4 replies to this topic

[hamadzzz]

I am process engineer working in petrochemical company .and we deiced to decommission (VCM Vinyl Chloride Monomer) tank , its storage temperature -13.5 C° the reaming liquid Qty is 500 mt .This is first time we take this tank for internal inspection .and would like to share with all of you the flowing point :
1-is there any formal or method to estimate how long it take to bring this tank to atm temperature ,if this system have close refrigeration systems ,that mean we cannot reverse the refrigeration to enter hot gas to the tank to heated up ,this tank equipped cold insulation to mitigate heat ingress and it is connected to the flair to burn access VCM vapor .
2-how much N2 Qty needed to inert to be ready for inspection .

HGA
hamadzzz@maktoob.com

[mishra.anand72@gmail.com]

If you will do PDP cycle of 0-2-0K and each cycle takes 1.5 hrs and tank capacity is 1000 M3 and there is no constraint on N2 supply and there is no VCM liquid but VCM vapours, this will take 8 hrs. If you are unable to supply N2, PDP cycle will get delayed and so the hrs required. After each three PDP cycle, you will give sample to lab so that hydrocarbon level is known. You will know that Tank is free of VCM vapours only after getting two consecutive readings of less than 1%. If lab instrument least meesure count is 0.1%, you can continue PDP upto getting two consecutive readings of 0.1%. Temperature will achieve atmospheric in these activities and refrigeration unit is shut down.

Another resignation is in pipeline.

[hamadzzz]

If you will do PDP cycle of 0-2-0K and each cycle takes 1.5 hrs and tank capacity is 1000 M3 and there is no constraint on N2 supply and there is no VCM liquid but VCM vapours, this will take 8 hrs. If you are unable to supply N2, PDP cycle will get delayed and so the hrs required. After each three PDP cycle, you will give sample to lab so that hydrocarbon level is known. You will know that Tank is free of VCM vapours only after getting two consecutive readings of less than 1%. If lab instrument least meesure count is 0.1%, you can continue PDP upto getting two consecutive readings of 0.1%. Temperature will achieve atmospheric in these activities and refrigeration unit is shut down.

Another resignation is in pipeline.

thanks for informative information Mr.mishra ,

My concern here VCM is consider as heavy material and will take time to evaporate at ambient temperature ..

Here I would like to share with you and with people have experience of this type of preparation in order to estimate the time required to decommission the tank if we consider the major time will be taken when bring the tank to ambient temperature and during N2 purging .


Let’s take first point
to bring tank -13.5C° to 40 C° :
Is there and technical or method to calculate the time if we keep tank at steady state condition and by shut down the refrigeration system :
If the we assume that the remaining qty is 140 m³ , and
At Liquid phase pritory
Liquid density (1.013 bar at boiling point) : 970 kg/m3
Liquid/gas equivalent (1.013 bar and 15 °C (59 °F)) : 365 vol/vol
Boiling point (1.013 bar) : -13.8 °C
Latent heat of vaporization (1.013 bar at boiling point) : 333 kJ/kg
Vapor pressure (at 20 °C or 68 °F) : 3.4 bar
If can use the latent heat of vaporization to know how much KJ need to heat up 140 m³ of VCM liquid base on the the sun heat that will pass through the tank insulation at absent of cooling system at ambient temperature 35Cº.


The second point is N2 qty .
If we 140 m³ VCM liquid mt is converted to vapor this will give :
Volume of the gas phase : 51100 (m3)

My question here how much time need to purge the tank if the flair capacity is 2500 kg/h and the N2 supply is 100 kg/hr many .


Thank for your kind cooperation

HGA
hamadzzz@maktoob.com

[djack77494]

I suspect that Mr. Mishra's reply was based on the complete drainage of all liquid VCM prior to purging. That is certainly a required step since you will want to absolutely minimize the flaring of VCM for environmental, safety, and financial reasons. Please plan on doing a thorough effort in this regard before proceeding with worrying about the vapor space. If you successfully remove all or nearly all of the liquid, low temperatures should not be much of a concern, since they are caused mainly by the liquid VCM heat of vaporization.

[hamadzzz]

I suspect that Mr. Mishra's reply was based on the complete drainage of all liquid VCM prior to purging. That is certainly a required step since you will want to absolutely minimize the flaring of VCM for environmental, safety, and financial reasons. Please plan on doing a thorough effort in this regard before proceeding with worrying about the vapor space. If you successfully remove all or nearly all of the liquid, low temperatures should not be much of a concern, since they are caused mainly by the liquid VCM heat of vaporization.

Dear Mr. djack

thanks for your reply really it is reasonable .
the tank does not have full drain point in other meaning these is design issue, that why it not possible to remove all Liquid ,the remaining quantity which is equal 10% of tank capacity I have to cover it vapor this will tank time from as mentioned before . and also it will cost me lot .The with limitation of flexibility of the system I need to estimate the time of this operation in proper way .

one again thanks too much for Mr Djack and Mr.mishra