Boiler Shutdown, Standby and Storage - Courtesy of Rentech Boiler Systems
When the unit is taken out of service, good care of the boiler during the idle periods is mandatory to prevent unnecessary corrosion damage. Gradually reduce load and run the fuel system to low fire position. When the unit is at the low fire position, blow down the boiler along with water column gage glass and the feedwater control valve. Following manufacturer's recommendations, remove from service all combustion equipment. The until is to be post purged with the fan for at least 5 minutes and then allowed to cool on natural draft after removing the fan from the service.
The steam pressure should be allowed to drop naturally without opening vents or other intentional means of taking steam from the unit to hasten the lowering of steam pressure. A cooling rate exceeding 100°F per hour to 150°F per hour by excessive opening of vents should be avoided to limit the drum distortion and resulting strains on tube joints. Vents can be fully opened when the steam pressure drops to 15 to 20 psig. Close the stop valve on the steam line and open the drain (if provided). When the steam pressure has dropped to approximately 20 psig, open the drum vent valve to prevent a vacuum from forming within the boiler. The boiler is drained only after the water temperature drops below 200°F.
To insure that the safety valves are maintained in good working order they can be hand lifted or popped at periodic intervals. It is recommended that the valves be lifted just prior to a planned boiler shutdown, as a check to determine whether repair work is necessary.
When removing a boiler from service for storage, inspection or cleaning the circulation of water ceases. This causes suspended solids in the water to settle out on the boiler surfaces, cake and dry to an adherent sludge, which can be mistaken for scale during normal operation.
Many boilers used for heating or seasonal loads or for standby service may have extended periods of non-use. Special attention must be given to these, so that neither waterside or fireside surfaces are allowed to deteriorate from corrosion. Corrosion can be more serious during this down time than when the boiler is actually in service. The key factors responsible for corrosion are water, oxygen, and pH. Elimination of either moisture or oxygen will prevent appreciable corrosion.
Two types of storage systems are widely used; these are wet and dry storage methods. Wet storage is adopted for short duration lay up and dry storage is used for shut downs exceeding approximately 30 days. No unit should be wet-stored when the temperature could drop below the freezing point.
In situations where the boiler is "headered" with other steam generating units it may be possible to maintain the boiler in idle storage by means of treated blowdown water or steam flooding from an operating unit. If these alternate lay-up methods are considered feasible the Owner should assure that his water treatment consultant firm has reviewed the water chemistry aspects of the alternate method and agreed to it's implementation as a means of protecting against corrosion.
As the boiler is being shut down and as the pressure subsides, but before steaming stops, add chemicals to the boiler to scavenge oxygen and to control pH, per the recommendations of the Owner's water treatment consultant.
When the boiler pressure gauge indicates about 10 psig completely flood the system with deaerated treated water with the drum vent open. Close the drum vent after it begins to overflow. The steam stop valve should already be closed. Water should be added through the feed pump until 10 psig is maintained as indicated by the drum pressure gauge.
Close the feedwater valves. Observe the steam drum pressure gauge and maintain pressure. If pressure builds, it may be due to leaking feedwater valves, which may not be properly seated. A pressure loss (after the boiler is cool) indicates leakage, so check all fittings and valves for leakage.
Frequent water samples should be taken and analyzed by the water treatment consultant. If the analysis indicates a need for additional chemicals, the level in the boiler steam drum should be lowered to normal level and chemicals added. The boiler should be then be steamed to circulate the solution, and the process of wet storage repeated as previously described.
Close all gas side access doors. Isolate the system to prevent cold air from reaching the heating surface. Periodic inspections of the external surfaces of the pressure parts should be made to guard against condensation and subsequent corrosion.
During storage, steps should be taken to protect the exterior components from the possibility of rust or corrosion. These parts should be coated with a rust inhibitor and protected from moisture and condensation. Electrical equipment should likewise be protected. Keeping the control circuit energized may prevent condensation from forming in the control cabinet.
When the unit will be idle for a considerable length of time and a short period can be allowed to prepare the boiler for return to service, the dry storage method is recommended.
1. The unit should be cooled down and then completely drained. Make sure that no pockets of water remain in the drum, piping, water column, etc. Open all vents t to assure complete drainage.
2. Open all the manways and thoroughly wash the water side surfaces to remove any sludge deposits. Mechanically clean residual particulate from the gas side metal surfaces and inspect the system thoroughly.
3. Dry the system thoroughly. If an air hose is used, be sure condensate from the air tank is not blown into the tubes and that the air is oil free.
4. Place flat wooden trays of moisture absorbent, such as quick lime or silica gel,
inside the drums to absorb any moisture that will be trapped when the unit is
closed up. The trays should be placed on supports to allow air to circulate under them. For recommendations on quantity of moisture absorbent, the Owner's water consultant should be contacted. The trays should not be more than 3/4 full of the dry absorbent to prevent overflow of the corrosive liquid that has been absorbed.
5. After the entire system is dried the boiler system should be pressurized with nitrogen to approximately 5 psig though the drum vent. Close the steam outlet, drain valves, and feedwater block valves. Ensure that the system has been purged completely before pressurizing.
6. Close all the openings to prevent water, steam or air leakage into the unit. Leave open the code required drain valve between the process steam stop valves to prevent back-leakage of condensate from any downstream header that may be pressurized.
7. Isolate the boiler to prevent moisture from reaching the heating surface.
8. Keep the boiler room dry and well ventilated to reduce possible surface corrosion.
9. If it is suspected that moisture cold have been absorbed by the refractory, follow the recommended Refractory Dry Out Procedure.
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