Gas pipe sizing

[doitall]

i divvent spell rongly bonny lad that's ow i speek ask DIA

marra

If you mean can I understand him then not a cat in hells chance

A very nice guy, just talks funny like.

 

[kevplumb]

EXACTLY i rest my case

now then marra

 

[doitall]

Back to the problem, No 28mm isn't large enough.

You have 4.9m³/hr on 45m of 28mm pipe, whereas the actual discharge for that length allowing for bends is 3.8m³/hr

 

[chrishutt]

Anyone remember this?

http://www.slumberlands.freeserve.co.uk/gasflow.htm

DIA, I'm afraid your wrong. The flow rate through the 45 metres of 28mm is not 4.9 cu.m/hr.

That flow rate only applies to the first 10 metres (pressure drop 0.35 mb). The next 20 metres takes 4.7 cu.m/hr (pressure drop 0.6 mb) and the final 15 metres just 3.7 cu.m/hr (drop about 0.3 mb). Total pressure drop is 1.25 mb, only marginally over the notional 1.0 mb allowed and perfectly OK in practice.

 

[ChrisR]

You could try the formula derived from the tables, which is:

Flow = 34.15(pressuredrop/length)^0.5847 for 28mm pipe.

Which gives flow for 1mbar drop over 50m as 3.467 m³/hr.

= 'bout 38.5kW

But we all know the tables are miles out. Allow 50% - 100% more drop than they say.

 

[doitall]

Anyone remember this?

http://www.slumberlands.freeserve.co.uk/gasflow.htm

DIA, I'm afraid your wrong. The flow rate through the 45 metres of 28mm is not 4.9 cu.m/hr.

That flow rate only applies to the first 10 metres (pressure drop 0.35 mb). The next 20 metres takes 4.7 cu.m/hr (pressure drop 0.6 mb) and the final 15 metres just 3.7 cu.m/hr (drop about 0.3 mb). Total pressure drop is 1.25 mb, only marginally over the notional 1.0 mb allowed and perfectly OK in practice.

Yes I know what you are saying Chris, but the 28mm is still too small for the total load.

the total length of 45m will discharge 3.8 m³/hr, or sufficient for the last appliance only, when you add the other 2 appliances the first 10m needs to be 35mm.

 

[chrishutt]

DIA, with the 28mm pipe the pressure drop will be 1.25 mb or Thereabouts. Are you seriously saying that such a drop is unacceptable?

Sure, ideally the first 10 metres would be 35mm, but as you know that presents a problem for us domestic guys, so we'd just live with the appliance inlet pressure at 19.75 mb. instead of 20 mb.

ChrisR may have a point about the tables being inaccurate, but that's a separate issue.

 

[kevplumb]

DIA, with the 28mm pipe the pressure drop will be 1.25 mb or Thereabouts. Are you seriously saying that such a drop is unacceptable?

Sure, ideally the first 10 metres would be 35mm, but as you know that presents a problem for us domestic guys, so we'd just live with the appliance inlet pressure at 19.75 mb. instead of 20 mb.

the rules have changed chris

 

[chrishutt]

You mean we can do 35mm now? I heard a rumour, but where do I find chapter and verse on that?

 

[doitall]

Not at all Chris, I wouldn't worry about a 1.25 drop, but I disagree with your calculations

 

[kevplumb]

You mean we can do 35mm now? I heard a rumour, but where do I find chapter and verse on that?

i always could marra

 

[doitall]

A-B 1MB.
B-C 0.62MB.
B-D 2MB.
D-E 1.2MB.
D-F 1MB.

PLEASE NOTE B-D,

ITS UNEXCEPTABLE.

 

[bster]

how did you get the mb figures.

I have read up about converting the pipe lengths for .33mb pressure drop (3 X section length = Xmeters). but what do i do to get the pressure drop in mb (as you have).

cheers.

 

[doitall]

The chart you are looking at is for a permitted drop of 1mb. You have 3 legs; therefore the permitted drop per leg is 0.33, a you said.

The length from A-B is 10m, and has a total gas rate of 4.9m³/hr.

Now for the tricky bit

because the chart is for 1mb you times the length of pipe x 3.

10m x 3 =30m

according to the chart 30 will discharge 4.8m³/hr, = 1mb drop

All the legs are calculated the same except B-C which only has 2 legs so its 0.5 and times 2.

 

[chrishutt]

DIA, if 30 metres of 28mm pipe with 4.8 cu.m/hr has a pressure drop of 1.0 mb, then 10 metres (1/3 of 30m) of the same pipe at the same flow rate will have a pressure drop of 0.33 mb (1/3 of 1.0 mb).

You get more or less the same figure if you use the nomogram that I linked to. Have a good look at it.

Also it doesn't matter what the permitted drop is on each leg, provided the total for each appliance is not (much) more than 1.0 mb. In practice pressure drops of 2 or 3 mb to an appliance are common and are within the operating parameters of gas appliances.