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Basis of Momentum Criteria for Flare System
Learn Syan Lee
Process Design Engineer
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Dear friends,
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As per clause 6.5.1 of NORSOK Standard P-001 (Process Design), "all flare lines
shall be designed to keep the rho.v2 < 200,000 kg/m.s2 criteria (where rho is the
fluid density or mixed density for two phase conditions in kg/m3 and v is the
velocity in m/s)."
As per clause 13.5.1 of TOTAL GS EP ECP 103 (Process Sizing Criteria), "(flare)
headers: 0.7 Mach max and rho.v2 < 150,000 Pa considering the maximum flow
rate due to process limitations."
Appreciate if anyone from the group can shed some light regarding the following
queries:
Ghasem Bashiri
Process Lead Engineer at EIED
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• Basis of limiting flare system momentum to < 150,000 or < 200,000 kg/m.s2. Is it
based on industrial experience or experimental results to maintain mechanical
integrity of piping?
• If the estimated momentum exceeds 200,000 kg/m.s2 (say 250,000 kg/m.s2) due
to tie-in of a higher flare load to an existing system, what is the implication of
exceeding the momentum criteria (e.g. excessive piping vibration)? Is it mandatory
to increase the tie-in line size or can it be considered acceptable by providing
proper supports/bracings since relief load to flare system is considered as
intermittent service?
Thanks in advance for sharing your experience and thoughts on this topic.
Regards,
Syan
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 Vinay Kumar likes this
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S M Kumar
Process Design Consultant
SM
Syan: Please google FIV Study + AIV Study. Flow Induced vibration/ Acoustic Induced Vibration
and read all about it. Usually branch connections break and release HC inside the plant, rather
than take it all the way to flare tip.
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Mohd Fitri bin Mohd Lias
Process Engineer at MMC Oil and Gas Engineering Sdn. Bhd
hi, you may refer to aiv assesment and protection guideline by energy institute. its help
Mohd Fitri
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Hi, Syan The criteria holds good only for the single phase, not for two phase flow. More over if the
rho.v2 is higher you will have to given more concrete support for the vibrationa and has a
tremendours impact on the welded joints.
As a good practise you can go upto 1 mach in tail Pipes, but not on the Sub-header and Main
Headers.
Check for the conditions at the main header and sub-header and speak to your piping department
for the inputs on the cost of support, generally this has a huge financial Impact. For these reasons
we recommend the limits.
More over check for the generally Line sizing criteria. for the lines for two phase.
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as said by others these values are related to maximum stress and vibrations on lines,
for single and two phase
F = P*A + Rho*A*V^2 / g
gives the force on a specified section, there is a static (P*A) and dynamic (Rho*A*V^2 / g)
contribute,
since you know the mass flow and conditions (for a PSV I simulate as isentropic nozzle with
PRODE PROPERTIES) you can calculate density and velocity,
for two phase I generally prefer the HEM model (i.e. gas and liquid traveling at the same speed
and at equilibria),
from the above you can calculate the maximum stress,
for mach number you need the two-phase speed of sound,
again you can calculate the speed of sound for two phase flow with a simulator (I use PRODE
PROPERTIES) and then calculate the mach number
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