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Preferred partner

Dynamic modelling and simulation of

Flare Systems

Flow assurance and Dynamic Simulations Seminar, Stavanger 23.10.2012

Henriette Raddum | Process Specialist Engineer, Aker Solutions MMO

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Typical flare system

■ Purpose: ■ Protect equipment against overpressure from system malfunction or fire ■ Collect hydrocarbon gases and liquids from relief and depressurisation ■ Conduct relieved fluid to a location were it can be safely discharged

■ Typical relief sources: ■ Pressure Safety Valves (PSVs)

■ Blowdown valves (BDVs)

■ Pressure Control Valves (PVs)

Flare

KO Drum

Flare tip

Flare sub header

Pressure

Safety

Valve

(PSV)

Pressure

Control

Valve (PV)

Blowdown

valve

(BDV)

From various process equipment

Flare main header

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Typical simulation of flare systems

■ Steady state (SS) considerations traditionally used for flare system

load evaluations (Flarenet)

■ Steady State (SS) simulations: ■ Initial peak rate “goes on forever”

■ Flare system volume not

accounted for

■ No accumulation in system

■ Peak rate in = peak rate out

■ Why dynamic simulation (DS)? ■ More realistic

■ Less conservative

■ Includes initial accumulation due to volume in flare network

■ Peak flow out typically delayed and smaller than peak flow in

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Rules and regulations

■ API Standard 521 / ISO 23251 (Pressure-relieving and Depressuring Systems) about Dynamic Simulations (DS):

■ § 5.22 (Single system): ■ DS can be used to calculate transient pressure increases ■ DS can be used to calculate relief rates (PSVs) ■ Conventional methods are conservative and can give oversized relief and flare

system designs. ■ Dynamic simulations are better to define the realistic relief loads and improves

the understanding of what happens during relief ■ If the physical phenomena are not well understood, the dynamic model shall

include conservative assumptions ■ At steady-state conditions the dynamic model shall closely

match the steady-state

■ § 7.1.4.2 (Disposal system load): ■ DS is particularly useful in analysing existing flare systems ■ DS allows user to predict timing of individual system peak loads ■ Peak load for various parts of disposal system can occur at different times

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Typical flare simulation tools used by Aker Solutions

■ Flarenet (steady state)

■ OLGA (dynamic)

■ Hysys Dynamics (dynamic)

■ K-Spice (dynamic)

■ Selection criteria – choice of tool: ■ Number of sources

■ Compositions

■ Gas or multiphase flow

■ Customer preference

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Ongoing study

■ Background: ■ Modification project on existing platform with limited flare capacity

■ Total Platform depressurisation → maximum gas to flare

■ Sequential depressurisation introduced

■ Tie-in of new fields → new equipment → increased blowdown volumes

■ Increased Inlet separator pressure → higher initial blowdown rate

■ Static calculations → Flare KO drum pressure close to max pressure

allowed by design code (design pressure + 10%)

■ Main purpose – dynamic flare model: ■ Include initial “packing effect” in flare system

■ More realistic rates and pressures in different parts of flare system

■ Include effect of flare back pressure on segment blowdown times

■ Minimise Project Execution cost (modification requirement)

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Ongoing study – methodology

■ Flare model built in Hysys (v7.3)

■ Built “from scratch” (isometrics)

■ 67 Blowdown Valves (BDV)

■ 87 Pressure Safety Valves (PSV)

■ 12 Pressure Control valves (PV)

■ Headers and lines downstream Blowdown Valves modelled in detail

■ Lines without Blowdown Valves simplified

■ Lines <4 inch not included unless connected to Blowdown Valve

■ Blowdown segments modelled as vessels

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Ongoing study – Flare network in Hysys

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Ongoing study – Flare network in Hysys

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Ongoing study – challenges

■ Large model – many sources

■ Large amount of input data required

■ Choice of settings and parameters in Hysys

■ Existing input data for blowdown segments incomplete

■ Existing Flarenet model has errors → challenging to tune the

dynamic model

■ Model detail level

■ Simulation time

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Ongoing study – current status

■ Flare network and KO drum / flare tip modelled in Hysys

■ Test / verification of dynamic model against Flarenet model

■ Aspentech Support consulted for advice and trouble-shooting

■ Compared “Depressuring Utility” (Hysys SS) with depressuring of equivalent vessel in Hysys Dynamics

■ Re-calculated several blowdown segments

■ Tested blowdown of 5 identical “dummy segments” connected to flare network

■ Re-run of Depressuring Utility in Hysys SS for all BD segments

■ Dynamic modelling of vessels representing blowdown segments ongoing

■ 1st priority: Depressuring without heat input from fire

■ Depressuring with fire may be included later

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Ongoing study – preliminary test results

■ Result from “dummy test”

■ 5 identical segments connected at different locations in flare network

■ Blowdown valves opened simultaneously

■ Graph shows mass flow from BD valves (blue) and out of flare tip

(pink)

Steady State mass flow

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Ongoing study – “the End”

■ What we hope to achieve: ■ Find available margins to maximum allowable pressure in KO drum

■ Find available flare capacity for blowdown of future tie-ins

■ Clarify need for sequential depressuring on new or existing segments

■ Better understanding

■ Reduced modification scope and cost for future tie-ins

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Slide 14 6 November, 2012