Steam condensate line sizing question

[jari001]

Hi everyone,

I was trying to reproduce the result of the 4" condensate line size that users Chemtech2 and 25362 got for the 30 psig back pressure scenario here: thread124-86721

I couldn't find a resource that suggested formulas so I went to the following places:
1)

http://cpinc.com/nicholson/SizingCondensateReturnLines.pdf

2)

https://beta.spiraxsarco.com/learn-about-steam/condensate-recovery/sizing-condensate-return-lines

Using the charts at both places I come to different answers. From Ref 1, following their graphical method, I come up with ~15k lb/hr but from Ref 2 I come up with ~15k kg/hr.

Later I found a website (

https://www.tlv.com/global/US/calculator/pipe-sizing-recovery-line-velocity.html?advanced=on)

that had a formula but they don't reference the derivation (and I haven't been able to figure out the derivation myself).

I used 150 psig/10.3 barg as the steam system pressure and 30 psig/2.0 barg as the condensate system pressure. Could anyone help me figure out where I went wrong?

 

[casflo]

To size and design condensate and lines, it is necessary take into account the piping length, likewise the piping diameter and the critical conditions in the valve and at the end of the piping.
I suggest to visit the Research Gate site to find papers about the flow of saturated wáter.

 

[quark]

You need to check one point. Usually, condensate inlet pressure to trap is considered equal to that of steam, unless there is greater pressure drop across the equipment. Condensate pressure is what you maintain at flash vessel. They seem to be matching for a test case of 100 psig, 0psig and 4.5" pipe size. Check with your unit conversion. Casflo has a good suggestion for you.

 

[jari001]

To clarify, I'm not looking to analyze a situation where a back pressure valve is installed to alter the hydraulics of the condensate line. I am narrowly focusing on the 6th and 7th post where Chemtech2 and 25362 state that a 4" condensate line can handle a condensate flow of 15000 to 16000 lb/hr when the downstream (i.e. condensate system) pressure of the line is 30 psig. So I used the two graphical methods by assuming that the condensate has the driving force of 150 psig (steam system pressure) to 30 psig (condensate system pressure). If the situation I want to analyze is 150 psig condensate draining to a flash tank at 30 psig, then I should be able to reproduce Chemtech2's and 25362's results using Refs 1 and 2 right - or am I missing the obvious? If there is a different way to analyze these situations?

The two references I used are both graphical methods, so after double checking my conversion from psig to barg and my line drawing abilities, I get the same issue where the digit part of the result are the same for both methods but the units are not.

 

[quark]

You are doing it right and no problem with that. There are two major assumptions with both the charts. First is acceptable velocity and second is percentage of flash steam vs condensate. The first one considers 50fps (15m/s) and 20:1. Where as in Spirax chart, it can be 20m/s (65.6 fps) and 'according to proportion of flash steam formed'. With your given conditions, the actual flash percentage of flash steam vs condensate is 50:1


% flash steam is (781-584)*100/(2732-584) = 9.1%

Flash steam to condensate ratio is 0.524*0.091/0.001*.91 = 50. (specific volume x percentage, of both fluids). The calculator seems to be conservative and right one as it includes pressure drop of piping system.

 

[jari001]

@quark
I missed the difference in assumptions, thanks for pointing that out. I will re-do the problem and see if I can set myself straight.