DESIGN Fired Heaters Training 1 -3 days

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FIRED HEATERS - DESIGN

Mar 2014

AcknowledgmentAPI Standard 560/ RP 530

Project Standards/ SpecificationsPictures from many sources, suppliers, internet

DEDICATED TO:

Lynn Evans and Bill Handel, Foster Wheeler, UK

Robert D Reed, John Zink, USA

Today we are able to see further and farther standing on the shoulders of such giants

The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location.

Fired Heaters Training 1 - 3 days

� Introduction (100 slides) - 6 ~ 8 hours

� Design (40 slides) - 3 hours + 4 hours in Worksheet

� Operations (45 slides) - 2 ~ 4 hours

Design

� Firebox Heat Transfer - A Primer

� Radiant Section Sizing

� Convection Section Sizing

� Coil Design

� Stack

� Burners

� Refractory System

Operations

� Safety Alerts

� Firebox Explosion

� Excess Air Control

� Draft Control

� Fuel & Firing Control

� Heater Control

� Coking & Hot Spots

Since

1976197619761976 38

� Introduction

� Process Fired Heaters

� Plant energy flow; Fired heaters - Source of high temperature heat

� Vertical Cylindrical, Box, Cabin, Multi-cell� Fired Reactors

� Ethylene Cracker; Steam Hydrocarbon Reformer; EDC Cracker; Visbreaker; Delayed Coker

� Other Types

� All Convection; Water Bath� Firing

� Single or double sided; Floor up-fired, End or Side wall fired; Multi-level fired and Roof or Down-fired

� Burners

� Gas or Oil or Combination; Low NOx� Draft

� Natural, Forced, Induced and Balanced� Components

� Tube & Tube Supports; Soot blowers, Stack, Refractory� Waste Heat Recovery:

� Steam Generation; BFW Heating, Air Pre-heater, Gas Turbine Exhaust WHRU

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Contents

� Firebox Heat Transfer - A Primer

� Radiant Section Sizing

� Convection Section Sizing

� Coil Design

� Stack

� Burner, Fuel Systems

� Refractory System

Design Steps: There are 2 loopsLoop 1: Assume Coil Size, No of Passes, to calculate coil velocity, heat transfer coefft etc

Loop 2:Assume Radiant Duty, Flux, Size Firebox, Find Firebox Temperature, Do heat balance; Verify assumed radiant duty, Repeat, if required

Size convection Section; Check pressure dropRepeat Coil size/ passes, if required

FIREBOX HEAT TRANSFER - A PRIMER

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Radiation - Bulk from gas; not flameThe linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location.

� Radiation: heat transfer across space without physical contact.A surface phenomena related to surface temperature

� Solid radiation is in full spectrum. Surface phenomena.Gas radiation - Thickness or Depth related. Limited range.

� Wavelengths: Shorter as energy level/ temperature rises� Sun is hot. Much of its radiation in shorter wavelengths, visible light

� It may appear that radiation is from flame,� But bulk is from hot CO2 and H2O in flue gas, in infrared range

� 90% from gas; 10% from flame� Note: H2 has no visible flame. Oil flame appears luminous as it cracks to

soot during combustion

IR: 0.7 to 400 µ. Heater 0.7 to 14 µ


Gas Radiation - H2O + CO2

Why only CO2 and H2O?

� Symmetrical molecules O2 + N2 do not absorb or radiate in firebox IR range

� Binary H2O and CO2 molecular bonds bend and vibrate. � Kinetic energy� H2O emissivity is the highest

� Gaseous fuels radiate better

� Gas emissivity εG depends on gas temperature, partial pressure of H2O + CO2 + SO2 - “p” and depth of gas cloud or mean beam length, L. High Xs air reduces εG


Fuel H2O/ (H2O+CO2)Heavy oil 1/3 (0.3-0.42)Distillates 1/2 (0.42-0.5)Natural Gas 2/3 (0.6-0.7)

