Boiler Blowdown: Calculation, Rates and Control

Boiler blowdown is the controlled removal of concentrated water from a steam boiler, stated as a percent of feedwater, that keeps the dissolved and suspended solids left behind by steaming inside the boiler-water limits for the operating pressure; rates run from below 1 % of feedwater with very pure feedwater to above 20 % with poor feedwater. How is that percent calculated, which limit sets it, and how much of its heat comes back?

Blowdown is where a boiler program meets the drain: the water treatment chemicals fed to a boiler leave with it, and its rate decides how hard every other part of the program works.

This page covers the chloride-ratio calculation with metric and US worked examples, the ASME limits by pressure, continuous and bottom blowdown, conductivity control, heat recovery, treatment chemistry and US discharge rules.

The four key figures of boiler blowdown are listed below.

• Blowdown range: below 1 % to above 20 % of feedwater (Veolia Water Technologies Handbook, chapter 13).
• Cycles: 5 % blowdown at 20 cycles, because percent blowdown = 100 / C.
• Automatic control: within 5 % of set point, about 20 % less blowdown.
• Flash steam: 24.5 % of blowdown from 450 psig (31 bar) flashing at 10 psig.

Every value on this page is checked against primary sources; see our methodology and fact-checking process.

What Is Boiler Blowdown and Why Do Steam Boilers Need It?#

Steam boilers need blowdown because steam leaves the drum almost free of solids, so every non-volatile salt that enters with the feedwater stays behind and concentrates until a share of boiler water is removed. Turbine steam carries only 10 to 30 ppb of total solids at 300 to 1,500 psig (Veolia handbook, chapter 16).

Feedwater equals steam plus blowdown, so blowdown is the only exit for the concentrated solids. What goes wrong at either end of that balance?

Too Little Blowdown: Scale and Carryover; Too Much: Lost Heat and Chemicals#

Too little blowdown lets boiler-water solids and alkalinity rise until the water foams and carries over into the steam, and too much blowdown throws away heated water together with the treatment chemicals dosed to protect the boiler. Foaming rises with alkalinity and dissolved solids.

Carryover takes five forms: spray carryover, priming, foaming, leakage carryover and selective vaporous carryover. Silica is the vaporous case: it passes into steam from 400 psig (28 bar) and is usually not a problem below 900 psig (62 bar). Their causes and remedies are covered under boiler carryover, foaming and priming.

Too much blowdown wastes heat because the water leaves at boiler conditions: 24.5 % of it flashes to steam when a 450 psig boiler blows down to 10 psig. Chemicals leave with the same water; in a coordinated phosphate program every blowdown lowers both phosphate and pH.

Too little blowdown also drives deposits. The chemistry of scale formation in water systems explains why calcium, magnesium and silica precipitate as they concentrate.

How Is Boiler Blowdown Calculated?#

Boiler blowdown is calculated from one mass balance, feedwater equals steam plus blowdown, and one concentration ratio, the chloride in boiler water divided by the chloride in feedwater.

Percent Blowdown and Cycles of Concentration in a Boiler#

Percent blowdown equals blowdown flow divided by feedwater flow times 100, which is the same as feedwater chloride divided by boiler-water chloride times 100 (Veolia Water Technologies Handbook, chapter 13). The three working formulas, calculated from these definitions, are listed below.

• Cycles: C = boiler-water chloride / feedwater chloride = 100 / % blowdown.
• Blowdown: blowdown = steam / (C - 1).
• Feedwater: feedwater = steam x C / (C - 1).

Chloride works as the tracer because it neither precipitates nor volatilizes in the boiler. Percent blowdown is always a share of feedwater, never of steam; a "percent of steam" figure understates the share.

The same ratio is called cycles of concentration in cooling towers, where evaporation replaces steam. The general cycles of concentration calculation and its limits sit on their own page.

Worked Examples: Blowdown, Feedwater and Cycles#

A boiler making 2,000,000 lb/day (907 t/d) of steam at 50 cycles blows down 40,816 lb/day (18.5 t/d), 2 % of a feedwater flow of 2,040,816 lb/day (926 t/d); Veolia's Table 13-4 rounds the same case to 41,000 lb/day. The two worked examples below apply the same three formulas to a large plant boiler and a mid-size process boiler.

