Blowdown rate

Feedwater The feedwater for a steam cycle must be purified. The degree of purity depends on the pressure of the boiler. Higher pressure boilers require higher feedwater purity. There is some trade-off between feedwater purity and boiler blowdown rate. However, increasing blowdown rate to compensate for lower feedwater purity is expensive, because blowdown water has been heated to the saturation temperature. Typical feedwater specifications for utihty boilers are given in Table 4. To some extent turbine steam purity requirements determine the feedwater purity requirements. The boiler-water siUca required to maintain adequate steam purity for higher pressure steam turbines is considerably less than the boiler could tolerate if deposition in the boiler were the only issue. 

Scale control can be achieved through operation of the cooling system at subsaturated conditions or through the use of chemical additives. The most direct method of inhibiting formation of scale deposits is operation at subsaturation conditions, where scale-forming salts are soluble. For some salts, it is sufficient to operate at low cycles of concentration and/or control pH. However, in most cases, high blowdown rates and low pH are required so that solubihties are not exceeded at the heat transfer surface. In addition, it is necessary to maintain precise control of pH and concentration cycles. Minor variations in water chemistry or heat load can result in scaling (Fig. 12). 

Phosphate Boiler dosing Consumption depends on blowdown rate for boiler... 

While vitally necessary, blowdown can be expensive in terms of lost heat. Therefore a point will be reached when it is economical to install a blowdown heat recovery system. Generally, the heat content in the blowdown water for a shell boiler will represent only about 25 per cent of the heat content in the same percentage of steam. Therefore, if a blowdown rate of 10 per cent is required this represents an approximate heat loss of 2.5 per cent from the boiler capacity. This differential reduces and eventually becomes insignificant on high-pressure watertube boilers. 

Example 23.1 A small package fire-tube boiler has makeup water that contains 500 ppm dissolved solids. The steam system operates with 50% condensate return. Estimate the blowdown rate. Assume that the maximum limit for the TDS is 4500 ppm. Assume that there are no solids in the evaporation or the condensate return. 

Improving control of cooling tower blowdown (see Chapter 24) for evaporative cooling water circuits to increase the cycles of concentration and reduce the cooling tower blowdown rate. 

Figure 3.62 Saturated water blowdown rates through tubes. (From Lahey and Moody, 1977. Copyright 1977 by American Nuclear Society, LaGrange Park, IL. Reprinted with permission.)... 

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. 

The blowdown rate, B, in this case, is 5% of the circulating water ... 

Feedwater rate = Steam rate + Blowdown rate... 

Water treatment capacity, membrane stacks, gal. /day Hydraulic and rectifier treatment capacity, gal./day Blowdown rate... 

Mechanical losses include leaks, in-process applications of cooling water, and drift. To maintain a certain concentration factor, the required blowdown rate is... 

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. 

The deaerator balance (Table 16.12) has two unknowns, namely, treated water as makeup and condensate return. Boiler feed water is provided for three boilers, steam generation in convection sections of furnaces H-9001, F6001 and F-5001 as well as two WHB s. Assume blowdown rate of 2% in average, the total amount of BFW can be calculated as BFW rate = (394 + 64 + 54+18 + 63) x 1.02 = 605klb/h. 

BFW use increases by 10.2klb/h in which 0.2 is extra blowdown rate. 

The main steam line flow limiter, a flow restricting venturi built into the RPV MSL nozzle of each of the two main steam lines, limits the coolant blowdown rate from the reactor vessel to a (choke) flow rate equal to or less than 200% of rated steam flow at 7.07 MPa (1025 psig) upstream gauge pressure in the event a main steam-line break occurs anywhere downstream of the nozzle. 

Since the weight fractions x and x f are proportional to the corresponding ppm values, the blowdown rate is... 

Let s assume that we are generating 100,000 Ib/h of 150-psig steam. In the economizer section, the effluent from the deaerator, at 250°F, would be heated to 350°F. As the specific heat of water is 1.0 Btu/[(lb)(°F)], we would need 100 Btu/lb of water. However, to produce 100/100 lb of steam, we might need a 10 percent blowdown rate to control TDS. This means 110,000 Ib/h of boiler feedwater is needed. Therefore, the economizer heat duty would be... 

In. systems where the sulfur is reslurried and sent to a melter, there are no filtercake solution losses, but some blowdown is necessary to purge salts from the system. Since the required blowdown rate depends on the process tendency to form undesirable by-products, mainly thiosulfate, the selection of operating conditions that lower by-product formation will push the optimum iron content of the solution toward higher concentration values and lower circulation rates (Hardison and Ramshaw, 1990). 

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