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Economizer Steaming..the Basics    V.Ganapathy

One of the problems frequentlyencountered in gas turbine HRSGs is economizer steaming or steam formation in economizers,particularly at low loads or low steam generation levels.This may result in vibration,noise problems,deposit formation inside tubes and consequent fouling and poor performance.In order to solve the problem one should first understand why and how this occurs.
Figure below shows the gas/steam temperature profiles for a gas turbine HRSG at two loads,namely full load and at part loads of the gas turbine,both being unfired conditions.

The evaluations were performed using the  HRSGS software . The HRSG consists of a superheater evaporator and economizer generating steam at 600 psig.For the design case,a pinch and approach point of 15 F was used when the gas turbine operates at 100 % load at the design ambient temperature. Steam generation is 24,600 lb/h and exit gas temperature is 333 F.
Now using the HRSGS program,the performance was evaluated at 40 % load of the gas turbine,when the gas inlet temperature is 712 F,exhaust gas flow remaining nearly the same.As a result,we see the gas/steam temperaure profiles on the right.
The steam generation decreases to 10,650 lb/h due to the lower inlet gas temperature
The steam temperature also is lower due to the low inlet gas temperature
Most importantly,the feed water temperature leaving the economizer is at saturation. In fact,the % steam formed in the economizer is computed by the HRSGS program to be 2.1 %.
The exit gas temperature from the HRSG is also higher, 411 F vs 333 F.
The basic problem can be traced to the ratio of gas to steam flows. Exhaust gas flow from a gas turbine typically remains constant even at low loads(unless we have a multi-speed machine,whose exhaust gas flow decreases at low loads).As a result,the gas side heat transfer coefficient(which affects U,the overall heat transfer coefficient) does not fall off much with GT load.
Due to the low inlet gas temperature to the HRSG(712 F vs 1019 F),the exit gas temperature from the evaporator is slightly lower as well as the duty or energy transferred to superheater and evaporator.Hence lesser steam is generated. Then at the economizer,due to the combination of low steam flow(and hence small water flow) and same gas flow as at full load and the nearly same overall U,the enthalpy absorbed by water is more;thus water leaves the economizer at saturation conditions with steam formation.The duty however is less at the economizer due to the smaller water flow compared to full load.Hence the exit gas temperature leaving the economizer is also higher.Similar situation arises even at full load of the gas turbine operating at low ambient temperatures. Here,the gas flow may be even higher but at a lower inlet gas temperature,resulting in smaller steam generation and consequent economizer steaming.Hence in cold locations,it is desirable to check the HRSG performance in unfired cold ambient conditions and at part loads as well as at low steam pressures.
When the steam generation is increased by auxilliary firing,the problem goes away as the enthalpy of water absorbed in the economizer is lower.The HRSGS program may be used to evaluate these situations.
In conventional boilers or steam generators,steaming is not a concern. The ratio of gas flow to steam remains nearly constant at all boiler loads,as we reduce the air flows when steam demand or load decreases.Hence the economizer duty is such that the enthalpy absorbed at low loads is lower.The figure above shows the boiler vs HRSG water exit temperatures as a function of steam generation. In conventional,fired boilers,if the economizer does not steam at full load,it will never steam at low steady loads. On the other hand,the HRSG has to be checked at low steam generation conditions for steaming.
Methods  of minimizing Steaming Concerns in HRSGs
At low exhaust gas inlet temperature conditions,reduce the gas flow entering the HRSG or the economizer using bypass dampers.Bypassing the gas flow around the economizer(but not the evaporator) reduces the energy losses.This will reduce the economizer duty and thus the enthalpy of water absorbed.
The economizer may be bypassed fully on the water side but this may cause problems when we want to put the economizer on line while it is hot..flashing of water,vibration may occur,unless we are planning to shut down and restart the unit.
If the amount of steaming in the economizer is small(as determined by HRSGS program or similar software),we may design the last leg of economizer coils as having vertical flow in order to ensure that the steam bubbles flow smoothly up.Downward motion of steam bubbles can cause flow stagnation and flow instabilty problems.The economizer may be designed with multipasses to accomplish this.See the author's Waste Heat Boiler Deskbook.
If steaming occurs for a very short duration only,the situation can be handled by increasing the continuous blow down,though it is not recommended for continuous operation as treated water is wasted.
Remember that the steaming problem is associated with low steam flows in the HRSG. Hence if you have auxilliary firing capability,use it to increase the steam flow when steaming occurs.
Hint!Using simulation methods such as the HRSGS program,evaluate the HRSG performance at various modes and understand the HRSG behaviour at different ambient conditions,gas turbine loads and steam pressures.I would recommend all plant engineers be aware of their HRSG characteristics.If the HRSG supplier has not provided this information,you may use the HRSGS program to evaluate these characteristics by yourself.

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