4 replies to this topic

[TNCX34]

Hello,

 

I am very confused with Methanol storage tank venting requirements.  The issue is not the calculation of breathing requirement according to API 2000; that is very easy, but It brings me other issues that I have not found till now.  I am trying to explain the issues that I have faced in the following;

 

1. I am starting with most important and general question: How storage tank design pressure is determined, according to vapor pressure of liquid?  All these below questions are related to that subjected query, I can not find the design pressure of storage tank.  But I have the storage tank GA drawing and design pressure is mentioned as atm.  I need certain reply regarding to determining of design pressure according to API 650 storage tank because there is no certain definition in API 650 standard as well. 

 

2. I am going to size PVRV for methanol tank at 60 C degree. Storage tank has the IFR as well. IFR has the double wiper seal. According to my investigations, there needs to use floating roof having vapor pressure of liquids between 1.5 to 11.1 psia. According to EPA regarding true vapor pressure is determined as 4.525 psia, proving the IFR usage at 100F degree(37.8C). Now if we are going to check 60C degree vapor pressure is equal to 12.24 psia. That amount is out of tolerance and maybe need usage of recovery system, that is another issue and maybe discussing by another topic.

 

However, what I do not understand is  methanol vapor pressure 12.24 psia is gauge or absolute in storage tank? If it will be assumed that it is gauge pressure then it is highly over API 650 design pressure because internal pressures can not more than 17.2 kPa (21/2 pounds per square inch) gauge. If I assume that 12.24 psi is gauge pressure, first of all, regarding design code is wrong, is it true approach? 

 

3. Make it easy, talking over vapor pressure 100 ^oF degree methanol, how can it be determined whether IFR seals are working in enough capacity and sealing perfectly?  These are just some kind of rubber and according to EPA calculation methodology you can estimate the evaporation losses as weight but there is no explanation wether seals can achieve that vapor pressure development or not.  Why I am asking because when the vapor pressure is developed at that temperature  vapor area of regarding type (pantoon type) IFR will be filled and pressurizing by the methanol gas.  Accordingly, after that moment, no more liquid can be stopped in that vapor area and all them are begining to be lost.  I just need from you to hear that I am wrong and why? 

 

4. When the gas is now over the IFR area and there will be gas transfer/development in this area till the vapor pressure of gas is achieved. Again the same question of point #1. What will be the design pressure? Because set pressure of the PVRV would be arranged according to getting basis of design pressure. If it will be achieved how the 12.24 psig storage tank can be adopted as design pressure because it is mentioned that storage tank is designed according to API 650.  I have found some information in this forum and other forums "both API 650 and API 620 can be usable for methanol storage".  But API 620 is not the part of this questions because we are talking about existing storage tank.

 

Finally, I would like you to kindly help me in this regards.  Thanks.

[fallah]

Hi,

 

Long post! anyway below you can find my 2 cent...:

 

1- If you can specify the exact design pressure of the atmospheric storage tank; try to do per Appendix F of API 650. To do so, you can obtain a design pressure of maximum 18 kpag otherwise you should get maximum 1 kpag as tank design pressure.

 

2- The vapor pressure is in absolute unit; the maximum TVP at storage temperature in a floating roof storage tank is around 11 psia but In practice floating roof tanks for liquids of TVP up to 13 psia have been designed and constructed.

 

3,4- The matters aren't so clear...

[breizh]

Hi ,

A few document to support your work .

 

At 60 C the vapor pressure of methanol is 84 KPa (absolute)  or 12.2 PSI  (absolute) !

 

Hope this is helping you.

 

Breizh

Edited by breizh, 05 January 2019 - 08:03 AM.

[TNCX34]

Hi,

 

Long post! anyway below you can find my 2 cent...:

 

1- If you can specify the exact design pressure of the atmospheric storage tank; try to do per Appendix F of API 650. To do so, you can obtain a design pressure of maximum 18 kpag otherwise you should get maximum 1 kpag as tank design pressure.

 

2- The vapor pressure is in absolute unit; the maximum TVP at storage temperature in a floating roof storage tank is around 11 psia but In practice floating roof tanks for liquids of TVP up to 13 psia have been designed and constructed.

 

3,4- The matters aren't so clear...

 

Dear Fallah,

 

Thanks for your kind replies,

 

I think it is safer way to accept storage tank design as 1kpa. For other issues first of all, indeed, I asked for calculation methodology of seals of IFR that proving how much it could hold the pressure inside? What I know generally wiper seals approximately have 5-8 N/m load on shell however I am not sure whether I can balance the forces developed by IFR vapor pressure with this sealing force getting reference of that value. I am wondering whether I can assume like gaskets used in breathing seals, I think I need to realize that work and see the results.

 

Secondly, Why I am confused at related to the pressure units because of EPA is mention that vapour pressure of methanol is psi absolute. I think that determined absolute pressure will be accepted as gauge pressure in storage tank meaning over 1 atm but I am still not sure. Anyway I am trying to find solutions in the light of the given information and assumptions that I have explained. Thanks.

[fallah]

I think it is safer way to accept storage tank design as 1kpa. For other issues first of all, indeed, I asked for calculation methodology of seals of IFR that proving how much it could hold the pressure inside? What I know generally wiper seals approximately have 5-8 N/m load on shell however I am not sure whether I can balance the forces developed by IFR vapor pressure with this sealing force getting reference of that value. I am wondering whether I can assume like gaskets used in breathing seals, I think I need to realize that work and see the results.

 

 

 

Secondly, Why I am confused at related to the pressure units because of EPA is mention that vapour pressure of methanol is psi absolute. I think that determined absolute pressure will be accepted as gauge pressure in storage tank meaning over 1 atm but I am still not sure. Anyway I am trying to find solutions in the light of the given information and assumptions that I have explained. Thanks.

 

 

Hi,

 

There is normally rim vent to release any accumulated vapor in tank's annular space; hence no need the tank's peripheral seal holds the pressure inside.

 

As i mentioned, i repeat the vapor pressure is in absolute unit.