Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ASME CRTD-34-1994 — Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers
🔥 ASME CRTD-34-1994 is a consensus operating-practices document prepared by the Feedwater Quality Task Group for the Industrial Subcommittee of the ASME Research and Technology Committee on Water and Steam in Thermal Power Systems. It presents a full set of recommended feedwater and boiler water chemistry control values for modern industrial, high-duty, primary-fuel-fired boilers (oil, gas, or coal). An expansion and revision of the 1979 original, the document is aimed at minimizing the penalties of severe corrosion or deposition, frequent cleaning requirements, and unscheduled outages in steam generator systems and their auxiliary steam users. It remains one of the authoritative references for industrial boiler water-chemistry operation.
💡 This document is the origin of the ASME “consensus” series on water chemistry; the later consensus documents for combined-cycle chemistry control and for sampling and monitoring build directly on it.
1. Scope and Boiler Classes 📋
The document establishes chemistry control limits for six classes of boilers (Tables 1 through 6):
- Industrial watertube: high duty, primary-fuel-fired, drum type, with superheaters and turbine drives and/or process restrictions on steam purity (excluding HRSGs installed in gas turbine exhaust systems);
- Industrial watertube: without superheaters and/or process restrictions on steam purity;
- Industrial firetube: high duty, primary-fuel-fired;
- Industrial coil-type watertube: high-duty, primary-fuel-fired rapid steam generators;
- Marine propulsion watertube: oil-fired, drum type;
- Electrode type: high-voltage, recirculating jet type.
The chemistry values apply to steam generators in continuous or relatively steady-state operation, including current-design boilers with locally high heat fluxes up to 1.5 × 105 Btu/hr·ft² (473.2 kW/m²), potentially uncertain circulation, relatively small steam drums, and relatively small furnaces. Special operating conditions — startup, shutdown, rapidly fluctuating loads, or initial operation of new boilers — may impose greater water chemistry restrictions. The document explicitly excludes mobile locomotive boilers, boilers of copper or other unusual materials, immersion-type and low-voltage electrode boilers, heating boilers of special construction, waste heat boilers of unusual design, firetube boilers with superheaters, hot water boilers, and oil field steam flood boilers.
⚠️ The document does not recommend specific makeup-water pretreatment, condensate treatment, or internal chemical treatment programs — but the requirement for such treatment is clearly implied by the suggested feedwater quality values. If the feedwater targets cannot be met, pretreatment and dosing systems must be provided.
2. Objectives of Water Treatment 🎯
Proper treatment of makeup and feedwater is necessary to prevent scale, other deposits, and corrosion in the preboiler, boiler, steam, and condensate systems, and to provide the required steam purity. The document emphasizes that where a choice is available, the reduction or removal of objectionable constituents by pretreatment external to the boiler is always preferable to, and more reliable than, managing those constituents inside the boiler by internal chemical treatment.
3. Water Chemistry Parameters and the Six Limit Tables 📊
The consensus chemistry controls for the six classes are presented in Tables 1 through 6, categorized by operating pressure range. Pressure is the prime factor because it dictates the internal water-chemistry regime, the normal cycles of feedwater concentration, the silica volatility, and the carryover tendency. The density difference between steam and water decreases with increasing pressure and temperature, making phase separation in the drum harder; the tendency to carry over is greater at higher pressure, so boiler-water concentrations must be kept lower to meet the same steam purity target.
| Table | Boiler Class | Basis of Categorization | Key Parameters |
|---|---|---|---|
| Table 1 | Industrial watertube with superheaters, turbines | Pressure range | Steam purity, SiO₂, phosphate, pH, conductivity |
| Table 2 | Industrial watertube without superheaters/turbines | Pressure range | Hardness, alkalinity, pH, TDS, suspended solids |
| Table 3 | Industrial firetube | Pressure range | Alkalinity, TDS, sludge control, phosphate |
| Table 4 | Industrial coil-type watertube | Pressure range | Hardness, SiO₂, dissolved oxygen, cycles |
| Table 5 | Marine propulsion watertube | Pressure range | Congruent phosphate-pH control, steam purity |
| Table 6 | Electrode, forced circulation, jet | Pressure range | Conductivity, pH, dissolved solids |
The text notes that industrial boilers using high-purity demineralized or evaporated makeup water should be operated with a minimum of 1% blowdown (100 cycles of feedwater concentration) to avoid excessive concentration of trace contaminants and possible deposit formation. Congruent phosphate-pH control, coordinated phosphate-pH control, volatile treatment (AVT), chelants, polymers, and volatile amines are suggested where these are commonly accepted practice or applicable, with the applicability described in the table notes.
