externally dried, and then closed up tight to exclude both moisture and air. %rays of lime, silica gel, or other moisture absorbent may be placed in the drums to draw off the moisture in the air trapped by the closing up of the boiler. %he following general procedure is recommended when placing a unit into dry storage. )tep ire the boiler according to the normal start+up procedure and establish upto #.1+ kg2cm!3+drum pressure. )top firing. )ecure the boiler and when the pressure decays to -.# kg2cm!3, immediately drain the boiler and headers under air. s soon as possible, open the drums to allow air to circulate for drying of all internal surfaces. %his step is included for a unit that has been in service and is to be placed into storage. or a unit that has never been in service, start with )tep !. )tep ! If the unit is full of water and cold, drain the unit under air. ll non+drainable boiler tubes should be blown with compressed air. If an external source of heat is available such as a steam coil air heater, portable heaters, etc., operate these heaters to assist in drying the internal boiler surfaces. Install trays (of non+porous construction and capable of passing through the drum manhole$ containing the moisture absorbent (silica gel is preferred$ into the drums. Insert the trays into the drum being certain that none of the absorbent comes into contact with the metal surface of the drum. %o insure against an overflow of corrosive liquid after the moisture has been absorbed, the trays should not be more than 4 full of dry absorbent. %he amount of absorbent can vary but the recommended minimum is one pound of absorbent per -/// pounds per hour steam flow capacity of the unit. )tep # "pen the isolation valve for nitrogen connection, on the steam drum, close all other vents and drains and pressuri'e the boiler to /.# to /.5 kg2cm !3 with nitrogen. %he amount of nitrogen required will vary according to the volume of the unit. )tep 6 ith the boiler pressuri'ed, alternately open all boiler drains to purge air from the unit until pressure decays to 'ero. It may be necessary to repeat this process several times to reduce the amount of oxygen left in the unit to a minimum. %he unit should now be stored under /.# to /.5+kg2cm !3 nitrogen pressure maintained at the steam drum. %o maintain the nitrogen pressure, all connections and valves should be blanked or tightly closed. 7heck gas pressure daily to ensure protection. e would recommend that periodic inspection of the unit be performed every # months to assure that no corrosive action is taking place and to replenish the absorbent as required. )ince air will enter the unit during this inspection, it will be necessary to repeat )teps # 8 6 to expel the air. 79%I": %he unit should be properly tagged and the appropriate warning signs attached noting that the boiler is stored under nitrogen pressure and that complete exhaustion of the
nitrogen must occur before anyone enters the drum. Before entering drums test to prove that the oxygen concentration is at least -;.1 <. %he above procedure is intended to include the economi'er.
WET STORAGE
%he advantage of employing the wet storage procedure is that the unit is stored completely wet with the recommended levels of chemicals to eliminate a wet+dry interface where possible corrosion can occur. It is suggested that volatile chemicals be used to avoid increasing the level of dissolved solids in the water to be used for storage. In preparing a unit for wet storage, the following procedure is recommended. %he unit should be filled with deaerated, =emineralised water treated with !// ppm hydra'ine (:!06$ for oxygen removal and sufficient ammonia (:0 #$ in order to attain a p0 of -/ (for demineralised water, this will require approximately -/ ppm ammonia$. e strongly recommend pre+mixing of the chemicals with the water to insure a uniform mixture entering the boiler. %his can be accomplished by the blend+fill method. %he blend+fill method consists of blending the chemicals with the demineralised water at a continuous rate such that a uniform mixture is entering the boiler. )imply introducing the chemicals through the drum after establishing water level will not insure adequate dispersion of chemicals to all internal surfaces, unless sufficient heat is delivered to the furnace (i.e. firing the boiler$ to induce natural circulation throughout the boiler. ill the unit with the treated demineralised water to the normal centerline of the steam drum. )top filling further. Back+fill the with treated =emineralised water until a rise in steam drum level is noted. 7ontinue filling until water exits from the steam drum vents. fter filling, all connections should be blanked or tightly closed. source of low+pressure nitrogen should be connected at the steam drum to maintain /.# to /.5 Bar 3 to prevent air from entering the unit during the storage period. 79%I": %he unit should be properly tagged and the appropriate warning signs attached noting that the boiler is stored under nitrogen pressure and that complete exhaustion of the nitrogen must occur before anyone enters the drum. Before entering drums test to prove that the oxygen concentration is at least -;.1<. If storage continues into winter, ambient temperatures below the free'ing point of water create a real ha'ard to the boiler pressure parts and it will be necessary to provide a means of keeping the unit warm to avoid damage. t some later date when the unit is to be placed into service, the boiler can be drained to normal start+up water level and placed into operation.
In some cases, an expansion tank or surge tank (such as a 11+gallon drum$ above the steam drum elevation may be required to accommodate volume changes due to temperature changes. %his tank is equipped with a tight cover and sight glass and contains properly treated water. %he tank should be connected to an available opening, such as a vent line at the top of the steam drum in order to create a hydrostatic head. %his tank will provide a ready, visual check of water level or in leakage during lay up. source of low+pressure nitrogen should be connected to the surge tank to maintain /.# to /.5 Bar 3 to prevent air from entering the unit during the storage period. %he treated demineralised water should be analy'ed weekly, and when necessary, sufficient chemicals should be added through the chemical feed line, to establish the proper levels recommended. )amples of the treated water can be taken at the continuous blowdown line or any suitable drain connection. :o unit should be stored wet when there is any possibility of a temperature drop to the free'ing point unless sufficient heat can be provided to the unit to eliminate the danger of water free'ing and subsequent damage to pressure parts.
NITROGEN BLANKET
:itrogen can be introduced at the following locations %hrough the steam drum %hrough the fuel gas line %he nitrogen required to seal the drainable components may be supplied from a permanent nitrogen system or portable tanks located near the vent elevations. =ue to differences in plant layout, the owner should choose his own method of piping the nitrogen, either from their permanent system or from portable tanks, to the vent (or drain$ locations listed. 79%I": %he unit should be properly tagged and the appropriate warning signs attached noting that the boiler is stored under nitrogen pressure and that complete exhaustion of the nitrogen must occur before anyone enters the drum. Before entering drums test to prove that the oxygen concentration is at least -;.1 <. BOILER LAY UP PROCEDURES
%>&? " )09%=": SHORT OUTAGES
6 =>) "R @?)). 9:I% :"% =RI:?= %>&? " )09%=":
&R"7?=9R? aintain the same hydra'ine and ammonia concentration as present during normal operation. ?stablish /.# to /.5 kg2cm !3 nitrogen cap on the steam drum &R"7?=9R?
SHORT OUTAGES
6 =>) "R @?)). 9:I% I) =RI:?= LONG OUTAGES
@":3?R %0: 6 =>) 9&%" -1 =>). 9:I% I) =RI:?=
LONG OUTAGES
"R? %0: -1 =>) + 9:I% I) =RI:?=.
=rain and open only those sections require repair. Isolate remainder of the unit under /.# to /.5 Bar3 nitrogen pressure where possible. aintain the same nitrogen and ammonia concentration for water remaining in the cycle ill the boiler with &olish water having !// ppm of hydra'ine and -/ ppm of ammonia to maintain p0 -/. ?stablish nitrogen cap of /.# to /.5 kg2cm !3 over the steam drum. =ry storage of boiler with nitrogen alone is preferred procedure. :itrogen cap of /.# to /.5 kg2cm !3 to be maintained on the steam drum. Installed silica gel tray in the steam drum to soak moisture if any present in the drum atmosphere.