0.35

0.30

0.25

0% Xs Air 20% 40%

H2

CH4

10°Oil

Pa

rtia

l P

ress

ure

or

Mo

l Fra

ctn

H2

O+

CO

2

0.7

0.1

0.2

0.3

0.4

0.5

0.6700°C

900°C1100°C

pL, atm.m

Ga

s e

mis

sivi

ty,

ε G

0.0 0.5 1.0 1.5L = 3.6*V/Ae, Wimpress

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Firebox - All 3 modes

� Flue gas recirculates - between tubes and refractory - thermo siphon effect - and transfers heat by convection to the front + back of tubes and refractory

� Heat to tubes/ fluid� Front: Flame + gas cloud radiation + convection

� Back: Convection + Refractory radiation

� By conduction thru tubes; by convection from tubes to process fluids

� Heat Exchange between gas components� 70% heat in non-radiating O2 and N2. Exchange

heat by convection with CO2 and H2O that radiate

� In firebox - all 3 modes of heat transfer active� Radiation from gas cloud; convection between

gases, to tube and refractory; tube and heated fluid. Conduction thru scale and tube wall

Bulk of heat transfer in radiant section is by convection between gases

Gas cloud remains at residual or equilibrium firebox temperature, receives heat from flame and radiates it to tubes and refractory

Flue gas recirculation velocity increases with firebox height, giving convection coefft hc = 10 to 15 W/~, 10-15 kCal/~, 2-3 Btu/~)

Re-circulating Flue Gas

Re

fra

cto

ry Gas Cloud

Firebox - Refractory’s Role

� Refractory re-radiation enhances gas emissivity + heats tube backside

� Larger the tube spacing: or more gap between tubes, less direct radiation and more reflected radiation� Tubes usually on 2D spacing� Tube to wall 1½ D. Beyond 1½ D, rear side absorption �

� Av Radiant Flux:� Based tube outside area = πDL� In analysis, area of receiving plane, viz αSL taken, viz.

effective area of Cold Plane or αAcp

Reflection boosts radiation transfer. Effectiveness:Direct 75%; Once reflected 68%; Twice 61%; Thrice 54% Take 100 rays from a single point

Direct 40 rays x 0.75 = 30 units

Once reflected 30 rays x 0.68 = 20.4

Twice reflected 15 rays x 0.61 = 9.2

Thrice reflected 15 rays x 0.54 = 8.1

Total 67.7 units

Re

fra

cto

ry

S

D

0.8

0.6

0.4

0.2

0.021 3 4

1.0

Total = Direct +

Reflected

Direct

Tube Spacing Vs. Fraction

absorbed α c.f. parallel planes

Spacing S in terms of tube dia d, S/D

Fra

ctio

n a

bso

rbe

d α

Acp = nSL if single side fired

Acp = 2nSL if double side fired,

but “α” direct only

S

S

L

L

Re

fra

cto

ry

SAcpAcp

Acp

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Firebox - Refractory’s Role

� 3 Zones:1. Source = Gas cloud + Flame

2. Sink = Tubes; and both enclosed by

3. Refractory that absorbs no heat but reradiates back to source, to sink and to itself

� Refractory� Enhances heat to tubes - back and front side

� More effective - close to tube + high emissivity; short distance (square-of-distance-effect)

� Contribution more in side-wall and central/ bridge-walled fired heaters; but mean beam length and flue gas recirculation is low

� Once refractory’s role is understood, 2nd row of tubes abandoned. Doubles the cost but adds 25% more area. Also abandoned efforts on polished reflecting surface on refractory

0.90.80.70.60.50.40.30.20.1

0.2 0.3 0.4 0.5 0.6 0.7

Ar/αAcp =531

0

Gas emissivity, εG

Ove

rall

Exc

ha

ng

e F

act

or,

F

Ar = Uncovered refractory area = Total Refractory area, At minus αAcp

Re

fra

cto

ryX

X

Rgt

Rgr

Rrt CCRra

Cgr

Cgt

Flux & Temperature Gradient

� Gradient in Tube and Gas Temperature based on:� Size and shape of firebox

� Squat firebox: Less gas temperature gradient� Long cylinder more gradient

� Proportion of tube covered wall area

� Flow of fluid w.r.t flue gas flow

� Tube to tube spacing

� Tube to wall spacing and

� Type of firing & firing density impacting recirculation

� Single sided firing: Max front or fireside 180°flux 50% higher than average flux on 2D spacing; front 60° is 80% more - 45% more on 3D

