Saturation steam flow through pipeline 1

[Mike4chemic]

Dear All,
Do I have to suppose that when the saturated steam flows through a pipeline it expands (due to friction losses, in my case about 2 bars) according to the isenthalpic-adiabatic model similar to the saturated steam expansion through a pressure control valve or an orifice?
Is the saturated steam becomes superheated along the line?

Thanks in advance, Mike

 

[quark]

No.

 

[Mike4chemic]

quark,

Can you please explain why no ?

 

[quark]

Frictional loss is not isenthalpic (nor adiabatic). Energy is lost from the fluid due to friction and the energy gets dissipated out of the control volume. So, you can't model it as isenthalpic expansion.

As for superheat, since I never thought of this and came across first time, I would give it some thought before responding. So far, I assumed saturated conditions.

 

[katmar]

Mike, what generally happens in a steam line is that it would be designed for relatively low pressure drop (because the user wants the pressure at the other end!) but because it is hot a significant amount of heat can be lost through the pipe wall and insulation. In normal practice steam traps have to be installed along the line to remove the condensate generated from the lost heat. In this case the steam remains saturated all along the length of the line.

It is conceivable that if the line was very well insulated but had a high pressure drop then the pressure could drop faster along the line than the corresponding saturation pressure due to heat loss. In this case you could get superheat developing but I would say that this is very much a non-typical situation.

To get to the specific behavior in your application you would have to work out how much heat is transferred through the pipe wall and insulation, and then knowing the heat content of the steam at the downstream end (from this calculation) and the pressure (from the friction pressure drop) you can work out whether you have had some condensation or else generated some superheat.

Katmar Software - Engineering & Risk Analysis Software

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[quark]

...or check the steam temperature at reduced pressure.

 

[Mike4chemic]

Gentlemen,Thanks

It is very interesting issue.

I did an estimation of the pressure drop of the saturation steam through the about 1 km pipeline using the WinSim simulator.

Based on the WinSim that used adiabatic model,steam at the pipe outlet is superheated. Is this make sence?

If J-T effect is applicable for this case?

In which case the isenthalpic-adiabatic model can be used?

 

[katmar]

Of course if you have an adiabatic process and you reduce the pressure of a saturated vapor then it is going to get superheated. But it is not reasonable to assume a 1 km pipeline is adiabatic. If it were, Armstrong and Spirax would have gone out of business long ago. You have to take the heat losses through the insulation into account if you want an accurate estimate of the exit condition of the steam.

In general, it is not a bad estimate to assume the flow of steam in a header or pipeline is incompressible. If you are working with high pressure steam and relatively low pressure drops then the steam density is virtually constant over the length of the pipeline. You can treat it as an adiabatic or even isothermal compressible flow if you want to be a bit more accurate in your calculation of the pressure drop. But you cannot then use that assumption to determine the exact condition of the steam at the exit. These assumptions are OK for determining the density changes because they are so small, but not OK if you want to determine the exact condition of the exit steam.

As the steam flows down the pipeline it will cool because of heat losses. This increases the density. But the pressure decreases because of friction. This decreases the density. Overall you will find only a small change in density unless you have a very non-typical installation.

Katmar Software - Engineering & Risk Analysis Software

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[Mike4chemic]

Katmar,

Thanks for your explanation.


I would like to ask you a last question.

Why a steam expansion through the orifice is differ from the saturated steam expansion through the pipeline from the thermodinamic point of view.

Why the 1st process is isenthalpic and 2nd not.

 

[katmar]

It's simply a question of available heat transfer area. An orifice or valve will have a small heat loss relative to the pressure drop when compared with a steam pipeline. The heat loss is small enough that the valve/orifice case can be regarded as isenthalpic.

Katmar Software - Engineering & Risk Analysis Software

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"