Running gas pipe

[megawatt]

Agile wrote:

Chris and Sanj, its pretty obvious that a transition from 28 mm to a smaller 22 mm pipe would be expected to set up a considerable turbulance and I would expect an additional pressure drop.

Has this become a personal forum Tony ... Do you not feel anyone else has anything to contribute?

I'm racking my brains here back to my gas turbine theory days (GT's work primarily on con-di/di-con nozzles ... Bernoulli again ... and manipulate velocity and pressure across the cycle for those who may not know).

Boyles law may have a bearing on what we are discussing

The law states that if the temperature of a confined gas is not changed, the pressure will increase in direct relationship to a decrease in volume. The opposite is also true — the pressure will decrease as the volume is increased.

As the main thing we are trying to do is to minimise pressure drop end to end shouldn't we always be trying to reduce volume across each stage to maximise pressure i.e. transition from larger to smaller pipe CSA's?

To clarify Chris's reply which arrived before I finished this one you get turbulence (non-laminar flow) at either transition con-di and di-con.

MW

 

[megawatt]

Need to go and do some work ... I'll be interested to see the outcome of this

 

[chrishutt]

To clarify Chris's reply which arrived before I finished this one you get turbulence (non-laminar flow) at either transition con-di and di-con.

I wonder if all the flow would be turbulent anyway since flow velocities will be quite high when gas pipes are operating towards the limit of their capacity. I think laminar flow would be associated with the sort of low velocities you might find in a river. We might be talking about degrees of turbulence rather than laminar/turbulent transitions.

And what is con-di supposed to mean? I think the design of the venturi is the key to understanding this.

 

[GeckoGas-com]

Errrr.... What the hell is this thread about?

If I'm reading this right I can have a block of flats with a meter on the ground floor; run the gas pipe in 15mm to the top flat and then have an inch of 22mm connected to it and apparently I'll get the right pressure? ******!

When has theory ever been relevant to the practical side of life; back to the original question - 28mm; then reduce to 22mm.


For all the rest - When you've solved this can you solve the chicken and the egg question for me please?

 

[gas4you]

Must admit I agree Gecko, this is all way above my head I'm a gas engineer end of, not a physicist, as long as I get my 20mBar at the appliance with only a 1mBar drop I don't care what goes on in the middle

 

[sooey]

Must admit I agree Gecko, this is all way above my head I'm a gas engineer end of, not a physicist, as long as I get my 20mBar at the appliance with only a 1mBar drop I don't care what goes on in the middle

so you're not interested in knowing the square root of a beetroot either then Dave.

 

[gas4you]

Spring onion maybe, beetroot no

 

[sanj.varah]

megawatt, the convergent divergent nozzles on the back of GT's are to make the flow choked. You then get more thrust from the choked flow. When the flow is subsonic, converging the nozzle increases velocity, you get mach 1 at the throat and then to accelerate a supersonic flow, you need a divergent nozzle.

Sorry I am digressing.

To sort of combine a few of the queries that people have mentioned:

Because gas is a compressible fluid, theory has it that if you went from the meter in

15 -> 22 -> 28mm

it would be the same pressure drop as

28mm -> 22 - > 15mm.

In reality, scenario 2 will always produce less pressure drop. There are some technical reasons for this but they are due to Reynolds numbers, fanning friction factors and the shape of the geometry change.

In scenario 1, the end user is pretty much in the lap of the gods as far as recovery goes, a venturi will give you a fair amount of recovery but its not 100% effective. Its better to go from a big bore to a small bore and compress streamlines.

Agile, if you go from the meter in 22mm and then switch over to 28mm, the main problem will be the loss in the reducer, you'll have more of a loss in the reducer than if you went from the meter in 28mm and then switched over to 22mm.

Looking through the copper org's website, I can't find anything about reducers. Will keep looking.

hope this helps.