� Double sided firing - 25% more on front 60°

� Larger tube spacing evens out maximum flux rate� Example: Peak film temp- 2D = 450°C 3D = 433°C

Double sided firing

Single sided firing

60°

180°

Impact of T4 relationship:if T2 < ½T1, flux gradients minor. Take a Crude Oil Heater. Tin = 300°C Tout = 370 Tav = 335 Tt = 370 Tg = 870°, Tt4 is about 10% of Tg4. 10% variation in Tt has little impact on Tg. Some correlations ignore Tt.Inlet : Outlet flux = 1.05:1.0

In case of fired reactors T1 and T2 are close

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Summary

� Emissivity, ε and Absorptivity, α depend on temperature/ wavelength. Higher the temperature, lower in metals; exception non-metals like refractory

� Polished and clean metal surfaces - low ε; oxidized and rough metal surfaces - high ε. Usual tube ε = 0.9

� Simple heat transfer analysis - stirred tank or single zone. Computerized methods - multiple small zones� Temperature gradients expected in wall firing tall and narrow fired reactors

� Conduction and convection decided by temperature difference or ∆t. Radiation decided by temperature level. WB = σT4. Higher sensitivity to temperature

� Net exchange between two surfaces:q1�2 A1F σ(T1

4 - T24)/(1/ε1 - 1/ε2 -1) + convection part, hc Ao(T1 - T2)

� Multiple sources + multiple receptors @ temperature gradients in different firebox zones - can be solved by a network analysis as in an electrical circuit with differing potential

Rgt

Rgr

Rrt CCRra

Cgr

Cgt

Literature

� Wilson, Lobo & Hottel (1932)� Qr = Qin/ (1+G(Qin/αAcp)½/4200). Only Xs air. Firebox Ar, pL, Tg, Tt not in

� Hottel (1938)� Qr = αAcpφoverallσ(Tg

4 - Tt4). pL, εgas,εeffective ,firebox geometry considered

� Mekler (1938)� Qr = 1.74e-8CSe(Tg

4 - Tt4). pL, εgas ,εeffective ,firebox geometry not considered

� Lobo & Evans (1939) - Widely used� Qr = 1.73e-8αAcpφ(Tg

4 - Tt4) + 7φ(Tg

4 - Tt4)

� pL, εgas ,εeffective ,firebox geometry + convection considered. εtube = 0.9

� 85 tests; 19 different furnaces; Xs air = 6-170% q/A = 3,000 to 54,000 Btu/h.ft2; Ar/ αAcp = 0.45-6.65. Error 5-16%. Modern: Xs air = 5-40% Ar/αAcp = 0.5-1

� Hottel & Sarofim (1967)� Qr = Acp σ εeffective (Tg

4 - Tt4)

� pL, εgas , firebox geometry not considered. εeffective based on tube pitch and εtube free to input

Qr = Radiant dutyQin = Heat input into fireboxφoverall = Overall exchange factorC = Empirical coefft based on TgSe = Equivalent effective area

Tg

Tg = Based on both heat transfer and heat balance

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Literature

� Literature “bookish” correlations require� Firebox layout to find Tg; as if equations

are good for rating only

� Not true. One can assume a radiant duty split / duty, lay out firebox and tubes, calculate Ar, αAcp, Tg, and check duty

� Which tube area?� Projected area, At = DL

� Projected cold plane, Acp = SL = 1.8DL

� Flux specified on tube OD area, Ao = πDL

� At = Ao/π = Acp/1.8

� Acp = 1.8*At = 1.8Ao/π. Ao ≈2 αAcp

Acp

S

S = 12/6.625 = 1.81S = 8/4.5 = 1.77, say 1.8 D

S

L

D

Ao ≈ 2αAcp

hc = 10 W/~, kCal/~ , 2 Btu/~ units based on Ao.