Calculated: C = boiler-water Cl / feedwater Cl; blowdown = steam / (C - 1); feedwater = steam + blowdown; 1 kg = 2.2 lb, 1 lb/day = 0.4536 kg/d.

Worked example: a boiler making 10,000 kg/h (22,000 lb/h) of steam, with 5 mg/L chloride in the feedwater and 100 mg/L in the boiler water, runs at 20 cycles and 5 % blowdown: 10,000 / 19 = 526 kg/h (1,158 lb/h) of blowdown. The inputs are illustrative, not typical values.

Halving the cycles roughly doubles the blowdown: at 10 cycles the same boiler blows down 10,000 / 9 = 1,111 kg/h (2,444 lb/h). Which rates are normal?

Typical Boiler Blowdown Rates: From Below 1 % to Above 20 %#

Boiler blowdown runs from below 1 % of feedwater when the feedwater is of very high quality to above 20 % when it is poor, according to chapter 13 of the same handbook.

A German boiler-water engineering summary (EWT) reports three European operating modes. High-TDS operation, with feedwater conductivity above 30 µS/cm, runs about 3 to 10 % blowdown; low-TDS operation, at 30 µS/cm or less, runs 1 to 2 %; and no-TDS operation, with cation conductivity below 0.2 µS/cm, runs 0.5 to 1 % or less. EN 12953-10:2003 is the shell-boiler water standard behind these modes. The table below compares blowdown rates by feedwater situation.

European operating modes as summarized for EN 12953-10:2003 shell boilers; ranges, not limits.

Which limit decides where in that range a boiler runs?

Which Limits Set the Blowdown Rate? Conductivity, Silica and Alkalinity by Pressure#

The blowdown rate is set by whichever boiler-water limit is reached first, and for an industrial drum boiler the ASME consensus limits fall with pressure: specific conductance from below 3,500 µS/cm at 0 to 300 psig (0 to 20.7 bar) to 100 µS/cm or less at 1,501 to 2,000 psig, silica from 150 to 1 mg/L as SiO2. The ASME consensus values reproduced in chapter 13 of the Veolia Water Technologies Handbook give the three boiler-water limits that most often set blowdown.

ASME consensus (CRTD-34, 1994) as reproduced in the Veolia Water Technologies Handbook, chapter 13; high-duty drum boilers with superheater or turbine drive and deaerated feedwater; above 473.2 kW/m² (150,000 Btu/ft²·h) heat flux use the next higher pressure range. Bar values converted from the MPa bands in the source. Guidance, not regulation.

These values are consensus guidance (first published 1979, CRTD-34 of 1994), not legal limits, and apply to high-duty drum boilers with deaerated feedwater. Feedwater oxygen, iron, copper and hardness by pressure are listed under boiler water limits by pressure.

Older ABMA guidance from 1982, reprinted as Table 16-1 of the same handbook, allowed 700 to 3,500 ppm of dissolved solids in boiler water at 0 to 300 psig and 200 to 2,000 ppm at 601 to 750 psig; it is historic guidance.

Silica becomes the controlling limit at high pressure because it vaporizes into steam from 400 psig (28 bar). Its reactive and colloidal forms and their tests are covered under silica in water.

What Are the Types of Boiler Blowdown?#

Boilers use 2 types of blowdown: continuous blowdown, which removes dissolved solids at a controlled steady rate, and bottom blowdown, a series of short manual blows that removes the sludge settled in the mud drum.

Continuous Blowdown for Dissolved Solids#

Continuous blowdown removes concentrated boiler water all the time at a rate set by the conductivity or chloride target, so dissolved solids stay at their set point instead of rising between blows.

Continuous blowdown is the stream that automatic control regulates and that feeds flash-steam and heat recovery. With continuous blowdown in service, bottom blowdown drops to about one short blow per shift.

Bottom Blowdown for Sludge: How Often and How Long#

Bottom blowdown works best as frequent short blows, about one short blow per shift where continuous blowdown is also in service, and waterwall headers are not blown while the boiler is steaming (Veolia Water Technologies Handbook, chapter 13). Three kinds of sludge leave the boiler through bottom blowdown.

• Calcium phosphate (hydroxyapatite, Ca5(OH)(PO4)3), formed as a non-adherent sludge at a boiler-water pH of 11.0 to 12.0.
• Tannin-conditioned fluid sludge, which settles in the mud drum.
• Other precipitated solids conditioned by the internal treatment.

