9 replies to this topic

[tribleprince]

I am working on Compressed air delivery system. Here are some details

Compressor delivery pressure: 30bar

Compressor flow rate : 0.45m3/h

Air storage cylinder capacity: 250 liters at 30bar

My problem is that how can i size a proper pipe for this system, (i.e. pipe inner or outer dia) so that pipe delivers my required pressure and flow rate.

Here is the simple formula for line sizing

volume flowrate, Q = AxV

While A = required area and V= air velocity

but i cant determine air velocity in pipe, or i should assume air velocity. what should be the maximum velocity of a gas in pipeline? (some say 10m/s)

Thanks

[breizh]

Consider this standard , it might helps .

additional info :
http://www.engineeri...ipes-d_852.html
Breizh

Edited by breizh, 13 July 2012 - 01:37 AM.

[ankur2061]

tribleprince,

Refer the link below for an excellent article on compressed air systems:

http://www.chemicalp...es/2005/12.html

Hope this helps.

Regards,
Ankur

Edited by ankur2061, 13 July 2012 - 02:40 AM.

[Art Montemayor]

This involves an exercise in practical, common sense engineering. In this application, mechanical strength and ease of installation is what is important. This involves an obvious laboratory-scale size of compressor that is very small.

For an air flow rate of less than a cubic foot per min (0.6 cfm) at 30 bar, it does not make any sense to make calculations knowing that the answer is a 1" schedule 80 carbon steel pipe. The reason for the obvious answer is that 1" is the smallest, available size that can be supported and welded with relative ease and safety. I am assuming this is NOT an offshore platform installation where NORSOK standards could be justified. Otherwise, you would be forced to use a 2" schedule 80 pipe.

[Sherif Morsi]

I agree with Art but if i may add something.....

Sometimes in projects philosophies, the project team agrees that the minimum line sizes for process & utilities is 1.5" and other times 2".
This is due to the fact that line with sizes less than 1.5" or 2" are weak and can be broken easily and also for the fact that sometimes stress analysis calculations are not conducted for lines below these two sizes.

I personally believe that for your case it's recommended to study this line stresses due to the fact it's a discharge line from a rotary equipment.

Sherif

[tribleprince]

Thanks to all honorable members for taking interest in my query.
I am using copper tube for compressed air and this system ll be used in a small recreational ship.

[Art Montemayor]

Now that you have fed us another crumb of basic data, I strongly advise you NOT TO USE COPPER TUBING on this application.

You are installing this 30 barg application on a recreational vessel and human beings - especially young ones - will be in the surroundings. Copper tubing is very weak and structurally very hazardous when subjected to the movement, vibrations, and stresses on a ship. It is a cheap and quick way to obtain a temporary solution, but it presents hazards.

I recommend you employ Stainless Steel (or even carbon steel) tubing for this application instead. It is much stronger and will withstand more abuse.

[kkala]

1. I am familiar only with local copper tubes (~1.5 m chunks, sometimes leaking at joints) used in potable water services. These are judged not suitable for the 30 barg air service.
2. Having looked at Perry (7th edition), Process Plant Piping in Section 10, thickness of copper tube / pipe is as follows for e.g. 1" nominal diameter.
- ASTM 888, tube, Type K, 1.65 mm
-ASTM B280 (dia 1.125"), tube, 1.27 mm
-ASTM B42, pipe, extra strong, 4.62 mm
-ASTM B302, pipe, 1.65 mm
Allowable stress for copper pipe / tubing can roughly be 6000 - 11200 psi (say 400 - 800 kgf/cm2) (Table 10-49, p. 10-112 of Perry), so for a design pressure of (say) 36 barg ~ 36.5 kgf/cm2 and outside diameter = 33 mm, required copper thickness (seamless pipes) would roughly be between 36.5*0.033/ 2/800*1000 = 0.75 mm and 1.5 mm, plus corrosion allowance and wall thickness tolerance (Perry, p. 10-103) . This indicates availability of some grades for the service, other ones can be unsuitable.
3. Joints can be a weak point of a piping system (Perry, Table 10-48, p. 10-108, for example), yet joints complying with design pressure of straight pipe are reasonable to find in the market.
Copper is more flexible than steel, http://www.matter.org.uk/schools/content/youngmodulus/introduction.html.
4. On the other hand "Copper tubing is restricted by JIC standards to low-pressure stationary applications and air circuits because it acts as an oil-oxidation catalyst and tends to work harden when flared. In addition, copper tubing has poor resistance to vibration", http://www.pneumaticchina.com/news/pipe-nbsp-and-nbsp-tubing.htm, JIC = Joint Industry Council (of USA?). This alone might exclude copper tube / pipe of 36 barg design pressure from ships, but more specific data would be appreciated from somebody having expertise.
5. Neglecting para 4, reason favoring use of copper tube / pipe in ships might be the high corrosion rate of carbon steel by humid air above sea (also observed by kkala). Copper offers good corrosion resistance, http://www.corrosionist.com/copper_pipe_corrosion.htm.
Note: copper tube is easily "beveled" in comparison to stainless or even carbon steel, but such "beveling" (applied to tubes of para 1) is judged not applicable for 36 barg air service.
7. In order to have more clarifications / knowledge, tribleprince is kindly requested to clarify following to the extent possible:
α. Whether a standard / practice is applied in ships, about compressed air transfer in copper tubes of such pressure.
β. Reasons of intending to use copper tubes for the service (apparently there are reasons against it).
γ. By the way, is the compressor specified for oil free air out?
8. Since above is not based on experience, comments / advise from others willing for it are welcomed.
After post No 6 (informing of copper tubing) I had a look at Perry and thought available copper tube thickness could be adequate for 30 Barg operating pressure of air. But then post No 7 revealed that other factors should be also taken into account. Hopefully above considerations are useful, especially in case there are replies (paras 7 and 8).
It is noted that allowable stress for carbon steel used in pipes is 11000 - 24000 psi, versus 6000-11200 psi for copper (Perry). All references to Perry concern the 7th edition (1997).

Edited by kkala, 18 July 2012 - 12:29 AM.

[kkala]

...Sometimes in projects philosophies, the project team agrees that the minimum line sizes for process & utilities is 1.5" and other times 2". This is due to the fact that line with sizes less than 1.5" or 2" are weak and can be broken easily and also for the fact that sometimes stress analysis calculations are not conducted for lines below these two sizes... Sherif

Agreeing with Sherif, it is further noted for local refineries that minimum pipe size for interconnecting pipes (pipes out of the units) is 2". Policy differs from refinery to refinery inside their units. One refinery uses 2" min size in its units. Another uses 1" minimum size in its units.Above concerns process pipes (not e.g. steam tracing).
Since the point of minimum pipe size was touched, this additional note may be useful.

[tribleprince]

thanks to all respectable members especially kkala, ART and Sherif.
I am intended to use copper-nickle alloy Cu/Ni 70/10 for tubing according to "NES" (Naval Engineering Standard), and reason behind using Cu/Ni is obvious i.e. good corrosion resistance, bends easily, using compression fiitings and more important "NES" advice.
yes compressor delivers oil free air and for more safety air receiver is installed.

please looking for your kind reply

thanks again