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Gas service pipe size |
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| message from Set Square on 18 May 2004 |
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?
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| Christian McArdle replied to Set Square on 18 May 2004 |
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.
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| Set Square replied to Christian McArdle on 18 May 2004 |
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?
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| tarquinlinbin replied to Set Square on 18 May 2004 |
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.
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| Ed Sirett replied to Set Square on 18 May 2004 |
Yes, phenomenally.
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| BigWallop replied to Set Square on 18 May 2004 |
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.
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| Set Square replied to BigWallop on 18 May 2004 |
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.
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| Andy Hall replied to Set Square on 18 May 2004 |
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
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| tarquinlinbin replied to Set Square on 18 May 2004 |
Hi,
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.!
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