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singlaatul
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Free Draining, Slope or No-Pocket

05/14/2009 9:55 AM

There are three cases i have come across.

1. Free draing

2. Slope

3. No-Pocket Requirement

I would be grateful if anybody could explain under what conditions we go for these different conditions or combination of these.

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 Atul singla
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PennPiper
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#1

Re: Free Draining, Slope or No-Pocket

05/14/2009 5:14 PM

These terms are common in the process plant engineering and design profession.

They appear on the P&ID (Piping and Instrument Diagram) printed under specific lines that require that special design treatment. These terms are instruction notes from the Process engineer to the Piping Layout Designer.

1. Free Draining - Any such line with this note needs to "step" down from the high point to a lower point i.e: it would leave the origin point and then drop down then (possibly) run horizontal then maybe drop down then enter the destination point. The line does not need to be sloped but it must not be pocketed.

2. Sloped - Sloped means just that it must be sloped. However the Process Engineer must include the required slope angle.

3. No-Pocket - This is sometimes misunderstood and applies to almost every line in a process plant except lines in a pipe rack with loops. A "No-Pocket" or Do-Not-Pocket" line can leave vessel "A" and go up then horizontal then up again then horizontal then down to vessel "B" or to an exchanger. But it cannot go up then down then up then down, this would create pockets

Does this help?

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singlaatul
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#2
In reply to #1

Re: Free Draining, Slope or No-Pocket

05/26/2009 9:45 AM

What would u say on this??? Is it free drain or is it pocketed??

What i think it is pocketed but still fulfilling the requirement of free draining..what do u say???

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PennPiper
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#3
In reply to #2

Re: Free Draining, Slope or No-Pocket

05/26/2009 1:17 PM

That configuration:

- IS pocketed

and

- is NOT free draining

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singlaatul
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#4
In reply to #3

Re: Free Draining, Slope or No-Pocket

07/10/2009 12:52 AM

Why do u think it is not free draining... pressuse head developed is more than the pressure head loss..so it is fulfilling the concept of free draining...

What do u say??

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singlaatul
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#5
In reply to #3

Re: Free Draining, Slope or No-Pocket

08/07/2009 12:46 AM

I guess it is a case of free draining. what do u say???

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PennPiper
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#6
In reply to #5

Re: Free Draining, Slope or No-Pocket

08/07/2009 11:59 AM

That configuration is not free draining!

Let us look at this descriptively. Starting on the left side there is a horizontal run with a flow arrow. We will all this H1. Then there is a vertical run (V1). Then a second horizontal run (H2). Then a second vertical run (V2) then a third horizontal run (H3). And, finally a third vertical run (V3).

Now we can have two basic types of flow in this line. We can have a Fluid or we can have a Vapor.

Example #1. Vapor flow (Steam, wet gas, etc) A Low Point Pocket - The vapor can flow through this configuration just fine to start with. However, after the vapor condenses you will start to have flow problems due to Slug Flow. You will be develop a liquid plug in the low point of "H2' and the liquid can build up to the level equal to the length of "V2".

Example #2. Hot Liquid flow (Condensate, Hot Oil, etc) A High Point pocket - The liquid can flow through this configuration just fine to start with. However, after a while the hot liquid will tend to give off vapor. This vapor will tend to rise and collect in the high points such as the run "H3". This can cause a condition called "Vapor Lock" which will result in slug flow.

So again, I say the example configuration IS pocketed and it IS NOT free draining

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singlaatul
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#7
In reply to #6

Re: Free Draining, Slope or No-Pocket

08/08/2009 4:04 AM

that was really nicely explained.

Does that mean the figure shown above is a case of free draining for low temperature services, i mean where there is no chance of condensate or vapour formation..though in that case there will be no free draining or no pocket reuqirement mentioned in P&IDs. But concept wise can we call that a "free draining" case??

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PennPiper
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#8
In reply to #7

Re: Free Draining, Slope or No-Pocket

08/08/2009 12:19 PM

NO!

The configuration you have shown above is "Pocketed" and "IS NOT" free draining,no matter what the commodity is or what the pressure or temperature is.

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singlaatul
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#9
In reply to #8

Re: Free Draining, Slope or No-Pocket

08/08/2009 2:28 PM

what i feel is if the temp is not high (i.e. there is no chance of formation of vapour or condensate) and pressure is good enough at the starting point, then flow will definetely flow freely..then why isnt it the free flow.... what else is the extra requirement... pressure head difference is -ve here for this case...

it looks like a free flow...

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PennPiper
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#10
In reply to #9

Re: Free Draining, Slope or No-Pocket

08/08/2009 4:04 PM

Singlaatul,

Please do an experiment and see if you still believe it will "Free Drain".

Get some transparent (clear or colored) hose.

Against a fence or other vertical surface mount the hose to replicate the configuration above. Add an extra vertical run and a removable funnel before "H1" so you can pour water into the hose.

Now pour water into the hose until you have flow out the bottom end.

Block the bottom end and continue to fill with water until the water backs up out the top.

Stop adding water.

Now remove the blockage from the bottom end and let the water drain out.

When the water stops flowing, look and see if there is any water remaining in "H2".

Now remove the funnel and blow into the hose at the end where you poured in the water.

Look again and see if there is any water in the hose.

Now tell me if the configuration is free draining.

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Anonymous Poster
#11
In reply to #10

Re: Free Draining, Slope or No-Pocket

09/10/2009 12:53 AM

Beautiful explanation by PENNPIPER
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Razali
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#14
In reply to #10

Re: Free Draining, Slope or No-Pocket

06/10/2012 2:39 AM

Dear Pennpiper,

Thank you for the clarifications. It was clearer explanation for pocketed and no-pocket piping (free drain). Good discussion

Thanks again
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Anonymous Poster
#12

Re: Free Draining, Slope or No-Pocket

03/07/2010 12:54 PM

Great discussion and liked it very much
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Anonymous Poster
#13

Re: Free Draining, Slope or No-Pocket

07/27/2010 8:02 AM

The figure shown above is gravity flow, you can add it as the fourth case
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Anonymous Poster (3); PennPiper (5); Razali (1); singlaatul (5)

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