Last week’s blog provided steps for proper dry boiler storage. While dry storage methods are most recommended, wet storage is an acceptable option in the event of a more temporary standby. This week’s blog provides you with proper steps in wet boiler storage. Please contact your Watertech representative for further specifics on how to properly layup your boiler.
When using the wet storage method, take at least one of the following precautions to avoid freezing:
- Auxiliary heat
- Cascade lay up boiler with hot boiler water from an operating boiler
- Light firing of the boiler
- Use of a 50/50 water/ethylene-glycol mixture (this however, requires that the boiler be drained, flushed, and filled with fresh feedwater prior to start up)
Even though dry storage methods are preferred, wet methods are acceptable whenever a boiler is on emergency standby.
- The boiler should be filled to the normal water level with deaerated feedwater. Then add either:3-4 pounds caustic soda and 1.5-3.0 pounds sodium sulfite per 1000 gallons of water contained in the boiler. A minimum 400 ppm (mg/L) phenolphthalein alkalinity as CaCO3, and 200 ppm (mg/L) sulfite as SO3 should be maintained at all times, OR..
- 4-5 gallons of diethyhydroxylamine (DEHA) per 1,000 gallons for oxygen control and 1 gallon morpholine per 1,000 gallons to raise the pH to 10.0-11.0.
- With vents open, heat should be applied to boil the water for 1 hour. This will allow for complete mixing of the water and chemical.
- If the boiler is equipped with a non-drainable superheater, fill the superheater with condensate or demineralized water treated with 4-5 gallons of DEHA per 1000 gallons and 1 gallon morpholine per 1000 gallons. Then fill the boiler with deaerated feedwater.
- If the superheater is drainable or the boiler does not have a superheater, the boiler should be allowed to cool slightly after firing. Before a vacuum is created, completely fill the unit with deaerated feedwater.
- After filling the boiler, connect a surge tank, such as a 55 gallon drum, containing a solution of treatment chemicals or nitrogen tank at 5 psig (0.35 km/cm2) pressure to the steam drum vent. This supply will compensate for volumetric changes as a result of temperature variations.
- The drain between the non-return and the main steam stop valve should be left wide open. All other drains and vents are to be left closed tight.
- The boiler water should be tested on a regular basis, with additional treatment being added whenever necessary to maintain the minimum treatment levels.
- When chemicals are added, the boiler water is to be circulated by means of an external pump or by reducing the water level to the normal operating level and steaming the boiler for the short time. If steaming is necessary, the boiler should be filled completely in keeping within the above recommendations.
Alternative Wet Storage (Cascade method):
Continuous blowdown from an operating boiler is used to keep the idle boiler filled with hot, treated water.
- If the unit to be laid-up has not been cooled and drained, connect the continuous blowdown line from an operating unit to a convenient bottom drain line or blowdown line of the laid-up boiler.
- If refilling an idle boiler, first open a vent line and close it only after completely filling the unit with surface blowdown water from the operating boiler. Then open the continuous blowdown line of the stored boiler to allow a flow of water from the idle boiler. The standby boiler must have no air pockets in the upper portion of the boiler.
- Enough hot surface blowdown from the operating boiler must be available on a continuous basis to keep the standby boiler warm and the exterior surfaces of boiler tubes and other boiler components warm enough to prevent condensation from forming on those surfaces. Proper care should be taken to ensure chemical levels are properly maintained in the operating boiler.