Saturated Steam becoming superheated steam 1

[nitpicker]

Many opinions as to superheated steam.
Question please?
Scenario-100 PSI saturated steam from boiler. Boiler fed steam thru a regulator to a 6000 gal accumulator (horizontal and half full of water) at a 27 PSI pressure in the accumulator.
Steam supplied from 27 PSI accumulator to a vessel with controlled pressures ranging from 1.5 PSI to 8 PSI. Steam is control vented so as to allow a volume of steam to pass thru the vessel while sustaining these back pressures.

?Is the steam from the 27 PSI accumulator, as it passes thru the regulator to the vessel at reduced pressures superheated at these low pressures until pressure is stabilized in the vessel? Does steam temperature at 27 PSI reduce from 269.78 deg F gradually to vessel pressure temperature of 216.94 at 1.5 PSI. If so is this superheated steam at the point of entry of the vessel?

 

[TD2K]

I haven't worked with acummulators so I'll use your basis that the steam leaving the accumulator is saturated, that is, the 338F 100 psig saturated steam from your boiler has enough residence time in the accumulator to saturate with the liquid.

The 100 psig steam from the boiler that is let down to the 27 psig accumulator pressure will definitely be superheated when it enters the accumulator, hence my query above.

The 27 psig steam from the accumulator, assumed to be saturated, will leave at about 270 deg F. There will be some temperature drop due to the expansion of the steam through the valve to 1.5 psig but this should be minor (you can estimate this assuming the valve is an isenthalphic expansion). The steam will start to cool off from this initial temperature, approximately 270F, due to heat losses from the piping as it travels to the vessel. If the heat losses from the piping are minor, then the steam will enter your vessel at 1.5 psig at close to 270F. If the heat losses are high, you could lose all the superheat (and condense some of the steam) causing it to enter the drum at about 216F.

 

[TD2K]

I did some quick checking and the steam leaving the accumulator should be 'close' to saturation if not at saturation.

The water in the accumulator will be maintained at saturation since steam will condense into the water if it's less than saturation.

The steam leaving the boiler saturated at 100 psig will be about 338F. When you let this steam down across a control valve to 27 psig, it will cool off to about 307F just due to the expansion. This isn't much above the saturation temperature of 270F at 27 psig so with some heat losses from the system, I can easily see the steam leaving the accumulator either saturated or with minor amounts of superheat.

 

[25362]

The full analysis, in the end, would depend on the degree of steam-water contact or heat diffusion from the steam to water in every vessel, water temperatures, steam flow rates and heat losses. I think, in a qualitative manner, TD2K is absolutely right.

 

[chicopee]

You do not tell us about steam quality leaving the boiler as enthalpy of steam leaving boiler will be dependent on steam quality. However assuming steam quality is 95% enthalpy at that point will be assumed to remain the same when steam reaches accumulator and at that lower pressure of 27 psig, steam is still saturated with a slightly higher quality. When that steam mixes with steam in the accumulator which is now the control volume, steam quality will significantly drops perhaps to the 50% quality level( I estimated and did not do calculations). When that steam leaves the accumulator at that 50% quality level with a corresponding lower enthalpy level and travels toward the pressure reducing valve and assuming delta h =0 thru reducing valve,may still be around the 50% level.
The above reasoning is based on a sketch and of a check of the steam table, however I did do the calculation of steam quality in the accumulator and at inlet/outlet of reducing valve as I didnot have the mass flow rates from boiler and to the reducing valve.