Running gas pipe

[sanj.varah]

Even boilers with zero gas pressure where the gas and air mix cannot really be said to "suck" gas from the gas supply pipe since the appliance inlet pressure should still be at least 18 mbar, although they will operate equally well at lower pressures (which is perhaps what you meant?).

Its difficult to describe this unless you get into absolute pressures and static/dynamic/total pressures. and i don't want to bore the tits off everyone in here.

 

[Onetap]

Just to clarify something else. Bernoulli's equation does not apply to gas, it only applies to incompressible fluids which are also inviscid (have no viscosity/friction). Although having said that, it is readily applied to water calcs. The governing equations for gas flow are Navier Stokes.

Really?
Well, that is a shock !

I was so surprised to hear that, that I hoiked out the first fluid mechanics textbook I could find (Douglas, Gasiorek & Swaffield) and looked up Bernouilli in the index. It referred me to Page 347 for Bernouilli's equation "in compressible flow". It doesn't mention Navier or Stokes (except Stokes' theorem in relation to 2D ideal flow).

Bernouilli's equation is about the conservation of energy, so could be applied to any flow in a pipe or duct.

Its difficult to describe this unless you get into absolute pressures and static/dynamic/total pressures. and i don't want to bore the **** off everyone in here.

Does that mean you were talking bolleaux?

Do try, we're all eager to learn from you.
Are you stating that the fan will generate a negative gauge pressure at the end of the gas train?

 

[Agile]

Although what they teach in college or whatever states that it makes no difference where the bigger bore section of pipe is, in reality due to the low pressure of gas, its always better to have 28mm at the source and then getting smaller with branches. This is because you never ever get 100% recovery.

I'd just also point out, that the calcs the OP hyperlinked allow for a pressure drop of 0.1mbar. Agile stated that pressure drop is 1-2mbar in a combustion chamber, that more than negates that effect.

I really do have difficulty in understanding why a fixed restance like a straight length of pipe can make any difference where it is in the run.

You also seem a little confused because you go on to talk about a pressure drop of 0.1 mB. With Nat Gas we are allowed 1 mB pressure loss on a nominal 21 mB supply pressure.

The pressure drop in a combustion chamber caused by the exit fan is 1-2 mB relative to the atmosphere. The gas valve contains a constant pressure regulator supplying about 10 mB and I would take the view that that depression has little effect on the supply pipe.

We are interested in these things.

Tony

 

[sanj.varah]

I was so surprised to hear that, that I hoiked out the first fluid mechanics textbook I could find (Douglas, Gasiorek & Swaffield) and looked up Bernouilli in the index. It referred me to Page 347 for Bernouilli's equation "in compressible flow". It doesn't mention Navier or Stokes (except Stokes' theorem in relation to 2D ideal flow).

Bernouilli's equation is about the conservation of energy, so could be applied to any flow in a pipe or duct.


Does that mean you were talking bolleaux?

I didn't make myself clear. But the standard form which was previously stated by "onetap" cannot be used for compressible flow. Purely because density is not constant. The Compressible version of bernouilli's equation is really no good in this instance because you can't get a pressure term out. I am not denying you cannot apply it to a flow in a duct, its just it doesn't work with compressible fluids because the concept assumes a barotropic fluid (a fluid whose density does not change with temperature). I can't think of a fluid that when heated, does not get less dense!? can you?

The limitations are discussed in the following books:

Fluid Mechanics - FM White
Eastop and Mckonkey - Engineeing thermodynamics
Intro to fluid mechanics - Fox

here's the link on wikipedia.
http://en.wikipedia.org/wiki/Bernoulli's_equation

Navier Stokes equations are widely accepted as standard issue for modelling compressible flow etc.

http://en.wikipedia.org/wiki/Navier_Stokes

As for your other question. There is a concept of "total Pressure".

Total Pressure = Static Pressure + Dynamic Pressure.

Pt = Ps + 1/2 x rho x V^2

Assuming no work is done on a gas, then the total pressure in the system is fixed.

If you crank the fan, then the gas starts to move, it has a velocity V. therefore the Static Pressure drops to keep the equation valid.

hope thats sufficient clarification

 

[sanj.varah]

I really do have difficulty in understanding why a fixed restance like a straight length of pipe can make any difference where it is in the run.

You also seem a little confused because you go on to talk about a pressure drop of 0.1 mB. With Nat Gas we are allowed 1 mB pressure loss on a nominal 21 mB supply pressure.

The pressure drop in a combustion chamber caused by the exit fan is 1-2 mB relative to the atmosphere. The gas valve contains a constant pressure regulator supplying about 10 mB and I would take the view that that depression has little effect on the supply pipe.

We are interested in these things.

Tony

sorry the 0.1/1.0 was a typo.

In the grand scheme of things the difference of 1.0mbar-2mbar will make small difference. But as a percentage its 10% which is significant.

Don't understand about the fixed lengths of pipe? care to elaborate.

 

[Onetap]

I didn't make myself clear.

No, you didn't. You'd said Bernouilli's equation doesn't apply to gas.
Your Wikipedia link gives a version applicable to compressible flow. The version I'd given was for incompressible flow, but it's been a long time.

I can't think of a fluid that when heated, does not get less dense!? can you?

Yes. Water, between 0 to 4 degC.
So there! Nah nah na na-nah!

