4 replies to this topic

[udhay]

Hello,

I am about to design an hazardous open drain system.The set-up is arranged in such a way that there will be drain box with overflow line to sea and drain collection line to a new open drain header will be routed to open drain caisson via seal pot.

Wondering how to size the overflow line from drain box.Shall i have to consider froude number (less than 0.3).But when i checked the NORSOK P-001,they mentioned that froude number is not to be taken into account.

I am wondering whether my understanding of standard is not right.Can anyone help me out.

NB!
Overflow capacity to be routed to sea is 45 m3/h.If I consider froude number,I will end up using 10" line.

Regards,
UKD

Attached Files

•  Pages from P-001e5.pdf   27.13KB   194 downloads

[ankur2061]

UKD,

The equation 2 as given in P-001, edition 5, Sep. 2006:

Q = 8855.8*D^2.5/sq root (f)

where:

Q = flow in m³/h
D = inside diameter of pipe , m
f = Moody friction factor = as defined in eqn 3 of P-001 edition 5

This equation could be used for sizing of your overflow pipe for deluge water flow from the drain box.

Have you tried to size your overflow line based on this equation.

Regards,
Ankur.

[udhay]

Hei Ankur,

Thanks for your reply.

I tried.Please find the attached spreadsheet and I got high frode number.

Regards
UKD

Attached Files

•  Overflow line.xls   325.5KB   161 downloads

[ankur2061]

UKD,

The calculated flow rates & velocities are practically not possible to implement for the given line sizes. Maybe there is something wrong in the equation.

Suggest you size your overflow line considering a maximum velocity of 0.8 m/s or 1.2 m/s (if solids are present). Some company standards (BP, Shell) recommend this as the maximum velocity for drain line sizing.

Hope this helps.

Regards,
Ankur.

[Art Montemayor]

UKD:

You do well to furnish us with an Excel sketch. However, you should give as much detail as you know of. You don't furnish us details on the specific overflows and how they are going to be piped.

It is extremely important to show ALL the details on a simple fluid mechanics problem. It becomes important because nothing is as simple as it seems. Gravity vertical flow is NOT SIMPLE. It just looks simple. And that is why you should study your fluid mechanics - especially vertical gravity flow. NORSOK has good reason for specifically citing the fact that the Froude Number is not applicable in 100% full liquid flow in a vertical pipe. They do so because the Froude Number is not meant for that. If you studied your Fluid Mechanics and knew the derivation of the Froude Number you would know.

I have contributed an Excel Workbook on Fluid Flow to our Forums and in it you will find a worksheet dedicated to the application of vertical gravity flow. It is an article by Hill and has been used by such engineering houses like Bechtel as a primer for their design manual. Look for my workbook in past threads by using our SEARCH machine.

It is very important to identify and recognize the type of fluid flow you are employing. Your seal design and piping configuration both play an important factor in setting and defining what kind of flow you will get in your drains. Drain pipes don't necessarily run 100% full of liquid simply because you need them to, you think they do, or because they have to. You have to carefully analyze what you have as flow rate, piping configuration and type of flow resulting.