7 replies to this topic

[THTee]

Dear all,
I would like to know how to design the pipe size for self-venting gravity flow system, I can use Manning Equation (Q = 1/n x A x R^(2/3) x S^(1/2)to do the calculation but I did not know what is the best water depth (distance from pipe level to water serface) to use that applicable for self-venting circular pipe.
I also came across this statement : " A properly designed drain line with gravity flow will be self-venting if the liquid
Froude number (dimensionless parameter),is less than approximately 0.3", is it correct ? If yes, then I can cross check the answer abtained from Manning equation.
Thank you very much.

[Art Montemayor]

I have addressed this important subject in one of the many Excel workbooks I have posted for downloading in these Forums in the past. I included a detailed article by Hill in one of my workbooks titled¨"Art's Fluid Flow". If you use the Search function, you should find it.

[breizh]

THTee
An other source will be piping standard design from NORSOK you can download from internet .
let you try google : NORSOK STANDARD P-001 .

hope it helps
Breizh

[katmar]

THTee,

Art Montemayor and Breizh have both given you good pointers, but before you apply any design methods you need to get clear in your mind whether you are talking of a vertical pipe (eg barometric leg), or a near horizontal sloped pipe (eg storm water drain), or an over-flow outlet at the top of a tank. Each instance needs to be addressed slightly differently.

[THTee]

Dear Mr. Art Montemayor, Mr. breizh and Mr. katmar,
Sorry that I did not state my problem clearly. I had work out my problem in the spreadsheet and also drawn the process flow to explain further on my question. I got two main problems here : 1. Can I assume water depth y = D/3. 2. For self-venting system (Slope pipe) Froude number should be equal or less than 0.3 ?

Dear Art Montemayor,
I did download the "Gravity flow.xls" that posted by you earlier. But I suppose it is dealing with Vertical Gravity piping system.

Dear Mr. breizh
I had downloaded the NORSOK STANDARD P-001 from internet. It stated that for liquids flow by gravity, the downward slope should be 1:100, however, this requirement will be cost more to construction because i need a deeper tank and pipe to cater for the slope. We are using the old plant to convert into die casting plant. The water tank and trench is constructed earlier and the slope allowed is about 1:200 without major construction.

Thanks again for your kind reply.

Best regards,
THTee

Attached Files

•  Gravity flow pipe design.xls   1.29MB   1466 downloads

[daryon]

Hi THTee,

I read a paper on "Designing piping for gravity flow" but P.D. Hills, at ICI in the paper he suggests that for near-horizontal piping that is self-venting a minimum slope of 1:40, to provide static head to overcome frictional losses. This may be a bit conservative.

Hills also says "To aviod having the liquid carrying gas foward, adequate free area must be left in the pipe to allow gas to pass backward. For pipes 200mm dia. liquid depths should not be more that half the pipe diameter. For large pipes, depths up to three-fourths of the diameter may be possible".

Yep... he also says Fr < 0.3

[demank]

After reviewing your spreadsheet,
I agree with 8 in. pipe, If using 8 in. pipe, the slope will be 1 : 880 in my calculation.

[katmar]

THTee, I like the thorough way you have set out to do this design, but like all engineering design the final decision is an economic one and not a purely technical one. If the distance from your casting machines to the hot well is short, say < 20 metre, then I would say rather than waste too much time designing and optimizing just put in the 300 mm pipe and be done with it.

However, if the distance to the hot well is substantial then the cost differential between installing a 300 mm pipe and a 200 mm pipe can be substantial and then it would be worth spending money on design to save money on construction. Also, the practical implications of building a long line with a constant slope needs to be considered. If your sloped line has a low point anywhere and the line is forced to run full at that point then you could get slugging and you will have lost the benefit of self venting design.

Please see the attached sketch as an alternative.  Alternate self venting layout.xls   18KB   764 downloads

If, for example, the distance to the hot well is 200 metres and the allowable slope is 1:200 then you have a height difference between the start and end of the line of 1 metre and therefore a head of 10 kPa. The head required to get a flow of 1000 litre/minute in a 200 mm pipe running full is only about 3kPa, which is less than the 10 kPa you have available. In this case I would install a short piece of vertical self venting pipe and then reduce the diameter to 200 mm and run a horizontal line to the hot well. This would be cheaper and it is much easier to construct a horizontal line than a sloped one. The syphon break is important to prevent the U-leg being drained and causing intermittent cycling. Depending on your actual height differential you may even get away with a 150 mm line.

Edited by katmar, 03 September 2009 - 03:40 AM.