What determines boiler saturation temperature?; Feed pump discharge pressure or others?

[takiyasamsama]

Hi,

I am very new when it comes to boiler system, design and construction but I knew a bit of steam generation.

What interest me is what is the operating pressure of the boiler? Does it the same as discharge pressure of its feed water pump?

For example for heat exchanger, when the inlet pressure of water is at 5 barg then the required steam temperature shall be around 159 degC. Does the same concept applies to boiler? But the example I given just now if the heat exchanger application is DX-evaporator (direct-exchange evaporator).

But when it comes to boiler, now we have the steam-water drum (water-tube boiler) and this makes me wonders if this had anything to do with the water volume instead of feed pump discharge pressure not that I am in the assumption that the volume affects the boiling point.

 

[Compositepro]

The feed water pressure must be greater than the boiler pressure for water to flow into the boiler. The pressure in the boiler is controlled by the firing rate and steam consumption.

 

[davefitz]

boiler saturation temperature is defined by the equation of state, there is only one saturation temperature for each pressure. Therefore the current operating pressure determines the current operating saturation temperature.

The water at the feedpump is "subcooler" whihc meens it is below the saturation temperature. When the cold water is heated in the furnace it boils , and thus achieves saturation temperature.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[MJCronin]

Also keep in mind that boilers (and feedwater valve trains) have significant pressure drop.

The pressure at the main steam outlet of the boiler is typically 30 - 100 psig less than the BFP outlet.

MJCronin
Sr. Process Engineer

 

[takiyasamsama]

Dear gentlemen,

I truly appreciates your inputs and answers.

From all the responds, what I now understand is for example I needed 30 barg steam (boiler outlet) therefore I would need a BFP (boiler feed pump) of at least 40 barg or 50 barg? When it is said that pressure loss in boiler is far much significant that would means the water holding pressure inside the tubes, mud drum and steam drum had to be 30 barg?; then will the water be boil up to it's respective saturation temperature, is this true?

Sorry but I keep imagining of simple water kettle because the kettle had a safety valve, so when it reaches boiling temperature then the water valves raises to release the steam at around 99.6 - 100 degC and I know that pressure inside and outside the kettle is at 1 atm. Unless if the kettle is connected by a pump at discharge pressure of 5 barg then the boiling point would be different, wouldn't it? or is it still 99.6 - 100 degC? Unless the kettle does not have any steam outlet (closed containers) then the boiling point would be different?