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Compression Fittings VS. Weld Fittings

  1. Jul 25, 2011 #1
    I need to install 4 bar stainless steel piping system (40 meters length, 20 Tee, 40 Elbow) . do you recommend using compression fittings or welded fittings.

    I know that compression fittings is easy to assemble but they have leakage problems.

    Butt-Welded fittings on the other hand would make leakage-free pipeline but it need precise TIG (GTAW) welding (to have high quality welding orbital TIG welding is used) and if the welding is performed manually (by welder) it I may end-up with fittings been misaligned to the pipeline.

    So what do you recommend?
  2. jcsd
  3. Jul 25, 2011 #2
    Is your material toxic or flammable? If so, go with the welded connections.

    If not, compression should be fine, especially at only 4 bar pressure.

    Carefully follow manufacturer recommendations for compression fittings (e.g. hand tighten then tighten another 1.25 turns, ensure the tube is fully installed in the fitting, and so on).

    I would stick with name brand compression fittings. Some of the other stuff on the market has poor hardening coatings on the ferrules and degraded chemical resistance.
  4. Jul 25, 2011 #3


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    You haven't mentioned what the particular application is for this stainless piping system. There might be code requirements which must be satisfied.
  5. Jul 26, 2011 #4
    The stainless steel pipeline is carrying CO2 for Incubators (In-Vitro Fertilization) through roughly 50 meters with approximately 30 bends. The flow rate is about max 70 Lpm, Is there any code for this?

    Would using compression fittings be a hazard if any leakage occurs?

    Does Socket-Weld fittings perform well? I hope so as they are easier to weld than Butt-Weld fittings?

    Also, Could you advise the tube diameter to be used (fluid: CO2 , Piping material : stainless steel tubes 316L, equivalent length:40 meters , Flow Rate : 70 Liters)
  6. Jul 26, 2011 #5
    Compression fittings should work - install per manufacturers guidelines as I mentioned above.

    Socket weld performs good and is faster than butt weld.

    For diameter, check pressure drop at your flow. Start with a line size that gives a velocity around 60 to 120 ft/sec.

    Dual rated 316/316L is a good choice and easy to buy.
  7. Jul 26, 2011 #6


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    If in the US, use ASME B31.3.

    I'd agree that compression fittings are fine, socket weld is even better but not required.
  8. Jul 28, 2011 #7
    Thank you all for assistance

    I have estimated the pressure drop through the pipeline and it is within the acceptable loss range and it was about 36 Kpa loss (less than 10% for 400 KPa).

    I have used Darcy equation since I stated that the pressure drop would not exceed 10% of the inlet pressure (400 KPa) and I guess this would give a good estimation

    I need someone that could enhance these results so I could proceed. could you please estimate the pressure loss for this system:

    Gas : Carbon Dioxide
    Inlet Pressure : 430 KPa (4.3 Bar)
    Total equivalent length : 70 meters
    Flow Rate : about 70 Lpm (2.47 scfm)
    Temp. : 25 C
    Pipe material: Stainless steel 316L
    Pipe Outer Diameter : 12.7 mm
    Pipe Wall Thickness : 0.89 mm

    Please advise, Is this or I should use bigger size?
    Last edited: Jul 28, 2011
  9. Jul 28, 2011 #8
    I calculated a value close to your number of 36 kPa. Your velocities are reasonable also.
  10. Jul 29, 2011 #9

    I was wrong my results for OD=12.7mm and thickness=0.89 was Pressure loss= 6.8 KPa

    36KPa pressure loss is for OD=9.53mm and thickness=0.89

    Could I know how did you calculate it? do you use software, or you just calculated it manually?

    I am using pipe flow expert v5.12 , Is there another better software that has more functions?
    Last edited: Jul 29, 2011
  11. Jul 29, 2011 #10
    I use formulas from Crane Technical Paper 410 and the Cameron Hydraulic Data Handbook.

    For OD = 12.7mm, t = 0.89 mm, what velocity did you get? What density did you use?
  12. Jul 30, 2011 #11

    Density of CO2 at T=25C and P=420 KPa is 9.25 Kg/m3

    calculated velocity = 2.52 m/s

    pressure loss=6.8 KPa
  13. Jul 30, 2011 #12
    I think your velocity of 2.52 m/s is low. You listed your flows as 70 Lpm (2.47 scfm). These volume flows are equal, so your value of 70 Lpm is standard conditions? If so, use the density at standard conditions to figure your velocity from mass flow.
  14. Jul 31, 2011 #13
    Here is what I did :

    Firstly I converted the 70 Lpm to scfm (@1 Bar T=20C) to acfm (actual flow @4.2 Bar T=25C) these results in :

    70 Lpm = 2.47 scfm

    2.47 scfm@1 Bar = 0.52 acfm@4.2 Bar

    Density using Ideal gas equation : P=rho*R*T


    P=4.2 Bar
    T=25C = 298 Kelvin

    ==> rho = 9.25 Kg/m3

    then I used Pipe Flow Expert solver (using Darcy equation) then I have got:

    ID = 12.7 -(2*0.89) =10.92 mm
    rho=9.25 Kg/m3
    Inlet Pressure = 4.2 Bar
    stainless steel surface roughness=0.045mm
    length = 70m

    ==> velocity = 2.52 m/sec
    mass flow = 0.0022 Kg/sec
    friction loss= 6.38 KPa
  15. Jul 31, 2011 #14
    Using an actual flow of 0.52 acfm and flowing conditons of 4.2 bar @ 25C, I was able to match your numbers. In my first calc, I fouled up the denisty.
  16. Aug 1, 2011 #15

    Could you please attach the equation that you are using (do not write it as it will appear as talisman) or just type of the page number (Crane Technical Paper / Cameron Hydraulic Data Handbook).
  17. Aug 1, 2011 #16
    See Equation 1-4 in Crane TP 410, 25th Printing or Page 3-3 of Cameron Hydraulic Data 19th Edition. They are both the Darcy-Weisbach equation.
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