Dr. Wayne Monnery, Ph.D., P.Eng.

Flare Systems

PROJECT MANAGERS

✓ Solid comprehension of process scope

✓ More confidence in technical aspects

✓ Expand your network!

PROJECT ENGINEERS

✓ Learn shortcuts and troubleshooting tips

✓ Expand knowledge in specialized areas

✓ Expand your network!

JUNIOR POSITIONS / EIT

✓ Gain a solid foundation of process engineering

✓ Get ahead of your competition

✓ Expand your network!

ABOUT WAYNE MONNERY, Ph. D., B.Sc, MSc, P.Eng

• Professor at University of Calgary

• 30+ years of hands-on experience

• Works published in SPE-Oil and Gas Facilities, Chemical Engineering & Technology, Oil & Gas Journal, etc.

• Worldwide project execution w/ focus on North American gas plays

TODAY’S PANELISTS

FORREST CHURCHILL

VP OperationsSIGIT USA

forrest.churchill@SIGIT.us

JEFF CARLSON

Process LeadSIGIT

jeff.carlson@SIGIT.us

General Housekeeping

Questions?

• Use the Q&A feature at the bottom of your screen to ask questions – refer to slide #.

• We will try to answer all Q&A at the end of the class.

If your Q does not get answered

We will provide a Q&A summary to all attendees.

Chat

Feel free to chat amongst yourselves however it will not be thoroughly monitored.

Dr. Wayne Monnery, Ph.D., P.Eng.

Flare Systems

Flare Systems

• Pressure relief systems required at facilities to avoid overpressure

• Elements of relief systems are• Relief devices (PSVs, rupture discs, etc.)

• Flare header system (tailpipes, sub & main headers)

• Flare knockout drum

• Flare stack

Flare Systems 7

Flare Systems

• Definitive reference is API 521

• Several types of relief scenarios can occur• Blocked Flow• Fire (Vapour generation and expansion)• Thermal Expansion• Control Valve Failure• Tube Rupture (large difference in design pressure between shell and tube side in

heat exchanger)• Utility Failure (cooling water, electric power)

• In general, do not consider two unrelated events occurring at the same time (double jeopardy)

Flare Systems 8

Flare Systems

• Pressure Level Relationships

Flare Systems 9

Flare Systems

• Back Pressure = Superimposed (constant) + Built-Up (from flow)

• Relieving Devices• Most common are conventional and balance relief valves (PSVs)

• In PSVs, pressure directly opposed by spring & disk area > nozzle area

• Conventional PSV: if bonnet vented to atmosphere back pressure acts with vessel pressure; if bonnet vented to valve discharge, back pressure acts with spring

• Back Pressure up to 10% Set Pressure

Flare Systems 10

Flare Systems

• Relieving Devices• Balanced PSV designed to minimize effects of back pressure

• Piston or bellows type

• In piston type, guide is designed & vented so that back pressure of both sides of disk cancels out

• In bellows type, bellows area same as seat area and attached to valve body so that no back pressure can act on that part of disk, balancing the effect on both sides of disk

• Back pressure up to 50% Set Pressure

Flare Systems 11

Flare Systems

• Relief Valves

Flare Systems 12

Flare Systems

• Relief Devices• Pilot Operated Valve: consists of main valve & pilot; inlet pressure routed through

pilot to hold piston down (no spring, piston is held down due to more area on top than bottom) & when set pressure is reached pilot opens and reduces pressure on back of piston allowing valve to open

• Back Pressure up to 50% Set Pressure

• Rupture Disk or Burst Plate: thin plate between flanges designed to rupture and relieve pressure

Flare Systems 13

Relief Load Scenarios

• Blocked Flow

• Fire Case – Vapour Generation• Determine vessel wetted area

• Heat Duty to Vessel Q = 21,000(F)(Aw)0.82, F from GPSA Fig 5-16, F=1 for no

insulation, 0.3 for insulation

• Determine Heat of Vap

• Relief Load W = Duty/Heat of Vap

Flare Systems 14

Relief Load Scenarios

• Fire Case – Vapour Expansion A = (F’)(A’)/P10.5, F’ = (0.1406/(C1 Kd))((Tw-T1)

1.25/T10.6506)

• Tw = 1100 F for CS, T1 = Top (P1/Pop) ; N/A for Super Critical Dense Phase

• Control Valve Failure• Determine max vapour flow through valve at 100% fail open, normal P

• Tube Rupture• Calculate relief load from orifice type equation

• W2 (lb/hr) = 288.4*P1*d2*((1-(P2/P1)

2)*(MW/T)/(24*(fL/d-ln(P2/P1))

• W1 (lb/hr) = 1154*d2*(*P)0.5

• W1 = tubesheet flow, W2 = tube flow

• d, inch; L, fr; P, psia; , lb/ft3; T, R

Flare Systems 15

Relief Load Scenarios

• Thermal Expansion • gpm = BQ/(500*SG*Cp)

