Dear sir

We have machine GE frame9FA during running an alarm came "FUEL GAS SYS LOWER DUE TO WOBBE INDEX" AND UNIT START TO RUNBACK DUE TO THIS ALARM during runback fuel gas control valve D5 start to open and alarm vibration came and reach to shutdown value in the turbine bearing no.2 i need to know this is normal to open D5 as soon as the alarm " Fuel sys lower due to wobbe index"

Thanks,
Mohamed salem

 

Hi Mohamed,

Origin of the alarm you mentioned is in most cases too low gas temperature. Burning the combustible gas in such conditions can lead to excessive combustion dynamics. In that case the run back command is issued by code (what you mentioned) that reduces the turbine load very quickly (typical rate is 1.32%/min so it would take apr 3 min to go down from Base Load to zero MW). If the run back originated in the low gas temperature (what I assume), run back would be stopped for TTRF1 lower than 1750F (K26LFTG constant, check constants on your site) what transferred to MW would mean that turbine would be un-loaded till app 35MW. During the turbine un-loading sequencing, D5 valve opens at the instance that turbine operating mode goes out of Premix i.e. at apr. 120...110MW (check settings on your site).

Conclusion: opening of the D5 valve is part of the run back process.

Mentioned by you high vibrations might be the consequence of the rapidness of the turbine un-loading. Burning the gas with low temperature (what I assume was the origine of the run-back) do produces high combustion dynamics what could also contribute to higher vibrations. Do you record high vibrations also at normal (i.e. slow) turbine shut down?

Hope this helps.

Regards

 

Dear sir

thanks for your reply put the stringer things is that after make trend and analyze what is the happen? i found the D5 start to cut in service and PM4 and PM1 start to close at approximately 217MW this change in combustion mode i think its make a high shutdown vibration alarm beside the wobbe index alarm for unit i need to ask again this change in combustion mode is normal at this load if no what is the causes for the change in mode at this load

Thanks
Yours Mohamed Salem

 

Dear Sir,

Please confirm with code on your site: when wobbie index alarm is present you loose permissive to run on Premix (L3FXMS) and at the same time runback begins (turbine un loading).

As result D5 (G1) will open (L3G1 =1)and at the same time G2/G3 will begin closing as consequence of turbine de-loading and second reason as result of D5 opening (all three valves share the total requested gas flow).

Thanks and Regards

 

Dear sir

thanks for your replay after i checked the logic i found that the value of wobbe index control constant between the ranges 43.8 n/d and 48.44 n/d.

if the value of the wobbe index more than 48.44n/d or less than 43.8 n/d the alarm is coming and affect on premix loading permissive
L3FGW_PERM = 0 and going to reduce load L70L (Runback). but it is not clear if this runback give command to D5 to cut in or not please if u have any answer please reply

thanks
Mohamed salem

 

> but it is not clear if this runback give command to D5 to
> cut in or not please if u have any answer please reply

Hi,

You did not provide details so I assume that this is 9FA turbine with DLN 2+ system that has three gas valves, not DLN 2.6 with four gas valves.

Please compare below description with code on your site.

You write that as consequence of wobbie index alarm, gas valve D5 has opened so I assume that before the issue began, turbine was operating on Premix: G2 and G3 were opened and G1 (D5) was closed and the gas line for this valve was under purge.

Valve D5 did not start opening as consequence of run back but as consequence of transfer to Piloted Premix being the consequence of the lost permissive to run on Premix. Wobbie index out of limits initiated two processes: transfer from Premix to Piloted Premix and turbine unloading (runback).

With typical configuration, when wobbie index is out of permissible limits (L3FGW_PERM becomes 0) it is lost request to operate on Premix (L3FXMS becomes 0 and L3FXHS becomes 1). As consequence purge process on G1 line is stopped: L20PG1ON is disenergized and when both purge valves (PG1 and PG2) are closed, confirmed by their limit switches (L33PG1C and L33PG2C both become 1) prefill of G1 (D5) will begin and finally L3FXHC becomes 1 so L83FXH becomes 1, and all three valves will take position requested by Piloted Premix mode: D5 will open and G2 and G3 will remain opened but to different positions than before the event.

At the same time it should appear alarm: ‘High CRT for Piloted Premix’ because TTRF1 at this instant is above the thresold for transfer out of Premix.

When wobbie index is out of limits, the run back process starts so all three valves continuously will be driven down by reduced request for gas flow.

Regards