Does anyone else have a suggestion? The pipers are saying either 1/32"/foot or 1/8"/foot, but I'm not sure which. Obviously the larger number is more conservative but is the larger number the *right* number? I don't konw.
ChEMatt,
You do not tell us the size of this flare header. You also do not refer to yourself as a piper. Maybe you need to find one and spend some time in conversation about this.
My thoughts are this. You need to consider the total Flare system route. If you select 1/8" per foot slope then you will drop 1" (inch) for every 8 feet of run. So let's say the header will run 2400 feet from the first inlet (the one farthest from the flare Knock-Out Drum) to the KO Drum.
2400 divided by 8 equals 300, that is 300 inches.
300 inches divided by 12 equals 25 feet.
You might be entering the KO Drum at or near the top with a tangential nozzle. If the (horizontal) Flare KO Drum is 15'-0" in diameter and setting on a 7'-0" high foundation you are starting about 20' - 0" above grade. I am using 24" as a guess at a header size.
So starting at a BOP at the KO Drum of 20'-0" above grade then 25'-0" of slope you end up with the far end of the Flare Header at or near 45'-0" above grade.
Now consider the structural support design and the cost involved in those supports.
You must also consider what this might be doing to the piping systems back at the starting end of the Flare Header. Every PSV must be located higher than the Flare header so the discharge line will drain to the Flare Header. This might mean extra cost to add raised platforms to provide support and maintenance access to the PSV's.
As the car ad says, "Behind every detail there is another detail"
At the layout stage of the project, it might be worthwhile to consider cleanouts at elbows and changes indirection.
I have worked with clients who have required small stainless steel manway/cleanouts on thier flare piping due to the probablility of collecting liquids and sediment.
The cleanouts have the same pressure rating as the piping system
The cleanouts are not cheap, but also enable visual inspection of the pipe interior.
ChEMatt
The slope needed for a pipe to drain depends on several factors. Notionally any slope will drain but a common convention is to have at least 0.2% available slope.
For that, bear in mind that there is another factor which is often overlooked, that being the sag between the pipe supports. All pipes will sag but smaller pipes have a greater potential to pocket. I suggest that you look at the potentially worst slope within the longest span between supports, for each line size and thickness and add at least 0.2% to that.
The greatest slope within a span is easy to calculate from standard formulae. Just ask your friendly structural engineer for the encastre beam solutions.
