Plastic gas main

In a road near me, the put in a new yellow gas main, by running it down the old iron gas main. It is a smaller bore of course.
I noticed they did the same across the gardens, running what is the equivalent of 1/2" yellow plastic through 3/4 and 1" iron main pipes to each house. One house has what looks like a 1 1/4" iron main pipe (looks like original 60s or 70s), at the meter it has what looks like a 1 1/4" to 1" reducer, a small short piece of 1" steel pipe, then a 1" to 3/4" reducer, with two, what looks like a 1/4" plugs in the 1" to 3/4" fitting, with then short piece of 3/4" steel pipe and then a brass maintap on that.
Does anyone one know what the "two" plugs are for? How did they connect the plastic onto the house meter? Connecting onto the 1 1/4" steel pipe under the ground?
Will a house that has a large combi using a 1" main gas pipe be OK with a 1/2" mains plastic gas pipe? 1/2" looks way undersized to me. If ant of these house upgrades to a larger higher water flow combi will they be in problems?
All this plastic gas mains looks like a cheap way of upgrading the mains for the gas company's benefit, not for the benefit of the house who may be short changed on a gas supply.
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Bear in mind that the pressure is higher than it is in your house. There is a regulator after your meter to reduce the pressure.
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On Fri, 1 Oct 2010 09:27:01 +0100, John wrote:

Thought the regulator was before the meter so it is measuring the volume of gas consumed at a known pressure rather than the street pressure that varies with local demand....
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Dave.




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On 01/10/2010 10:13, Dave Liquorice wrote:

Correct, although AIUI typical street pressure is still only marginally over atmospheric (37 mb?), so it won't make a huge difference.
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Andy

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I would assume the 37mbar is on an old large bore steel pipe gas mains. Hopefully it would be towards 75mbar when converted to plastic with small bore pipes..
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On Fri, 1 Oct 2010 05:57:19 -0700 (PDT), Bay Man wrote:

marginally
I am surprised that street pressure is that low. The 35 to 75mb being banded about is only 0.5 to 1 psi. Guess they must have higher pressure trunk distribution to other regulators for the street distribution.
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Dave Liquorice wrote:

WE'RE JUST HAVING A GAS SUPPLY DONE AT WORK, MAPS ARE ALL WRONG SO sgn CONTRACTOR PUT FORK THROUGH 32MM YELLOW PLASTIC PIPE,
We're just having a gas supply run in at work. Maps are all wrong and as a consequence the contractor making the roadside connection put his fork through a 32mm plastic pipe to an adjacent house, the jet sandblasted him, he said the medium pressure pipes were 2 bar.
AJH
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Yes. Actually, the pipe capacity of the high pressure natural gas network is a significant proportion of the country's gas storage.
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Andrew Gabriel
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Some of the fittings used have wire coils embedded in the plastic. To join them they apply current through the coil to weld the joint from the inside. I suspect these plugs are the connection points.
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I have seen them on the blue water plastic pipe. I think it is called electro welding. They heat up the plastic fitting connecting at the two terminals, which the fitting has copper wires running through it, welding the fitting to the plastic pipe.
This is a steel reducer from 1" to 3/4". Can they weld the plastic to the steel fitting?
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What is the maximum volume of gas these new small plastic gas mains can supply? Can it supply the maximum mater capacity? Do they have to ensure the volume of gas supplied was as before?
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It's not so much a maximum VOLUME you're interested in, but a maximum SUPPLY RATE (i.e. m3 per hour). As to what is the maximum supply rate for a gas main of a given size this would depend on the pressure of the main, which nobody here will be able to tell you, as it varies from main to main.
Regardless of the pressure of the main, the regulator in your meter will reduce the pressure to 20mbar working pressure. Therefore, regardless of your own consumption rate (up to a certain maximum usually detailed on the meter) , and regardless of the mains pressure, your meter will supply gas at the correct pressure.
Yes, if the pressure in the main were to drop significantly such that the mains pipes were unable to sustain the required supply rate, then you would end up with a pressure drop at the appliance end. But it is probably a safe bet that when the replacement mains were laid, calculations were made as to the pipe sizing bearing in mind the prevailing pressure of the gas main. It may have involved a rise in the mains pressure as a result of the replacement mains, as the new plastic pipe will be able to withstand a far higher pressure than its cast iron counterpart.
Luke
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Thanks Luke. I looked at the meter and its says 6 cubic metres an hour. I assume that is the maximum "volume" of gas it can pass. From what you say they have increased the mains gas pressure to compensate for the small 1/2" plastic pipe from the road to the meter.
From 1" steel down to 1/2" plastic to me seems a hell of a lot of downsizing. The gas pressure before the meter regulator must be substantially higher to compensate. I fear that when many people go over to these bigger combis the gas mains will not cope, whereas before using the old steel pipe it could. The gas company could be storing up problems for the future.
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Well... 6 cubic metres an hour is not a measure of volume. It is a measure of RATE, i.e. the volume the meter can supply *over a given time period*. To talk in terms of volume alone is not relevant, without including a time element to the calculation. What is means if that if you disconnected all your appliances from the meter and let the meter piss out gas into the air, over 1 hour, 6 cubic meters would piss out at a pressure of 20 mbar. So do you see that talking in terms of volume alone does not relate to the capacity of the meter?
Regarding the concern over the main sizing. Almost certainly the reason why the main was upgraded was twofold a) to replace ageing pipes which may fail and b) to allow an increase of pressure to be applied to the main due to increased gas consumption rate brought about (partly) by people installing large combination boilers.
You might be able to allay your fears if you contact the distribution company directly, but frankly, you are worrying needlessly. The dist. co. aren't going to upgrade the main without doing a detailed calculation of the expected demand, both present and future.
Luke
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Thanks Luke. I just did search and found this. http://webcache.googleusercontent.com/search?q=cache:GE3uaw2huWoJ:myreader.co.uk/msg/1391140830.aspx+%22uk.d-i-y%22+gas+main+plastic+%22ed+Siritt%22+cynic&cd=1&hl=en&ct=clnk&gl=uk
Someone had problems "after" a new plastic gas main was installed. I suppose it all depends if the gas engineers do their job properly in sizing up the gas mains.
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According to a recent National Grid visit the smallest alkathene domestic service pipe in the UK is 15mm, a fraction over 1/2". I did not enquire what the length restriction was on that (it might be around 20m or 32m for that size). Most people get a 20mm pipe because that will fit through most existing service pipes.
The street supply is something like 35 to 75mbar (min to max).
When you consider you can have a fair length of 15mm pipe inside and have only a 1mbar pressure drop, you can see there is a large amount of leeway between the high grid supply pressure and your meter regulator 21mbar output.
When I asked why gas pressure seemed to drop in the 2009 winter locally, he said that was almost always due to an undersized local grid supply combined with local emergency maintenance. The local grid was in fact upgraded in 2010 in the same street for just that reason (well partly, the sub-contractor went bust as it happens so only half got done).
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Much of the old street main cast iron and steel gas pipes were laid back in the days of town gas which had a lower supply pressure and hence a larger pipe was required. The change to natural gas at a higher pressure caused a lot of early failures but where the network was good and remained so it was kept in service. Add to that the friction pressure loss per metre in a steel pipe is greater than that in a smooth bore plastic tube and it is entirely possible the use of a smaller replacement pipe will deliver as much as and maybe more gas than the old pipe did when installed. Connection of plastic to metal pipes is a trivial exercise and can be achieved by flanged joints for large sizes, screwed threads, compression type sockets etc. You will not find plastic pipe used above ground within a domestic dwelling for fire resistance reasons although it may or may not be jointed where it emerges from the ground via a fibreglass or other protective sleeve. If the two plugs you mention are in a metal piece fitted between other metal pipes they are probably for pressure drop test purposes. Did you ask the installers?
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Thanks. The two plugs are about an inch apart. It was done a while back. Why two? They are not test nipple s as on the gas meter. They look like 1/4" threads.
Would they have connected the 1 1/4" steel pipe to the small plastic pipe under the ground? The ground looks undisturbed. If the 1 1/4" pipe had an elbow in it under the ground they could obviously not run that directly into the meter cupboard.
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On 1 Oct,

The original pipe was almost certainly sized for town gas distributed at low pressure. About when north sea gas first arrived the mains pressure was increased so that existing pipes would handle greater volume. Before then there was always a reduction of pressure prior to Sunday lunch due to the increased demand. The higher pressure and regulators by the meter have made this problem nonexistent now.
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So these meter regulators can take in 35 to 75mbar and give out a constant 21mbar?
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