Once-through cooling water

The process of using almost once-through cooling water as a FW source for HR is termed ebullient cooling. 

In addition to these effluent standards, the EPA has also established separate BPT, BAT, BCT, and NSPS standards for ballast water and BPT, BAT, and BCT standards for contaminated storm water (40 CFR Part 419). Once-through cooling water is allowed for direct discharge if the total organic carbon concentration does not exceed 5 mg/L. 

Once-through cooling water. Recirculating cooling system blowdown. Ash transport water ... 

In a steam electric power plant, cooling water is utilized to absorb heat that is liberated from the steam when it is condensed to water in the condensers. The cooling water is withdrawn from a water source, passed through the system, and returned directly to the water source. Shock (intermittent) chlorination is employed in many cases to minimize the biofouling of heat transfer surfaces. Continuous chlorination is used only in special situations. Based on 308 data, approximately 65% of the existing steam electric power plants have once-through cooling water systems. 

Chemical additives are needed at some plants with recirculating cooling water systems in order to prevent corrosion and scaling. Chemical additives are also occasionally used at plants with once-through cooling water systems for corrosion controls. 

Therefore, the amount of blowdown required to control scaling can be reduced. Chemicals added to once-through cooling water to control corrosion or to recirculating cooling water to control corrosion and scaling is usually present in the discharges. Chromium and zinc are the active components of most of the popular corrosion inhibitors. 

Table 1 Summary of Priority Pollutants in the Steam Electric Industry Once-Through Cooling Water... 

Seawater Although seawater is widely used for once-through cooling water applications, it is being used increasingly as a raw water source for industrial water, due both to a lack of suitable alternatives in some areas of the world and to constantly improving RO water production/cost ratios. Seawater TDS levels vary around the world from approximately 36,000 ppm to more than 45,000 ppm. As TDS levels increase, the RO applied pressure requirement may increase from 800 to 1000 psi or more to maintain recovery rates (usually 25 to 35%). 

If a once-through cooling water system is used, the velocity through the jacket is very small (0.032 m/s), which gives such a small h = 68 J s 1 K-1 m-2 that the design is unfeasible. Reducing the jacket clearance from 0.1 to 0.01 m, which is probably too small for a 5-m-diameter vessel, gives a ha = 682 J s 1 K 1 m-2. 

If an internal cooling coil is used (as discussed in the next section) with a 0.0763 m diameter, the velocity in the tube is 3.7 m/s and the film coefficient is ha = 3200 J s-1 K-1 m-2, giving a TJ = 800 J s 1 K 1 m-2. ft is clear that the once-through cooling water system is by far the most inferior of the three systems. 

Chlorine has been a preferred biocide for many years on account of its effectiveness and its relatively low cost. It has been used particularly in once through cooling water systems where large volumes of water are used, in order to minimise the cost of treatment. It is effective against micro- and macro-organisms. 

Continuous ferrous sulphate addition has been used for many years to reduce water side corrosive attack of steam condenser tubes. It is usually applied to once through cooling water systems because of its low cost, to provide an iron-rich protective film on the tube surface. For recirculation systems other more expensive, corrosion inhibitors are generally employed. Two phases of the ferrous sulphate treatment programme may be recognised. The first phase involves the initial laying down of the protective film. The second phase involves the maintenance of the film, which would be otherwise destroyed by the shear effects of flow. 

Continuous injection of corrosion inhibitors is practiced in once-through systems where slugs or batch treatment cannot be distributed evenly through the fluid. This method is used for water supplies, oil field injection water, once-through cooling water, open annulus oil or gas weUs, and gas lift wells. Liquid inhibitors are injected with a chanical injection pump. These pumps are extremely reliable and require little maintenance. Most chemical injection pumps can be adjusted to deliver at the desire injection rate (Chen et al. 2010). 

Once-through cooling waters (usually obtained from rivers, lakes, or wells) usually cannot be treated chemically, both because of the large quantities of inhibitors required and because of the problem of water pollution. Sometimes, additions of about 2-5 ppm sodium or calcium polyphosphate are made to help reduce corrosion of steel equipment. In such small concentrations, polyphosphates are not toxic, but water disposal may continue to be a problem because of the need to avoid accumulation of phosphates in rivers and lakes. Adjustment of the saturation index to a more positive value is sometimes a practical possibility. Otherwise, a protective coating or metals more corrosion resistant than steel must be used. 

In the unlikely event that both power sources should be lost slmul-taneously the reactor will be scrammed and when the coolant flow and temperature drops will be automatically cooled with emergency once-through cooling water. This transient will not be as severe as that discussed In Section 4.2.4 "Spurious Operation of the Dump Valves". 

Once-through cooling water—cooling-tower blowdown or boiler blowdown... 

An assessment of the generic impact of cooling water discharge on the marine enviromnent is provided in UKP-GW-GL-034 (Reference Error Reference source not found.). This assessment assumes that the APIOOO will be cooled by a direct (once through) cooling water system this being the option that maximises cooling efficiency and requires less overall use of sea water. 

The index has proven a useful tool in predicting the aggressiveness of once-through cooling waters. The Larson-Skold index might be interpreted by the following guidelines ... 

Once-through cooling water One-day amount Ozonation... 

---