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ASME B133.6-1978 — Procurement Standard for Gas Turbine Ratings and Performance
📊 ASME B133.6-1978 is the standard in the American B133 family of gas turbine procurement standards that specifically governs ratings and performance. The purpose of the B133 standards is to provide criteria for the preparation of gas turbine procurement specifications and to facilitate response to such specifications. This Standard applies to open-cycle, closed-cycle, and semi-closed-cycle gas turbines with conventional combustion systems for industrial, marine, and electric power applications. Gas turbines applied to earth-moving machines, agricultural and industrial-type tractors, automobiles, trucks, buses, and aeropropulsion units are excluded. For gas turbines using unconventional or special heat sources (such as chemical processes, nuclear reactors, or furnaces for supercharged boilers), the Standard may be used as a basis with appropriate modifications.
💡 The core value of B133.6 is that it establishes a completely defined ratings framework, so comparisons among alternative gas turbines can reflect significant differences in power and efficiency rather than being obscured by inconsistent definition practices.
1. Overview and Scope 📌
Gas turbine ratings and performance are developed by the manufacturer to match site conditions supplied by the user. When complete site conditions are unavailable, the standard conditions defined herein may be used. The ratings framework covers performance under both standard conditions and site conditions and is compatible with ISO 2314, Gas Turbines — Acceptance Tests, and ISO 3977, Gas Turbines — Procurement.
The body of the Standard has four sections: Section 1 (Scope), Section 2 (Ratings), Section 3 (Standard Reference Conditions), and Section 4 (Definitions), plus two appendices — Appendix A provides sample performance curves and Appendix B sets out provisions for measurement of performance. This structure lets buyers and manufacturers communicate around a common terminology and a common set of reference bases.
⚠️ B133.6 defines how ratings are specified and declared; it does not prescribe specific power values that any turbine model must achieve. Each manufacturer declares performance for its own design. The key to evaluating bids is to compare declared values on the same basis, under the same qualifying conditions.
2. Components of a Rating and Influencing Factors 🔧
Per para. 2.1, the rating of a gas turbine includes the output power, the thermal efficiency, and the specified control conditions under which that power is produced. Only when power, efficiency, and their qualifying conditions are given together is a rating complete and comparable.
The main physical factors influencing ratings are:
- Absolute ambient pressure: at a given turbine inlet temperature, output power is generally proportional to absolute ambient pressure.
- Compressor inlet air temperature (normally the outside dry-bulb temperature): output is highly dependent on this temperature; warmer inlet air reduces mass flow and output.
- Turbine inlet temperature: for a given inlet air temperature, output depends on the turbine inlet temperature, which is set by fuel input.
- Thermal efficiency: affected by the same variables as output power but generally independent of absolute ambient pressure.
| Rating Element | Key Point | Main Influencing Factors | Procurement Significance |
|---|---|---|---|
| Output power | Electrical or shaft power under stated conditions | Ambient pressure, inlet air temperature, turbine inlet temperature | Basis for sizing and capacity matching |
| Thermal efficiency | Ratio of output to fuel heat input | Same drivers as power; nearly independent of ambient pressure | Fuel cost and operating economics |
| Control conditions | Speed, load limits, and control settings for the rating | Control system logic and settings | Ensures the rating is reproducible |
| Qualifying conditions | Ambient and operating conditions tied to each rating | Site temperature, pressure, duct losses, fuel | Prevents misleading bare-number comparisons |
💡 Practical rule: ratings must be declared in pairs. The manufacturer must establish at least two sets of operating conditions — a site rating and a standard rating — each fully qualified by the associated operating or reference conditions. Never request just “one power number” in the inquiry.
3. Standard Ratings and Operating Modes 📋
The manufacturer shall declare standard ratings under the standard reference conditions of Section 3, for the following operational modes, stating the type, frequency, and degree of inspection and/or maintenance required:
| Operating Mode | Hours per Annum | Hours per Start | Typical Duty |
|---|---|---|---|
| Standard Mode A (Peak) | 1,250 hours/year | 5 hours/start | Grid peaking, peak load, standby power |
| Standard Mode B (Base) | 8,000 hours/year | 800 hours/start | Continuous generation, cogeneration, industrial drive |
This time-frequency-life combination is very practical: the same machine may be rated differently for peaking versus base-load duty because frequent starts accelerate thermal fatigue and component degradation. By declaring ratings for both modes together with maintenance requirements, the buyer can select the appropriate declared value for the planned duty.