Based on αAcpTaking εeffective = 0.57, 10*2/0.57 = 35 W/~, kCal/~, 7 Btu/~ units,

Literature

� Qr = αAcp εeffective σ(Tg4 - Tt

4) + hc Ao(Tg - Tt) or Qr/αAcp εeffective = σ(Tg

4 - Tt4) + 35 or 7 (Tg - Tt)

� Graph of Ttube Vs various Tgas against Ao flux

� Convection in a Crude Heater firebox, say Tg=870°; Tt=370°C = 10*500 = 5,000 W/~ or about 16.6%

� Correlations that ignore convection give a higher gas or Bridge Wall Temperature + lower radiant duty split� Example F = 30,000 W/~ Tt = 370 Tg = 870 / 925°C

69/66%

Tube Metal Temp

950°C

850°C

750°C

Av

Flu

x -

OD

Ao ≈ 2αAcp

hc = 10 W/~, kCal/~ , 2 Btu/~ units based on Ao.

Based on αAcpTaking εeffective = 0.57, 10*2/0.57 = 35 W/~, kCal/~, 7 Btu/~ units,

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Case Study - 3 MW

Parameter

WLH Method

100% Load

20% Xs Air

100% Load

60% Xs Air

67% Load

20% Xs Air

Stack Gas Temp, °C 385 405 340

Efficiency, % 80.2 74.0 82.7

Radiant Split, % 67.4 62.5 72.0

Bridge Wall Temp, °C 900 825 800

Arch Draft, mm WC -1.5 +4 -1.5

Burner Draft, mm WC -6.3 -1 -6.3

Method Radiant % BWT, °C

Simple F = 0.35 on OD. - Convection � 69 875

GPSA WLH - No Convection 65 930

Nelson WLH - No Convection 67 900

Hottel & Sarofim (Add Convection) 65 (68) *935 (890)

Lobo & Evans - With Convection 65 *940

Chart = 890°C

* BWT by trial & error. Need to assume firebox layout first to determine heat transfer in fireboxT-ambient

T- Stack

BWT

Flame Temp

Radiant efficiency

Overall efficiency

Xs air reduces flame temp; Air-preheat increases flame temp

RADIANT SECTION SIZING

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Fuel & Flue Gas

� Verify heat duty = ẃ(Hout-Hin)

� Select heater type

� Select stack temp = Inlet fluid temp + 30 ~ 150°C

� Check if inlet fluid temperature results in

� Flue gas water dew point (≈ 60-70°C) and/or

� Acid dew point (≈ 150°C). Stack gas T > Dew Point

� Find thermal efficiency based on stack gas

temperature, Xs air and 1-3% casing loss

� Find fuel fired, flue gas and air demand

� Duty = 3 MW

� VC Heater

� Tin / Tout = 250/ 450°C

� Stack Gas ∆T = 140°C

� Stack Gas T = 390°C

� No dew pt issue

� Fuel = Refinery Gas

� Ex Air = 20%

� Thermal eff = 82.2 %

� Casing loss = 2%

� Fired Duty = 30/0.802 =

3.74 MW

� Fuel (LHV 46,250 kJ/kg) =

291 kg/h

� Air = 5,427 kg/h

� Flue gas = 5,718 kg/h

NHV Air Air kg/

Fuel kCal/kg kg/kg 10,000 kCal

H2 28,670 34.8 12.10

C 8,110 11.6 14.30

S 5,650 4.4 7.80

Methane 13,290 17.3 13.00

Ethane 12,420 16.1 13.00Combustion Air Demand. Magic Number

3.4 kg/10,000 kJ;14.0 kg/ 10,000 kcal; 7.8 lb/ 10,000 Btu

Coil Design

� Based on flow and allowable ∆P,

select tube size (4 or 6”) and passes

� Find velocity and heat transfer coefft

� Vacuum heater: Limit outlet velocity

to 80% sonic to avoid shearing liquid

� Ensure flow regime in vaporizing

service gives wetted wall i.e. bubble,

froth, plug, slug, and annular

� Check ∆P after heater sizing - total

equivalent length

� If calculated ∆P is higher than

allowed, change tube size / passes.

