Steam Pressure Reducing Station Superheat "Dissipation Distance"

[BronYrAur]

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?

 

[don1980]

There's no rule-of-thumb answer to this question. Each case is vastly different. Without a de-superheater, superheat will be present until sufficient heat has been lost from the pipe to the environment. In a low-flow case, with a small reduction in pressure and a small pipe, the distance may be relatively short. With a high reduction in pressure, a high flowrate, and a large diameter insulated pipe, the distance can be huge.

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.

 

[BronYrAur]

Here is where I am going with this:

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.

 

[rusty_sim]

Hi

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

 

[quark]

Superheated steam doesn't effect working of sterilizer but rather the very process of sterilization. 1 Kg of steam, when condensed, releases 2200 kJ of heat (which produces the lethat effect to kill living organism) where as 1 kg of steam produces only 2.18 kJ when its temperature drops by 1 deg.C.

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.

 

[BronYrAur]

I couldn't find my own thread because I didn't realize I posted it in the chemical engineering section. I meant to do so in the heat transfer section. Anyway thank you all for your replies. I guess I'm going to now have less superheat which is a good thing particularly for the sterilizers. So I will continue investigating what it will take to drop system pressure from 140 PSI to 125 psi. I'm looking into pipe sizing, steam trap sizing, and prv station sizing. Since everything Downstream at lower pressures won't change, other than the small change to superheat, I see no issues there.

 

[chicopee]

What you have not stated is the range of steam temp. and pressure needed for the sterilizer to operate properly, then work backwards on whether or not the pressure reduction will work.

 

[BronYrAur]

I don't have specific specifications on the sterilizer but I know it receives 80 psi Steam

 

[quark]

I couldn't find my own thread

That is easy. Go to my stuff on top right corner and click on my threads/my replies.