Manoir Industries centrifugally cast and

internally FINNED TUBES

Star tubes

The ethylene industry has been looking for means to improve furnace efficiency throughout the years. ► Options have been focused on metallurgies, by designing alloys to push furnaces operating limits. ► While most of the industry uses circular smooth bore tube, increasing the heat exchange surface between feedstock and furnace tubes can be achieved by switching to a non circular bore. The tube ID developed surface is therefore increased. Those tubes are commonly called finned tubes.

► The concept of internally finned tubes for ethylene steam cracking furnaces is not new. (old patent dated 1961) (link) idea: to increase thermal exchange across the tube thanks to an increase of tube inside surface without a too much prejudice on pressure drop, this through the whole tube length. ► This concept has been first applied to extruded tubes but with following disadvantages: - impossibility / difficulty to extrude high carbon grades (C > 0,4%) - fine microstructure resulting from recristallization of grain => lower creep properties than cast tubes. - very poor inside roughness => this also implies a loss of carburization resistance compared to a machined smooth surface

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ep

ca

rbu

riza

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Star tubes

2012 Typical extruded tube structure

Typical cast tube structure

Fine grains due to

extrusion process

2012

Alternative ways to obtain a finned tube

• Because of the limited amount of metallurgies available, and hence of their properties, alternative ways have been subsequently develloped.

• ECM(Electro Chemical Machining) is available, but this process is expensive, slow, and has no positive effect on the tube ID surface. This process can generate electric arc strikes along the tube lengths and it is very difficult to accurately inspect the complete tube ID after completion to detect those.

• Our method, consisting of « chips removal », is much faster, and has a definite positive effect on the carburization resistance of the tube due to the enhancement of the surface by the machining operation itself.

2012 Star tubes

Startube ECM Extruded

Creep properties High High Low

Carburization resistance

High Medium Low

Manufacturing method

« Conventional » machining

Electro chemical machining

Extrusion

Manufacturing possibilities to obtain a finned tube

Star tubes

► The use of a finned tube can either : - allow to produce equally with a reduced coil TMT( with positive impact on coking, run length and tube life, and energy savings), -or produce more by firing the same or more.

2012

Effect of type of surface preparation on

corrosion resistance

Met

al

wa

sta

ge

du

e to

met

al

du

stin

g (

mg

/cm

²/h

)

0 2 000 4 000 6 000 8 000 10 000

Source : Klower, H.J. Grabke, E.M. Müller-Lorenz

10-2

10-3

10-4

10-5

10-6

601 material

Electropolished

Machined or ground

Black

Exposure time in hours

2012

Material behavior vs. surface preparation

0 5 10 15 20

6

4

2

0

Time in hours

Ma

ss g

ain

in

mg

/cm

² (

carb

on

pic

k-u

p o

n m

eta

l su

rfa

ce)

Source : H.J. Grabke, E.M. Müller-Lorenz

electropolished

Cold rolled

Machined or ground

25/20 material

Star tubes

Structure shows here having been

pulled by machining tool

Working effect of the machining operation on carburization resistance

Tool moving in this direction

A machined surface is harder and enhances Cr diffusion, slowing down Carburization progression

► Proposed profiles: - 5 to 3 mm radius top and bottom of fins to prevent coke concentration at bottom of fins and improve decoking in these areas, - 35 mm smallest tube ID at top of fins, - various profiles available - ID of finned tubes is calculated to keep the same feedstock cross section as previous bare tube, - OD is calculated to keep the same MSW at bottom of fins as previous bare tube.

Star tubes

► Proposed minimum profiles:

w

r = 5

r = 5

Star tubes

► large dia examples of profiles:

Star tubes

26 fins, 487,1 mm developped length 8 696 mm² cross section

26 fins, 506 mm developped length 9 715 mm² cross section

Star tubes

► Tube to tube welding:

Chamfering on 10 mm on tube

ID in order to weld on a constant

wall thickness. Protusion cannot

be ground.

Either Or

Welding through varying wall

thickness with EBW.

Protrusion can be ground on

the tips of the fins only.

2012

References

Customer Country Mat’l Furnace Design

Comments Number Of Furnaces

Polimeri Italy Man XM-25/35

Lummus SRT4

Inlet tubes 1 full furnaces + subsequent spares

Formosa plastics

USA Man XTM-35/45

KBR millisecond

All tubes 2 full furnaces

Dow chemical Canada Man XTM-35/45

Technip SMK All tubes

4 test coils + 1 full furnace ( 24 coils)

Borealis Sweden various - Test tubes N/A

Basell France Man XM-25/35

Lummus SRT5

Replacement of extruded inlet tubes by cast

partial

Exxon Chemicals

France Man XM-25/35

own Outlet tubes 1 complete furnace

Daqing petrochemicals

China Man XTM- 35/45

Score All tubes 2 coils worth

Exxon chemicals

USA Man XTM- 35/45

own Outlet tubes 1 full furnace worth

2012

Finned Tubes Characteristics

• Max Length

• Max OD

• Min ID

• Number and depth

of the fins

• Material

4 m

125 mm (*)

35 mm

combination to increase the exchange surface by 25% minimum

Manaurite XM, XTM, 40X0, and others

(*) larger dia can be accommodated against OD skinback and cropping

2012

Machining Capacity Today

• 2 Machines

• 3 shifts a day – 7 days a week

• 6 to 12 finned tubes per day

(ex. Furnace Formosa USA = 528 tube sections)

2012

Star tubes Shop

2012