The handbook states the practice, not a duration in seconds or minutes; the plant's operating procedure sets the length of each blow.

How Is Boiler Blowdown Controlled?#

Boiler blowdown is controlled on a measured boiler-water value, usually conductivity, checked against chloride, silica and alkalinity tests, either by an automatic valve or by an operator.

Conductivity Control: Measurement Basis and Set Points#

Conductivity controls boiler blowdown because it follows dissolved solids, and the Veolia Water Technologies Handbook (chapter 13) converts it at 0.55 to 0.90 ppm of dissolved solids per µS/cm in boiler water, after the hydroxide has been neutralized with gallic or boric acid.

Worked conversion: a set point of 3,000 µS/cm corresponds to about 1,650 to 2,700 mg/L of dissolved solids (3,000 x 0.55 to 0.90); dilute condensate converts at 0.5 to 0.6 ppm per µS/cm.

Conductivity is reported in µS/cm at 25 °C (77 °F) under SM 2510 B and ASTM D1125-23. The ASME limits in the table above are stated as unneutralized specific conductance, so the set point and the limit are compared on the same basis. Units, temperature compensation and the TDS estimate are covered under conductivity and TDS in water treatment.

Chloride, Silica and Alkalinity Tests That Check the Blowdown#

Three boiler-water tests check what the conductivity signal controls: chloride proves the cycles and the percent blowdown, silica guards the high-pressure limit, and total alkalinity guards the low-pressure limit. The three checks and their standard methods are listed below.

• Chloride, mg/L as Cl, by SM 4500-Cl- B or ASTM D512-23.
• Silica, mg/L as SiO2, by SM 4500-SiO2 C or ASTM D859-16(2021)e1.
• Total alkalinity, mg/L as CaCO3, against the limits in the ASME table.

Boiler water is tested at least once per shift for phosphate (if used), P-alkalinity or pH, sulfite (if used) and conductivity. Test lists and frequencies are covered under boiler water testing.

Automatic Versus Manual Blowdown Control#

Automatic blowdown control holds boiler-water conductivity within 5 % of its set point and, according to chapter 13 of the Veolia Water Technologies Handbook, saves about 20 % of the blowdown a manually set valve wastes.

Worked saving: in the 10,000 kg/h example, about 20 % of 526 kg/h is about 105 kg/h (232 lb/h) of hot water no longer discharged.

The loop opens a blowdown valve on the conductivity signal. Sensor, timer and valve logic are covered under conductivity control for water treatment controllers.

How Is Heat Recovered from Boiler Blowdown?#

Heat is recovered from boiler blowdown in 2 stages: a share of the hot blowdown flashes to low-pressure steam, and the remaining water passes a heat exchanger that warms the makeup.

Flash Steam Recovery#

Blowdown from a 450 psig (3.10 MPa, 31 bar) boiler flashing at 10 psig (about 0.7 bar) turns 24.5 % of its own mass into steam, according to the flash-steam example in chapter 13 of the Veolia Water Technologies Handbook. The 24.5 % holds for this pair of pressures only.

Worked number: if the 10,000 kg/h example boiler ran at 450 psig, its 526 kg/h of blowdown would give about 129 kg/h (284 lb/h) of flash steam.

Heat Exchangers, Blowdown Valves and Blowdown Tanks#

A blowdown heat exchanger brings the discharged water to within 10 to 20 °F (5 to 10 °C) of the incoming makeup temperature, the terminal difference given in chapter 13 of the same handbook.

The blowdown valve is the actuator of the control loop, and the blowdown tank receives the water that is not recovered; their types, sizing and code requirements belong to equipment references.

How Do Phosphate, Tannin and Antifoam Programs Change Blowdown?#

Internal treatment changes blowdown in 3 ways: phosphate programs turn hardness into a non-adherent calcium phosphate sludge that leaves by bottom blowdown, tannin conditions sludge so that it settles in the mud drum, and antifoam lets the boiler hold higher concentrations with less blowdown. The table lists each treatment chemistry, what it does to the boiler water and what it means for blowdown.

Doses and residual windows are covered on the program pages; none are stated here.