Repeat to match specified ∆P

� ∆P, allow = 3 bar

� 2 pass - 4”

� Velocity = 17.7 m/s

� hi = 1,088 W/m².°C

� hio = 983 W/m².°C


Reaction Heater: Coil size to suit soaking volume

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Typical Radiant Flux

Service Av Flux, W/m² Av Flux, Btu/h.ft²

Crude, Charge/ Feed Heater, Reboiler, Hot Oil 32~38,000 10~12,000

Vacuum (Fuel), Coker 32,000 10,000

Vacuum (Lube) 28,000 9,000

Visbreaker, Raffinate, Extract 22,000 7,000

Asphalt 19,000 6,000

Ethylene Cracker; Hydrogen Reformer 64,000 20,000

Radiant Section

� Assume a radiant split (% total duty in firebox) 60-80%, find fluid inlet temperature and average temperature in firebox

� Take average radiant flux given by client/ assume� Higher the flux, heater size �; TMT and coking �

� From “hio” and tube thermal conductivity, calculate average tube metal temperature (Tt)

� For the flux and Tt, find Tg, the gas cloud radiating temperature� Hottel, Wilson, Lobo & Evans, Mekler correlations

� In a well stirred heater, Tg ≈ Bridge Wall Temperature (BWT) or radiant exit gas temperature to perform heat balance and radiant section efficiency. In tall fireboxes Tg > BWT by 100-150°C

� Fluid Tin = 250°C

� Tout = 450°C

� Radiant 67%; Convection 33%

� Radiant Tin = 250 + 0.33*200 = 316 °C

� Fluid Tav = (316+450)/2 =383°C

� Av Flux = 30 kW/m²

� Assume 2D (8”) pitch

� Front 180° Flux = 45

� Tube = 315 + 45/ hio + ∆tm =435°C

� Tgas = 900°C (WLH)

� Radiant Eff = 54 %

� Split = 0.54*3.74/3 = 67%

� Assumed split OK

Measured Firebox Temperature could be less by 50-100°C, due to heat given up to shock tubes

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Radiant Section

� Based on radiant duty� calculate tube area and length

� Assume number of tubes, get tube length, tube circle dia or width and L/D ratio or b:h:l ratio

� A L/D ratio of 1.5 to 3 or b:h: l ≈ 1: 2-3: 5. Smaller heaters L/D ≈ 1 or b:h: l ≈ 1: 1-1: 2

� L/D lower with more number of burners, as D gets bigger with bigger Burner Circle

� Tube to Burner distance specified in API 560

� Radiant Q = 0.673*3 = 2.02 MW

� Radiant Area = 2,200/Flux 30 = 67 m²

� 4” NB 4.5” OD

� Coil Length = 188 m

� Assume tubes = 36

� Even no/pass. Top out

� Each Tube L = 5.2 m. Credit 180° bend 4.9m

� 2D (8”) pitch. Tube Circle Diameter = 36*8”*/pi() = 2.3 m

� L/D Ratio = 5.2/ 2.3 = 2.2 OK

� Say 3 burners. 1.5 MW @ 120% BCD = 0.7m. H-clear = 0.8m V 0.9m

Heat, MW Vertical, m Horizontal,

m

1.5 5.6 0.9

2.5 8.3 1.2

5.5 11 1.4Extract. Oil Firing. Full table in RP 560

L

D

cH

BL

CONVECTION SECTION SIZING

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Convection Section

� Shock Tubes - Standard correlations� Bottom 2 - 3 rows, bare to avoid high rates.

Firebox radiation + Hot gases transfer. ho governs

� Bottom half may get firebox radiation, if located above fire box. Area not counted in radiant area

� 3 modes: Gas convection, radiation and refractory radiation

� Gas convection: hoc = k*Cp*G^0.667*T^0.3/Do^0.33 k = 0.018 in SI units; 1.6 in British units

� Gas radiant: hr = x*T-y x= 0.092 y = 34 in SI units; 0.0025 and 0.5 in British units with t

� Refractory radiant: hrc= 5-15% of hc + hr

� Total = 1.1*(hc+hr)

