10 replies to this topic

[Afshin445]

Dear Experts,

 

Currently I am facing with a design problem. I am going to design an overboard line for a new potable water maker system  which dumping 10.4 gpm seawater to sea and need to in existing platform.The overboard line in first section is horizontal and finally will become vertical downward. Based on Norsok recommendation for designing of self-venting vertical gravity downward pipes based on Froude number<0.3 minimum 3" pipe size is required.

 

In other hand, there is existing 2" overboard line which currently using by platform existing water maker with dumping rate of about 7 gpm. Client proposed so we can using existing overboard line for new water maker due to existing water maker is going to demolish after installation of new water maker.Now I need to do adequecy check for existing overboard line to see can be used for overboard line for new water maker.

 

Based on new seawater dump line (10.4 gpm) for 2" line Frouds number is about 0.5 which is more than self venting requirement.

 

The question is what is the consequence if existing overboard line will be used for new water maker ? Is any pulsation and air entrainment  in vertical section of pipe? Any modification in existing overboard piping system is necessary, e.g. installation of Siphon breaker in piping high point?

 

Any advise in this regards is highly appreciated.

 

Afshin

 

 

 

 

 

 

 

 

 

 

[Ajay S. Satpute]

Hi,

 

Please go through below link, especially Art's excel sheet. All the answers can be found there.

http://www.cheresour...vity-flow-line/

 

Quote from above excel sheet;

 

Three approaches to the design of gravity drainage systems are possible:                           1. For full flow, with the outlet piping size based on single-phase criteria;     2. For self-venting, with the liquid velocity in the outlet pipe kept low enough to allow gas to   flow counter-currently to the liquid.           3. For gas entrainment, but with the system designed to accommodate it.    

 

Thanks and Regards

 

Ajay S. Satpute

[Afshin445]

Hi Ajay,

 

Thank you for your prompt responce.Actually I tried to download excel file but after downdload can't open.Appriciated if you send me the file.

 

My Email Address: Akhamisabadi@gmial.com

 

Regards

Afshin

Edited by Afshin, 19 November 2013 - 07:41 PM.

[katmar]

The flow of 10.4 gpm through a 2" pipe results in very low friction losses - of the order of 4 mm of water column per metre of pipe. This means that the flow is going to want to accelerate as it falls down through the pipe. If the bottom of the pipe were open air would try to flow into the open space around the accelerated flowing water, but I guess the bottom of the pipe is submerged and this is not possible.

 

What will happen (if the top of the pipe is sealed) is that the system will try to initiate a siphon and you will get a zone of low pressure at the top of the pipe and if the pipe is long enough the pressure at the top could go low enough to cause the water to boil and cause cavitation and vibration.  The solution to this is to put a siphon break at the top and allow air to be entrained.  The two-phase flow of air and water down the pipe will also cause some vibration, but probably less than if the top were sealed.  I don't know of any proven method to predict the level of vibration.  I have attached a diagram from the 1968 article by Simpson (Chem Eng. June 17, 1968) which shows a very similar situation to yours.

 

If I were in your situation I would push for the 3" pipe, or maybe a 3/4 " pipe which would switch the flow to the other side of Simpson's Figure.

Attached Files

•  Fig 11.gif   70.48KB   45 downloads

[Afshin445]

Katmar,

 

Thank you for your reply.The existing overboard line is not submerged and is open to atmosphere, but there is no any vent or siphone breaker in top of pipe.With new folwrate (10.4 gpm) froud number in this pipe is about 0.5.Also pipe vertical section height is about 14 m.With this information still you think low pressure zone will be apperead in the top of the pipe with new flowrate in 2" overboard line?

 

In this regards, actually I don't have access to simpson article now, but i try to find it.As I understand you suggest to we are using a new 3" overboard line with 3/4" vent (in high point ) and existing 2" overboard line can't be used anymore.Please confirm.

 

What will happen if we are using existing 2" overboard line with new vent in high point?

 

 

Thanks.

Afshin

Edited by Afshin, 22 November 2013 - 03:29 AM.