Coordinated phosphate control rests on the reaction Na2HPO4 + NaOH → Na3PO4 + H2O: monosodium phosphate consumes caustic, trisodium phosphate adds it, and blowdown lowers both phosphate and pH (Veolia handbook, chapter 11). The phosphate residual and the blowdown rate are therefore adjusted together. Residual windows and pressure limits are covered under phosphate treatment programs for boilers.

Phosphate and chelant feed is set on feedwater hardness, and oxygen-scavenger feed on dissolved oxygen plus chemical demand (Veolia handbook, chapter 35), so program and blowdown form one balance.

Which Rules Apply to Boiler Blowdown Discharge?#

Three sets of rules reach boiler blowdown in the US: the NPDES permit when it is discharged directly, the pretreatment rules when it goes to a municipal sewer, and the separate limits for cleaning wastes that must not be mixed up with it.

NPDES Permits and Additive Disclosure#

A plant that discharges boiler blowdown directly to a water body does so under its NPDES permit (Clean Water Act section 402, 40 CFR 122), which requires the treatment additives in the discharge to be disclosed on the application and controls them through effluent limits and whole-effluent toxicity testing. Additives such as scale and corrosion inhibitors, biocides and dechlorinators are disclosed.

Phosphorus from a phosphate program counts toward any phosphorus limit in the permit. Disclosure and WET testing are covered under NPDES permits.

National Pretreatment Program Limits for Hot Blowdown to a POTW#

Blowdown sent to a municipal sewer falls under the general and specific prohibitions of 40 CFR 403.5, which include heat that would raise the temperature at the POTW treatment plant above 40 °C (104 °F) unless the approval authority allows it, and local limits that can set an upper pH. The same section prohibits discharges below pH 5.0 unless the plant is designed for them.

Blowdown leaves at boiler temperature and alkaline, so cooling and heat recovery also serve compliance. Local limits and categorical standards are covered under the National Pretreatment Program.

Blowdown Versus Boiler Chemical Cleaning Wastes#

Boiler chemical cleaning wastes are not blowdown: at steam-electric plants they are chemical metal cleaning wastes under 40 CFR 423.11(c), limited to 1.0 mg/L copper and 1.0 mg/L iron under 40 CFR 423.13(e). Both values apply as daily maximum and 30-day average and bind steam-electric power plants only. Solvents and their disposal are covered under boiler chemical cleaning and descaling.

How Does Blowdown Fit into Boiler Water Treatment as a Whole?#

Blowdown is the exit of the boiler water treatment program: makeup quality decides how many cycles a boiler can run, and internal treatment decides what form the solids take when they leave.

The full program, from pretreatment to condensate, is mapped under boiler water treatment by pressure.

Demineralized makeup is what moves a boiler from the 3 to 10 % range toward 1 % or less, as described under boiler feedwater treatment.

Boiler Blowdown Versus Cooling Tower Blowdown#

Boiler blowdown is a percent of feedwater, while cooling tower blowdown is calculated from evaporation as E / (C - 1), so the two formulas are never interchanged.

Rate calculation and control methods for evaporative systems are covered under cooling tower blowdown. In HVAC, hot-water heating boilers at about 93 °C (200 °F) recirculate the same water as a closed loop and are treated as described under hot water heating system treatment.

How often should a boiler be blown down?#

Continuous blowdown runs all the time under conductivity control, and bottom blowdown is done as frequent short blows, about one per shift where continuous blowdown is in service, with waterwall headers never blown while steaming.

What is CBD and IBD in a boiler?#

CBD is continuous blowdown, the steady controlled removal of dissolved solids, and IBD is intermittent blowdown, the short manual bottom blows that remove sludge from the mud drum.

How much water does a steam boiler use?#

A steam boiler uses feedwater equal to its steam rate plus its blowdown, steam x C / (C - 1): at 20 cycles a 10,000 kg/h (22,000 lb/h) boiler needs 10,526 kg/h (23,158 lb/h) of feedwater (worked example). At 50 cycles, the same boiler needs about 10,204 kg/h (22,449 lb/h).

Who Runs Boiler Blowdown and Water Treatment Programs?#

Blowdown set points, controllers and the internal treatment are usually specified together by a boiler water treatment program provider, so a request should state pressure, steam rate, makeup type and current blowdown.

How to compare providers is covered under water treatment companies.