Intermittent blowdown

AS—Air Supply BD—Blowdown BF—Blind Flange CBD—Continuous Blowdown CD—Closed Drain CH-O—Chain Operated CSO—Car Seal Open CSC—Car Seal Closed DC—Drain Connection EBD—Emerg. Blowdown Valve ESD—Emerg. Shutdown FC—Fail Closed FO—Fail Open HC—Hose Connection IBD—Intermittent Blowdown LO—Lock Open ML—Manual Loading NC—Normally Closed NO—Normally Open OD—Open Drain... 

The second method could be automatic intermittent blowdown. With this, a timer-controlled valve is installed at the bottom of the boiler prior to the main blowdown valve. A program is then designed to operate this valve in short bursts, which disperses any sludge and controls the levels of solids. This method is preferred for boilers having internal treatment. 

A second method is a time-controlled valve allowing regular intermittent blowdown of the boiler. Again, regular checks need to be carried out to monitor the quality of water in the boiler. 

Using a skimmer, the BD water is taken from close to the top surface of the BW, either from the side of the FT boiler shell or WT boiler top drum or from a top-mounted sparge pipe extending down 3 to 6 inches below the waterline (but above the top set of tubes in a FT boiler). Blowdown is provided on either a continuous blowdown or an intermittent blowdown basis. 

INTERMITTENT BLOWDOWN - The blowdown is taken from the mud drum, waterwall headers or the lowest point of circulation. 

The blowdown system for System 80+ (Section 10.4.8) is designed for intermittent blowdown of up to 10% of feed flow. The system is effective in removing particulate accumulations at the tubesheet. Access for sludge lancing is also provided via the 8 handholes. 

The TDS content of BFW should be minimized by upstream treating. In former days, hot lime water softening was employed. In most modern process units, ion exchange resin is used to demineralize BFW. "Blowdown" is the water that is drained out of the boiler to control the accumulations of TDS or silicates in the boiler. My old design value for this flow was 10 percent of the makeup BFW flow. Two types of blowdown from a boiler are used continuous blowdown from the steam drum, and intermittent blowdown from the mud drum both will reduce silica. Note that with the continuous blowdown, heat recovery equipment may be economically justified. 

Intermittent Blowdown— This blowdown from the mud drum of a boiler is usually once a shift via manually operated valve. If this is automated it is important to have the valve full open for short periods to keep the valve clear and unblocked. 

Intermittent blowdown—Used to control suspended solids from mud drum of power plant boiler. 

As the water evaporates into steam and passes on to the superheater, soHd matter can concentrate in a boHer s steam dmm, particularly on the water s surface, and cause foaming and unwanted moisture carryover from the steam dmm. It is therefore necessary either continuously or intermittently to blow down the steam dmm. Blowdown refers to the controHed removal of surface water and entrained contaminants through an internal skimmer line in the steam dmm. FHtration and coagulation of raw makeup feedwater may also be used to remove coarse suspended soHds, particularly organic matter. 

Blowdown (Purge) The continuous or intermittent wasting of small amounts of circulating water. Its purpose is to prevent an increase in the concentration of solids in the water due to evaporation, normally expressed as a percentage of the water being circulated. 

The third and most automatic system is the conductivity-controlled blowdown. This constantly measures the level of solids in the water and instigates an automatic variable blowdown on a continuous or intermittent basis. 

Intermittent, surface BD arrangements are controlled automatically and are not very suitable for smaller boilers with high percentages of returning condensate (i.e., those boilers with only a very low blowdown volume requirement). 

Blowdown is the continuous or intermittent discharge of a certain amount of water from the tower to prevent the buildup of solids in the water due to evaporation. It is 2.5-3% of the water circulated to limit salt concentration. 

Pumps (both regular and spare) must be started up to flush out suction and discharge lines. Heat exchangers, side product lines, blowdown and emergency lines, and any connections used intermittently should be flushed and drained during this step. Circulation should be established and maintained in pumparound lines (296). Instruments should be flushed and be fully operational before this step... 

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