 

[sanj.varah]

Sanj.varah, I've very grateful for your explanations. I think I follow most of it, but there's at least one matter that still puzzles me.

If a certain length (including allowance for fittings, etc.) of 22mm pipe has a pressure drop of say 0.5 mbar for a given gas flow and a much longer length of 28mm pipe has the same pressure drop of 0.5 mbar, then when the two pipes are joined end to end the total pressure drop for the given gas flow will be 1.0 mbar.

However you seem to be saying that it will make a difference if the 22mm pipe is upstream of the 28mm pipe rather than vice versa (due to "recovery"). I can't see this, since the resistance of each pipe is exactly the same. Is it due to greater turbulence arising at the junction between the two pipes where the smaller (higher velocity) pipe leads into the larger (lower velocity) pipe?

If the transition from higher velocity to lower velocity causes more resistance than vice versa, would it not depend very much on the smoothness of the transition and would it actually be significant for a typical in-line reducer? Also where the outlet from the gas meter is 22mm anyway would it then make any significant difference which pipe came first?

Yep spot on, the position of the reducer has a marked effect. Due to the nature of the copper fittings, its much better to have the reducer getting smaller than bigger.

Pressure drop is always greater when you are "expanding" the flow. You get flow separation which causes the pressure drop. Here's a picture, have a look at the top picture.

 

[megawatt]

Rather a long-winded way of saying what I said earlier about Boyles law Sanj

Chrishutt: Apologies for the abbreviations ...
di-con = divergent-convergent
and I'm sure you'd now get
con-di = convergent-divergent

Sanj: The gas turbine doesn't rely on di-con at the nozzle to produce thrust anymore, this was the design of single spool noisy beasts, ramjets and rockets and GT's rely on a combination of con-di and di-con throughout the cycle ... I'm sure you're aware also that in modern turbofans > 70% of the thrust is produced by the fan and only a minimal amount is generated at the nozzle.

MW

 

[Garthzog]

Hi,

Many thanks for all the replies etc - there has certainly been some interesting stuff in this thread!

For what it's worth, I was CITB gas trained in 1990 and my gas notes from then contained CORGI stuff (I just checked - CORGI 1984). I had to be trained up since I had to maintain and repair oil and gas fired boilers in large buildings as well as all aspects of building maintenance including lifts and refrigeration (amongst other things). Since then we have moved away from the heating/lifts side of the work so my gas ticket was allowed to expire. I still do refrigeration and brazing though so I can solder pipework, so I was hoping that would go someway to making me competent(ish).

I do know someone indirectly who is CORGI registered, and I was hoping to get him round to tell me what I could do with regards to the install etc in order to cut costs, but also wanted to get a good idea in my head of what I may have to do in advance so that I know what could be done and give him the confidence to let me do the vast majority of the work before he did his commissioning (and whatever else he preferred to do).

I have fitted my own combi boiler and central heating in the past, but don't have the experience you guys have with condensing combi's, which is why I thought I'd ask here first about pipe sizing, since your experience has more value than the chart in this case judging from what I'd previously read here.

I haven't yet decided on the exact location of the boiler which is why I wasn't sure on the exact run, but from your comments it looks like 28mm will have to be used and probably to a greater degree.

Anyway, I just did an exact measure of the pipework out from the meter as it was asked for:

There is about 0.1m (4ins) from the meter outlet flex to the first iron elbow which is 3/4" (add 0.5m for the elbow), then another 0.3m to the proposed 22mm copper elbow (+0.5m) which is measured straight through the iron T (+0.5m) which tees to the hob (that goes down to 1/2"). I estimated that would be equal to around 1.9m there.

The run to the boiler location will be around 6.1m with 4 bends (2 to get the pipe under the floor, and two out and up to the boiler) - if it was 22mm 3 of which can probably be done with the bender but if I use 28 then I'll have to use elbows so that looks like around 8.1m worst case. I can probably add another 1.5m for where I'd prefer to put the boiler.

