Boiler 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. 

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. 

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. 

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. 

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... 

All these methods will require careful monitoring initially to set up and determine the correct rate of blowdown once the plant is operating. In order to take the necessary sample from the boiler the boiler(s) should be fitted with a sample cooler. To automate the continuous blowdown a conductivity-controlled system may be installed. Here a controller continuously compares the boiler water electrical conductivity with a value set in the controller. Depending on whether this is above or below the set rate, it will automatically adjust the blowdown flow rate. 

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. 

Continuous, surface blowdown arrangements employ a multistage nozzle valve that permits the BW to expand and flash gradually and safely across each successive orifice and chamber with almost no noise. This effect reduces the flow velocity and virtually eliminates the risk of wire drawing. The BD valve is provided with a regulating lever and calibrated dial (or an electric actuator) for either manual or automatic BD rate adjustment. Continuous blowdown arrangements are entirely suitable for incorporation into FSHR systems. They are commonly employed for WT boilers. 

If the rate of blowdown is too high, heat (fuel) and treated water are wasted and the boiler water chemistry may be less than desirable for optimum waterside control. 

The use of FW that has been inadequately pretreated makes the mechanical operation of a boiler and the control of boiler water conditions innecessarily difficult. Additional quantities of internal treatment chemicals and higher rates of blowdown (BD) are usually required, which reduces boiler plant efficiency and raises the cost of generating steam. 

A chemical inhibitor must be added to the water in a boiler to avoid corrosion and scale. The inhibitor concentration must be maintained between 4 and 30 ppm. The boiler system always contains 100,000 kg of water and the blowdown (purge) rate is 15,000 kg/hr. The makeup water contains no inhibitor. An initial 2.8 kg of inhibitor is added to the water and thereafter 2.1 kg is added periodically. What is the maximum time interval until the first addition of inhibitor ... 

On the other hand, 30 percent condensate recovery as a percentage of the steam generation rate is rather bad and represents sloppy operations and bad condensate recovery design practices. Very approximately, 10 percent of the cost of steam generation could be saved by recovering the condensate. The condensate contains few silicates, thus no blowdown (explained below) is needed, no chemical treatment or deaeration is required, and no energy that would otherwise be used to preheat the boiler feedwater will be wasted. 

Certain disadvantages are implicit in the use of sodium sulfite. One is that it can decompose to form SOjOr HjSin high-pressure, steam-generating equipment, thus appreciably increasing corrosion rates in the steam-fed water cycle. It is believed that limiting concentration to 10 ppm, sulfite decomposition occurs in 61 bars (900 psi) boilers. Another disadvantage is increased total dissolved solids in the boiler water, which requires more blowdown. 

---