Author Topic: Steam feed tubing question  (Read 793 times)

Online crueby

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Steam feed tubing question
« on: January 14, 2019, 04:58:36 PM »
Hi all,

I am getting close to putting the tubing/piping in to feed the engines on my Marion build, and have a question about brass tube vs copper tube.


There is a much larger variety of sizes available for brass tube in the model marketplace, but the walls are usually pretty thin - either 0.014" or 0.029" thick wall. There are some sizes of copper available at 0.040" thick.

Question is, what kind of pressures can these tubes take? This will be for the sections away from the boiler, so they will not be subject to high heat, but need to handle a working pressure of 60 to 80 psi. On this model the piping is all straight sections with elbows, no bent sections needed.

What do you think? What is the safe wall thickness to go with, and does it matter if copper vs brass? The brass is stiffer but thinner.

Thanks!
Chris

Online Roger B

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Re: Steam feed tubing question
« Reply #1 on: January 14, 2019, 05:34:19 PM »
These small tubes are quite strong. My fuel injector is connected with 1mm bore 2mm OD annealed copper tube so 0.040 bore 0.020 wall. This withstands over 100 Bar at normal temperatures so 1500 psi. I don't think you need to worry too much over 60 to 80 psi.
Best regards

Roger

Offline Gas_mantle

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Re: Steam feed tubing question
« Reply #2 on: January 14, 2019, 05:45:32 PM »
Chris what size OD of tube are you thinking of?

I usually use 5/32" copper at 22 swg for my smaller engines and that carries ample steam at 80psi. (for my larger vertical engine I'm going to go for 1/4" at 18 swg.

0.04" is 19" swg so that compares with the kind of stuff I use without any problem. 0.019" is 30 swg and I wouldn't like to comment on how safe it is but it does seem to be thinner than the usual steam pipework.
« Last Edit: January 14, 2019, 05:54:53 PM by Gas_mantle »

Online crueby

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Re: Steam feed tubing question
« Reply #3 on: January 14, 2019, 05:53:00 PM »
The main lines are around 3/16" OD, some a little less, to match the scale on the real engine.

The .014 brass seems light to me, but how about the .029??

Offline Jasonb

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Re: Steam feed tubing question
« Reply #4 on: January 14, 2019, 06:37:20 PM »
What sort of fittings are you using? American stuff seems to use more threaded fittings so you may need a thicker wall to take the thread. If just soldered nipples and nuts then that is not such an issue.

Thicker wall will be easier to bend without collapse and copper will bend easier than brass. Also there are soft and hard brass tubes, the softer will be better for bending.

Online crueby

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Re: Steam feed tubing question
« Reply #5 on: January 14, 2019, 06:50:48 PM »
What sort of fittings are you using? American stuff seems to use more threaded fittings so you may need a thicker wall to take the thread. If just soldered nipples and nuts then that is not such an issue.

Thicker wall will be easier to bend without collapse and copper will bend easier than brass. Also there are soft and hard brass tubes, the softer will be better for bending.
The fittings will be home made to match the scale original, and the tube sections will be straight. Fittings will be silver soldered in place.

Online crueby

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Re: Steam feed tubing question
« Reply #6 on: January 15, 2019, 01:04:56 AM »
Been doing a lot of digging, and heard from some, looks like the thinner .019 is a no go, but there is the stuff made for AC and refrigeration equipment that in copper (labelled ACR I think) straight and in coil that is the right alloy and thickness to do the job very safely. There's a good hardware store cross town that carries lots of plumbing etc that I will go get some from. Thanks very much for the information!

Offline MJM460

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Re: Steam feed tubing question
« Reply #7 on: January 15, 2019, 12:30:47 PM »
Hi Chris, sounds like I am a bit late to the party, but have not been able to reply earlier today.

The short and simple answer to your question is that any of the tubes you have mentioned are more than adequate for your 80 psig steam, so use the thickness you prefer, or the one you can get.

In more detail, I have referred to the Australian code for Copper Tubes for plumbing and drainage.  There is another code for engineering and refrigeration duties which has more stringent requirements but I donít have a copy at the moment to check if different pressures are allowed.

Your 80 psi gauge steam has a saturation pressure of 330 F, assuming no superheat, and this temperature determines the allowable stress. 

Now I need to revert to SI calculations.  The code gives allowable stresses and safe working pressure for the standard thicknesses defined in the code,  it more importantly it gives a formula for safe working pressure, assuming brazed joints.  Obviously threaded fittings would reduce the actual wall thickness and hence reduce the safe working pressure.  Brazing or silver soldering leaves the metal annealed, so the allowable stress is specified on this basis. Tubes are then tested to 1.5 times this safe working pressure.  The formula is the standard thin shell formula.

The allowable stress at 50 C and below is 41 MPa, while at 330 F/165 C it is only 28MPa.

To avoid a cloud of figures just a summary of the results of this calculation for 3/16 tube.

0.014Ē wall thickness, 0.3556 mm gives safe working pressure of 4521 kPa or 656 psig
0.019 w.t.           0.4826 mm.         6318 kPa or 892 psig. 
And so on, the safe working pressure increases with wall thickness.

One thing to watch is that tubes have both a nominal thickness and a minimum thickness.  The minimum has to be used in the formula.  But the labelling, here anyway, is based on the nominal thickness, not the thickness you would use in the formula.  The formula also only applies to straight tubes, bending is likely to cause some flattening of the tube, and this will reduce its pressure capacity.  But your straight tubes with silver soldered end fittings will be fine.

Clearly a good margin over your 80 psig, but the figures do not apply to higher pressure steam, as that would have a higher temperature, and hence lower allowable stress, so needs recalculation.  However the safety margin provides enough margin for minimum wall thickness, minor defects, ovality or other defects which are inevitable even with good workmanship at your pressure.

If you want to pressure test your piping with cold water, you would use 80 x 41/28 x 1.5 = 175 psig. based on the temperatures and allowable stresses above.

This is the standard calculation to stress the pipe under test to the same proportion of the allowable stress at room temperature when it is intended for duty at higher temperature.

I hope that helps

MJM460
The more I learn, the more I find that I still have to learn!

Online crueby

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Re: Steam feed tubing question
« Reply #8 on: January 15, 2019, 02:05:41 PM »
Thanks MJM! Very good summary. Combined with what I heard elsewhere, confirms that if I use the copper ACR products I am well within safety limits, and can take that tube right up to the boiler outlet valve.


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