I've seen some installs where 22mm has come out of the existing pipework near the meter to a new gas valve, and the pipework has gone to 28mm after that. That has already been suggested here and sounds like a good option for ease of getting from the meter to 28mm and a straighter run.

As for the pipe work going from 22 to 28 then back to 22 - if the chart gives us resistance to flow as pressure drop, can't those values can be calculated and added together (assuming they were accurate of course)? So if you used the chart you'd know how much your combinations of 15, 22 and 28 would drop for example. That would preclude you from running a long run of smaller pipe and a short run of larger because the calculation for the smaller pipe would already tell you that you've exceeded the allowable amount of drop.

It was said here that quite often solder flow into the pipe can reduce flow, so does that mean that compression fittings would ensure a flow closer to optimum and be a better option?

Thanks again

G

 

[Onetap]

Er, no. In this case you're measuring the static pressure at the meter and the appliance with all the appliances running. The static pressure would drop if the column of gas started to move, but it's already moving here, so the loss of pressure is solely due to friction or maybe changes in the velocity due to pipe size.

I don't understand what your saying. But to reiterate, 20mbar is a static pressure that comes out of the governor. When everything is switched off your total pressure = static pressure (because dynamic pressure is zero).

When you start an appliance up the total pressure is still 20mbar, but the static pressure will drop to lets say 18mbar, because the gas flow is moving. the 2mbar is dynamic pressure.

don't know if that actually answers the question but i tried!

Yes, agree with that; if you measure the gas pressure at no flow and the gas then started to flow, you would expect the measured static pressure to drop, to compensate for the energy in the increased velocity pressure.

However, it doesn't apply in this case because you're only measuring the static pressure at the meter test point with the gas flow at the maximum and the static pressure at the appliance, also with the maximum gas flow. There are velocity pressures, but they don't come into the pressure loss measurement.

Two other points which aren't relevant to the previous.

The 20 mbar measured at the meter should remain constant, whether or not the gas is moving, bacause the governor (pressure reducing valve) should open up to maintain the 20 mbar at it's outlet as the gas flow rate increases.

Also, any negative pressure generated at the end of the gas pipe by a boiler fan wouldn't affect the pressure loss along the pipe. The pressure at the governor outlet should still be 20mbar and the pressure at the appliance should still be 20 mbar minus the pressure losses due to friction along the gas installation.

 

[megawatt]

I've seen some installs where 22mm has come out of the existing pipework near the meter to a new gas valve, and the pipework has gone to 28mm after that. That has already been suggested here and sounds like a good option for ease of getting from the meter to 28mm and a straighter run

The crux of the debate you've followed has been focused on the legitimacy of this ... IMO there is no benefit.

It was said here that quite often solder flow into the pipe can reduce flow

No competent installer would make such a mess of the soldering to have any negligable effect on flow IMO. If this is an issue for you, rather than opt for compression fittings opt for someone competent to do the work for you instead.

MW

 

[Onetap]

Regardless of what the fluid dynamics expert thinks the plain fact is that people who regularly measure the pressure at a gas appliance find that its always less than the calculated value. Usually 20-70% less.

Tony

I find that quite mind boggling; I don't have much involvement with gas so I have no reason to doubt what you say is correct.

I have had numerous occasions to check pressure losses in water systems (usually because the pipes/pumps are undersized). I've found the design estimates can deviate from the actual pressure losses by something like 30% because the designer can't anticipate all the amendments required on site. If you work through an actual installation, the calculated preessure loss is usually spot on (+ or - about 5%) with the CIBSE guide, unless there's something strange going on (Coke cans in the pipes, spades left in flanges, etc.). The tabulated pressure losses are the results of actaul measurements, not theoretical calculations .

If you can't rely on the design data, how can you produce a working design?

 

[megawatt]

There is no reason for it, physical laws are invariant and I would suggest that it says something about the competence of the person doing the calculations