• B = Liq Expansion Coefficient in GPSA

• Heating by solar radiation to calculate Q (250-300 BTU/hr-ft2) x surface area

• More detailed calculations of liquid expansion in literature

• Utility Failure• Depends on utility, details in API 521

Flare Systems 16

Sizing Relief Valves

• Based on maximum flow case of relief scenarios

• Calculate required orifice area & choose standard orifice designation (GPSA Fig 5-7)

• Vapour Relief – Critical Pressure• Pressure at which downstream flow becomes sonic

• PCF = P1 (2/(k+1)) (k/(k-1))

• Typically about 52-58% of P1

Flare Systems 17

Sizing Relief Valves

• Vapour Relief• If P2 > PCF→ Subcritical, P2 < PCF→ Critical (sonic)

• Critical:

• Kd from manufacturer, preliminary 0.975

• Kc = 0.9 if rupture disk installed, otherwise 1.0

• C1 can also be obtained from GPSA Fig 5-8, 5-9

Flare Systems 18

Sizing Relief Valves

• Vapour Relief – Kb conventional

Flare Systems 19

Sizing Relief Valves

• Vapour Relief – Kb balanced bellows

Flare Systems 20

Sizing Relief Valves

• Vapour Relief – Subcritical

• F2 from GPSA Fig. 5-12

Flare Systems 21

Sizing Relief Valves

• Steam equations in GPSA

• Liquid Relief – Turbulent

• Factor Kw – GPSA Fig. 5-14

Flare Systems 22

Sizing Relief Valves

• Viscosity Factor Kv from GPSA Fig. 5-15

Flare Systems 23

Sizing Relief Valves

• Liquid Relief – Laminar• Iterative –

1. assume A,

2. calculate Re

3. obtain Kv (viscosity factor) from GPSA Fig. 5-15

4. calculate A’ = A/Kv5. repeat from 2 until convergence

Flare Systems 24

Sizing Relief Valves

• Mixed phase relief• Determine vapour & liquid relief areas and add them – old method

• Newer methods: Homogeneous Equil Method (HEM) and Homogeneous Non Equil Method (HNE) that assume a pseudo single phase with averaged properties; lots of literature on applicability

• Inlet Piping – limit pressure drop to max of 3% of set pressure at relief flow(API RP 520, ASME)

Flare Systems 25

Sizing Relief Valves

• Outlet Piping – Maximum pressure • 10% of set pressure for conventional PSV

• 50% of set pressure for balanced PSVs

• Valve Cycling• Inlet pressure drop too large or back pressure too high

• Also caused by oversized PSV

• Good software available for relief valve sizing (ex. Farris, Crosby)

Flare Systems 26

Flare Header Design

• Typically done with software but basics must be understood

• Can have HP (PSV relief) & LP systems (tanks, LP amine/glycol flash, amine regen)

• Determine relief loads for various scenarios for each header system and consider blowdown

Flare Systems 27

Flare Header Design

• With relief loads, initially design PSVs

• Add up relief loads for scenario

• Total load goes to FKOD and stack

• To design headers, work backwards with pressure drop calculations (with known lengths) from flare stack tip, limiting velocities to Mach 0.2-0.5 for tip and 0.5 for piping

• Piping Press Drop calculations for 1 and 2-phase in API 521

• Tip pressure drop depends on type

• Establish back pressures at PSVs to confirm initial design (conventional versus balanced)

• If back pressure too high, must increase header diameters

• Slope to drum

Flare Systems 28

FKOD & Stack

• Flare Knock Out Drum• 2 Phase Horizontal Separator Calculation

• Sized according to API RP 521 using 300 – 600 micron droplets

• Limit velocity to allowable with K = 0.40-0.5 ft/s

• Flare Stack• Height to limit ground level radiation to 1500 BTU/hr-ft2 (4725 W/m2), including

solar

Flare Systems 29

FKOD & Stack

• Stack height methodology in GPSA and API RP 521 (with latter more conservative flame lean; different flame length)

• Fraction of heat radiated in GPSA incorrect (should be about double); also a function of velocity

• If sour, must check ground level SO2 versus allowable and adjust height if necessary (involves using Screen 3 type or better software)

Flare Systems 30

Flare System

• Purging• Flare system must be continuously purged to prevent ingress of air

• Small amount of fuel gas into end of each main header

• Target velocity is 0.5 – 1.0 ft/s (0.15 – 0.30 m/s), much lower if stack equipped with seals

• Ci=% component, K=const(2.328 for C1), O2=6%, y=25 ft, D=inches stack dia, Q scfh

Flare Systems 31

Flare System

• Some Interesting issues:• Comparison of GPSA and API stack heights

• How does the heat of vaporization change as a fire progresses?

• How to deal with supercritical relief

• Let’s look at some calculations

Flare Systems 32

QUESTIONS