Copper pipe metric/imperial compatibility

[Chris Green]

I am about to undertake some changes to the hydraulic steering (not power assisted, so relatively low pressure) on my boat. It's quite an old boat and the existing system is a Hynautic one, they're a US company and all the information I can find out about their systems suggests they're all imperial sizes for the pipework.

However I measured the pipes with vernier calipers when I was last aboard the boat and the sizes I got were just over 10mm (i.e. 10.1 to

10.3mm) and just over 6mm (mostly 6.1mm). These don't seem to me to be 3/8" and 1/4" as I would expect as 3.8" is just over 9.5mm and 1/4" is 6.35mm.

The boat was built in the Netherlands so it's possible that the pipework is metric, with metric to imperial fittings where it goes into the BSP type threaded fittings on the pumps and so on.

What *actual* sizes are these sizes of copper pipe? Can any of them be interchanged - e.g. would 3/8" pipe work in 10mm compression fittings? Or, could one put 3/8" olives and pipe into 10mm fittings?

Presumably 1/4" pipe won't fit into 6mm fittings as the 'sleeve' part of the brass casing will be too small. Or is there enough clearance?

[Chris Green]

I think I may be a bit confused about the meaning of imperial sizes, they seem to be *inside* diameters when talking about imperial pipes. That's why 3/4" pipe is close enough to 22mm to fit in metric 22mm fittings.

So I'm even more confused now. :-)

[John Rumm]

Remember that with imperial pipe the specified size is usually that of the bore, not the outside diameter. Hence 1/2" is about 12.7mm bore, then add on a couple of pipe wall thicknesses to get to a little bit under 15mm.

So 1/2" pipe will normally go in 15mm compression ok.

3/8" is close to 12mm, and 1/4" 8mm.

See:

BES will have all the more esoteric sizes:

[Chris Bacon]

As far as I remember, 3/4"/22mm and above needs an adaptor, 15mm/1/2" does not.

[Roger Hayter]

Complicated by the fact that in the imperial system wall thickness alters the OD. For the same reason I have no idea if the 30 year old oil piping around my bouse is 10mm (which it is very tight in) or some imperial size.

[Tim+]

I could be wrong but I thought the convention was that ?pipe? sizing was measured on the outside, hence 15mm plastic or copper pipe will all go in the same fittings. ?Tube? sizing though was based on internal diameter.

Tim

[Michael Chare]

For those sizes of pipe you might be able to join a metric pipe to an imperial pipe using a flared fitting.

For oil fired boilers you are not supposed to use soldered joints in the oil supply pipe as thy may fail. I did have a soldered joint for a few years and in that time it did not leak.

[NY]

I've always wondered why the imperial measurements were of the *internal* diameter of the pipe. The *external* diameter is the important one because the pipe has to fit *inside* compression and solder fittings.

I hadn't realised that imperial and metric pipes were close enough in external diameter that an imperial pipe would make a water-tight seal in a metric fitting: I'd always heard that imperial pipe needed imperial compression or solder fittings, and metric pipe needed metric fittings. I'm sure when my dad was adding extra pipe to the imperial pipes in their holiday cottage, he tried a metric T piece and found that either an imperial pipe was too big to fit or else left too much of a gap for solder to seal the joint. I think he had to use a compression fitting wherever there was a joint between old imperial pipe and modern metric pipe, relying on the compression of the olive (and a lot of PTFE tape) to make up the difference.

[Robin]

On 15/10/2021 10:00, NY wrote: <snip>

FSVO of "important". If you are bothered about how fast stuff will flow through your pipe then the internal size is what matters.

[NY]

Yes, but the difference in flow rate between an internal diameter of 12 mm and 13 mm is fairly small: all that matters is that it is *about* 12 mm (suitable for mains-pressure water) as opposed to *about* 20 mm (suitable for the pathetic dribble of hot water that you get from a header tank only a few feet above bath). On the other hand, the outside diameter of a pipe and the inside diameter of a fitting must be matched much more accurately if the joint is not to leak. Of course, if the pipe thickness is accurately controlled and is the same for all copper pipe, then the i/d is fine because it also defines the o/d, but if one manufacturer makes his pipes with slightly thinner walls, then the o/d will no longer be constant for all pipes of the same nominal i/d.

It's like the convoluted system used for defining car/bike tyre sizes. If I was designing a tyre measurement system, I'd specify wheel diameter, tread diameter (or tyre depth) and tyre width, each as numbers in the same units (*). But instead we have: width in mm, i/d in inches, depth as a proportion of width. What kind of a brain comes up with a system like that? Depth as a proportion of i/d would make more sense than depth as a proportion of width.

(*) Preferably all in mm, but I'll accept all in inches. The crucial thing is not to change units half way through.

[charles]

They fitted at 15mm/½inch - but not at 22mm/¾inch

[Chris Green]

After further thought I think it's 1/2" imperial that is close enough to 15mm metric to work reasonably well.

[Robin]

The difference between 12 mm and 13 mm in Poiseuille's Law is 38%. And wall thickness isn't always such a small proportion of radius. And

[Steve Walker]

They aren't actually, although they are close.

It probably all comes from piping (rather than tubing).

Pipes would have been steel, cast iron, lead, etc. - before copper or stainless steel tubing.

When you choose a pipe size, the first consideration is the flow rate - so the internal diameter is what matters.

Pipes have a "Nominal Bore". So a 1" Schedule 40 pipe is 1.315" OD and

1.049" ID, while a 1" schedule 80 pipe has the same OD, but is 0.957" ID. The differences are small enough that the flow (including a safety margin) is the same.

You'd choose your pipe NB, then the Schedule (wall thickness) for pressure (or noise reduction downstream of valves) but for that NB, there will be a small variation of actual bore, while the OD remains constant (which it has to so that it can be threaded for screwed fittings or welded to slip-on flanges, etc.)

Imperial tube simply followed the same idea and was named after the bore.

Tube generally has much thinner walls and so there is not too much difference between ID and OD, so with more modern metric tubing it is normal to specify the OD and wall thickness (for pressure rating).

1/2" and 15mm are so close that they are interchangeable. 3/4" and 22mm are a little too far out. IIRC, you can put a 3/4" pipe into a 22mm compression fitting, using a 3/4" olive. To go the other way, you need an adaptor fitting.