Condensate in a superheated steam line 4

[onuigbo]

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?

We deliver steam as if your life depends on it.

 

[norzul]

Theoretically, there should not be condensate in the superheated steam but as protection measure we do install the steam trap. As example in our current project we have installed some steam traps at strategic location i.e. low point, elbow area where pressure drop is expected.

 

[onuigbo]

As long as there is air, oxygen and other dissolved gases in feedwater, there will be pressure-loss-induced condensate, whether the steam is saturated or superheated. The feedwater will always contain air. It can also contain bicarbonates and carbonate alkalinities, which oxidize to carbon dioxide at elevated temperatures. The oxygen and carbon dioxide together combine to produce acidic carbonization that has a corrosive effect. Deaeration and pretreatment of the feedwater does not remove all the air, some of them are carried over to the steam. This is a malignant fact associated with steam generation.

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.

 

[sailoday28]

onuigbo (Mechanical)states that the line was partly insulated. I think this speaks for itself with regard to the possibility of condensation.
Also, is the insulated portion correctly specified and installed properly?
Regards

 

[jproj]

Besides the "partly insulated" line, I think the important thing to look at would be the flowrate through the line. If you are continually using steam (24 hours a day with velocities of 6,000-10,000 ft/min), I have a very hard time believing condensate would accumulate to the point of plugging the line. On the other hand, if there is minimal or no flow for periods of time, there will be condensation that would accumulate (especially if your traps are closed). Of course, if your lines are too small and / or sloped in the wrong direction, that could be the source of your condensation. In that case, however, the condensate would be from saturated, not superheated steam.

Regards,

jproj

 

[TBP]

Pretty much the only time you should see condensate in a superheated steam line, is on start-up. Any air in the line is heavier than the steam, and gets kicked out by the traps. Once the line is in service, and up to operating conditions, it's hard to make superheated steam condense. If you send superheated steam to a heat exchanger, it will behave as if it is airbound. You have to get past the superheat, which is sensible, to get to the latent heat, which is where all of the heat transfer action is. Is this line missing a HUGE chunk of insulation, combined with a very low steam flow?

There wouldn't be a desuperheater in this system, would there?

 

[toothless]

You will have very, very, very little air in the steam. The deaerators typically take the oxygen out to below 10 ppm. Oxygen scavengers take care of the rest.

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.

 

[25362]

Waterhammer induced by condensate collection in steam lines is dangerous. Suggest you read:

http://www.kirsner.org/pages/condInduceWatHamText.html

 

[Latexman]

If the absence of insulation (and weather barrier) allows the inside diameter of the steam line to cool down to the saturation temperature or below, there will be local condensation. If unprotected from the weather, it will worsen during times of precipitation, wind, non-sunny days and at night.

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

 

[TBP]

How long is this steam line, and what is the diameter? Steam source is from a boiler, or is it turbine extraction/exhaust? Is it supplying a turbine? If it's not feeding a turbine, why is the steam supply superheated?

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?

 

[sailoday28]

onuigbo (Mechanical)I am losing track of your original question. Does your line contain superheated or saturated steam?
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.

 

[zdas04]

Condensate cannot survive in a superheated line. Period. This is not a function of line size or velocity, the existence or absence of steam traps, or the slope of the line. It is a function of the conditions that describe superheated steam--when steam is heated in the absence of a coherent steam/liquid interface the temperature and pressure will rise independent of the latent heat of vaporization. This means that the steam will get very "dry" (i.e., very far from the conditions that would allow condensation).

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

http://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

 

[norzul]

Dear zdas04,

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

 

[sailoday28]

norzul (Mechanical-Presence? The original question was related to "the line was full of water"
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.

 

[TBP]

If the superheated steam line in question was dead-ended for a period of time, with a lot of un-insulated surface, then it would eventually condense - not right away, but perhaps overnight - especially if it's an outside run. But that's sure is a lot of goofy circumstances to come together.

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.

 

[zdas04]

Let me make myself clearer--as long as the steam in the system is superheated then you can't have an accumulation of condensate.

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

http://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

 

[GenB]

I would trap before the steam is sent to the superheater to assure the superheater will get dry steam.
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.

 

[zdas04]

I got an e-mail from the original poster that said

You said there can be no condensate in a superheated steam line PERIOD, then explain why we have tons of water (not during start-up) in a 26 inch 300 psi line delievring 500,000 pounds per hour steam.

The word " PERIOD" means under no circumstance, and that is absolutely incorrect.

My response to him was:

Just because the label on the line says “superheated steam” doesn’t make it so. As long as steam is superheated (i.e., the temperature/pressure combination is well into the “gas” portion of the phase diagram) then liquid water cannot accumulate. PERIOD. If you have a line that started out with some amount of superheat and then cooled it down through ambient losses then you have a SATURATED STEAM line that can easily accumulate liquid.

You don’t say what the temperature of your 300 psi line is, but if it approaches 420°F at any point then you don’t have a superheated line. I haven’t done the calcs on your system, but on a 300 psi superheated steam line I’d probably raise the steam temperature (out of contact with a coherent steam/liquid interface) to something over 600°F. In that case I would never see liquid water in the line.

I just thought this exchange would round out this thread.

David

 

[sterl]

Superheated steam and condensate in thermal equilibrium condition is not possible.

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.

 

[25362]

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.