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Naphta Heater Problem


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#1 sudheer

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Posted 01 August 2007 - 02:23 AM

Our CCR - NHT - Charge Heater is around 5Mmkcal/hr natural draft heater. The heater heats feed from 258°C to 303°C. Presently we are finding it difficult to load the heater to a COT of 281°C at design feed. The skin temperatures have gone high. No much change is observed in the delta presure across the coil ( within design limits) . We Suspect a inner layering of coke. Will steam spalling help ? Or any other method .Please let us know

regards
Sudheer Pai/Avinash

#2 Milutin

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Posted 01 August 2007 - 05:20 AM

Why you are suspecting coke formation?
If layer of coke is formed it will prevent heat transfer to naphtha, and tube metal under layer of coke will be much hotter than metal without coke layer. Appearance of coke should be visible as glowing red patches on tube.


Regards,

Milutin

#3 sudheer

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Posted 01 August 2007 - 05:37 AM

Thank you very much for the reply.
This heater we are operating since ten years and we are facing this problem of high skin temperature since last six months. Hence we suspect there can be some problem of coking.
Other reasons if any, can be kindly put across so that better results can be brought out.
regards
Sudheer/Avinash

#4 indy

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Posted 01 August 2007 - 08:36 AM

I suppose that after ten years, if you are experiencing high skin temperatures, and you find difficult to reach process design feed of 281ºC, your heater duty is becoming poor due to a coke layer inside heater tubes. Even small thickness of inner coke layers such as 5mm represents a decrease on heater duty of more than 30%. Steam air decoking is a method which requires skill experienced operators and good laboratory logistics to determine oxygen and co2. Another method of heater decokefication is through pigging.

I hope it helps

Luis marques

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#5 indy

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Posted 01 August 2007 - 08:41 AM

I have to add that a layer of coke of 5mm has not much impact on in delta pressure across the heater piping.

Regards

Luis marques

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#6 Milutin

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Posted 01 August 2007 - 12:39 PM

How you identified coking?

Usually this particular service heater is not prone to coking, because of low coil outlet temperature. I would expect coking in VDU or visbreaking heaters, not on NHT heaters. Usually heaters prone to coking have designed and installed decoking installation, I suppose you don't have one.

Nevertheless coil can coke if you had some incidents like firing heater without naphtha flow, so you should check heater history data from moment when you noticed high skin temperatures.


What about your convection section? If it is heavily fouled it can be very inefficient and in that case radiant section must be additionally fired to get proper COT. This also can be reason for high skin temperature. This can be checked by measuring heater draft, but suppose you firing fuel gas so no heavy fouling is expected.

You should check your heater inlet temperature is it ok or lower than expected , also how far are you from design throughput?

Regards,

Milutin

#7 sudheer

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Posted 01 August 2007 - 10:50 PM

Thank you very much for replying
We do not have provision for steam air decoking. Hence we are interested to do steam spalling which require very very less changes in the system. Whether only spalling may help.
Again whether anybody has experience of coking on similar service.
regards

#8 indy

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Posted 02 August 2007 - 04:26 AM

Recently we experience coke on our HN (Naphtha Hydrobon) charge heater which works at temperatures similar to yours. This heater was not decoked since 10 years ago.
During internal inspection we detect a layer of coke about 6 mm thick with Rx spots.
Without air and only with steam you will not be able to burn the coke. We have decokified our heater through pigging.

Regards

Luimarques

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#9 smalawi

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Posted 12 August 2007 - 01:31 AM

hi,

we have similar setup with our naphtha heater and termal decocking works fine. however, spalling will only remove the softer part of the carbon layer in the heater.

if you dont have the setup for thermal decoking then try pigging. the later is much more efficient than thermal decoke. it's also safer,

we use thermal decoke in our visbreaker, VDU's and others. were we dont have the setup we used pigging. pigging requires U type bends to be effective. if you have mule plugs it will be hard to do pigging and mechanical tube reaming or hydroblasting is more effictive.

carefully chose your piging company. some pigs scratch the tubes and do more damage,

cheers,

#10 Satyajit

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Posted 18 September 2007 - 01:44 AM

Sudhir,
All these fired heaters need decoking for regaining its thermal efficiency. Higher hydrocarbons crack at 520 Degree C and above and lays carbon on heating surfaces. The decoking operation is the only solution in this case. High skin temperature of the tubes is because of extra firing than design to get the required outlet temperature.

Decoking will take about 10-12 hours . First with steam at a flow rate of 5te/hr and then together with a small quantity of air ( maintain a ratio of 0.2 )intermittently is required to burn all the carbon . You can observe the completion by looking into flue gas colour. Atleast one burner is necessary to be online at minimum firing .
Please remember that the heater tubes have also reached 100,000 hours of operation and reaching its end of life and so the heater should not be over-fired otherwise Tube leak /rupture etc are foreseen .
Best wishes
satyajit




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