[katmar]

If the discharge point of the pipe is not submerged then all the Froude numbers are meaningless. In order to calculate the Froude number you need the velocity of the liquid in the pipe. In the Fr No calculation it is assumed that the liquid fills the cross section of the pipe. This would give you a velocity of about 0.14 m/s in the 3" pipe and 0.30 m/s in the 2" pipe. However, if the pipe end is not submerged then the liquid will accelerate as it falls/flows down the pipe and will definitely not fill the pipe and the velocities will be wrong.

I suspect that we all, and me in particular, worry too much about the Froude analysis sometimes. With a small flow like this in a relatively tiny pipe performing a simple discharge to atmosphere duty not much can really go wrong. If the old duty was working acceptably in the 2" pipe without any vent or siphon break then I would be tempted to just connect the new system to it. I know that I recommended the 3" pipe in my previous post, and if it were a totally new installation I would probably do it that way, but in this situation I would accept the 2" pipe. In the old configuration the flow (even though the theoretical Fr No was < 0.3) was not working in self-venting mode because the bottom was open and there was no vent at the top.

If you do experience some vibration with the new duty you could add a siphon break at the top, but I think it will be OK just as it is.

[Afshin445]

Katmar,

 

Thank you for your prompt reply.I am totally agree with you which froud number we calculating is theortical and can't prove anything.In this regards, because of uncertainty of proper working of old overboard line with new flowrate after installation of new water maker, I suggested to client to we also keep all of existing line from old water maker nozzle flange to tie-in point to new line from new water maker with isolation valve and blind flange at the end.After starting-up

of new water maker if operator feeling any vibration in overboard line he just can open the isolation valve and blind to allow air can enter to system.In final if really necesary they also can install a gossneck in top of flange.

 

I also checked pressure drop through this 2" pipe from new tie-in point to old water maker nozzle still is less than 5 mbar which recommended by Norsok.

 

What is your opnion about this proposal?

Edited by Afshin, 21 November 2013 - 05:11 AM.

[curious_cat]

I suspect that we all, and me in particular, worry too much about the Froude analysis sometimes. 

 

Have you encountered practical piping situations with gravity discharge that had problematic vibrations?

[katmar]

To Afshin - yes, I think that is a reasonable proposal.  Please come back and let us know how it works once you have implemented it.  Then we will all have one more data point to base our future designs on.

 

To curious_cat - In my personal experience the problems have been more of a process nature than bad pipe vibrations.  The Simpson article I referenced ealier gives a case study of a large cooling water return line that vibrated badly.  The problems I have encountered with air being drawn into cooling water return lines have been more to do with maldistribution of the water to the cooling tower nozzles.  The nozzles can vibrate quite badly as slugs of air and water come out.  You have probably seen what happens with domestic water systems when air gets in the line.  In process piping (not water) I have seen many problems with systems that were not self venting when they should have been.  The results there were process problems or vapor locks rather than vibration.

[curious_cat]

To curious_cat - In my personal experience the problems have been more of a process nature than bad pipe vibrations.  The Simpson article I referenced ealier gives a case study of a large cooling water return line that vibrated badly.  The problems I have encountered with air being drawn into cooling water return lines have been more to do with maldistribution of the water to the cooling tower nozzles.  The nozzles can vibrate quite badly as slugs of air and water come out.  You have probably seen what happens with domestic water systems when air gets in the line.  In process piping (not water) I have seen many problems with systems that were not self venting when they should have been.  The results there were process problems or vapor locks rather than vibration.

 

Thanks for the insights! One classical example I remember reading about was in one of Kister's distillation troubleshooting case studies where a gravity reflux line was vapor locked due to being undersized for a self venting flow. 

[Afshin445]

To Afshin - yes, I think that is a reasonable proposal.  Please come back and let us know how it works once you have implemented it.  Then we will all have one more data point to base our future designs on.

 

Sure.The new water maker is going to install most probably this month and I think we will inform about the results very soon.

Edited by Afshin, 21 November 2013 - 09:14 PM.