Pressure Drop (Condensate)

[C26M13]

Hi,

I would like to have some ideas about pressure drop calculation for gas (condensate). Does the elevation (the different elevation is 18 m) and the sudden enlargement and contraction affect the pressure losses along the pipe? if it does, how to determine the total pressure drop?
Can somebody please give me advise?

Thanks,

 

[Latexman]

My advise is to buy CRANE Technical Paper No. 410

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[C26M13]

Thank you Latexman for the information

 

[georgeverghese]

Also note that heat transfer effects can affect pressure drop. If the condensate gas cools down to below dewpoint at the operating pressure, then 2phase flow may affect the pressure drop if the liquid fraction in the gas is significant. For 2 phase flow, the hydraulics behaviour is much more complicated than that for single phase gas. For pressure drop in such situations, you may have to resort to a simulation program. The Crane paper helps with single phase flow pressure drop, but not for 2 phase flow.

 

[C26M13]

@georgeverghese: it's a water steam. Since we can assume it a single phase gas, how can I consider the elevation and sudden enlargement and contraction on the system?

Thanks

 

[georgeverghese]

You've got to have information on a few things to run this pressure drop calc, and am not sure yet if you can consider this to be single phase flow :

a) What is the total straight length of this line ?

b) What is the equivalent length of all fittings and elbows in this line other than this sudden enlargement and contraction ?

c) Is this steam flowing up or down this line?

d) What is the operating press and temp of this line ?

e) What is the min ambient temp ( outside of this pipe) that you wish to consider ?

f) Any insulation on this pipe ?

g) Line dia , flowrate in kg/hr

Run a heat loss calc first and see how much steam will condense. Then check the 2 phase flow maps to see which regime you are in. This may sound a little over to the top for you; if so, ask some one more experienced to help.

 

[C26M13]

@georgeverghese: Thanks a lot for your explanation. Really helpful for me. I am a new engineer, so I really appreciate you can share your ideas and experience.

Regards,

 

[BigInch]

If you keep pressure higher than the vapour pressure of the condensate, you will have single phase liquid flow. If the pressure at any point drops below vapour pressure, gas will boil off and flow will become two phase.

Pressure differences in single phase flow which are caused by elevation changes are calculated by density of the fluid X change in elevation, using consistent units.

 

[LittleInch]

Orlando,

please be a bit more precise about the fluid in the pipe, your response above says "it's a water steam. Since we can assume it a single phase gas" assuming a typo here and you meant water stream, how can this be single phase gas?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

If you are running uphill on this sat steam line, a steam trap may be required at the low point on this pipe. Once that is installed, then it should be possible to approximate this as single phase flow.

 

[C26M13]

@BigInch: Thanks for the idea. It means that delta P = rho x g x h, Am I right? if the elevation is 18 m, doses it affect much the pressure drop (gas)?

@LittleInch: Thanks for correct me. I meant, medium pressure steam to be used as a heating medium in NGL Stabilizer reboiler.

@georgeverghese: At the lower point we have a condensate collector (condensate pot)instead of steam trap.

 

[BigInch]

The change of pressure with elevation is directly proportional to the density of the fluid. If close to the density of water, that will be about 0.42 psi/ft of elevation change. Steam isn't so dense and it's change of pressure with elevation will be much less.

Pressure drop due to friction in one phase flow is affected mostly by viscosity, however in two phase flow friction will be affected most by the liquid's viscosity and the percentage of the liquid making up the flow stream. Elevation changes in two phase flow can trap liquid, increase pressure by the weight of the liquid trapped and otherwise severely affect pressure due to accelerations of the liquid as it moves within the steam or gas stream. Two phase flow is not a simple or accurate analysis. There are various two phase flow formulas that attempt to predict the results. Be careful with all. 50% variation from prediction is not uncommon under certain circumstances.

 

[georgeverghese]

Show us

a) the P& I D for this MP steam supply to the NGL stab reboiler and indicate which line segment you want to size ( which also shows the condensate collection pot)
b) the general elevation / piping path taken in this segment and where this collection pot is.

