3 replies to this topic

[Mahdi1980]

Dear friends

I am designing an Inert-gas blanketing system for Naphtha storage tanks which type is fixed-roof.

As mentioned, the liquid stored in these tanks is Naphtha with a flash point of -21°C, vapor pressure of 80 kPaa (worse case), IBP of 25°C and density of 0.72 .Although it is recommended to store such a liquid in floating-roof tank as per NFPA 30 and API 650, existing tanks are fixed-roof type and my employer intends to use these ones and as you know, many references suggest a nitrogen blanketing system to avoid contacting air to fuel while tank unloading.

There are two tanks each has the capacity of 2000 m3 and Max pump in/pump out flow is about 180 m3/h. The working temperature and atmospheric pressure of site are 35°C and 0.87 bara (due to much higher height to see level) respectively.

As the tank is atmospheric and its design temperature is 20 mbarg, I have considered a blanketing pressure of 10 mbarg and have calculated breathing capacity and sized PVSV and Emergency valve  according to API 2000 (7^th edition) and API 526.

Calculations show that the required flow of Nitrogen for blanketing is about 280 Nm3/h for unloading of each tank and total capacity of N2 is about 3500 kg for unloading of each tank to avoid collapse conditions.

Please let me know if I need a nitrogen generation package for such conditions or using several high pressure cylinders of N2 will satisfy the requirement although it is maybe not a practical decision. Moreover, if I have to utilize a generation package, what is the minimum required capacity for it

.Also, do I have to consider such a package for simultaneous unloading of both tanks or I should take into account a coincident factor to obtain the optimum capacity. Which choice is more economic?

Please also recommend that if I can use a closed loop system to re-circulate and recover the Nitrogen during out-breathing of the tank concerning this inert gas contains a low concentration of naphtha vapor.

It would be my pleasure to know your previous similar experiences and apply them into my project.

Thanks

Edited by Mahdi1980, 01 October 2015 - 03:49 AM.

[Mahdi1980]

Dear friends

My  first question has remained without answer.

Anyway, please let me know what is the maximum allowable concentration of oxygen (%W or %V) in N2 blanketing mixture for A fixed roof Naphtha storage tank. NFPA 69 cites that MOC for Gasoline is 10% oxygen. Is it applicable or the purity of Nitrogen blanketing to be much higher?

[Chemitofreak]

@ Mahdi

 

Nitrogen supply options include delivered liquid nitrogen stored in bulk tanks as well as delivered gaseous nitrogen stored in large banks of cylinders. In addition to that nitrogen can be generated by cryogenic plant, PSA or Membrane technology.

 

The choice of delivery method depends upon requirement for purity, usage pattern, profitability, space available for set-up of nitrogen generation unit.

 

NFPA 69 requires operating at 60% of LOC (Limiting Oxygen Concentration), which for many materials is about 10 vol%. Thus using 94 vol% purity Nitrogen shall suffice the requirement. To be on a safer side use 25% of LOC, this will require about 97.5 vol% pure nitrogen. 

 

I, have never seen nitrogen purity less than 97 vol% in any of the project handled. 97% purity can be easily achieved by membrane or PSA. 

 

Revert in case of any query.

[Mahdi1980]

Chemitofreak

Thank you for your reply