The limitations are discussed in the following books:

Fluid Mechanics - FM White
Eastop and Mckonkey - Engineeing thermodynamics

I have Eastop and McConkey on my desk. Whereabouts? I can't find it. It doesn't mention Bernouilli or compressible flow.

If you crank the fan, then the gas starts to move, it has a velocity V. therefore the Static Pressure drops to keep the equation valid.

hope thats sufficient clarification

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.

 

[Agile]

"""The pressure drop in a combustion chamber caused by the exit fan is 1-2 mB relative to the atmosphere. The gas valve contains a constant pressure regulator supplying about 10 mB to the burner and I would take the view that that depression has little effect on the supply pipe."""

Can you explain how exactly how, in this context, you conclude that the small negative pressure at the delivery port of the gas valve will make a difference to the pressure loss in the pipe.

Surely the pressure loss in the pipe is as a result of the flow rate which for a boiler is about 2.5 m³ per hour.

I dont understand why the slight negative pressure in the combustion chamber becomes relevant. Please Explain!

Tony

 

[sanj.varah]

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.

There are additional losses such as pipe friction which would contribute to more terms in the equation. so you would have

Total Pressure = Static Pressure + Dynamic Pressure + Skin Friction Losses + Geometry Losses

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

 

[sanj.varah]

"""The pressure drop in a combustion chamber caused by the exit fan is 1-2 mB relative to the atmosphere. The gas valve contains a constant pressure regulator supplying about 10 mB to the burner and I would take the view that that depression has little effect on the supply pipe."""

Can you explain how exactly how, in this context, you conclude that the small negative pressure at the delivery port of the gas valve will make a difference to the pressure loss in the pipe.

Surely the pressure loss in the pipe is as a result of the flow rate which for a boiler is about 2.5 m³ per hour.

I dont understand why the slight negative pressure in the combustion chamber becomes relevant. Please Explain!

Tony

Yeah I can explain this, when you induce a slight negative pressure in the combustion chamber. You are actually doing work on the gas, you're putting energy into the system. Therefore the pressure drop gets less.

 

[chrishutt]

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?

 

[megawatt]

Chris,

Any given static pressure will support a maximum flow of a fluid (or gas) based on the cross-sectional area (CSA) of the vessel carrying it (other factors understood but CSA is the major).

If the smaller CSA is upstream then this will be the limiting factor to flow rate and irrespective of how big you make the CSA of the pipework downstream surely it cannot increase the flow rate upstream once its capacity has been reached ... At least not without increasing static pressure?

If this wasn't the case installations could be run in 15mm ... I appreciate some bodgit and leggit installers do run in 15mm but then they invariably p1ss around with the governor to increase the pressure

Is this a purely theoretical discussion or have you actually ever come across installations where the pipework is configured ...

Meter -> 15mm -> 22mm -> 28mm - 22mm -> Boiler?

I certainly haven't and would've been quite puzzled if I had

MW

 

[chrishutt]

Is this a purely theoretical discussion or have you actually ever come across installations where the pipework is configured ...
Meter -> 15mm -> 22mm -> 28mm - 22mm -> Boiler?

But there's no reason in principle why such a configuration shouldn't deliver a given flow rate of gas with a 1.0 mbar pressure drop. Let's say for the sake of argument that each of the 4 pipe sections has a drop of 0.25 mbar - total 1.0 mbar! Obviously the 15mm section will be very short to impose no more than 0.25 mbar drop for the sort of gas flow rate required by say a combi boiler.

I'm sorry but the earlier part of your post doesn't make a lot of sense to me. In the example I gave both sections of pipework - the shorter length of 22mm and the longer length of 28mm - have exactly the same resistance and pressure drop. Why should it matter which section comes first?

 

[Agile]

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.

Conversly I would expect a 22 mm tube discharging into a 28 mm tube would not apparently create any additional resistance.

Please could you explain this.

Tony

 

[megawatt]

I'm sorry but the earlier part of your post doesn't make a lot of sense to me. In the example I gave both sections of pipework - the shorter length of 22mm and the longer length of 28mm - have exactly the same resistance and pressure drop. Why should it matter which section comes first?

Maybe because it's Sunday morning and my brain's still fried from last night but the simple answer to your question is ... I don't know ... But it just doesn't feel right

Have you ever seen what you are suggesting though is still my question ... In other words, are we debating a mute point and would there ever be a practical reason to do as you suggest?

MW

 

[chrishutt]

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.

Conversly I would expect a 22 mm tube discharging into a 28 mm tube would not apparently create any additional resistance.

Please could you explain this.

Tony

I think it might be the other way around, Tony (although I'm hoping Sanj. will give us a more authoritative response). If you think about the shape of a venturi, or an aerofoil, the leading edge has a sharper transition than the trailing edge. This is because there tends to be more turbulence after a constriction than before it so a smoother transition downstream is required to minimise the turbulence and the frictional loss.

By that logic the larger pipe should come upstream of the smaller pipe so that bore changes are like the leading edge of the venturi rather than the trailing edge, since a typical reducing fitting has quite a sharp transition.

However if the outlet from the gas meter is already 22mm nominal size there will have to be one step up from 22mm to 28mm with its attendant greater resistance so it probably doesn't matter where it is. At least that's how I understand it at the moment.