What bore is alkathene pipe?

[Dave]

My rising main is black alkathene (I think), the outside diameter is close to 21.5mm - how can I find out what the bore is without removing the fittings?

[Bob Minchin]

Hi Dave,

The only bit of alkathene (or MDPE), I have laying around here is the blue stuff. It is 20mm OD and 15mm bore. I'm not sure if alkathene was in use in the days of imperial plumbing, if so maybe yours could be imperial size hence the different OD. If I had to guess I'd expect imperial plastic pipe to have had the same bore as the iron pipes that it would have replaced. I've just mesured a piece of 1/2" iron pipe and this is 21.5mm OD and

15.25mm bore. maybe your black pipe is similar bore?

Hope this helps

Bob

[Steven Briggs]

In message , Dave writes

Well if you'd asked yesterday.... Just this morning I put a new stop tap onto a the end of an c. 1970 alkathene pipe. As in a previous thread, I'd measured the pipe (with a vernier) in situ at 22.0, but when I cut the end off this morning, a

22mm olive was a loose fit, so it was probably 21.5mm. Wall thickness was about 3mm. I might be able to find the off cut of the scrap pile tomorrow if you need more accurate than that. I successfully re-used the copper insert that was in the end of the pipe.

[Dave]

Thanks Bob. I'm about to replace the internal cold "main" with 22mm and was wondering whether it was worth replacing the present alkathene_to_15mm coupling; I think I'll leave it and just go from 15 to

22mm.

[Dave]

Thanks Steven, that ties-up with BobM's info.

[Simon Stroud]

Hi,

I have an extremely similar rising main to this. Black plastic, very close to 21.5mm diameter.

If it's the same as mine then it's what they used to call "half inch" which presumably must relate to the INSIDE diameter. I had a long but in the end embarrasing heated discussion about this with the chap in the local Plumb Center (sic) and in the end it turned out that he did in fact know what he was talking about.

I wanted to replace the old stopcock (rather restrictive one with 15mm copper out) with a 22mm one.

I used a "Talbot" brand push fit coupler, available from Plumb Centre. One of the ones they do is a "reducer" adaptor which takes the following:

At one end:

Modern blue plastic 20mm (O/D) pipe OR old fashioned black "1/2 inch" plastic pipe (they supply the correct hard plastic ring and O ring to suit one or the other of these.

At the other end:

Modern blue plastic 25mm pipe OR probably some old fashined imperial of a similar size.

You can see info on the Talbot range at

see they appear to have been taken over by Tyco now.

I've used this to convert from the black "1/2 inch" to 25mm blue plastic (only a few inches of this). And then I've put a big stopcock on the end of this (again a Talbot pushfit part) and that has a 22mm copper output. Then all my new mains pipework to the big combi boiler, showers, etc. etc is done in 22mm copper.

It made a big difference to the flowrate compared to the old rubbish 1/2" to

50mm stopcock and all 15mm internal mains pipework.

The icing on the cake would be to replace the black plastic with modern 25mm blue plastic all the way from the main, but that's a project for another year.

Regards, Simon.

[Doctor Evil]

Sounds like LDPE (black) rather than the current metric MDPE (blue) Two main types were used: Class C and Class D. Class C has a thinner wall than class D. You may find the type printed on the pipe.

In both 1/2" and 3/4" pipe, internal and external diameters differ.

BES

sell fittings to convert from Class C 1/2" to black or blue MDPE.

In your case the bore will be 1/2" (15mm). A standard 22mm compression joint will go over the pipe, as long as you still use the existing pipe insert. Wrap PTFE tape around the olive before tightening. If replacing a stop c*ck use a 2mm compression full-bore valve.

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[Doctor Evil]

I left out. Use the existing olive on the black pipe, as that will be old imperial 3/4". This can be used inside a 22mm compression fitting on LDPE pipe.

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[Dave]

Thanks (and to SimonS). If the bore of the main (about 15m to the external stopcock) is only 1/2" it seems that there's no advantage changing the fitting at the house end(?) Currently there's a compression fitting to a short length (20cm) of 15mm and then into the house stopcock, I'll just replace the 15mm stopcock with a full bore 22mm lever valve and run 22mm thereafter - unless someone tells me otherwise.

To estimate the impact of changing pipe bores I'm planning on using an ohm's law analogy and "series resistance". Can I just say that the relative "resistance" caused by 22mm and 15mm pipe is directly related to XSA (a factor of approx 2) of are the surface effects significant? It's a long time since I did any fluid mechanics, any Bernoulli experts out there? ;-)

[Andy Wade]

IANA fluid dynamicist but ohms law for pipes is a bit different to the electrical one. For a given 'resistance' (pipe type, size & length) flow is proportional to the square root of the pressure drop, or 'head loss' as it tends to be known. Or, put the other way round, head loss is proportional to the square of the required flow rate. For a given head loss per unit pipe length, flow seems to be roughly proportional to pipe area.

a pipe sizing chart (nomogram) which you might find useful.

[Dave]

Thamks Andy, that's a useful link.

[nrjc86]

Does anyone know the C-factor for blue alkethene pipe ?

[tabbypurr]

is it not smooth bored?

NT

[John Rumm]

It is, although there is some variation in flow resistance for different materials. PVC is something like 140 - 150 ISTR, so I would expect something similar.