Thread: Pipe measurements

I think the inside diameter is a nominal 25mm (1 inch)
That means it is not really 25mm
Hope that is confusing enough. (I don't know that this is correct, just my guess)

Pipe sizes are a nightmare. NOMINAL size is typically the equivalent size of a smooth-bore (imaginary) pipe for calculating flow characteristics, with friction absent. No such thing, of course.

To make matters worse, metric conversions are sometimes "soft," not exact. And pvc pipe is typically the same dimensions of iron pipe, even though their friction values are different.

A nominal 25mm pipe would be nominal 1" (I think). OD and ID are 1.315" and 1.049" respectively. Exact conversion to metric would be 33.4mm and 26.6mm. If your measurements are close to those, Bob's your uncle; if not, best consult a plumbing supplier, because I'm in deep water here.

The foregoing is for Standard Weight (Schedule 40). For Extra Strong and Double Extra Strong, the OD's stay the same, but the ID's are smaller (thicker walls), calculations be damned.

The discrepancy between equivalent and actual diminishes in very large pipes, because the skin friction is a smaller component of calculations. At 12" Standard Weight, it vanishes. At very small sizes, skin friction dominates, and the nominal size seems to be created out of thin air.

Cheers,
Joe

Kevin,
if it's black poly pipe (smooth, black, slippery stuff with screw together fittings) then there's a few different grades with different fittings.

Pressure poly is typically black with a blue stripe, or sometimes blue, but the older stuff was all black. It has a fairly thick wall and can withstand mains pressures. It uses compression poly fittings with a couple of sections which sit over the pipe.

Rural grade poly is now usually sold with a green stripe, but again, the older stuff was just plain black. It has a much thinner wall than the pressure poly. It has similar fittings to the pressure poly but there is a piece that fits inside the pipe to support it and to prevent the compression ring from crushing the pipe. It won't withstand mains pressure, at least not for long.

There is a third grade of poly which is used for garden irrigation systems which is thinner again. It's usually joined with black poly fittings which slip inside the pipe and are held in place with either metal hose clamps or plastic clips or clamps. This third system is very thin wall and is usually attached to a garden tap. Because it has a number of outlets and is usually set up so it can't be pressurised (ie, the outlets are always open) it doesn't need to be able to withstand high pressure.

Hope this helps.
BTW, there is a black pipe used inside houses now which is PEX. It's high pressure and usually uses crimp on fittings although push together fittings are also used. Not usually found in gardens.

Mick

Adding to what Joe & Mick said, if it's 25mm pipe, it will be the nominal inside diameter, if it's 25mm tube, it will be the nominal outside diameter.

Thanks for your reply Bradford. I'm getting the drift of what's going onwith pipes. Small wonder that plumbers charge so much.

Thanks for the time you've taken Mick. I'm getting the idea now. I wonder if plumbers and pipe-makers conspire to make things complicated the better to charge us more. Just a thought.

Thanks again Mick.

Regards,
Kevin

Thanks Joe for the time you've taken. I'm getting the drift now.
I guess our nails and screws are really nominal in their dimensions too. I've tended to be cynical about plumbers and their pipes. But things are clearer now.

Again, thanks.

Regards,
Kevin

Thanks Alex.

Regards,

Kevin.