4. Site Ratings: Generation and Mechanical Drive 🏭
Site power ratings must reference the specified conditions of the installation — ambient pressure and temperature, duct pressure losses, and the operating modes and fuels under which the plant is intended to run in service. Output is defined differently by application:
- Generating plant: the electrical power at the generator terminals. In addition, the manufacturer shall specify auxiliary loads and losses needed to produce that power; auxiliaries driven directly by the gas turbine independent of the generator are not part of the site power rating.
- Mechanical drives: the shaft power at the gas turbine output coupling(s), again with auxiliary loads and losses specified; auxiliaries supplied directly by the gas turbine are excluded from the site power rating.
- Separate gas generators: site power is expressed as gas power — the power from a reversible adiabatic expansion of the gas-generator exhaust (using total pressure and total temperature) to ambient atmospheric pressure under the specified site conditions.
⚠️ Deterioration allowance: if a specific allowance is to be included in the performance ratings for deterioration of rated power and thermal efficiency (e.g., due to fuel and/or air contaminants at a particular site), it must be stated and agreed between user and manufacturer, with reference to ANSI B133.10. Ratings not so qualified, as well as standard performance ratings, apply to the new and clean condition of the gas turbine components.
5. Engineering Practice Essentials 🛠️
- 📋 Inquiry preparation: require the manufacturer to declare output, thermal efficiency, and qualifying conditions for both Standard Mode A and Mode B, and to furnish performance curves in the format of Appendix A.
- 📊 Ambient correction: when comparing bids, correct all manufacturer data to the same standard reference conditions first, then examine the effect of site temperature swings on output.
- 🔧 Acceptance testing: conduct performance acceptance tests in accordance with ISO 2314, using the instrumentation, probe placement, calibration, and data processing provisions of Appendix B so field results remain comparable to declared values.
- 📈 Deterioration tracking: after commissioning, establish a performance baseline per the agreed inspection/maintenance plan; periodically re-measure to quantify degradation trends and schedule overhaul windows.
- 🔄 Coordination with related standards: use B133.7M for fuel-quality effects and B133.2/B133.3 for general procurement requirements as applicable.
6. Frequently Asked Questions ❓
❓ Q: Why must the same turbine have two declared ratings?
A: Because duty modes differ in life consumption and maintenance. Peaking (1,250 h/yr, 5 h/start) and base-load (8,000 h/yr, 800 h/start) yield different declared power and efficiency values; declaring both lets the buyer size the machine for the actual operating plan.
A: Because duty modes differ in life consumption and maintenance. Peaking (1,250 h/yr, 5 h/start) and base-load (8,000 h/yr, 800 h/start) yield different declared power and efficiency values; declaring both lets the buyer size the machine for the actual operating plan.
❓ Q: How does ambient temperature affect rated power?
A: At a given turbine inlet temperature, output is proportional to absolute ambient pressure and strongly dependent on compressor inlet air temperature. Higher inlet temperature lowers air density and flow, reducing output — hence inlet-air temperature correction, and sometimes inlet cooling, are essential in hot climates.
A: At a given turbine inlet temperature, output is proportional to absolute ambient pressure and strongly dependent on compressor inlet air temperature. Higher inlet temperature lowers air density and flow, reducing output — hence inlet-air temperature correction, and sometimes inlet cooling, are essential in hot climates.
❓ Q: Does thermal efficiency depend on absolute ambient pressure?
A: The Standard states that efficiency is generally independent of absolute ambient pressure, although it is affected by the same variables as output. When relocating a unit between highland and coastal sites, correct power for pressure but treat efficiency corrections as secondary.
A: The Standard states that efficiency is generally independent of absolute ambient pressure, although it is affected by the same variables as output. When relocating a unit between highland and coastal sites, correct power for pressure but treat efficiency corrections as secondary.
🔍 Q: How does B133.6 relate to ISO 2314 and ISO 3977?
A: B133.6 establishes standard reference conditions compatible with ISO 2314 (acceptance tests) and ISO 3977 (procurement), so domestic U.S. procurement and international trade use consistent performance declaration practices.
A: B133.6 establishes standard reference conditions compatible with ISO 2314 (acceptance tests) and ISO 3977 (procurement), so domestic U.S. procurement and international trade use consistent performance declaration practices.
