Condensate in a superheated steam line
Condensate in a superheated steam line
(OP)
Can we have condensate in a superheated steam line?. I run a 300 psi superheated steam line that was sufficiently trapped and partly insulated. One of my service guys shut all the traps and the next day the whole line was full of water. Then somebody told my operator that the water was not as a result of shutting the traps, that there can be no condensate in a superheated steam line. What do you guys say?
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RE: Condensate in a superheated steam line
RE: Condensate in a superheated steam line
In gas mixtures or multiphase flows, each gas assumes a part of the total volume or pressure. This is referred to as the law of partial pressures. The partial pressure of each constituent gas depends upon its proportion of the total pressure of the mixture. This is true regardless of whether the gas is ideal or Van der Waal.
Consider a 100 psi line consisting of 80% quality steam and say 20% air, the total steam pressure would be 80 psi and the total air pressure 20 psi. Consequently, the line would have 80 psi steam in a 100 psi line at a temperature of 312 degrees instead of 327 degrees at 100 psi, a difference of 15 degrees between the two pressures. In addition, there will a change in BTU’s. Though the 80 pounds steam has more enthalpy of latent heat per pound than the 100 pounds steam, it has less by volume. In other words the air has displaced a portion of the enthalpy needed, by displacing a portion of the steam. An air film 0.04” thick has the same resistance to heat transfer as water 1” thick, or iron 4.3” thick or copper 43“ thick. As a film it acts as an insulator, in solution with steam it chokes the steam of its heating potential, moisturizes the steam and corroding pipe and equipment. The effect is more pronounced in superheated steam line for one additional reason, air flowing at a velocity of 10,000 fpm will induce pressure drop in the line (velocity and pressure are inversely proportional, one increases at the expense of the other), as a consequence of the decreased pressure the system will seek an equilibrium temperature to sustain the pressure – temperature relationship within the superheat regime, causing a drop in temperature, hence loss in latent heat.
Condensate will accumulate in a superheated steam line if (1) it is not properly trapped (2) if drip legs are not properly located at reasonable intervals and (3) if the pipeline is not properly pitched. Since it will be easier for the condensate to flow with the steam, the line should be sloped down the direction of steam flow. If the flow of condensate was against the flow of steam we would have the following scenario: steam moving with a velocity of say 6000 to 10000 fpm, the condensate would tend to eddy into pools trying to overcome the friction of the steam on its flow to the low point upstream (in other words the steam – water surface shear force will tend to overcome the force of gravity causing the water to flow downhill so that water builds up and plugs the pipe). As condensate builds up in this manner the velocity of the steam will cause it to be picked up and introduced into the steam as a mist, essentially humidifying the steam and lowering its heat content.
We deliver steam as if your life depends on it.
RE: Condensate in a superheated steam line
Also, is the insulated portion correctly specified and installed properly?
Regards
RE: Condensate in a superheated steam line
Regards,
jproj
RE: Condensate in a superheated steam line
There wouldn't be a desuperheater in this system, would there?
RE: Condensate in a superheated steam line
The uninsulated portion of your line along with the flow rate vs. line size is the key. The steam will cool the entire length of the line. Lack of insulation greatly increases the cooling. If the line is oversized, now you have a large heat transfer area and a slow flow rate allowing plenty of time to cool to saturation.
RE: Condensate in a superheated steam line
Waterhammer induced by condensate collection in steam lines is dangerous. Suggest you read:
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RE: Condensate in a superheated steam line
Now, if the area that is "partly insulated" is not too large, there may be enough superheat to re-vaporize any condensate that formed. If not, that is another reason why dry leg steam traps are put on steam headers, besides start-up.
Good luck,
Latexman
RE: Condensate in a superheated steam line
Reverse slope on steam lines is indeed not the preferred lay-out on systems like this, but it's easy enough to do. Size the line so the velocity stays down, and install drip legs at shorter intervals than you would with a forward slope.
Why did the service guy valve the traps out - were they blowing through? What kind of traps are installed? They're not bucket traps, are they? (Bucket traps & superheated steam don't go together.) People aren't mistaking superheated steam (which will be invisible) for air if they crack open drain or vent valves, are they?
RE: Condensate in a superheated steam line
You give an example---"Consider a 100 psi line consisting of 80% quality steam and say 20% air, the total steam pressure would be 80 psi and the total air pressure 20 psi. Consequently, the line would have 80 psi steam in a 100 psi line at a temperature of 312 degrees instead of 327 degrees at 100 psi, a difference of 15 degrees between the two pressures."
I hope you don't consdier the steam in this example to be superheated.
RE: Condensate in a superheated steam line
Occasionally an uninsullated portion of the line will cool the no-flow boundary down to below the saturation temperature and cause local condensation, but if the line is super-heated, the bulk flow temperature and pressure must re-vaporize the drops prior to accumulation. Condensation can only collect in a system that is at saturation conditions.
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
RE: Condensate in a superheated steam line
If your statement "Condensate cannot survive in a superheated line. Period." is correct then how to explain the presence of condensate in the superheated line as initially questioned by onuigbo?
Thanks
norzul
RE: Condensate in a superheated steam line
I don't believe we have a clear picture of the "superheated" line.
Could it be that the source was slightly superheated nd that via an approximate throttling process, high quality steam results? This coupled with poor insulation, etc?
More information should be supplied.
RE: Condensate in a superheated steam line
There are more questions than answers here. Under any kind of "normal" conditions, I agree that you cannot have condensate in a superheated steam line.
onuigbo - I'm sure you are very capable in many areas that I'm not, but I can tell from the way you're coming at this problem and wording your posts that you're no "steam guy". You really need to find one to look at your system before something or someone gets waterhammered to death.
RE: Condensate in a superheated steam line
If you do have an accumulation of condensate then you don't have superheated steam.
The steam in the line the OP is talking about may have started out as superheated, but it is no longer superheated if he has "a line full of condensate". Superheated steam cannot exist in contact with a coherent steam/liquid interface. Ever. That is why superheaters are located in piping removed from the boiler and you can't get superheated steam in one step (i.e., you have to start with saturated steam, move it to a vessel with another source of heat and no steam/liquid interface and then superheat it).
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
RE: Condensate in a superheated steam line
In your case there my be a leg full of water just before the trap, you shutoff the trap and the steam will condensate.
Also it seems that you are shutting off the superheated steam
at some point because if the superheated steam is flowing, it will evapporate any water condensed within the line,
always must be a relief/steam drain/bleeder valve when the steam is stopped from the mover.
RE: Condensate in a superheated steam line
My response to him was:
I just thought this exchange would round out this thread.
David
RE: Condensate in a superheated steam line
Pipes are not in equilibrium by nature. Flow requires differential of pressure.
In a pipe, superheated steam can run right over the top of subcooled condensate and come out the other end still superheated.
Its a heat transfer process which requires differences to drive it.
If there are differences, there is no equilibrium. Thats the reason to keep track of entropy.
Nuff already.
RE: Condensate in a superheated steam line
It takes time to reach equilibrium in direct gas-liquid heat and mass transfer. In general, it has been confirmed that when a gas is cooling down while a liquid is evaporating into the gas, namely heat flows opposite to mass (as in humidification or steam desuperheating) the heat transfer coefficient is 30-80% of what would be expected from heat transfer alone.
RE: Condensate in a superheated steam line
RE: Condensate in a superheated steam line