2 replies to this topic

[gleicecardoso]

I need to design a deluge drainage system in a turret of the FPSO that will be routed to overboard.
Several drain boxes will be installed on the floor, which has 2 outlets, one for the platform's open drain system, which is located at a lower level and the other, which is located at a higher level, routed to the sea (6 drain boxes with 4" outlet for open drain and 6" outlet for overboard, total overboard flow 300 m3/h). From the drainage boxes it will be interconnected in a 10" circular overboard collector header with 24 m length and slope 1:50 and later in a vertical line of 8" which length is 30 m, 13 m are submerged in the sea. With regard to this system, We have the following doubts: As these lines are coming from drain boxes, it is possibl the calculation of the vertical line be considered assuming 50% of the available head and not the Froude Number, according to Norsok P-002?
- Is it necessary to provide a vent line to break the vacuum, at the highest point due to the vertical section?

[chocobang]

A graph will definitely help more people to reply. 

 

The capacity usually of drain lines are calculated by Manning equation considering horizontal lines with slopes and the capacity of vertical line too (not sure on vertical line). 

[snickster]

Looking at just the overboard piping that you describe I have the following comments:

 

It appears that the 10" header size may be on the borderline.  Based on tables which are based on the Manning formula, at 1:50 slope for 10" I.D. pipe flowing full, the resulting flowrate would be 280 m3/hr.  Therefore the water will back up somewhat in the drain boxes to acheive 300 m3/hr.  Ideally storm drainage systems are designed to flow half full, but in your case with a deluge system you may be able to get by with drain boxes filling with water and maybe spilling over a little on the floor to be able to get the flowrate.  For a 2:50 slope and a flow of 300 m3/hr the 10" header pipe will flow a little under 3/4 full pipe.

 

The 8" vertical pipe will definitely not flow full but the water will just fall down continuouly picking up velocity as it travels down to the sea water level.  The head loss in a 8" pipe flowing full at 300 m3/hr is about 1 meter/30 meters but you have 17 meters of head available to the sea level.  Therefore the water will be in free fall in the 8" vertical line overboard.

 

If the pipes were flowing full then a siphon effect would develop.  Even with the 8" pipe flowing partially full a siphon would tend to develop due static head of the fluid downstream in the vertical line pulling down on the fluid upstream before it actually starts to separate.  However in your system there is really no concern if this is to occur.  In a plumbing system this is undesireable since any siphon effect will drain all water out of the P-traps which would expose the areas to sewer gas.  In your case if a vaccum develops it will just make the flow greater and evacuate the water in the drains faster and even make the flow a lot greater in the 10" pipe for a given slope so the flow would be much much much more than 280 m3/hr for 1:50 slope.

Edited by snickster, 03 August 2023 - 06:35 PM.