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EGT 20 Deg C Lower Than Expected
During cogen plant commissioning, EGT during full load is 20 deg C lower than expected, which results in less steam in HRSG simple recovery. Can the EGT be increased will IGV setting?

The cogen plant is running a small GT, which has rated output of 6560kW at site condition. At full load, the expected EGT is 543 deg C, with 24.7kg/s exhaust gas flow, and 13.62% O2 in exhaust gas. This is the design point of HRSG (simple recovery), to produce 16.5 ton/hr saturated steam.

However, during commissioning, actual EGT temperature is 524 deg C. Exhaust gas flow is not measured. O2 is at 15%. Based on HRSG vendor, their boiler cannot generate 16.5 ton/hr due to low EGT.

Based on GT OEM, they are planning to adjust IGV setting in order to increase the EGT. My question is, can the IGV setting increase EGT by 19 deg C? If it does, will air mass flow be decreased as well? Ultimately, can HRSG produce additional 1.7 ton/hr steam?

Thanks.

It's certain we don't know all the details of the situation. So about all that can be said is this: The IGVs can probably be used to increase the EGT, BUT that will likely cause the GT power output to decrease for the same fuel flow-rate.

>It's certain we don't know all the details of the
>situation. So about all that can be said is this: The IGVs
>can probably be used to increase the EGT, BUT that will
>likely cause the GT power output to decrease for the same
>fuel flow-rate.

thanks for the reply. currently the GT is running more efficient than expected. Hence the low EGT temperature. Expected site rated performance is 6230 kWe @ 29.4% efficiency. Measured result is 6260 kWe @ 31.7% efficiency.

we will try to adjust the IGV setting next week. My concern is the mass flow reduction when we introduce more restriction at IGV, temperature might increase, but how much the reduction of mass flow has impact on the exhaust thermal energy.

The different in exhaust thermal power between 543 deg C & 524 deg C is 525.45 kW. In order to generate 1.7ton/hr steam @ 16.6 barg, 910 kW heat is required. Maybe something off with the HRSG design.

>we will try to adjust the IGV setting next week. My concern
>is the mass flow reduction when we introduce more
>restriction at IGV, temperature might increase, but how much
>the reduction of mass flow has impact on the exhaust thermal
>energy.

Reducing the IGV angle will lower the air flow through the gas turbine, and hence the exhaust flow will decrease. However, since the exhaust temperature will increase, the heat input to the HRSG should be about the same. The higher temperature increases the enthalpy of the exhaust gas so the net energy input to the HRSG should be nearly the same. The superheated steam temperature should also increase.

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>during commissioning

Hmmmmm, so it's probably never read the expected temperature?

Are you sure that thermocouple cold junction compensation is enabled and functioning?

Whenever I see consistent low thermocouple temperature readings on the order to 20-35 DegC, I wonder whether the cold junction is enabled and functioning, because a failed or disable CJ produces low readings on the order of 20-35 Deg C (depending on the temperature of the thermocouple terminal blocks.

The CJ compensation is enabled. we have 18 typ-K TCs surrounding the exhaust, which are wired to 5 different TC modules. All reading is consistent with each other. And all are showing ambient temperature during engine stop.