Steam Pressure Reducing Station Superheat "Dissipation Distance"
Steam Pressure Reducing Station Superheat "Dissipation Distance"
(OP)
I know the answer to this question depends on factors such as insulation and mass flow rate, but I am just looking for a practical answer. When steam expands through a PRV from let's say 100 psig to 50 psig, it becomes superheated due to constant enthalpy. My question is, how long does this superheat last? In other words, at what point has the steam cooled down to saturation at the lower pressure? Obviously that depends on pipe insulation, etc., but has anyone ever measured it? I'm just looking for a practical understanding. Is the superheat gone a few feet downstream of the PRV, or could the steam still be superheated a considerable distance away when that steam finally hits a device?





RE: Steam Pressure Reducing Station Superheat "Dissipation Distance"
The rate of heat transfer from an insulated pipe to the atm is very low, which the necessary heat reduction to de-superheat the flowing steam can be very high.
RE: Steam Pressure Reducing Station Superheat "Dissipation Distance"
I have a hospital sterilizer running at 80 psig. The 80psig is stepped down from 140 psig. Distance from PRV to sterilizer is approximately 50'. There is talk of eliminating the 140 psig steam and replacing it with 125 psig. A new PRV station may be required, but ultimately the same 80 psig steam will be produced. The only difference would be in the amount of superheat (if any) still present when the steam reaches the sterilizer. I'm trying to head off any sterilizer issues and have a call into the manufacturer.
If the superheat was long gone before reaching the sterilizer, then I see no difference at all. That's why I asked about the distance to dissipate the heat. I knew there was no solid answer to the question.
RE: Steam Pressure Reducing Station Superheat "Dissipation Distance"
I did a quick check, for 125 to 80 psig superheat is about 6 degrees C but this assumes 100% dry 125 psig steam, which is practically impossible.
Even with a separator upstream of your PRV, you won't have 100% dry steam. See for yourself the effect of 99 or 98% dryness on the isenthalpic effect. At 99.5% dryness, I calculated 2 degrees C of superheat.
Superheat only becomes a problem at large pressure drops and exceptionally dry steam.
You'll have enough exposed flanges, valves etc. even with pipe lagging to lose the superheat.
If you're still concerned, why don't you ask the insulators to make a short section removable eg. 0.5 meter. You would then need to look at guarding
RE: Steam Pressure Reducing Station Superheat "Dissipation Distance"
Since, sterilization cycle is controlled based on temperature, the cycle may end without sterilizing the entire load.
Given your up and downstream pressure conditions, the degree of superheat is 5C, which is allowed (if you don't have local standards, you can refer HTM 2010 and EN 285). Further, 5C is superheat presuming inlet steam is 100% dry and saturated.
I don't know for hospitals, but it is common practice in industry to check DF, NCG and DOS for Pure Steam.
RE: Steam Pressure Reducing Station Superheat "Dissipation Distance"
RE: Steam Pressure Reducing Station Superheat "Dissipation Distance"
RE: Steam Pressure Reducing Station Superheat "Dissipation Distance"
RE: Steam Pressure Reducing Station Superheat "Dissipation Distance"