5 replies to this topic

[dowen]

G'day all,

I am a graduate engineer reading through a standard used by domestic plumbers to size gas pipes. The tables require the plumber to assume a pressure drop through the pipe (which will depend on the supply pressure available and pressure needed at the appliance), then design to that drop and the required flow rate of the appliances. From these, they pick the table representing their desired pressure drop, look up the length of pipe they need, look down the table for the flow rate they need (MJ/hr), then look across for the pipe diameter. I've attached a copy of one of these tables to show what I mean.

The thing that confuses me is that some areas of the table are not recommended - it hits a velocity threshold. At this point problems such as erosion and noise start affecting it. These elements are at the shortest pipe lengths and largest diameters. I would have thought velocity would be higher with smaller diameter pipes, and I'm not sure why length of pipe would affect it.

All I can think is that since the tables have been designed at constant pressure drop, large diameter and short pipes give unrealistically high gas flows. And it is these gas flows that cause a high velocity rather than any properties of short, wide pipes.

Can anyone think of another reason these problems would occur?

Edited by dowen, 10 January 2011 - 06:29 PM.

[daryon]

From these, they pick the table representing their desired pressure drop, look up the length of pipe they need, look down the table for the flow rate they need (MJ/hr), then look across for the pipe diameter. I've attached a copy of one of these tables to show what I mean.

You forgot the attachment

[Art Montemayor]

Dowen:

The topic of compressible flow is always fascinating and contains its customary ingredient of “intrigue” that always is subject to visualization and interpretation. Inherently, it is not an easy subject to understand completely at one sitting. But the challenge of continuing on and fully understanding the phenomena is intriguing.

Although, as Daryon has pointed out, you have obviously forgotten to push the magical “Attach this file” button (as I often have) you nevertheless have described your query in detail. I believe I understand your basic query and can start to address the issue(s) presented here:

You are basically correct in stating that the gas velocity would be higher with smaller diameter pipes – for a given fixed mass gas flow rate at a given pressure and temperature. However, you are straying away from the reality of compressible flow when you doubt that the length of a pipe would affect the same gas average velocity. It is the inherent nature of compressible (gas) flow to INCREASE ITS SPECIFIC VOLUME (decrease its density) WHEN IT DECREASES IN PRESSURE. And it is precisely that effect (a pressure drop) that occurs as the gas flows through the pipe. Note that here, I am presuming constant gas flow within a fixed diameter pipe in a given direction. This basis requires that there exist a pressure drop (a driving force) through the pipe as the gas flows. Otherwise, there can be no flow – as stated in Fourier’s Law.

Accepting the fact that there is a pressure drop associated with the gas flow establishes the fact that the gas density is decreasing as the gas flows downstream and differentially is getting lower and lower in pressure. That being the case, then THE GAS VELOCITY IS INCREASING IN THE DOWNSTREAM DIRECTION due to a differentially decreasing pressure since gas velocity = gas volumetric flow rate / flow area. Since the gas volumetric flow rate is increasing (lower pressure means larger volume) and the flow area is constant, the gas velocity is, indeed, increasing.

I do not agree with your statement (and the tables, if indeed they reflect what you state) that the highest gas velocities are realized at the shortest pipe runs and the largest pipe diameters. In actuality, the effect is usually the other way around. Since I don’t have the tables (yet), I can’t comment on what you are interpreting. However, I am confident that the tables (if they are correctly done) do not express what you state.

You are correct in stating that large diameter and short pipes give high gas flows. That is what large diameter pipes are for: to give a bigger gas flow capacity. They do this by offering less frictional pressure drop – due to the LOWER GAS VELOCITY existing in the larger diameter.

I will await your uploading of the gas tables.

[dowen]

Oops - first post and stuffed up the attachment Should have worked now.

The shaded areas on the left are the non recommended pipe sizes.

Thanks for the summary of compressible flow. That much makes sense. Makes me think my first instinct (surely short pipes shouldn't be worse than longer ones) was correct. I agree - I'm sure the tables are correct, it just seems very strange to me.

Attached Files

•  pipesize.PNG   62.43KB   46 downloads

[daryon]

Oops - first post and stuffed up the attachment Should have worked now.

The shaded areas on the left are the non recommended pipe sizes.

Thanks for the summary of compressible flow. That much makes sense. Makes me think my first instinct (surely short pipes shouldn't be worse than longer ones) was correct. I agree - I'm sure the tables are correct, it just seems very strange to me.

The tables show that you get less flow through a longer length of pipe (i.e. fisrt line shows a recomendeds energy flow of 617 MJ/h for 2m DN 15mm pipe & 188 MJ/h for 18m pipe of the same diameter), but it's not imediately clear why they report flows for small bore, short lengths of PVC pipe that aren't recomended for use. Why not present a lower energy flow rate that would be acceptable to use? Could you post paragraph F1?

[Art Montemayor]

Dowen:

I agree with Daryon's comments. I would appreciate it very much if you could upload - in a scanned, .pdf format - the complete F1 paragraph mentioned. Also, could you also tell us what is the heat content of the Natural Gas taken as the basis in order to show the flow rate as an "energy" flow rate? In other words, is the natural gas assumed to have 1.055 MJ/ft3? Here, we would need the "standard" conditions taken to define the volumetric flow rate.

I am attaching a transcribed copy of your submitted data in an Excel workbook. This is a much more readable version of the data and it can be confirmed easily by calculations if we all know the basis that was used to generate the flow figures given in the table. That is why I am anticipating that paragraph F1 will tell us all that -- I hope!

I have not had the time to check the data that I read off of your .png submittal and keyed into the spreadsheet. I would greatly appreciate it if we could have some peer checking from our members reading this thread and a confirmation that my product is accurate and correct as presented. This data expressed in this spreadsheet format can be of profitalble use to some member in the future.

Attached Files

•  Natural Gas Flow in PVC Pipe.xlsx   13.93KB   46 downloads