2/3/2015 Flare systems: Major thrust points for stress analysis | What is Piping

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Introduction:

Flare sy stem is a means of safe disposal of waste gases by burning them under controlled conditions. Flare piping generally

comprises of PSVs outlet piping, sub header piping & main header piping. Design conditions considered for stress analy sis are as

per P&IDs, line list and specific information related to flare if any by process.

A ty pical flare sy stem consists of:

PSV outlet pipes, sub header connected to main flare header , main

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20th December 201 4 want2learn flange leakage checking 0 Comments

Flare systems: Major thrust points for stress analysis

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flare header connected to knock out drum , outlet of flare knock out

drum to flare stack.

A knockout drum to remove and store condensable and entrained

liquids.

A single or multiple burner units and a flare stack.

The major thrust points which a stress engineer should consider

carefully are listed below:

Fluid Density : Fluid density has to be taken from list list (Process

Department). In absence of data, 1/3rd water filled weight can be

assumed for Caesar input.

Test Method: Flare lines are normally pneumatic tested. So, hy drotest weight is not required to be considered. However in

specific cases water filled weight has to be considered (Check with Process department).

Supporting: Flare lines are normally prov ided with slopes as per Project specification. So general practice is to support with

Shoe/saddle supports. Supporting span to be maintained such than sustained sagging should not exceed 3-5 mm. Structure

below pipe support / shoe height is to be planned to meet piping sloping/free draining requirement.

SIF: Normal industry practice is to take 45 degree/90 degree branch connections from Flare Header: Proper SIF (both

inplane and outplane) should be incorporated at branch connections while entering data into Caesar II. SIF s can be

calculated using Fe-SIF, Nozzle Pro, or some other ty pe of FEA software. Sometimes reinforcement may be required to reduce

SIF value.

Temperature Gradient: In a flare sy stem sometimes temperature gradient or profile may exist when the hot contents flow into

the subheader / main header which is at a lower temperature ( confirmation with process if required ).

Flexibility : Piping shall be evaluated for flexibility . If necessary expansion loops shall be prov ided. Expansion joints to be

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avoided. Flare piping loops are planned in horizontal plane (4D bends) in order to ensure pipe slope/free draining

requirement. However no of loops should be minimised as much as possible.

Flare line routing and supporting to be planned in such a way that forces and moments on flare knock out drum nozzle

connections are minimized.

Sometimes Flare line may consist of two phoase flow. So Vibration/Accoustic analy sis is required to be performed and

supporting to be strengthened.

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