Also provide responses to previous queries

 

[C26M13]

Thank you all for your comments and inputs.

I have built my ownspreadsheet and I have not completed it yet. Based on our discussion, I have provided this drawing so you could have the ideas about it.

 

[georgeverghese]

Believe this is the same system for which you raised an ealier query seeking advice on whether to locate the PSV at the same location as you have shown in this diagram ?

Our collective advice for this earlier thread was to relocate this PSV to be closer to the point of relief so that there wont be any restrictions in the flow path for the gas blowby stream from this LCV / TCV to the RV.

Moreover (a) believe this feedline to the condensate flash drum is also receiving 2 phase feeds from other sources in the plant (b) the condensate collection drum in parallel with the reboiler doesnt help with the concerns raised in this thread re 2 phase flow in the path to the RV.

Where is the design pressure break from MP steam going to the condensate flash drum - is it at the feed nozzle at this flash drum ? - if not, all bets are off on making this RV work at the preferred RV location at the flash drum.

 

[C26M13]

georgeverghese: Yes it is.

The reason why the PSV installed far from the condensate flash drum is cost and it is installed to protect multiple scenarios. in this system, it needs a line to the de-aerator to allow for any vapour formed to be vented. The problem is control valve in line to cond. flash drum fully open. potentially to overpressure the cond. flash drum.

I have got one example in Crane book (p. 4-6 example 4-10), they calculate pressure drop of steam for a 400 feet horizontal pipe. the steps are
a) Re Number
b)L/D for fittings
c)Sum of L for fittings
d)Total equivalent length
e)Delta P

Can I consider the steps above for my case?

 

[georgeverghese]

Okay, so the line from this control valve to the flash drum is fully rated for MP steam. Which is 24.82barg?

Think there is a typo error in your picture - the design press of the condensate flash drum should be 6.89barg and not 1.9barg?

Pls advise all other devices that may be in the steam blowby relief path from this control valve:

a) Is there a wiremesh demister on the vapor exit from the condensate flash drum ?

b) What is in the path of this relief stream as it rises up to the PSV in the de aerator - trays or packing? Or is the PSV actually on the main body of de aerator ?

What about any isolation block valves in this relief path ? Are these all locked open ?

Any piping low points between the steam exit piping out from the condensate flash drum and the entrance into the de aerator ?

 

[C26M13]

georgeverghese: Thanks for your correction. Cond. flash drum (MAWP = 9.9 barg)

a) It has a mist pad on the vapour exit from the condensate flash drum
b)It contains trays and the PSV is located 1m from the vapour outlet nozzle in the vessel

The isolation block valve is in FP (CSO)

The lowest point in this scenario is in between reboiler and Cond. flash drum (EL :100.350M).
In between cond. flash drum and dearator the low point is 104.600m

One question : Where should I take the point 1 (at the lowest point or at the reboiler (condensate outlet)

Regards,

 

[georgeverghese]

1)Am not sure to which query you responded to earlier ? Anyway, pls confirm again that the line from this control valve to the flash drum is fully rated at the design pressure of MP steam ?

2)It really is bad design to locate an RV downstream of a wiremesh demister pack, but you need to assess the risk of this wiremesh unit buckling with a high velocity gas blowby stream from this control valve- there are many incidents in industry where such devices have blocked exit lines and blocked the relief stream. At the least, the pressure drop through this during the relief scenario must be estimated.

3) Re your reponse to (b), is the RV below or above the trays ?

4) There is a low point pocket between the flash drum and the de aerator ? Not clear from your response. Pls check and advise if there is a low point pocket in this piping ? There should be no risk of steam condensing and liquid water collecting at low points in this line which can cause high backpressure.

 

[C26M13]

I am really new into this kind of job, because I'm a new engineer. To be honest I a kind of confusing.

Well, this line is fully rated for MP Steam and there is no low point pocket from the referred line (this line contains two phase flow). The RV is above the trays.

Thank you