Max BFW (Boiler Feed Water)Temp. at 800 psi

[newengr]

I am trying to heat the BFW at 800 psi, what is the maximum temperature BFW (water) can be at (liquid state, without any vapor)? What method was used to calculate Max temp. of water?

 

[TD2K]

Get a copy of the steam tables. 800 psi water starts to boil at about 520F.

 

[poetix99]

The maximum PRACTICAL temperature of the BFW should be less than the boiling temperature (known as the saturation temperature) of the water at a given pressure.

This "subcooling" of the liquid state will ensure against cavitation as the flow generates local low pressure spots at elbows, flange pairs, or other irregularities in the piping.

My guess is to design for at least 10 - 20 Fahrenheit degrees below saturation of the liquid, but some piping/process/boiler people should weigh in on this one.

 

[davefitz]

Eventually the heated water will be fed to a high speed boiler feed pump. Typically that pump will have a minimum permitted NPSH of about 60 ft liquid , that is, the pressure of the fluid at the pump's inlet nozzle must be at least 60 ft of liquid ( about 23 psi)above the pressure at which the fluid will boil . The pressure must be corrected for the effects of pressure drop thru the piping to the nozzle ,and the nozzle velocity head and any add'l turbulence due to upstream elbows, etc.

The best way to avoid insufficent NPSH is to ensure the liquid is subcooled below the boiling temperature, and a subcooling of 20 F is equivalent to allowing 130 psi pressure drop from the heat exchanger to the pump inlet nozzle.

The bigger issue is whether there are system upsets which will cause the pressure to drop below the 800 psi design presure ( ie, load changes or filter pluggage). If so, it is essential that the fluid heating system respond quicker than fast, because it only takes 30 seconds of inadequate NPSH to wreck a pump.

 

[poetix99]

It is worth noting in this discussion that it is typical of multi-heater turbine-generator installations for the "top" feedwater heaters to be after the feedwater pump and (of course) before the boiler.

No doubt this is related to the potential problems of pump burn-out due to cavitation.

"newengr" should note that this discussion illustrates the differences between (theoretical) optimal first-order design and the very important (seemingly secondary) considerations of safety, reliability, operating margin for transients, life-cycle costs,...

 

[newengr]

How do you quantify the savings for raising the BFW temp? For example, if BFW is now 300F, compared to 255F going to all boilers, how do you calculate the heat benefit?
(savings in $)? Also, any savings in raising pressure in deaerators from 3 psi to 15 psi? Shouldn't use as much oxygen scavenger? What is benefit, how do you calculate the savings ($)?

 

[davefitz]

assuming the plant heater cycle has high pressure FW heaters downstream of the BFP, the add'l heat supplied by the new heater tends to reduce the amount of steam extracted from the steam turbine to heat the next HP heater fed by the BFP. This steam can instead be used to produce electric power. If that extraction steam had a enthalpy of h* and a incremental steam flow of w*, the extra electic power to be produced might be estimated as

Delta power = w*(h*-ho), whre ho is the used energy endpoint of steam leaving the turbine to the condenser

 

[Sanjay1964]

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