This guide explains the main maintenance factors for gas turbines used in mobile and bridge power service, and how to count them. The exact limits belong to the manufacturer’s documentation for each model; this page explains the logic so that planners can ask the right questions.
Fired hours and starts: two clocks
Manufacturers usually define maintenance intervals in equivalent operating hours that combine fired hours with a weight for each start, trip and extra load. Fired hours measure time at temperature, which relates to creep and oxidation in hot section parts. Starts measure thermal cycles, which relate to fatigue and cracking.
A turbine that runs steadily accumulates hours with few cycles. One that cycles daily accumulates starts and may reach its inspection point sooner in calendar time even with low hours.
Why starts are expensive
During a start, parts heat quickly from ambient to operating temperature, and the thin parts heat faster than the thick ones. The difference creates stress, and repeated cycles cause cracks, particularly in combustors, nozzles and blades.
Aeroderivative turbines, which are the usual basis for mobile units, are designed for faster starts and more cycling than heavy industrial frames, but cycles still count. Always check how the manufacturer counts them.
Trips weigh more than normal stops
A trip is an unplanned shutdown from load, often due to a protection event. It subjects the machine to fast temperature changes and, in some cases, to rotor and casing stresses. Many manufacturers count each trip as several normal starts for maintenance purposes.
Reducing trips is therefore one of the best ways to extend intervals. Check the causes of every trip: fuel pressure loss, grid faults, vibration alarms or control issues, and fix the root cause instead of resetting.
Load and ambient conditions
Running at peak or overfire ratings increases metal temperature and shortens life, while part-load operation can reduce wear but may cost efficiency and emissions performance. Hot ambient conditions push the machine to work harder for the same output, and dusty or salty air accelerates fouling and corrosion in the compressor.
Mobile units move between sites with different conditions. Keep a record of where each unit has run and under what ambient and load profile, since the same hours at two sites can mean different wear.
Fuel quality and gas supply stability
Gas composition, heating value variation and contaminants affect combustion stability and deposits. Liquid carry-over in gas supply is dangerous for combustors and fuel nozzles. Dual-fuel operation brings its own maintenance factors for liquid fuel components.
Our note on mobile gas turbine fuel quality in the field explains the checks to make before connecting a new supply.
Counting and recording
Use the control system to log fired hours, starts, trips, load hours at each band and the date of each event. Export the data regularly, and store it with the unit’s maintenance record. If the control system resets or is replaced, preserve the history.
A simple monthly summary of hours, starts and trips per unit makes trends visible. It also lets the planner see when a unit is approaching an inspection.
Planning inspections around the cycle
Typical inspections include borescope inspection of the hot section, combustor inspection and hot section or gas generator exchange. For many aeroderivatives, the exchange of the gas generator module is faster than on-site repair, which is why the module swap approach is common.
Read our guide to a mobile gas turbine engine change-out for how it works in the field.
Balancing availability and cycling for bridge power
Bridge power for an industrial site or data center may require frequent starts at first and then long steady running. Plan the operating profile with the owner: units that carry base load should run continuously, while peaking units cover swings and accumulate starts.
Rotating the duty between units evens out wear, but it also means each unit needs the same preparation and spares.
A simple duty log that pays for itself
Create a one-page record for each unit that lists every site, the dates, the hours and starts at each site, the fuel type, the ambient range, the trips with their causes and the maintenance performed. Update it monthly. When the unit comes up for inspection, the log shows what the machine has really been through.
The log also helps in commercial discussions, since it gives a factual basis for anything that depends on how the machine was used.
Questions to ask a supplier before accepting a unit
Ask how the manufacturer counts starts and trips, how many equivalent hours remain before the next major inspection, what the unit’s recorded history shows and which components were replaced at the last inspection.
A supplier with a good answer to each can usually supply the documents that prove it.
Contracts and spares
Service agreements can be based on fired hours, equivalent hours or a fixed fee per period. The best fit depends on the expected cycle. Clarify who bears the cost of extra starts and trips, and keep critical spares such as igniters, fuel nozzles and sensors on site.
Review the contract if the operating profile changes. A unit that moves from base load to cycling duty may need a different plan.
Who writes this
Duty cycle planning for trailer-mounted turbines is a regular subject for the engineers who support bridge power. Why trust this? It comes from the USP&E turbine team, and USP&E signs EPC and O&M contracts itself. Figures that matter: 25,000 MW contracted since 2002; 150+ power projects in 45+ countries; 24+ years in the field. Learn about our operations and maintenance services for gas turbine fleets.
Offices: Colorado Springs (1755 Telstar Dr, Suite 300), Knoxville (800 S Gay St, Suite 700), London (162-168 Regent Street), Johannesburg (22 Kildoon Rd, Bryanston), Cape Town (31 Brickfield Road, Woodstock) and China (No. 1 Ningzhou Road, Wuxi). Regions: USA | AFRICA | UK | UAE | CHINA. ISO 9001 and ISO 45001 certified.
Questions about turbine starts and maintenance
Do starts matter more than running hours?
Both matter. Manufacturers combine them with weights into equivalent hours, and cycling duty can bring inspections forward.
Why are trips counted more heavily than normal stops?
Because of the sudden thermal and mechanical stress during an unplanned shutdown from load.
How can I reduce wear on a mobile unit?
Reduce trips by fixing root causes, keep fuel supply stable, avoid unnecessary starts and track every event.
Does ambient condition affect maintenance intervals?
Yes. Heat, dust and salt air increase stress and fouling, so site conditions should be recorded.
Planning bridge power with turbines?
Send us the expected duty cycle and site conditions, and our engineers will outline the maintenance plan and spares that go with it.
Phone +27 10 822 2324, email info@uspeglobal.com with your site data, or go through the turbine enquiry form. Turbines available now are shown in our gas turbine inventory.
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