Blowdown Calculations

One should perform an energy balance around the deaerator to estimate the steam for deaeration and then perform the blowdown calculations based on the conductivity (or total dissolved solids) of the boiler water, makeup water, and the feed water.

Figure 6.11 shows a simple deaerator scheme with one condensate return line. Saturated steam for deaeration is taken from the drum. Makeup water is added in the deaerator from which the feed water for the boiler is taken. By doing a mass, energy, and conductivity (or TDS) balance, we can estimate the deaeration steam.

The allowable boiler water TDS (total dissolved solids) is given by both ASME and ABMA, which are shown in Tables 6.9 and 6.10, and based on practice, one may select the appropriate value for boiler water TDS.

Example 6.15

A boiler generates 50,000 kg/h saturated steam at 20 kg/cm 2 g of which 10,000 kg/h

is taken for process and returns as condensate at 82°C. Makeup water is available at

21°C, and steam for deaeration is taken from the drum operating at 20 kg/cm 2 g. The deaerator operates at 1.76 kg/cm 2 a. Blowdown water has a TDS of 1500 ppm, and

makeup has a TDS of 100 ppm. Evaluate the steam for deaeration and the blowdown.

From mass balance around the deaerator,

, 10 000 + D + M = F = , 50 000 + B

Enthalpy of saturated steam in the drum from steam tables = 668.52 kcal/kg.

Enthalpy of makeup water at 21°C = 21.5 kcal/kg and that of condensate = 82.4 kcal/kg Saturation temperature at deaerator = 115°C. Enthalpy of feed water = 115.8 kcal/kg From energy balance around the deaerator,

, 10 000 × . 82 4 +× D 668 . 52 + M × . 21 5 = 115 8 . × F

To process

10,000 D

DA Boiler

Make up

To blow down

Pump

FIGURE 6.11

Blowdown scheme.

336 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

TABLE 6.9

ABMA Boiler Water limits and Associated Steam Purity at Steady-State Full Load Operation— Water Tube Drum-Type Boilers

Suspended Solids TDS Range, b,c Steam Drum Pressure

Range Total

(ppm) (Max (psig)

TDS Range, a Boiler

Alkalinity, b Boiler

Boiler Water

Water (ppm) (Max)

Water (ppm)

(ppm) (Max)

Expected Value)

50 n.a.

25 n.a.

0.05 1400 and above

15 Once-through boilers

n.a.

0.05 Sources: American Boiler Manufacturers, 1982, Boiler water guidelines.

0.05 n.a.

n.a.

Note: n.a., not available. a Actual values within the range reflect the TDS in the feed water. Higher values are for high solids in the feed

water, and lower values for low solids. b Actual values within the range are directly proportional to the actual value of TDS of boiler water. Higher

values are for the high solid, and in the boiler water, lower values for low solids. c These values are exclusive of silica.

d Dictated by boiler water treatment. From solids balance,

100 M = 1500 B

Substituting these and solving, we get M = , 35 625 kg/h B , = , 2 375 kg/h D , = , 6 750 kg/h and F = , 52 375 kg/h

Example 6.16

In a boiler plant if the conductivity of the condensate, makeup, and feed water are 800,

40, and 150 µmhos/cm, respectively, what is the approximate % of condensate returns in the feed water?

From mass balance around deaerator, C + D + M = F (C, D, M, F, B refer to various flows as seen in Figure 6.11). From conductivity balance, 800C + 40M = 150F. We have only two equations and three unknowns. Simplifying, we have

M F C D and =−− 800 C + 40 × ( F − C − D ) = 150 F

760 C − 40 D = 110 F

Since the question is regarding the ratio of C/F, make the practical assumption that D will be very small compared to F, and hence, neglecting the term with D, C = (110/760)F = or about 15% of feed water. (This incidentally is a quiz question!) Since the question is regarding the ratio of C/F, make the practical assumption that D will be very small compared to F, and hence, neglecting the term with D, C = (110/760)F = or about 15% of feed water. (This incidentally is a quiz question!)

