After a major overhaul, a more flexible GT

During a scheduled Major Overhaul on an European combined cycle power plant, Ansaldo Energia developed and implemented the “Lead Engine” project in collaboration with the user. 

 Below are excerpts on this from a paper titled, 'Upgrade in gas turbine flexibility: The Lead Engine project’ by Alfonso Palmieri and Davide Palermo presented at the recent Power Gen Europe. 

MEL - Minimum Environment Load

LE - Lead Engine

TIT - Turbine Inlet Temperature

AEN - Ansaldo Energia

The Lead Engine project was a co-operation project between Ansaldo Energia (AEN) and an important European energy utility within a Service long-term Frame Agreement between the two companies. AEN and the Customer jointly agree to implement some hardware and software modifications on a gas turbine located in Spain manufactured by AEN in order to improve the flexibility of that turbine.

GT has been chosen as Customer’s fleet “Lead Engine” (Main GT) taking advantage of a scheduled Major Overhaul that had to be performed at GT 25 kEOH in 2014. The hardware and software modifications implemented on the Lead Engine had been all tested by AEN in other power plants/test rig before being implemented on the GT. Both companies took advantages from this project.

The main advantages for AEN have been:

1. The possibility to introduce innovative tested modifications on a GT and to check the behavior of the machine from Genoa AEN headquarter since the power plant is connected to Genoa by AEN remote monitoring and diagnostic system (ADA);

2. The possibility to show the Client the benefits of this modification in order to potentially implement them on other Customer’s fleet GTs, manufactured either by AEN or by other OEM.

Main advantages for the Customer have been:

1. The possibility to introduce upgrades on one GT of its fleet at a very competitive price;

2. The possibility to improve GT flexibility by turning down its Minimum Environmental Load, increase GT maximum load, load ramps and secondary frequency reserve, increase Hot Gas Path maintenance interval from 25 to 33 kEOH (MEP33).

Lead engine project’s main objectives

All the modifications foreseen for Lead Engine project had the goal to improve GT flexibility. Current European market rules, indeed, require GT and combined cycles power plants in general to be more flexible, by increasing start & stop amount, reducing start time, enlarging available load range and operating with a higher load gradient during uploads/downloads.

The main objectives of the LE project were the following:

1. Turn down capability to MEL below 45% of Base Load;

2. Increase of Base Load power/efficiency;

3. Increase loading gradients from MEL to Base Load from 13 MW/min to 22 MW/min;

4. Increase HGP maintenance interval from 25 to 33 kEOH.

In order to achieve the goals above described, AEN developed some HW & SW modifications described in the following section of this paper.

Hardware and software modifications

One of the hardware modifications regarded the upgrade of the Secondary Air System (SAS) in order to recover air from cooling system of Annular Combustion Chamber and turbine blades & vanes. Air recovered from cooling air system participates to combustion.

The secondary air saving goal is to use air recovered from cooling air systems to improve GT combustion stability and combustion regulation, improving therefore GT behavior at high loads and pollutant emissions at Minimum Environmental Load, to increase load range.

The project foresaw the modification of annular combustion chamber outer shell and hub in order to reduce the amount of cooling air for combustion chamber. A spare hub has been used, while outer shell assembled originally on Lead Engine GT was sent to AEN workshop in order to perform the modifications.

Turbine vanes were modified too.

Regulation logics of 2nd and 3rd stage turbine vanes cooling valves were implemented on GT regulator to optimize cooling air flow to these stages.

Original burners installed on GT (diffusive pilot burners) were replaced with new design flexible burners. The new “hybrid” burners are equipped with two pilot lines: one is premixed pilot line, so called VeLoNOx (AEN trade mark which means Very Low NOx), while the second line is diffusive pilot line.

GT regulator logics were modified in order to allow the use of both pilot lines one by one or also together. Specific logics have been developed and simulated in AEN in order to guarantee 100% of flexibility to new combustion configuration. A change-over from one line to the other is possible and bidirectional at all loads and also when GT is at full-speed-no-load (FSNL) condition.

Due to a further agreement between AEN and the Customer, it had also been possible to test a third configuration of combustion system. This configuration, tested on an operating GT for the very first time, gives the possibility to inject fuel gas from another point of the burner, through a minor retrofit on fuel gas piping. The modifications are completely removable, if necessary. The aim of this modification is to improve combustion stability within all load range. The configuration was tested during a week and gives the Client the possibility to operate GT in 3 different configuration depending on plant operation mode and needs.

Other minor hardware modifications have been applied, as installation of AEN designed Helmholtz resonators on the 24 diagonal swirler (in order to improve combustion stability, especially during load ramps and at high load) and turbine blades 4th stage “tip timing” (in order to measure clearances between rotor and stator parts in that zone).

To improve GT flexibility AEN designed SW modifications have been implemented in GT regulator, as the so called “Tuning 4 Seasons”, which allows combustion regulation parameters to be diversified all-over the year without on-site tuning interventions and “Wide ramp management”, which allows high flexible management of GT regulation parameters especially during wide load variations (e.g. a ramp from MEL to hot base load) and at high load.

To improve GT protection from combustion instability ACC acceleration phenomena and to avoid the necessity to perform an ACC inspection, new improved thresholds on ACC acceleration have been introduced, during upload, download and base load. A threshold has been introduced also on acceleration moving average, to prevent acceleration deterioration in case of incipient combustion instability.

In order to make GT regulator able to receive and manage all the logics modifications, AEN and the GT regulator provider decided to upgrade GT regulator CPUs, by installing the last model of CPU available on the market. The new CPUs are able to run faster than the previous.

Additional instruments for GT monitoring and protection improvement have been installed on the engine and connected both to AEN remote monitoring and diagnostic system (ADA), directly connected to Genoa via MPLS network and to control system (DCS). Some instruments installed and connected have been: new annular combustion chamber ∆P transducer, new annular combustion chamber accelerometer, Helmholtz resonators thermocouples and dynamic pressure transducers, ACC tile holders thermocouples, tip clearance/tip timing on turbine blades stage 4th, turbine vanes stages 2nd and 3^rd thermocouples, high precision flowmeters on fuel gas pilot lines.

Lead Engine project engineering involved a team of about 4 service engineers for hardware modifications, software modifications and simulation, as-built scheme production and on-site cold and hot commissioning for about 1 year. During this period a team of about 5 R&D engineers worked on the modifications and additional instrumentation.

On site Major Overhaul activities took around 75 days of mechanical, I&C and cold commissioning activities with standstill GT and 15 days of hot commissioning activities and tests. Around 70 high specialized mechanical, I&C, engineering manpower worked together to achieve the expected results.