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Pressure Drop (Condensate)

Pressure Drop (Condensate)

Pressure Drop (Condensate)

(OP)
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,

RE: Pressure Drop (Condensate)

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.

RE: Pressure Drop (Condensate)

(OP)
Thank you Latexman for the information

RE: Pressure Drop (Condensate)

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.

RE: Pressure Drop (Condensate)

(OP)
@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

RE: Pressure Drop (Condensate)

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.

RE: Pressure Drop (Condensate)

(OP)
@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,

RE: Pressure Drop (Condensate)

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.

RE: Pressure Drop (Condensate)

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.

RE: Pressure Drop (Condensate)

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.

RE: Pressure Drop (Condensate)

(OP)
@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.

RE: Pressure Drop (Condensate)

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.

RE: Pressure Drop (Condensate)

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


RE: Pressure Drop (Condensate)

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.

RE: Pressure Drop (Condensate)

(OP)
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?

RE: Pressure Drop (Condensate)

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 ?



RE: Pressure Drop (Condensate)

(OP)
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,

RE: Pressure Drop (Condensate)

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.

RE: Pressure Drop (Condensate)

(OP)
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

RE: Pressure Drop (Condensate)

Okay. So here again, you have to make an assesment whether the trays will withstand the diff pressure created when the gas blowby stream blows through it.

So the calc is as follows, for a first pass estimate:

a) Obtain the wide open Cv for this control valve
b) Assume flow is critical across the control valve. Then compute the critical flow across the valve using the max normal MP steam pressure upstream of the valve.
c) Now check if the assumption for critical flow is correct. Tally up all the equivalent lengths. Then work backwards from the PSV relieving pressure ( set press + 10% ) for each line size to get the correspondinf upstream press. Keep moving backwards till you reach downstream of the control valve. At the appropriate locations, you need to add the additional press drop for the de aerator trays and the flash drum wiremesh demister.
d) Check if the total computed pressure obtained from (c) at the control valve is less than the critical pressure for steam for the given upstream pressure and the specific heat value for steam.
e) If the press downstream of the control valve is found to be > critical pressure, redo steps (b) to (c), using the other flow formula across a control valve for subcritical flow. As a first assumption for subcritical flow calc, you can use the pressure obtained from the critical flow assumption above.
(f) Check if the total recalculated backpressure at the control valve is the same as the one used in this trial. If not, redo the calc again ( step e ) until the backpressure at the control valve = the pressure used in this trial.

If you need help with any of these steps, ask a senior process engineer to help. Many of the formulas for some of these steps may be found in Crane publication 410, but you may need to search elsewhere for some others. When calcs are completed, you have to get the calc documented and checked / signed off. In the calc , you have to state the risks involved with the wiremesh demister and the trays. Ask your senior engineer to advise the mechanical engineers to devise some means to strenghthen the wiremesh pack and the trays to withstand the dp during gas blowby.








RE: Pressure Drop (Condensate)

(OP)
@georgeverghese: Thanks a lot for you kind support and input. I will let you know after doing the calculation (hand calculation and HYSYS)

Thanks again

RE: Pressure Drop (Condensate)

So. How many questions have YOU brought up to your boss or supervisor?

YOU are responsible for getting YOUR company's training and experience from their sources (ref library, shelf-in-the-back-closet, technical library in the next floor, headquarters building across the quad, or the shelf-behind-George's-desk). In no case should YOU be basing your calc's strictly on internet sources - no matter how much I trust and respect the guys giving YOU advice here! - because ultimately, your boss is responsible for your company's design to your company's customers about your company's future business and reliability.

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