6 replies to this topic

[Shah Khan]

What is the criteria of selecting control valve position on instrument air faliure. What shall be the basis for fail to close or fails to open for a control valve on instrument air faliure if it is upstream of a fracionator, column level controler or top product control valve. What importnt thinks are to be considered.

Regards

[SSWBoy]

What is the criteria of selecting control valve position on instrument air faliure. What shall be the basis for fail to close or fails to open for a control valve on instrument air faliure if it is upstream of a fracionator, column level controler or top product control valve. What importnt thinks are to be considered.

Regards

Safety is generally the main consideration. Picture a control valve on the steam supply to a reboiler, would you want it failing open or closed? Common sense would dictate to have it fail closed, the alternative being it failing open and leading excessive reboiler heat input.

Essentially, picture your control valve and work out which failure position will lead to the safest conclusion, note also that valves can freeze in their last position on IA failure.

[sailaja a]

Criteria is based on

to make process stable and avoid emergency condition.

for example..

1)column reflux control valve is air fail to open because if it closes column pressure increases suddenly and what ever the material in the column vaporises (heaviers).vapour builds up in the column.so there may be a chances of PSV POP up,. to avoid those scenarios it is designed control valve as air fail to open. so we can add reflux to column by adjusting manually.
2) reboiler steam flow control valve is designed as air fail to close. because to avoid large amount of heat duty to the column and to avoid overpressurisation.

Edited by sailaja a, 14 August 2009 - 06:48 AM.

[Qalander (Chem)]

It depends on
a thorough review of many factors and may not be a simple reply

since process safety has to be ensured either


through relatively lower pressure,temperature conditions at this 'failure onset' or


operator response/intervention also considered and some alerting meanse are made inherent to ensure the requisite process safety levels/conditions achievement.

Hope this helps in way forward.

[Shah Khan]

Dear All

Thanks for your replyes. As the process we are working on actually is a non refinery process, we are using five fractionation column with number of recycel to previous column, it makes the system complex either to close which valve on I.A faliure. However i have positioned the vavle on least severe condtion, to have least heat accomulation which may lead to high pressure, or where i may have the cushion of the operator intervention (as if it is taking more then 10 mint to prussurized).

Best Regards

[JoeWong]

What is the criteria of selecting control valve position on instrument air faliure. What shall be the basis for fail to close or fails to open for a control valve on instrument air faliure if it is upstream of a fracionator, column level controler or top product control valve. What importnt thinks are to be considered.

Regards

Always think of position lead to
- plant safety
- less trouble when restart
- less inventory loss

[Uday Bhatkande]

What is the criteria of selecting control valve position on instrument air faliure. What shall be the basis for fail to close or fails to open for a control valve on instrument air faliure if it is upstream of a fracionator, column level controler or top product control valve. What importnt thinks are to be considered.

Regards

Most important criteria is safety of your equipment and plant....
ex. All reflux control valves are fail safe to open and valves providing heat duty to the column are fail safe to close.
In special applications like molex process pressure control valves are fail safe to close whereas in most of the applications column pressure control valves are fail safe to open...column level control valves are fail safe to close as otherwise reboiler will run dry.