10 replies to this topic

[Zubair Exclaim]

For steam condensate line sizing i came across two sizng criterias can any one confirm


1- To calculate the mixture density then calculate mixture velocity (two phase velocity ) use a maximum limit of 80~ 85 ft/s and size the line. (This is the criteria i am using )

2-to calculate the separate velocities of liquid phase and vapor phase using hysis or other simulation software and use a liquid flow velocity criteria of 2 ft/S. (this criteria is proposed by my friend and in a one sizing he showed me liquid velocity coming around 3.5 ft/s and vapor velocity coming out 62 ft/s )


Now i am fighting him so we dont use two dfferent criterias for the sam project

[breizh]

Consirer this resource it may help
http://www.spiraxsar...calculators.asp

One more :

http://www.armstrong...te-return-lines


Breizh

Edited by breizh, 10 April 2012 - 01:39 AM.

[Propacket]

Exclamation

It seems from your description that the condensate is two phase. I think the 'Erosional Velocity' criteria as per API RP 14E will be valid here.

Edited by P.Engr, 10 April 2012 - 03:32 AM.

[fallah]

Exclamation,

Maximum limit of 80~ 85 ft/s for a mixture of steam and condensate seems to be too high. As far as i know it could normally be as high as 10-20 m/s.

Fallah

[Technical Bard]

I would propose you use something like Mandhane, Dukler or Beggs-Brill to determine both flow regime and pressure drop in the line. You want to stay away from the churn/slug flow regimes as this can cause serious piping vibrations, but you also need to stay below the erosional velocity (API RP14E is a good reference). Your first criteria seems too fast for two phase flow, but your friend's value of 2 ft/s liquid superficial seems too low (likely to be in slug flow).

Also remember to check the flow regime at both ends of the line.

[Zubair Exclaim]

thanks, all of you

i could drop the 83 ft/s although i found it in one of the practices,,, but the real question is the approach

• IS the approach of using the combined velocity of two phase a criteria

OR

• Are we supposed to use the single phase velocity of the two phase to decide the velocity

[S.AHMAD]

Exclamation
1. Below is my qualitative comments
2. Method 1 may leads to a wrong pipe size. It is ok for wet steam (the domain fluid is steam) but may lead to undersized for liquid domain system. We may use sliding scale single velocity that is depending on the vapor and liquid load. For example, for 100% liquid let say 3 m/s and for 100% vapor 20 m/s. Any mixture the velocity limit shall be in between these two limits (e.g. weighted average).
3. Method 2 sounds more reliable. If you are using sliding scale in method 1, I believe the result will be the same as method 2.
4. I am not very sure how the method 2 is, but my understanding is that use separate velocity limit for liquid and vapor. Calculate the cross-sectional area from the volumetric flowrate for each phase. Combine the area and determine the pipe diameter.

Edited by S.AHMAD, 12 April 2012 - 03:01 AM.

[kkala]

Following notes may be clarifying for some issues.
α. Rule of 2 ft/s must refer to the hypothetical condition of no flashing. We apply "size the line for 1-2 ft/s velocity, as if all condensate were liquid" (rationale not found). See http://www.cheresurc...id-line-sizing/ '> http://www.cheresurc...id-line-sizing/ , post No 9, attached "LS.xls". E.g. for 3200 kg/h of 200 οC condensate (3.7 m3/h), a 2" Sch40 line would result in 1.56 ft/s, which is acceptable. Next step would be to check whether condensate pressure (14.5 barg) can cover ΔP of the line.
β. Also look at http://www.cheresour...ate-line-sizing '> http://www.cheresour...ate-line-sizing , mainly post No 9.

[Zubair Exclaim]

Following notes may be clarifying for some issues.
α. Rule of 2 ft/s must refer to the hypothetical condition of no flashing. We apply "size the line for 1-2 ft/s velocity, as if all condensate were liquid" (rationale not found). See http://www.cheresur...d-line-sizing/ , post No 9, attached "LS.xls". E.g. for 3200 kg/h of 200 οC condensate (3.7 m3/h), a 2" Sch40 line would result in 1.56 ft/s, which is acceptable. Next step would be to check whether condensate pressure (14.5 barg) can cover ΔP of the line.
β. Also look at http://www.cheresou...te-line-sizing , mainly post No 9.

refering to my own post 9 i have the criteria of velocity substantiated:

1-one standard says 83 ft/s (assuming more steam less liquid phase)
2-othe say 73ft/s (assuming two phase)
3-one even says 65 ft/s (assuming two phase)
4-another one says 2 ft/s ( assuming all stabilized condensate no steam)


the problem is not with the velocity criteria, i have it and can be adopted as per flash conditions.

the clarity sought is the method.

A- to use combine mixed phase velocity (which i support)
B-to use vlocity of a single phase (liquid or vapor in a two phase mixture.)

see attached hysys file for reference it is the result of two phase pipe sizing utility

I dont kno whw it comes aup with this separate velocities for both phases though..........however the question is wether to use separate velocities or combined velocity as criteria

Attached Files

•  HYSYS-Print.pdf   256.33KB   154 downloads

Edited by Exclamation, 12 April 2012 - 09:11 AM.

[kkala]

1. We apply criterion No 4, post No 9 of present thread, as explained in post No 8, para α (not identical to criterion 2 of post No 1).
2. Saturated condensate line sizing seems to concern line sizing of corresponding flash steam, neglecting liquid. See http://www.spiraxsa...turn-lines.asp - sizing condensate lines from traps.
3. However method mentioned in para 1 seems simpler.
Note: I cannot tell whether actual velocity criteria (para 2) are "better". Ratio of true velocity in gas / liquid phase depends on the ratio of their specific volumes, but also on "void fraction" and "weight fraction of gas flowing", not quite clear to me (W Rohsenow, Handbook of Heat Transfer, MacGraw-Hill, 1973, Section 14, Two phase flow). Liquid at 2.4 ft/s and vapor 58 ft/s might indicate stratified or wave flow, needing to try a smaller pipe diameter. Having touched condensate lines, I felt them very quiet, so flow may approach mist flow (not speaking about short lines discharging to atmosphere). Sarco's web reference also gives this impression, assuming homogeneous flow model. Opinion would be welcomed on this specific matter.

Edited by kkala, 12 April 2012 - 02:30 PM.

[josh68]

We are facing  failure of condensate return line down bends  how can we explain this . the design flow is about 101000 Kg/hr tempt at 140 degC and pressure 4 Kg/cm2 and lie size is 10'' sch 20