Bsp pips - what fits inside what (as in ID vs OD)

[julianf]

So i need, probably, a mild steel pipe to fit within another stainless one.

Its hard to find pipe in all diameters off the shelf, so i thought "why not use bsp plumbing pipe"

So, i reckon the ID of a 2.5" (2 1/2") stainless pipe should be 2.5 inches, right?

ie 63.5mm


So then if i want another pipe to slide into it, with minimal gap, but be able to be removed by hand, i want an OD a little less than 63.5mm?


The major diameter of 2" bsp thread is 59.614mm - so about a 4mm gap (as the pipe will sit on one 'face' so all the slack will be in the same place, on the opposite face)

2 1/4" bsp major dia is given as 65.710mm so thats too large.



But...

My actual question is -

Am i right in trying to work it out like this? Can i really assume diameters of the pipe from thread data. Or am i wasting my time, and the only way to be sure is to go and buy some?



For background - this is for the burner on my furnace. I want the burner tube to slide into another tube, but be able to remove it without too much bother.

 

[julianf]

Hmm... just looking at the numbers, 3" would give an ID of, well, i assume, 3" (76.2mm) and 2.5" bsp major diameter is 75.184mm - ie a pretty good fit.

IF the numbers are to be believed....

 

[Turbo]

If you are using standard black iron bsp gun barrel pipe each size will usually be a nice sliding fit inside the next size up but you have to remove the internal seam.

 

[julianf]

Thank you.

I actually just ordered a 2.5" section so as i can measure the actual against my expected, and then ill go from there.

 

[julianf]

So, in follow up, the 2.5" section turned up.

My expected dimensions -

vs actual -

so a good 5mm out on the ID.

Which will make for a sloppier fit than i want.

From looking at various tables, i now realise that if i purchased sc40 in 2.5" then i would do better - 73.0mm OD and a specified 5.16mm wall, would give me a good fit when used with 2" (around 59mm OD)


My problem is that the 2.5" sc40 stuff is about a million pounds per mtr....

 

[CompoSimmonite]

Couldn't an old hydraulic ram off a pick-up or similar with seals removed give you something suitable ?

 

[Woody.v8]

 

[hotponyshoes]

So, in follow up, the 2.5" section turned up.

My expected dimensions -

View attachment 173366

vs actual -

View attachment 173367

so a good 5mm out on the ID.

Which will make for a sloppier fit than i want.

From looking at various tables, i now realise that if i purchased sc40 in 2.5" then i would do better - 73.0mm OD and a specified 5.16mm wall, would give me a good fit when used with 2" (around 59mm OD)


My problem is that the 2.5" sc40 stuff is about a million pounds per mtr....

I think thats the wrong way around.
I thought SCH pipe sized are the O/D sizes (and thats a nominal size rather than the actuall size)
So, what you thought was 2.5" I/D is actually 2.5" O/D.
The Diffrence between the 75.184mm and the 74.68 is within tolerance. Most stuff sold for water pipe or suce is going to have welds or solder or compression fittings on it so unless you start talking expensive it's not going to be a good fit.

For a forge/furnace I would just go for a bigger gap and pack with some heatproof rope (fire door seal) or something or fit a couple of plates over each tube so you can put some fiberglass gasket between them.
Or, allow a decent gap between the tubes then wrap some heat wrap around it once in place Stainless steel cable tie could be cut off easy enough if you need to remove it.
. If you make the tolerences too tight you are going to be stuffed once the heat gets into it as its going to distort all over the place.
I have a small ish propane forge and the burner tubes on that are machined inconel and not directly in the fire and you can only really use them once for initial set-up and after a few weeks of use you wont ever get them apart again.

 

[julianf]

Schedule 40S Sizes

Nominal size [inches] Outside diameter [inches] Outside diameter [mm] Wall thickness [inches] Wall thickness [mm] Weight [lb/ft] Weight [kg/m]
1/8 0.405 10.3 0.068 1.73 0.24 0.37
1/4 0.540 13.7 0.088 2.24 0.42 0.84
1/2 0.840 21.3 0.109 2.77 0.85 1.27
3/4 1.050 26.7 0.113 2.87 1.13 1.69
1 1.315 33.4 0.133 3.38 1.68 2.50
1 1/4 1.660 42.2 0.140 3.56 2.27 3.39
1 1/2 1.900 48.3 0.145 3.68 2.72 4.05
2 2.375 60.3 0.154 3.91 3.65 5.44
2 1/2 2.875 73.0 0.203 5.16 5.79 8.63
3 3.500 88.9 0.216 5.49 7.58 11.29
3 1/2 4.000 101.6 0.226 5.74 9.11 13.57
4 4.500 114.3 0.237 6.02 10.79 16.07
5 5.563 141.3 0.258 6.55 14.62 21.77
6 6.625 168.3 0.280 7.11 18.97 28.26
8 8.625 219.1 0.322 8.18 28.55 42.55
10 10.750 273.0 0.365 9.27 40.48 60.31


I know the above is hard to read : ( But, basically, 2.5 inch sc40 is 73.0 OD and then 73.0 - (2 x 5.16) ID = 62.68 (about 2.5 inches)


If i then get some normal 2" pipe, that should have an OD about 2mm less than the ID of the sc40 stuff.

What i want is to be able to remove the inner tube quickly, but not have it loose enough that it smokes a whole load.

In use, the burner tube does not really get hot, as it has such a volume of air running through it. I know this by the amount of soot over the end of my existing test part.

But, i know, if i left it in the furnace, without the air, it would get real hot - but then it will destroy all the connections to the burner tube also, so i have to remove it whilst its not running.

Im sure there are different approaches to different systems, and what im doing might not be right - its just what ive found from my own prototype setup - that i cant leave the tube in place, otherwise the heat comes back and melts the air ducting, so, as long as i can whip it out the way as soon as the air stops, i dont think it should get so hot.

 

[hotponyshoes]

How about you cut a big washer (or square plate with round hole) then weld onto the removable burner pipe.
So, metal washer "a" You could cut a fibre gasket or use rope on the inner side of it and it would seal up against surface "b"