How does altitude affect gas turbine output?

Gas turbine output decreases approximately 3.5% per 1,000 feet of altitude above sea level. This is because air density decreases with elevation, reducing the mass flow rate through the compressor section. At 5,000 ft elevation, a turbine loses roughly 17% of its ISO-rated power.

What is gas turbine derating and why is it important?

Derating is the reduction in gas turbine output from ISO base-rated conditions (59 deg F, sea level, 60% RH) to actual site conditions. Key derating factors include altitude (3.5%/1000 ft), ambient temperature (0.7% per deg F above 59 deg F), humidity, and inlet/exhaust pressure losses. Proper derating ensures the selected turbine can deliver required power at worst-case site conditions.

What is a typical gas turbine heat rate?

Simple-cycle gas turbine heat rates range from about 7,500 to 9,500 Btu/HP-hr depending on size class. Small turbines ( 20,000 HP) around 7,500 Btu/HP-hr (34% efficiency). Combined-cycle configurations can achieve overall efficiencies above 50%.

What is the difference between ISO 2314 and API 616 for gas turbines?

ISO 2314 defines gas turbine performance testing and acceptance conditions, establishing the ISO base rating at 59 deg F (15 deg C), sea level, 60% RH. API 616 covers gas turbines specifically for petroleum, chemical, and gas industry services, addressing mechanical design, materials, controls, and acceptance testing requirements for refinery and pipeline applications.

Gas Turbine Sizing Calculator | ISO 2314, API 616 Derating & Heat Rate

Application & Power

Application Type

Required Shaft Power

Driven Equipment Efficiency (gear losses)

Power Factor (generator applications)

Site Conditions

Site Altitude

Site Ambient Temperature

°F

ISO base rating: 59°F (15°C). Use site design max temperature.

Relative Humidity

Inlet & Exhaust Losses

Inlet Pressure Drop (filter, duct, silencer)

in.WC

Exhaust Pressure Drop (HRSG, duct, stack)

in.WC

Fuel Information

Fuel Type

Fuel Higher Heating Value (HHV)

Btu/scf

Typical pipeline quality: 1,000-1,050 Btu/scf

Fuel Price

$/MCF

Operating Parameters

Annual Operating Hours

hr/yr

8,760 = 24/7 operation. Typical: 8,000 hr/yr (91% availability)

Understanding Gas Turbine Sizing

Why Derate?
Gas turbines are ISO-rated at 59°F, sea level, 60% RH with zero inlet/exhaust losses. Real site conditions always reduce available power. Hot day + high altitude can derate output by 30% or more.

Key Derating Factors:
Altitude: ~3.5% per 1,000 ft
Temperature: ~0.7% per °F above 59°F
Inlet losses: ~0.5% per in.WC
Exhaust losses: ~0.15% per in.WC

Midstream Applications:
Pipeline compressor stations (most common), gas plant power generation, gas lift compression, refrigeration compressor drives, produced water injection pumps, and cogeneration with waste heat recovery.

Learn the Theory

Understand Brayton cycle thermodynamics, turbine types, derating methods, and gas turbine selection for midstream service

Read Engineering Guide →

Results

Required ISO-Rated Power

Site-Rated Power -

Total Derating Factor -

Heat Rate -

Thermal Efficiency -

Fuel Consumption -

Annual Fuel Consumption -

Annual Fuel Cost -

Exhaust Temperature -

Exhaust Flow Rate -

Waste Heat Available (WHRU) -

CO2 Emissions -

Formula

P_ISO = P_site / (f_alt × f_temp × f_hum × f_loss)

P_ISO = Required ISO-rated turbine power

P_site = Required power at site conditions

f_alt = 1 - 0.035 × (altitude / 1000)

f_temp = 1 - 0.007 × max(0, T - 59)

f_hum = Humidity correction factor

f_loss = Inlet/exhaust pressure loss correction

Standards & References

ISO 2314
Gas Turbines — Acceptance Tests
API 616 (5th Ed)
Gas Turbines for Refinery Services
ISO 3977
Gas Turbines — Procurement
GPSA Engineering Data Book
Section 15: Prime Movers
API 614
Lubrication, Shaft-Sealing, and Oil-Control Systems

Engineering Notes

ISO base conditions: 59°F (15°C), sea level, 60% RH, zero inlet/exhaust losses
Hot day design: Always size for site maximum temperature, not average
Inlet filtration: Self-cleaning filters typical 3-4 in.WC; static filters 2-3 in.WC
WHRU: Waste heat recovery can improve overall plant efficiency to 70-80%
Emissions: Natural gas CO2 factor ~117 lb/MMBtu (EPA AP-42)
Maintenance: Major overhaul typically every 25,000-50,000 operating hours

Quick Reference — Typical Sizes

Solar Centaur: ~4,700 HP, HR ~9,200 Btu/HP-hr
Solar Taurus 60: ~7,700 HP, HR ~8,600 Btu/HP-hr
Solar Mars 100: ~15,700 HP, HR ~8,300 Btu/HP-hr
Solar Titan 130: ~19,600 HP, HR ~8,100 Btu/HP-hr
GE LM2500: ~33,000 HP, HR ~7,600 Btu/HP-hr

Gas Turbine Sizing Calculator

Application & Power

Site Conditions

Inlet & Exhaust Losses

Fuel Information

Operating Parameters

Understanding Gas Turbine Sizing

Learn the Theory

Results

Formula

Standards & References

Engineering Notes

Quick Reference — Typical Sizes

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