Gas service pipe size

[Set Square]

Contractors working for Transco are currently renewing the gas mains in my street - including lining the service pipe from the main to each house.

My service pipe is about 20 metres long in all - because it doesn't join the main directly in front of my house. This was piped mainly in 1.25" (ID) steel pipe - reducing to 1" for the last bit.

The "lining" process consists of pulling a plastic pipe - having an OD of

20mm and an ID of about 14mm through the original steel pipe. The effective bore of my supply pipe has thus reduced from about 33mm to 14mm - representing a reduction in cross-sectional area of about 82%! [Just to avoid any doubt, the area of the new pipe is only 18% of that of the original]

Is this likely to be a problem? I am aware of discussions here about gas pipe sizing - often resulting in recommendations for boilers to be fed by

22mm, if not 28mm, pipe - whereas I've got the equivalent of 15mm supplying my house!

A possible saving grace is that this is on the mains side of the governor - and is presumably at higher pressure than the gas flowing between the meter and the boiler.

This gas pipe renewal business seems to be going on all over the country - so presumably they are using the same approach everywhere. Does anyone have any (informed!) comments?

[Andy Hall]

On the main side, the pressure is somewhat higher - I believe around

60mB+ - so the linear flow rate through the pipe to achieve what is needed on the house side is less.

A domestic service is supposed to be standardised to a certain level on the consumer side (IIRC 62kW), so AIUI, the flow has to be adequate for that on the main side.

I could see a potential for problems if you wanted to implement an IMM

2xcombi setup but otherwise there should not be a problem.

Nevertheless, it might make sense to call Transco and ask them about it. That is if you can find any sentient beings there.

.andy

To email, substitute .nospam with .gl

[Christian McArdle]

The supplier's lines are at high pressure, so don't need to be as big. The meter has a regulator in it that drops the pressure to a pretty tiny value (20mB), which is then used by the property.

It is a bit like (a seriously rural!) electricity supply, with the incoming supply at low current 11kV on thin wires and then a transformer (equivalent to the regulator) dropping down to the usable 230V at high current using thick wires. Obviously, you can't push the analogy too far, as with electricity, it is pressure (voltage) and flow (current) that determine the energy transfer, whilst with gas supply it is only really the quantity of compressible gas supplied that matters, provided the pressure is high enough to get the burners to work.

Also, not only does the high pressure mean that the gas gets through easier, it is also possible to have a much higher pressure drop as the input side of the regulator will take a large range of input pressures, whilst a house system is only allowed to drop the tiny 1mB increment.

Christian.

[Set Square]

Thanks for that - it's somewhat re-assuring. I did suggest in my original post that maybe it was ok because of the higher pressure on the mains side of the governor. What pressure is used in the mains?

One does wonder why - if 1/2" pipe is adequate for a domestic supply - it was originally piped in 1" or bigger. One possibility is that the area was originally on town gas - but only for a couple of years prior to being converted to natural gas in the 1960's. Would that make a difference?

[tarquinlinbin]

It is standard practice to renew domestic service pipes in 20mm PE pipe. The reasoning behind this is that a standard domestic meter passes a max of 6 cubic metres/hr and your 20mm PE service which will be running at a pressure well in excess of the required 21mb internal working pressure,is able to deliver that volume with no problem. Larger houses which may require a supply greater than 6M3/hr would then have to upgrade to the next available size gas meter which is u16 (about 512 ft3/hr). Of course the length of pipe is a factor also but it must also be remembered that the mains in the road AND your service are being renewed in PE pipe which has a smooth internal bore and therefore less surface resistance to flow.

The reason why there is wholesale mains renewal going on across the country is that for years the industry regulator OFGEM, supposedly there to gurad the consumers interests,has severely curtailed TRansco;s expenditure,thus preventing it from doing mains renewals. Transco have wanted to lay lots of new pipe for years ,OFGEM prevented it becuase less captial expenditure means more money for shareholders.

In the last few years there have been a series of dramatic and fatal mains failures leading to explosions,death and destruction of property. People say that Transco is becoming the next railtrack. It isnt Transco;s fault,it is the regulators fault.

Becuase of all these high profile cases splurged across the press, OFGEM panicked and certain directors of OFGEM ,fearing for their own jobs,allowed capital expenditure once again by Transco,hence all the digging. If its happening in your area, i know its messy and inconvenient but Transco has your interests and safety at heart,which is more than the regulator does,so let them work and get it over and cleared.!

[tarquinlinbin]

Thats exactly it,towns gas service pipes were larger becuase of the lower calorific value and lower supply pressures. Also,in the good old days,things tended to be "over engineered" and made bigger,stronger etc.

[BigWallop]

You might be missing the point of the thin liner material. Some of these liners are made to expand inside the pipework when treated with either a chemical, or simply in the form of high pressure steam in some cases, is applied to them. The liner then expands to form a skin on the inside of the original pipe. The liner is made to withstand the effects of the substance flowing through it better than the original pipe would, and so lasts a lot, lot longer. So the diameter of the original pipe may not be reduced by as much as you might be thinking, as gas flow, like water flow, needs to be of a certain volume per' to be effective.

[Ed Sirett]

Yes, phenomenally.

[Set Square]

Sorry, I don't buy that! What you are describing certainly applies to lining sewer pipes - where they use air pressure to turn a big long "sock" inside out. But the stuff they have used on my gas pipe is *rigid* plastic pipe - with a wall thickness of about 3mm. After inserting it, they injected expanding foam to fill the gap between the plastic and metal pipes - so there's no way the plastic can expand in the manner you describe.