Water blowdown quality

Further savings can be made by the use of a suitable blowdown (BD) water flash steam and heat recovery (FSHR) system. Such systems are standard equipment on large power boilers but are less common on smaller plant. However, the FS component (and often the HR component as well) can usually be justified for smaller systems because the capital cost payback of such equipment generally is less than 12 months and such systems continue to save fuel and high-quality water year after year. 

Secondary System. The water quality specifications for the feedwater and blowdown water in a recirculating steam generator (RSG) and the ... 

The American Boiler Manufacturers Association (ABMA) has established limits for boiler water composition. The limits are set to help assure good quality steam (for example, limiting silica in the steam to 0.02-0.03 ppm). Blowdown is normally based on the most stringent of these limits shown in Table 1. 

Blowdown on a boiler is mandatory. On small boilers, the required operation of the main blowdown valve may be sufficient to control the quality of water within the boiler. On medium and large plants, additional systems are employed. 

The simplest is a preset continuous blowdown valve to maintain a suitable water quality in the boiler. It is necessary for water quality to be checked frequently and the rate of continuous blowdown adjusted as may be found necessary. 

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. 

Wastewater streams from the utility functions include boiler and cooling tower blowdowns and waste brine and sludge produced by demineralizing and other water treatment systems. The quantity and quality of the wastewater streams depend on the design of the systems and the water source. These streams usually contain high dissolved and suspended solids concentrations and treatment chemicals from the boiler and cooling tower. The blowdown streams also have elevated temperatures. 

In modern high-pressure systems, blowdown water is normally of better quality than the water supply. This is because plant intake water is treated using clarification, filtration, lime/lime soda softening, ion exchange, evaporation, and in a few cases reverse osmosis to produce makeup for the boiler feedwater. The high-quality blowdown water is often reused within the plant for cooling water makeup or it is recycled through the water treatment and used as boiler feedwater. 

It is really the quality of the treated boiler feedwater that sets the blowdown rate. Deionized or demineralized water might require a 1 to 2 percent blowdown rate. Hot-lime-softened water might require a 10 to 20 percent blowdown rate. 

Can the cooling tower blowdown be reduced Often pretreatment of fresh water to the cooling tower to reduce calcium salts can be helpful to reduce the scaling and thereby reduce the blowdown. Ion-exchange or even more expensive options, such as reverse osmosis, may prove to be beneficial in this regard depending on the quality (hard or soft) of the feed water. 

Prevent cross-contamination of a higher quality recoverable product with a lower quality material whenever possible i.e., do not combine packaging area drainage with deodorizer cooling water blowdown. 

Wastewater standards are applied to process wastewater, storm water, and once through water, cooling tower and boiler blowdown. Wastewater standards set a standard (quality) for wastewater before it can leave the plant and be returned to rivers, streams, or bays. Water sources leaving the processing site to return to the environment are called outfalls. Outfall samples are collected to be analyzed prior to releasing wastewater back into the environment to ensure environmental compliance. Some plants have analyzers mounted on outfall piping or vessels because of the critical nature of environmental compliance. This places a great responsibility on analyzer technicians for environmental compliance. 

A boiler requires blowdown to remove concentrated dissolved solids and control the water quality. The lack of blowdown could result in a higher pH of boiler feed water (BFW) in the boiler, which could potentially lead to corrosion. Insufficient blowdown could also cause impurities to carryover to steam. On the other hand, excessive blowdown wastes energy, water, and chemicals. The optimum blowdown rate is determined by various factors including the boiler type and capacity, operating pressure, water treatment, and makeup water quality. Blowdown rate is 2—4% for relatively large boilers and 4—8% for small boilers. It can be up to 10% if makeup water contains high concentrations of solids. Industrial standards for blowdown are available and can be referenced that indicate the amount of blowdown depending on the type and pressure of the boiler. 

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