Duct burner system operation challenge and solution

1 Duct burner General

Duct burner can expand the operating range of HRSGs, It use supplementary firing to increase the heat energy of a gas turbine’s exhaust, making it possible to increase the output of a downstream heat-recovery steam generator (HRSG),

Gas turbine load increase, the more mass flow, decrease the gas temperature, and gas turbine decrease the less mass flow, less heat transfer will occur at the back end of the HRSG, raising gas temperatures, in the meaning time the downstream steam flow decreases to protect the tube so OEMs requests to trip the duct burner.

They are phosphate treatment, caustic treatment, all-volatile treatment, and oxygenated treatment programs.

2  ESEC duct burner

Figure 1 fuel gas valve

Figure 2 main gas valve and A scanner

3 Operation challenge and solution

3.1 Duct burner obsolete mechanical output modules

The ESEC duct burner systems on HRSG1 and HRSG2 have become unreliable, tripping frequently due to a “Critical Input Failure” alarm on the DB management system.  OEM  uncovered the fact that this is a known issue and has resulted in the upgrade from the existing mechanical relay to a solid state component

Replace the existing obsolete mechanical output modules (one on each DB system and one on the aux boiler Burner Management System) with new upgraded solid state output modules as recommended by COEN.  The new module is part #1756-OA16.  Replacement of this relay also requires the installation of a connector (part #1756-TBMH).

Additionally the main flame scanner logic will be modified to improve detection robustness. Improve the reliability of the duct burners and aux boiler burner management systems. This will alleviate the frequent trips of the duct burners and will also improve the reliability of the aux boiler. 

3.2 Flame impingement

Both unit 1 and 2 HRSGs have flame impingement occurring on runners 2 through 5 on the north (left) side wall.  Damage at present has not resulted in plate buckling significant enough to replace liners or loss of liner pins however this could occur if operation continues. 


Outage inspection will ensure that type A nozzles are installed in the outermost position as design intended.  If this is the as-found state the A nozzle will be relocated one position inboard (south or right) and the outermost nozzle plugged.  This will be done on all runners to remove the need to flow balance the runners.


Full load gas valve position is locked at 70% at present – IE, demand on gas valve to burners has not exceeded the 70% valve position.   This mod will close off one runner nozzle out of 63 (on each runner) if completed.  Verbally from Vogt no other actions are required.  Flame length may increase imperceptibly as same heat rate will be occurring over fewer nozzles.  No other changes are required, this is a standard “fix” to deal with this type of issue.  Verbally Vogt has confirmed ignition from A nozzle inboard one location will not be an issue.

It is possible flame impingement may occur on south wall if more flow is directed south – this will be monitored.

2016 update: After the changes implemented in October 2015, fall impingement on the north wall is still an issue. Upon further discussion with COEN/Vogt, they have recommended to blank an additional nozzle per side, moving the A nozzle inward one position on the pilot end. This change will be implemented in the October 2016 outage  Once a successful nozzle arrangement has been reached, the duct burner general arrangement drawing (DB-9094838-601 Sht 2/3) will be revised to indicate the final installation.

Long term operating reliability of DBU casing / Cost reduction for maintenance. Reduction or elimination of flame impingement on side wall liners

Work to be done from single swing stage on north wall.  Inspection first followed by contract Boilermakers changing nozzles.

3.3 Operation range

1DBVFV03711-DMD, 2DBUFV03711-BMD limited value fixed at 70% which is correct at winter time to limit the crossover pipe pressure not reach alarm point, but at summer time when gas turbine put less heat, 1DBVFV03711-DMD, 2DBUFV03711-BMD limited value fixed at 70% which is correct at winter time to limit the crossover pipe pressure not reach alarm point, but at summer time when gas turbine put less heat, 1DBVFV03711-DMD, 2DBUFV03711-BMD limited value fixed at 70% which is correct at winter time to limit the crossover pipe pressure not reach alarm point, but at summer time when gas turbine put less heat to HRSG, Duct burner can provide more gas to keep Steam turbine output at rated value, so it is necessary provide the manual input function for operator manual input the duct burner fuel demand, and locked out at winter time.

The DBU limits its firing rate to 70%.  We would like to have the ability to walk this value up 1% at a time – as we have to go slow and methodically to ensure we don’t reach OPPSD point on LP crossover all-the-while-STILL doing what we need to maximize steam turbine output.  The manual increase may be incorporated by a bias, a multiplier or simply providing the ability to raise that 70% limit to 71%, then 72% etc. etc.  The second part of the request is that we want the 70% default to re-initialize EVERY time the duct burners restart.  This will provide the protection we need to ensure we do not reach OPPSD while still giving us the ability to slowly increase duct burner firing as is prudent. 

can we look at airflow or tube temperature or some other sort of measurement which will possibly allow us to use duct burners at lower loads. frequency events will often drive us down on load and cause the duct burners to trip. A duct burner maximum output may also be worth considering for airflows below those corresponding to the 90% Baseload that is currently utilized. Mike Sterling has reached out to the DBU and the HRSG manufacturers to pose the question. would allow for reliable provision of supplemental energy in the required variety of dispatch ranges to allow us to be able to sell supplementary volumes with duct burner as well as providing increase, The HRGS control philosophy states:  

The Duct Burner should be only operated after the CT has attained at 100% of its unfired base load condition. If, during normal operation, the CT load is 90% of full CT load, the Duct Burner must be run back to 10% of full Duct Burner heat input. If during normal operation the CT load is 80% of full CT load, the Duct Burner must be tripped.

The trip point was changed from 90% to 80% as per the attached email.  Point G-I250_P01 had to be changed from 0.9 to 0.8 on both GTs to allow the ducts to remain firing until 80% load.  The logic already existed to ramp the ducts to minimum stable (16% position) when less than 90% load.

3.4 duct burner flame detector issue

. Duct burner the main flame detector has issue when duct burner running on the cold weather, the duct burner trip then the E runner always need to reset, to fix this problem is to replace new flame detector and adjust the flame detector position

 3.5 duct burner reliability

– If one duct burner has trouble lighting off and the other one is ramped up to help meet dispatch, then the second duct burner must be ramped up manually once lit. If the burner is put in auto, the valve will immediately open to the same position as the other burner. This causes a low gas pressure duct burner trip and adds more time to operational deviations or derates. If the master controller could be split to have separate or slave controllers for each unit, then the duct burner could be allowed to ramp up in auto to match the other unit.

– When issues arise and duct burners have to be operated manually, the master controller can get hung up. Recently there was an issue where one duct burner would not light, and the other duct burner was dispatched off from full fire during troubleshooting. Once both burners were off the master controller was still at 100%, and seemed to be tracking its own signal in a loop. If a duct burner is off then its controller should be at 0% in preparation for the next lightoff. 

Reduces downtime during upsets/ Improves startup reliability on equipment that provides 80 total megawatts for the plant.

3.6 High gas pressure alarm

Add a high pressure alarms to the fuel gas pressure for Duct Burners.  This currently only has a high range alarm- bad quality alarm at 1000 KPa.  1FGSPIT37006 and 2FGSPIT37006As a result of the upstream PV 37006 passing the RV -038 have lifted repeatedly.  This PT currently only has a bad quality alarm at full range of 1000 KPa and by that time the relief valve which has a set pressure of 1035 KPa has most likely already lifted.  If the alarm was set for 600 to 800 KPa it would at least give better awareness should PV 37006 pass in the future once it is repaired in the next outage.

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