🚨 Never use the limit tables without reading the footnotes. The notes define the pressure ranges, treatment-program assumptions, and exceptions attached to each value; using the numbers in isolation can produce incorrect control decisions.
4. Steam Purity 📈
Steam purity is a major subject of the document. Salt and silica carried over from boiler water contaminate superheaters and turbine blades, causing deposition and corrosion. The document discusses, in Section 5 and in Section 6.2 (iron, copper, hardness, and suspended solids), the question of using higher-purity water than required for the operating pressure — a practice that is not always justified. Control of carryover depends on limiting boiler-water concentrations, ensuring effective steam-water separation, maintaining adequate blowdown, and, where needed, using internal measures such as steam washing.
5. Chemical Control Analyses 🧪
The document prescribes recommended chemical control analyses covering boiler-water phosphate, pH, alkalinity, conductivity, hardness, dissolved oxygen, silica, iron, copper, suspended solids, and TDS, with guidance on sampling and testing. Analysis frequency should be matched to boiler load, feedwater quality fluctuations, and historical operating data. Because special conditions (startup, shutdown, fluctuating loads) may demand tighter chemistry, monitoring should be intensified accordingly.
6. Engineering Practice Essentials 🛠️
- 📊 Pretreatment first: external treatment — softening, demineralization, evaporation — is more reliable than internal chemical treatment; install it before relying on in-boiler programs.
- 📉 Read the right table: use the limit table matching the boiler class and pressure range; for older designs, limits for a lower pressure range are often sufficient.
- 💧 Blowdown management: maintain at least 1% blowdown on high-purity makeup units; combine continuous and intermittent blowdown to control boiler-water concentration.
- 📈 Trend your data: plot analytical results over time to detect corrosion and deposition risks before they become outages.
- 🧴 Match the program: phosphate, AVT, chelant, or polymer programs must match boiler type, pressure, and makeup purity; consult your water-treatment supplier.
7. Frequently Asked Questions ❓
❓ Q: Why are the limit tables categorized by operating pressure range?
A: Pressure determines the internal water-chemistry regime, the cycles of feedwater concentration, silica volatility, and carryover tendency. At higher pressure, steam-water separation is harder and carryover greater, so boiler-water concentrations must be lower to maintain the same steam purity.
A: Pressure determines the internal water-chemistry regime, the cycles of feedwater concentration, silica volatility, and carryover tendency. At higher pressure, steam-water separation is harder and carryover greater, so boiler-water concentrations must be lower to maintain the same steam purity.
❓ Q: Why do high-purity makeup units still need at least 1% blowdown?
A: Even demineralized water contains trace contaminants that concentrate in the boiler. At least 1% blowdown (about 100 cycles of concentration) prevents excessive trace-contaminant concentration and deposit formation.
A: Even demineralized water contains trace contaminants that concentrate in the boiler. At least 1% blowdown (about 100 cycles of concentration) prevents excessive trace-contaminant concentration and deposit formation.
❓ Q: Does the document recommend specific treatment chemicals?
A: No specific chemicals are recommended, but commonly accepted programs — congruent or coordinated phosphate-pH control, volatile treatment (AVT), chelants, polymers, and volatile amines — are suggested where applicable, with applicability stated in the table notes.
A: No specific chemicals are recommended, but commonly accepted programs — congruent or coordinated phosphate-pH control, volatile treatment (AVT), chelants, polymers, and volatile amines — are suggested where applicable, with applicability stated in the table notes.
🔍 Q: Can older boilers apply these limits directly?
A: The values target current-design boilers (local heat fluxes up to 1.5 × 10⁵ Btu/hr·ft²). For older units it is often sufficient to use the limits for a lower pressure range, particularly where operating experience has demonstrated success.
A: The values target current-design boilers (local heat fluxes up to 1.5 × 10⁵ Btu/hr·ft²). For older units it is often sufficient to use the limits for a lower pressure range, particularly where operating experience has demonstrated success.
