and hydrazine.

The procedures for layup of idle equipment fall into two general categories:
1. The wet procedure and
2. The dry procedure.
In general, with the exception for units on oxygenated treatment (OT), wet layup requires filling of most of the system with an alkaline reducing solution (ammonia and hydrazine) and preventing air ingress by pressurization with an inert gas (nitrogen).
Dry layup requires drainage while hot, and removal of all water followed by pressurization with a moisture-free inert gas or by use of dehumidified air to maintain a low moisture environment.

Wet Preservation:
Wet layup is a method of “protecting” a unit when it might have to be returned to service on relatively short notice. It generally involves filling the unit with demineralised water containing an excess of a reducing agent (oxygen scavenger).
Depending on the design, the oxygen the scavenger may be eliminated assuming a viable nitrogen blanketing system is available. Circulation may be maintained, a head tank may be used, or positive nitrogen pressure may be maintained throughout the shutdown with water at normal operating levels. Wet layup can generally be used for short periods.
Use of nitrogen blanketing is recommended in conjunction with wet storage, not only for the boiler but also with other heat cycle components.
During wet layup, the oxygen scavenger concentration is monitored. Also, the boiler and economizer waters should be circulated routinely to prevent stagnant conditions.


Author's Bio: 

Marcep Inc. is a division of Marcep Group which has a JV with some of the major projects over the world and has entered the Asian market in sharing knowledge to the industry.