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Low Flow in Pipes- posted in Ankur's blog

Sealing Flow/minimum Flow Required To Ensure A Full Line


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#1 ryn376

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Posted 22 August 2016 - 01:36 PM

According to some other forums and resources that I ran into researching gravity flow, the sealing flow rate seems to be given by the following equation: Q = 10.2 *D^2.5 where 'Q' is in gpm and 'D' is in inches. I believe this is the original article where the equation came from:

http://www.globalspe...ontal-run-pipes

 

This seems to be a very high flow rate. At the smaller diameters, this is close to the max pressure drop I would recommend per 100 ft and at the larger diameters above the max velocity I would recommend -- meaning most pipes I've sized and done pressure drop calcs on have been below the sealing flowrate. This leads me to believe that this is either an inaccurate equation or not very useful equation (since most pipe lines in chemical plants do not meet this criteria). What do you think? Is there a better equation?

 

I have attached a simple excel calculating the flows and velocities for small diameter pipes.

Attached Files


Edited by ryn376, 22 August 2016 - 01:38 PM.


#2 Bobby Strain

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Posted 22 August 2016 - 06:41 PM

I have designed lots of piping systems over 45+ years. Never have I encountered such a concern or requirement. Maybe Harvey can weigh in on the subject.

 

Bobby



#3 ankur2061

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Posted 23 August 2016 - 01:47 AM

Hi,

 

I had prepared a standard for a ME company wherein one of the topics covered was gravity flow. In a sub-topic related to near horizontal lines with certain slopes, the maximum flow was mentioned in a tabular form for 2 sets of pipe slopes (1:50 and 1:100) and various pipe sizes. The table assumes that the pipe filling is 75% and ensures enough sealing to prevent excessive pressure drops and vapor locks for gravity flow. The source of the table is the book "Fluid Mechanics with Engineering Applications by Finnemore and Franzini and has a basis in the Manning formula. I am reproducing the table for review.

 

Attached File  Flow_Capacity_Near_Horizontal_Pipes.png   11.68KB   16 downloads

 

 

Hope this helps.

 

Regards,

Ankur.



#4 katmar

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Posted 23 August 2016 - 04:35 AM   Best Answer

I have also felt that the Durand and Marquez-Lucero equation generates rather high flow rates but it is for a very specific situation that is rarely encountered in process plant design.  This equation is for open ended pipes discharging freely into the space above the liquid level in a tank or pond.  It is far more usual to find pipes that discharge into a tank below the surface level, or are kept full and hydraulically sealed by control systems.

 

And even when you do have a situation where the end of the pipe is not hydrulically sealed and does discharge above the liquid surface it would be unusual to require the pipe to be running full.  So I have to agree with Bobby Strain that it is not something we come across often.

 

I can recall only one instance where I have seen that it was relevant and important.  And although Bobby says he has never seen such a situation I think it is probably because his good pipe designing skills did not allow him to get into this bad predicament!

 

I was asked to trouble shoot a situation similar to that shown in the attached sketch.  These are not the real numbers - it was too long ago for me to remember them exactly but I have set up a similar situation to illustrate the principle.

 

Tank 1 overflows at a rate of 35 m3/h through 63 m of 100 NB HDPE pipe to the free space above the liquid in Tank 2.  The run from A to B is 50 m long and is horizontal.  Section B-C drops vertically for 3 m.  The final 10 m from C to D is horizontal.  The design engineer had calculated that the friction loss would be 980 mm of water column and since the actual difference was 3 m it was believed to be a safe design.  In practice Tank 1 overflowed and the line was not able to cope with the 35 m3/h flow.

 

As I said, Bobby would never design something like this and all of the experienced engineers reading this have already spotted the mistake.  For the sake of any inexperienced engineers I will show how the Durand/Marquez-Lucero equation can explain the problem.  This equation (and the table prepared by ryn376) gives the sealing flow for a 4" (100 NB) pipe as 335 USgpm (75 m3/h). This indicates that the horizontal section from C to D will not run perfectly full at 35 m3/h - although it would be more than 75% full according to Ankur's table.  It does not matter how full it is - as long as there is a continuous air layer above the water it means that the pressure at C is the same as at D.

 

If the horizontal section from C to D is not full then the vertical section from B to C is even less likely to be full.  And if there is a continuous air portion in the vertical section it means that the pressure at B is the same as at C.  So we come to the conclusion that the pressure at B (or just below B ) is atmospheric.  If we calculate the pressure drop from A to B (including the entrance and acceleration effects) we see that it is almost 1 m of water column and since the pressure recovery the design engineer expected to get from the drop between B and C does not exist the only way to get this head is to increase the level in Tank 1 and that is why it was overflowing.

 

So. even if we do not use this equation to design pipe systems that we actually build, it is useful in enabling us to avoid design mistakes.  It may be that the equation predicts values on the high side, but I believe that pipes designed this way will definitely be full.

Attached Files



#5 ryn376

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Posted 25 August 2016 - 12:03 PM

Gentlemen,

 

Thank you for your replies. I believe that we all agree that this formula isn't really useful for much other than saying, "never design a system that approaches this flow rate in a gravity drain pipe."






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