The primary reason for choosing wet layup is that the time to restart the boiler is considerably shorter than with dry layup as the unit does not need to be refilled. Control of water chemistry in wet layup typically involves dosing of excess alkalizing chemical to raise pH to the high end of the operating range specification and excess oxygen scavenger to maintain low oxygen levels in the system. Some plants do not overdose but maintain water chemistry within the operating band. In either case careful monitoring of pH and oxygen levels is required during layup to assure that these parameters remain in the target range. It also requires that the fluids be circulated occasionally to avoid buildup of stagnant zones where pH and oxygen levels are out of range. Typically a small “jockey” pump is used for this as fluids only need to be moved slowly or a few times per day. Many plants do not have this pump included in the design, therefore they are installed as a retrofit. Nitrogen pressure needs to be monitored as well to assure that the passive atmosphere remains intact.
Two major disadvantages for wet layup include the potential for freezing and the need to have nitrogen available to blanket unfilled sections.