7 replies to this topic

[nastaran]

Hi Dears,
I have to specify the essential instruments for a direct fired heater with 4 passes that fule oil in 45 degree centigrade and 874 CP enters to the coil and after passing the covection and radiation area exits in 65 degree centigrade and 57 cp .The fule in burner are both gas and oil.
is it necessay control of flow rate in evey pass or only one in overal input is enough?
is it necessary contorl of temprature in all passes in output?
can we omit the TI in skin tube beacuse the tempature is low?
Please help me about control system in this fired heater afterall.
thaks in advanced

[ankur2061]

Nastaran,

I haven't heard of fired heaters which don't measure skin temperature for the coils. Also, flow measurement (flow switch low (FSL) is practised for all the passes to shut down the heater on low flow conditions for preventing hot spots in any pass. Catastrophic failure has been regularly reported by the fired heater industry due to hot spots formed by flow starvation in any tube pass.

Besides this the regular burner management interlocks such as flameout and UV sensor detection as well as pre-purge and post-purge should in my opinion be mandatory.

Remember, fired heaters are among the most unforgiving piece of equipment operated in the chemical process industry. You just cannot consider taking any short-cuts for these equipment.

Hope this helps.

Regards,
Ankur.

[nastaran]

Dear Ankur,

Thanks so much for your favour. I am aware about the sensitivity of fired heater operation. would you please inroduce me a soursce for control of fired heater operation? I am not worried about burner management system beacuse the burner supplier will design the BMS. The process that happens in the fired heater is important for me. and about choking in the coil considering the high viscousity of oil .

[ankur2061]

[/quote]
Dear Ankur,
Thanks so much for your favour.I am aware about the sensitivity of fired heater operation .would you please inroduce me a soursce for control of fired heater operation?I am not worried about burner management system beacuse the burner supplier will design the BMS .The process that happens in the fired heater is important for me.and about choking in the coil considering the high viscousity of oil .
[/quote]

Nastaran,

Two very important standards/recommended practices related to fired heaters which should be studied for getting a deeper insight of fired heaters:

API STD 560- Fired Heaters for General Refinery Services

API RP 556- Instrumentation and Control Systems for Fired Heaters

These are the best international standards that I know regarding fired heaters.

Regards,
Ankur.

[Zauberberg]

Again, a very good source for getting the more insightful view into process equipment control, is Walter Driedger's website - and is available for free.

Particularly, the article related to control of fired heaters: Controlling Fired Heaters

[Majid-Process]

Hi,
The subject is quite interesting for me. Therefore I like to give you some feed back on raised issues.

For control requirements see API 560 and ISO 13705. In case you do not have access to them, flag up I will find a way to give a copy (The file size is more than 10 MB)

Control of flow input is enough since you will have the same flow on all passes unless you have a specific design with defferent flows on passes (any input or out put from passes)!

Temperature control on the last pass is fine. However the highest SIL certification is required. I beleieve voting configuration (2 of 3) should be applicable.

Any tube skin temperature sensor/transmitter shall not be deleted. In normal operating condition there should be nor problem but tube skin temperature will detect any obnormal temperature control or lack of flow in tubes.


Regards
Majid

Hi Dears,
I have to specify the essential instruments for a direct fired heater with 4 passes that fule oil in 45 degree centigrade and 874 CP enters to the coil and after passing the covection and radiation area exits in 65 degree centigrade and 57 cp .The fule in burner are both gas and oil.
is it necessay control of flow rate in evey pass or only one in overal input is enough?
is it necessary contorl of temprature in all passes in output?
can we omit the TI in skin tube beacuse the tempature is low?
Please help me about control system in this fired heater afterall.
thaks in advanced

[JMW]

In practise, when burning heavy fuel oils, temperature control of the heaters is often less effective than desired because of the variation in fuel oil quality which is often more pronounced in refineries where the fuel may not always be of commercial quality.

The strategy for controlling HFO heating is sometimes based on a combination of measures:

1) take periodic samples of the fuel for laboratory analysis in order to determine the EVT (Equi-Viscous Temperature i.e. the temperature at which the fuel viscosity is optimum for injection and combustion) The EVT value is the temperature set point value used in the control of the final temperature of the fuel as it is delivered to the nozzles.
Because of heavy fuel oil quality variation due to (a) the quality variation between fuel batches and ( stratification and separation in storage. Quality changes need not be progressive as, due to various factors, step changes can occur.
Periodic sampling can help manage this problem in conjunction with other management initiatives but not sufficiently so as to avoid all problems.

2) Excess oxygen: poor atomisation of the fuel due to the viscosity being other than optimum can result in burning taking place away from the optimum zone and fuel combustion being incomplete. Excess oxygen helps improve the combustion of the fuel.

3) Visual Flame Inspection: the operators must frequently inspect the flame and make such measures as are necessary.

Today some operators now use process viscometers for heater control.
These provide a real time measurement of viscosity and this value is when use for heater control.
The benefit is that as fuel quality changes the viscosity at the operating temperature will change and this is immediately detected by the viscometer and the signal used to modulate the heater control so as to maintain a constant viscosity in the fuel arriving at the burner nozzles. This has been a problematic measurement in the past as heavy fuel oils to burners are often not as clean as could be desired and viscometers could fail on a frequent basis.
However, some modern viscometers are able to operate even with the dirtiest fuels and are effectively self cleaning.
They can provide a very high on-stream factor. For example, at a power generation facility the EPA required that each burner had a duty and a standby viscometer in order that there should always be at least one viscometer functional. However, the original viscometers would fail frequently with a high burden on maintenance and periods when the standby would fail before the original could be returned to service (in which case heater control depended on the last good temperature value).
In trials of a new viscometer, one of the more modern types, the performance was such that the operator was not required to have standby viscometers but just one viscometer per burner.

There is an article relating to Huntsman Chemicals experience here:
http://www.engineeri...slm/slm106.html

For a useful spreadsheet to determine EVT visit http://www.viscoanal....com/page8.html and download the RMI ASTM D341 Rev01.xls Spreadsheet.

[Whit123]

You have not specified the volume of oil that would be heated. If it is a large quantity, then the dead times will be higher and the design will vary based on this.
IMHO you should have different temperature sensors for each pass, just one will not do. You can also troubleshoot easily if one of these is not working, which is not possible if you use just one set of sensors. If there are choking or other problems in the individual passes, these can be immediately noticed because of the multiple sensors.