TABLE 6.10

oi rC le

Boiler type: Industrial water tube, high duty, primary fuel fired, drum type Makeup water percentage: Up to 100% of teed water

alc u

Conditions: Includes superheater, turbine drives, or process restriction on steam purity

Drum operating pressure , MPa (psig)

Feed water Dissolved oxygen (mg/L O 2 ) measured

<0.007 before oxygen scavenger addition d

≤ 0.010 Total copper (mg/L Cu)

Total iron (mg/L Fe)

n.d. pH range at 25°C

Total bareness (mg/L CaCO 3 )

9.0–9 6 Chemicals for preboiler system protection

Use only volatile alkaline materials

As low as possible, < 0.2 Oily matter (mg/L)

Nonvolatile TOCs (mg/L C) e <1

As low as possible, < 0.2 Boiler water Silica (mg/L SiO 2 )

Total alkalinity (mg/L CaCO 3 )

<350 f <300 f <250 f <200 f <150 f <100 f n.s. f n.s. f

n.d. h n.d. h n.d. h n.d h Specific conductance (µmho/cm) at 25°C

Free hydroxide alkalinity (mg/L CaCO 3 ) g n.s.

≤ 100 without neutralization

<3500 i

<3000 i

2500 i

<2000 i

<1500 i

<1000 i

(Continued)

TABLE 6.10 (Continued)

S ASME Boiler and Feed Water Chemistry a

team G

Source: Adapted from ASME 1979 Consensus. Note: n.d., not detectable; n.s., not specified.

a No values are given for saturated steam purity because steam purity achievable depends upon many variables, including boiler water total alkalinity and specific

er at

conductance as well as design of boiler, steam drum internals, and operating conditions (see footnote i). Because boilers in this category require a relatively high

degree of steam purity, other operating parameters must be set as low as necessary to achieve this high purity for protection of the superheaters and turbines arid/

or to avoid process contamination.

b With local heal fluxes >473.2 kW/m 2 (>150,000 Btu/h ft 2 ), use values for the nod higher pressure range.

Boilers below 6 21 MPa (900 psig) with large furnaces, large steam release space, and internal chelant, polymer, and/or antifoam treatment can sometimes tolerate higher levels of feed water impurities than these in the table and still achieve adequate deposition control and steam purity. Removal of these impurities by external

pretreatment is always a more positive solution. Alternatives must be evaluated as to practicality and economics in each case.

ste H

Values in table assume the existence of a deaerator. e Nonvolatile TOCs are the organic carbon not intentionally added as part of the water treatment regime.

f Maximum total alkalinity consistent with acceptable steam purity. If necessary, it should override conductance as blowdown control parameter. If makeup is

demineralized water at 4.14–6.89 MPa (600–1000 psig), boiler water alkalinity and conductance should be that in table for 6.90–10.34 MPa (1001–1500 psig) range. g Minimum level of OH –

alkalinity in boilers below 6.21 MPa (900 psig) must be individually specified with regard to silica solubility and other components of inter-

:F rs

nal treatment.

h Not detectable in these cases refers to free sodium or potassium hydroxide alkalinity. Some small variable amount of total alkalinity will be present and measurable with the assumed congruent or coordinated phosphate pH control or volatile treatment employed at these high pressure ranges.

or P

Maximum values are often not achievable without exceeding suggested maximum total alkalinity values, especially in boilers below 6.21 MPa (900 psig) with >20%

makeup of water whose total alkalinity is >20% of TDS naturally or after pretreatment with soda lime or sodium cycle ion-exchange softening. Actual permissible

ss a

conductance values to achieve any desired steam purity must be established for each case by careful steam purity measurements. Relationship between conduc-

tance and steam purity is affected by too many variables to allow its reduction to a simple list of tabulated values.

n dP

la n tE n g in

ee rs

Miscellaneous Boiler Calculations 339

Blowdown Calculations