� Temperature change/ row: high in bottom rows

Bare

Studded

Finned

� Qc = 3 - 2 = 1 MW

� 4” NB 4.5” OD 2 shock rows; 6/row, 2.5m

� Tube Ao = 1o.8 m²

� Flue gas V = 3.m/s

� Assume q = 0.24 MW

� Tg out = 780°C

� LMTD = 530°C

� hc = 21.02 W/m².°C

� hr = 19.08 W/m².°C

� ho = 44.17 W/m².°C

� U = 41.57 W/m².°C

� Duty = 0.24 MW

900°C

316°C780°C

300°C

Convection Section

� Extended Surface - Studs or Fins� Standard heat transfer correlations; Radiation from hot

gas is low. Thinner gas cloud

� Fins are less expensive than studs� Fins: 0.05-0.1” thick, 2-5 fins per inch. Less on oil

firing; more on gas 1.25 - 2.5 mm thick, 80 - 200 fins per meter

� Studs ½” dia in 8-24 studs per plane with 19.2 planes/ft (63 per m)

� Height 0.5-1.25” (12 to 40 mm) based on tube spacing

� Max convection flux to may exceed max radiant flux -on bare surface basis due to high extended area ratio

� Options� Small dia tubes to increase outside coefft ho

� Tubes horizontal spacing: increase/ decrease gas velocity

� Higher velocity � higher ho � taller stack


� Qc = 1 -0.24 = 0.76 MW

� 4 finned rows 160

fins/m; 25 mm high 1.5

mm thick fins

� Tube Afinned = 226 m²

� Flue gas V = 3.4m/s

� Tg out = 385°C

� LMTD = 272°C

� hc = 175.46 W/m².°C

� hr = 0 W/m².°C

� U = 143.35 W/m².°C

� Duty = 0.76 MW

780°C

300°C385°C

250°C

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Coil Design

� Pressure Drop Calculations: � Standard friction loss method, for

single and two phase flows

� Packed bed method for catalyst tubes

� Two Phase: Wetted annular is desiredThe linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location. The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location.

Pressure Drop

� Re = 16,35,550 f = 0.017

� ∆P/100m = 55 kPa

� Eq L = 500 m

� ∆P = 220 Vs 300 allowed

Coil Design

� Two Phase Pressure Drop

� Use EFV Chart

� Typical vacuum heater chart

Equilibrium Flash Vaporization Chart

Temperature

Pre

ssu

re 0% Vaporized

60% Vaporized

07-04-2014

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Tube Design

� On internal pressure� Elastic deformation

� linear stress - strain relation

� once internal pressure is removed, metal gets back to original state. Stress at that point is called Yield Stress

� Plastic deformation, on further stress� remains permanent even when load is

removed

� Stress causing plastic deformation before fracture is Ultimate Tensile Stress

� Design stress = 2/3 Yield or 1/3 UTS. For CS plates it is about 17,000 psig (117 MPa)

� Yield point at 1.5*design pressure; deform at 3*design pressure.


• Steam coils to ASME

• Corrosion allowance, high

in initial life, indirectly

adds to design life

Tube Design

� Hot tubes undergo permanent growth or creep� Time dependent

deformation below yield strength at high temp

� Stress rupture - short term; Creep - long term failure

� Heater tubes are usually designed for 20,000 to 100,000 hours of creep-rupture life per API RP 530

This image cannot currently be displayed.




Limiting design TMT

07-04-2014

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Tube Design - Material of Constrn

Service 1¼Cr

2¼Cr

5Cr 9Cr 316 SS

317 SS

321 SS

347 SS

HK40 HP

Issues

CDU VDU X X X X Creep, External oxidation, Sulfidic/ Naphthenic Acid Corrosion

Delayed Coker

X X X Carburization,ErosionCreep, External oxidation, Sulfidic/ Polythionic Acid Stress Corrosion Cracking (TP 347)

Catalytic HDS

X X X X Creep, External oxidation, H2/ H2S/ Polythionic Acid Stress Corrosion Cracking (TP 347)

Tube Design - Material of Constrn

Service 1¼Cr

2¼Cr

5Cr 9Cr 316 SS

317 SS

321 SS

347 SS

HK40 HP

Issues

Cat Reformer X X X X Creep, External oxidation, H2 attack, Metal dusting, 1¼Crspheroidization

CO Boiler CSX

X X Internal corrosion, External dew point corrosion, Erosion, Creep, External oxidation

