Economiser Water in De-aerator

Deaerator - Wikipedia, the free encyclopedia

I know the principle of de-aeration using pegging steam. I wish to know how does economiser water serves the function of pegging steam in a de-aerator.

Check this out: https://en.wikipedia.org/wiki/Economizer

Just a guess, but perhaps the pressure of the water from the LP economiser is reduced enough when it is introduced into the deaerator that sufficient flash steam is produced to supply the deaeration. It's always best to have a functional description of the process to go with the diagrams to answer questions like yours. Does the steam turbine have any steam extraction?

The HRSG in question here is a two pressure hrsg. The LP economiser water is at a pressure of 17bar while de-aerator operates at 1.2 bar. I couldn't see any pressure reduction in the LP economiser water.
Downstream of the de-aerator is a condensate pre-heater. The feedwater from the de-aerator heats the condensate. The heated condensate is again introduced in the de-aerator. After extraction of heat from the feedwater the water is pumped in the HP and LP circuit (of HRSG) by the HP & LP feed pumps.
There are no turbine extractions. Generally we observe turbine extractions which serve as pegging steam in the de-aerator or in the feedwater heaters (in conventional coal based plants). But in this case their are not turbine extractions.

This is what we do in the HRSG I operate, we have a superheater, two High Pressure boilers, an Intermediate pressure boiler a high temp and a low temp economizers and a low pressure boiler.

the economizers preheat the feed water that comes from the deaerator and goes into the steam drums.

the LP steam is used solely to heat up the water in the deaerator.

the deaerator serves 5 purposes:

1-Water storage,

2-deaeration or removal of noncondensable gases,

3-head pressure to the feed pump,

4-preheater(with the help of Low Pressure steam)

5-chemical injection of oxygen scavengers to the feed water.

So in a much easier explanation we heat up the feed water using LP steam in the deaerator, then we pass that same water through the economizers and heat it up more. Why? Thermal efficiency.

I am not sure unless I see the P&ID , but some times there is no separate Deaerator in the circuit as it is in conventional system. Here Deaeration takes place in the Condenser.

Your second point - HP and LP superheated steam is directly sent to the turbine is correct - check the turbine should be double injection turbine a single module.

Just check- May be which you are referring as De-aerator could be referred as FWT- Feed water tank and heated by circulating water through the lowest heating section of the HRSG ? some times this is called as Condensate pre-heater that maintains the feed water temperature of economizer inlet.

Pegging steam you are referring to is generally used in start ups to provide sufficient NPSH for feed pumps.