Preservation & Maintenance Techniques of Boilers in Thermal Power Plant
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 a 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.
A 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 demineralized 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.
Wet Preservation with Nitrogen Cap:
Nitrogen Cap allows boiler and feedwater equipment to remain full, Requires no excessive addition of chemicals, Permits nitrogen to rush in when steam collapses & preventing oxygen from entering the system. The following procedures are utilized with a bulk nitrogen system
Main Condenser and Turbine
Nitrogen addition starts while the turbine is still spinning down
Nitrogen is added quickly at first, then slowly as the vacuum approaches zero (The condenser is the largest user of nitrogen)
Deaerator and Storage Tank
Nitrogen is added when the Deaerator is still hot
Nitrogen is purged for about 20 minutes followed by the maintenance of a small nitrogen positive flow
Nitrogen is supplied through a shell-side vent line
Nitrogen enters the drum through vent lines
Nitrogen feed is started while the drum is still hot.
The use of a nitrogen cap, as outlined above, improves startup chemistry, reduces layup corrosion, reduces boiler tube deposits and lengthens the time between chemical cleanings.
In many instances, it is necessary to drain off-load plant for plant overhauls, tube repairs, etc. In this situation, the preferred option shall be to use a dry preservation standard and procedure where possible.
The plant shall be blown empty from as high a pressure practicable (possibly up to 30 bar) and, once the plant is empty, all vents would be rapidly opened to promote air circulation and to evaporate any retained water.
When dry preservation system is established, it is by far the most convenient and flexible procedure to be used where a plant has to be opened to the atmosphere for a prolonged period.
Risk & its mitigation for dry preservation:
The residual heat in the plant should be insufficient to complete the dry-out.
There is also a serious risk of condensation occurring when the plant goes cold, therefore, if possible, the drying should be assisted by the circulation of dry air through the plant or by the use of low-level heat.
Dry Layup Using Dehumidified Air
The use of clean, dehumidified air to purge the boiler and auxiliary equipment during layup periods shall be practiced
The justification for the use of dehumidified air to protect ferrous surfaces is “corrosion can be mitigated by maintaining air in contact with corrosion prone surfaces at a relative humidity of 60% or less”.
- A desiccant dehumidifier commonly used for a layup of boilers and auxiliaries. T
- The dehumidifier consists of a wheel of ceramic material that has been corrugated, so air can pass lengthwise down the fluted corrugations. A desiccant is impregnated into the structure. Moisture is attracted from the air onto the desiccant as the air passes through the wheel.
- The power system components must be made as airtight as possible. The dry air circulation systems are then sized to provide ten air changes per hour for water/steam-side components, one air change per hour for flue gas-side components, and five to ten air changes per hour generating equipment.
- If the installation does not allow the systems to be air-tight, larger values are used. If the systems are exceptionally tight, smaller values can be used, or only a portion of the circulating air can be processed through the dehumidifier.
Short Term Preservation
Short-term layup presumes that the unit will be required to operate within a relatively short timeframe. In consideration of this, no major changes are required from normal operating conditions, with the exception being that the unit must be protected from air ingress.
- The condenser vacuum and turbine seals are maintained; the deaerator, heater shells, and boiler are inerted with nitrogen or steam; and, the feedwater chemistry is maintained according to the requirements of the treatment philosophy employed (phosphate treatment / AVT / oxygenated treatment).
- For short outages, a turbine steam bypass system will permit the boiler to operate at a low firing rate while taking the turbine off-line. This procedure may result in net energy savings while facilitating rapid return to service once load demand increases.
- To protect the condensate system from air ingress, maintain condenser vacuum and turbine seals.
- Provide auxiliary steam to blanket the deaerator. If auxiliary steam is unavailable, pegging steam from an adjacent unit or from the drum should be provided.
- For longer outages, nitrogen blanketing may be more convenient.
Long Term Preservation
The long term preservation of plant is a complex situation as consideration has to be given to all sections of the plant including electrical and mechanical equipment, as well as the pressure circuit. ‘Wet preservation’, using ammonia & hydrazine solutions and possibly nitrogen filling, can be used for preservation over quite extended periods. However, the first choice for the preservation & Maintenance of boiler, turbines, condensers and feed systems shall be ‘dry preservation’ with dry air circulation.
- When the plant is taken out of service boilers and feed systems should be blown down as soon as possible and header caps and air vents opened to promote air circulation.
- Turbines and reheaters can be blown free of residual steam using forced air cooling systems.
- If available, special air drier units are then used to circulate dry air through all sections of the plant continuously throughout the period of preservation.
- The relative humidity in the plant must be kept at <30% to prevent retention of water by hygroscopic salts to maintain a dew point 10-15 °C below the lowest temperature in the plant being stored.
- Dry air is preferred to warm air circulation as it is more effective in drying out plant, is an easier technique to sustain over long periods and uses much less energy to produce the desired effects.
In practice, this procedure has proved very effective in storing plant over prolonged period off-load.
Short Term Preservation (<3 days)
Intermediate-Long Term Preservation (>3 days)
Dehumidifier Feed Cycle