5 replies to this topic

[pklavins]

Problem: What is the minimum velocity in a condensate pipeline and which standard should I be following?

Fluid: Condensate ~ Pentane, Hexane, Heptane, Octane+, 4% Water Cut, SG=0.68 (liquid from a condensate tank at a compressor station) 

 

My client has asked me to check the suitability of using an older, over-sized pipeline for condensate from a compressor station to a larger oil battery (~18 km long, buried). According to API RP14E, the minimum velocity for liquid lines is 0.9 m/s (3 ft/s) to prevent deposition of sand and excessive corrosion. Is this the correct standard that I should follow? 

 

I'm getting a velocity of 0.3 m/s for the client's desired maximum flow rate (the client has also specified that they will likely be operating below this max). It seems to me that this pipeline will have both deposition issues and operating issues (especially with any check valves on that pipeline line). I'm a young engineer, so I would really appreciate any advice on this problem from experienced pipeline and facility engineers.

 

Thanks for your time.

[TS1979]

API RP14E is for offshore platform piping - Raw crude liquid. Your commodity is condensate. You should follow B31.3 for process piping.

 

When talking about minimum velocity, the concern is that the velocity inside the piping must be above the solids settling deposit velocity. In your commodity, you didn't indicate any solids in the composition. So, solids deposit velocity is not a concern.

 

With 4 wt% water content, it is unlikely that water will be separated from hydrocarbons - you don't have layer flow inside the piping. If the velocity inside the piping reduces further, you are likely in the laminar flow region which will increase the friction loss - this is a rare case for commercial piping. If your condensate contains acid, it will corrode the piping anyway.

 

Check valve in the piping should not be a concern.

 

Thanks

[syed_chemical]

Dear pklavins

 

What is your Condensate composition ? and Is it ..

 

a) Liquid at Bubble point ?

Non Boiling Liquid ?

 

What is your Source Pressure and what is your destination pressure requirement ?

 

Regards

Syed

[pklavins]

TS1979,

 

I originally was following B31.3 and CSA Z622-11 (I'm located in Canada FYI), but I couldn't find anything about low velocities. CSA Z622-11 (4.14.3.8) only states that "The designer shall avoid dead-ended piping unless corrosion is mitigated in such piping sections. Consideration shall be given to sizing piping to maintain a flow velocity sufficient to minimize the accumulation of water and sediment."

 

The lowest Reynolds number, I calculated was 2414, which would put my fluid in the transition region between laminar and turbulent.  

 

As far as acidity is concerned my condensate does have a small amount of H2S and CO2, but the water analysis I have says that it's pH is 7.7 with a TDS of 11,000 mg/L (mostly Na+ and Cl-).

 

 

syed_chemical:

 

The liquid is coming off of an atmospheric tank (with a VRU) so it would be a non-boiling liquid (Reid Vapour Pressure of 55 kPag).

 

The exact composition is:

 

Mole [Fraction] NITROGEN Trace CARBON DIOXIDE 0.0002 HYDROGEN SULFIDE 0.0007 METHANE 0.0053 ETHANE 0.0053 PROPANE 0.0099 ISOBUTANE 0.0049 n-BUTANE 0.0151 ISOPENTANE 0.0123 n-PENTANE 0.0225 n-HEXANE 0.0623 WATER 0.0408 C7+* 0.8208

 

 

Thanks for your inputs. Let me know if you need any other information about the system to help me solve this problem. 

[pklavins]

Sorry, the table didn't post properly:

NITROGEN                 Trace
CARBON DIOXIDE       0.0002
HYDROGEN SULFIDE 0.0007

METHANE                    0.0053

ETHANE                       0.0053

PROPANE                    0.0099

ISOBUTANE                 0.0049

n-BUTANE                    0.0151

ISOPENTANE               0.0123

n-PENTANE                 0.0225

n-HEXANE                    0.0623

WATER                        0.0408

C7+*                             0.8208

[Zauberberg]

As a rule of thumb, the velocity should be somewhere in the range 1.5 - 3.0 m/sec. This will ensure turbulent flow and reasonable pressure drop (you don't want to be in the laminar region), and still below velocities that can create electrostatic charge.