There are two basic ways of laying up boilers: wet and dry. In storing a boiler dry, trays of moisture-absorbing chemicals are distributed in the boiler drum (or drums), and the boiler is sealed. This removes the moisture and prevents premature wear on the boiler tubes that may cause future pitting and leaks. The alternate method, wet storage, involves forcing air out of the boiler by completely filling it (to overflowing) with water that has been specially treated. Nitrogen gas under a slight pressure can also be used to displace air and “blanket” the boiler open surfaces. Special consideration should be given to protecting superheaters during lay-up, particularly the non-drainable type, see your operating manual is this is your case.