H2 Reformer Ethylene Cracker

X Creep

Spheroidization: Aging of carbon and low alloy steels with loss of mechanical and creep strength when exposed to 440°C - 760°C (850°F - 1400°F) where carbide phases (the strengthening element of steels) become unstable and begin to agglomerate, which then results in loss of strength

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Tube Supports Design

� Tube Supports� Dead load and tube expansion

� Horizontal Support: @35D / 6m

� Vertical: 70D or 12m

� 25Cr-20Ni, 25Cr-12Ni, SS or CS. High vanadium + sodium in fuel oil may call for expensive 50Cr-50Ni to withstand fuel ash corrosion

� Cast Iron: 425°C (800°F)

� 25Cr-12Ni: 815°C (1,500°F)

� End tube sheets are CS lined with castable

Tube Wall Thickness

� OD = 114.33mm

� Design = 25 bar/540°C

� Allowable Stress - Elastic/ Rupture @ 100,000 hrs = 1,310/ 450 bar

� t calculated = 1.08/3.09mm

� Corrosion Allowance = 2mm (Note: Factored CA used)

� t min = 3.6/ 4.8mm

� t average - pipes ±12.5% = 5.49mm

� t provided = 5.52 mm

Stack

� Sizing based on:� Buoyancy effect of flue gas

� Frictional losses in convection bank, duct and stack

� Standard formulae for duct friction loss

� Stack draft affected by altitude/ atmospheric pressure at site

� Height to get draft or mostly suit GLC of emission� Due to heat loss, stack draft temperature <

Stack gas inlet temperature

� Structural design of firebox and stack based on:� Wind and seismic loads

� Stack dia, say = 0.6 m

� Arch draft min = 1.5mm

� Firebox T = 900°C

� Firebox ht = 5.5 m

� Firebox draft = 5 mm

(Draft for burner)

� Stack T = 390°C

� Draft/100m = 66 -12 =

54 mm

� ∆Pconvn = 2.6 - 0.85

� Damper loss = 3 mm

� Stack tip loss = 4 mm

� Draft reqd = 10.4mm

� Stack Ht = 20 m

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Burners

� Heater performance depends on burner performance

� Number of burners decided by flame length� Service, fuel, firebox layout. Flame length < 2/3 firebox

height. Gas flame 1-2 m/MW (1-2 ft/MMBtu/hr)

� Burner to tube clearance. Min 450mm (1.5’). See API 560

� 3 - in - Cluster burner for small heaters

� Burner to burner space: Must avoid flame merging to avoid unburnt fuel and longer flames; may increase NOx

� Excess Air: 5-10-20-30: based on fuel and draft (FD?)

� Burners, duct & stack sized for 110-120% firing and higher excess air

� Must avoid flame and hot gas impingement

Burners

� Low NOx Burners� Air or Fuel staging + Internal flue gas recirculation

� Extra 150 mm (6”) clearance to tubes; More burner to burner clearance for flue gas recirculation, min 250 mm (10”)

� Flames 25-50% longer. Retrofit difficult

� Avoid inner circle burners in VC; may not get cool flue gas recirculation �more NOx

� Fuel gas at 1.5 to 2 bar. Oil at 5-7 bar. Oil viscosity ≈ 25 cs� Atomizing steam at dP of +2 bar @ 25-45% oil flow

� Caution: Gas Mol Wt change can play havoc on pressure at burner; may lead to Xs firing on fuel switch over and explosion. Why?

Fuel - MW 11.6 25.8 42.6

HHV, Btu/SCF 700 1,400 2,100

HHV, kcal/kg 12,700 11.425 10,380

Press, barg HPPress, barg LP

2.11

1.20.5

0.90.35

Turndown Gone!

Combustion Air Demand. Magic Number

3.4 kg/10,000 kJ;14.0 kg/ 10,000 kcal; 7.8 lb/ 10,000 Btu

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Refractory / Insulation System

� Ceramic fiber and castable are common

� Fire bricks for fired-upon walls & hotter fireboxes - reaction heaters

� Design to limit casing temperature <100°C and < heat loss

� Insulation + air film resistance (hc + hr)� Casing surface temperature calculated at still air; heat losses @ high

wind

With wind, casing temperature fall off fast while heat loss increase marginally high

Sizing Firetube Heater

� Given� ID = 0.6m (2’) L = 0.9m (3’); Tg = 1,540°C (2800°F) and Tw = 150°C

(300°F). 20% Xs. Q = 1.2 MW (4 MMBtu/h)

� qr� F = Ac/At = πDL/(πDL + 2πD²/4) = 0.75

� pL = 0.24*0.6 = 0.144 at.m (0.48 atm.ft)

� εg = 0.12; εt = 0.79; (1/εg - 1/εt -1) = 8.6

� q/A = Fσ(T14 - T2

4)/(1/ε1 - 1/ε2 -1) = 53.3 kW/m² ~ (17,000 Btu/h.ft²)

� qc� hc = 7.45 W/~ (1.31 Btu/~). 10.34 kW/ kW/m² (3,275 Btu/ h.ft²)

� qr + qc = 63.6 kW/m² ~ (20,300 Btu/ h.ft²)

� Application: Water bath, steam bath, salt bath and reboiler� 150 - 750mm (0.5-3’) dia x 1.5 - 9m (5-30’) long

From: Engineering Data Book, GPSA, Section 8, Ex 8-6 and 8-13

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0

1

2

3

4

5

6

7

0 10 20 30 40 50 60 70

Fu

el &

Sn

uff

ing

Ste

am

, T

PH

Heat Liberated, MW

Utility Demand

Heater Utility

� Snuffing Steam� Purge firebox of unignited hydrocarbon, before light up to avoid an explosion

� Failure to purge - repeated cause of firebox explosions

� LP steam @ firebox volume in 5 minutes; additional purge connections to header boxes. Note: Use steam density at atmospheric pressure

� Use snuffing steam to put out any small fire caused by tube leak

� Run FD fan for few minutes, if no LP steam OR use steam ejectors

� Atomizing Steam� Atomizing steam - to shear and froth fuel oil. @ 25-45% oil flow

� Soot Blower Steam + Plant Air� Steam: 4,500 kg/h for retractable. Air: 80-100 Nm³/h for its pneumatic motor

AtomizingSnuffing

Heater Decoking

� Regular decoking will avoid hot spots and premature tube failure. Remove coke deposited inside tubes by� Gas oil circulation to soften and remove deposits. Not for heavy

coking

� Chemical cleaning - circulating inhibited acid or chemical + water wash for salt deposits

� Hydroblasting - high pressure water jet; abrasive grit. Shot/sand

� Steam: air decoking: In-situ combustion with steam and air� While tubes in 1 pass are decoked, tubes in other passes kept cool

with steam� Cooling steam is 5-15% of decoking steam flow

� Decoking air 5-15% of decoking steam flow

� Pigging - abrasive pigs. Less damage than decoking or acid wash

� Mechanical turbine thru Mule Ear Plugs



Decoking Pig

Intelligent Pig

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Recommended Reading




“Heat Transfer in Radiant Section of Petroleum Heaters” WE Lobo, JE Evans; Trans. AIChE, pp 748-778, 1939“Heat Transmission in Convection Section of Pipe Stills”, CC Monrad, Ind. Eng. Chem., Vol 24, 505, 1932“Generalized Method Predicts Fired Heater Performance”, N Wimpress, ChE, May 22,1978

� Furnace Operations, R D Reed

� Petroleum Refinery Engg, W L Nelson, Chapter 18

� Engineering Data Book, GPSA, Section 8

� API Publications� Spec 12K Specification for Indirect Type Oilfield

Heaters

� Std 530/ISO 13704 Calculation of Heater-Tube Thickness in Petroleum Refineries

� RP 535 Burners for Fired Heaters in General Refinery Services

� RP 556 Instrumentation, Control, and Protective Systems for Gas Fired Heaters

� Std 560/ISO 13705 Fired Heaters for General Refinery Services

� RP 573 Inspection of Fired Boilers and Heaters

THANK YOU

Stay Safe. The best for many years of safe and sustained operations


DESIGN Fired Heaters Training 1 -3 days

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