Design internal pressure is greater than maximum allowable working pressure (MAWP). AmeTANK ERROR. 2

[James Bonding]

Hi sirs,

I would like to seek your help. I am designing a Cone with internal floating roof tank which has a 22.94m ID x 14.52m H. Following information's also were indicated.
Design Pressure = 250mmH20 + Full Liquid
Design External Pressure = -60mmH20
Design Temp = 70 Deg C
Liquid Stored = Benzene

When I run this to AMETank I got an error as below:
Design internal pressure is greater than maximum allowable working pressure (MAWP).
Design external pressure is greater than maximum allowable working vacuum (MAWV).

I am thinking that design pressure is wrongly inputted in the datasheet. Please help me and enlighten me with this error that I encountered.
Thank you sirs.

 

[LittleInch]

Depends on what parameters you put into AME Tank for the tank and roof itself.

10 inches of water column is a little high, but depends on your tank and roof design.

I can understand for Benzene that you need to control venting, but if you've got an internal roof what type of venting / inerting do you have?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IFRs]

An internal floating roof tank is typically designd per API 650 Annex H to have circulation vents with huge open area and as such has zero working pressure and vacuum since free vents can't hold any pressure or vacuum. AMETank may be confused. Note that these vents are large enough to take care of emergency vent requireements and eliminate the need for a frangible roof-to-shell joint check. Also check the TVP of Benzene at 70 DegC (158 DegF) - it may be higher than the EPA allowable for an IFR tank, again possibly confusing AMETank.

Try removing the floating roof, AMETank should not show any errors. If the same error occurs, perhaps the top angle is not strong enough to withstand the internal perssure and/or the tank shell is not thick enough to withstand the external pressure / vacuum. If this is the case you may need to increase the thickness of the upper courses and/or add external stiffeners.

 

[James Bonding]

Dear sir @IFRs, thank you for your inputs. My AmeTank doesn't have a floating roof feature so no input with floating roof details.
What I did is I inputted the design condition stated on the datasheet- into the AMETank. I adjusted the shell section specifically the shell thickness. I got 6 courses (14mm, 12mm, 10mm, 10mm, 10mm, 10mm) lowest course to highest. I passed the shell thickness calculation but still the warning is still there.

Is there a possibility that the inputted design pressure and vacuum in the datasheet is wrong? Cos if that the case I will try to clarify it to CLient.

Thank you so much, sir.

 

[LittleInch]

For benzene they sound about right to me, if a little high, but depends on the design of your tank and whether it has free vents or not.

TVP of benzene at 70C is quite high so IFRS may be onto something there.

Is there someone local you can ask as its not easy to diagnose your issue remotely.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IFRs]

The internal pressure is small, no reason it should be a problem unless perhaps the empty tank needs anchors and you are not using them. Have you checked the need for anchorage from wind, seismis and internal pressure? Have you checked the adequacy of the top angle and roof-to-shell joint?

The external pressure is not much either but for a tank that size you may need intermediate stiffeners - have you checked for them?

What are the numbers for design and max allowable pressure and vacuum?

 

[James Bonding]

Dear sir @IFRs, AmeTank required 26min no. of anchor bolt, so I did add anchor bolt. Still the warning persist. I got some red warnings in my top angle (please see attached file) and I tried to increase the size. I also checking the Annex F of the API 650 especially the decision table but I really not sure on what is the cause of the error.

Regarding the stiffener, It doesnt required per AmeTank.
Max operating pressure is 150mmH2O as per data sheet.
For Wind Seimic no data provided by client so I let the default input from ametank. Does this affect the relationship of design pressure to MAWP? Thank you sir

Some errors on Compression ring
Top Member Data

- As per API-650 F.4, design internal pressure, 2.45 kPa, is greater than the maximum allowable pressure of 1.46 kPa for unanchored tank.
- As per API-650 F.5.1, top member region cross sectional area, 4,621.45 mm^2, is less than the required value of 11,308.16 mm^2.
- As per API-650 V.8.2.3.2, top member region moment of inertia, 1,329,276.73 mm^4, is less than the required value of 263.38 cm^4.

 

[Vetrivell]

Looks the selected shell to roof compression area is not sufficient. Select suitable shell to roof configuration from Figure F.2 ( May be Detail J or K) and increase the compression area to more than 11308.16 mm^2

 

[LittleInch]

The answers are staring you in the face.

From that list ametank doesn't think your tank is anchored and the roof to shell connection isn't strong enough.

250mm water column equates to about 1.5 tonnes force per meter of circumference trying to push the roof off.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[James Bonding]

Hi sir @LittleInch. Thanks for your input. I did increase the compression ring almost come up with 250x250x28 angle bar. And also increase my shell thickness just to pass the warning.

Please be informed that my ametank don’t have the floating roof features thus I dont have any input related to fr. I am thinking if I inputted some information to floating roof maybe there will be no warning( I’m not sure if its correct). I re-run the software using atm pressure and the warning disappear already.fyi this tank as per data sheet has a breather valve nozzle. Am I in the right track? Thanks sir.

 

[LittleInch]

In term of the tank itself, an internal floating roof doesn't really make any difference. an IFR does significantly reduce the amount of vapour that is produced from any volatile liquid, but is doesn't eliminate it and any pressure generated by the liquid still needs to be accounted for. The only thing it could be is that the weight of the IFR needs to be added to the liquid height to allow for this and therefore your actual pressure ABOVE the roof may be lower, especially if it is free vented. The program should either allow for that or substitute in a higher pressure in the vapour space above the internal roof.

Now given that this in benzene, which is a particularly nasty hydrocarbon, the designers may require a blanketing gas on top of the internal roof to prevent any vapours from escaping - I don't know, but 10 inches of water column is quite high for a basic API 650 tank as you're discovering, but not so high if you've got some blanketing gas.

A "breather valve nozzle" means nothing without knowing what is on the end of this nozzle / valve. Could just be an emergency vent / fire vent and not an open vent, so putting atm in the pressure may not be correct.

If your data sheet says 250mm water then that's what you need to put in.

If it means your tank has to get bigger and stronger then so be it or challenge the 250mm figure in the data sheet and ask if it really is required.

But I repeat, Benzene is horrible stuff and for a new tank you may not be permitted to vent any, even from an IFR tank and therefore your internal design pressure needs to be higher than atm in order to contain the vapour leaking past the internal roof

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[James Bonding]

Hi sir @LittleInch, thank you for your inputs.
Just I mentioned earlier, the tank has a nozzle in the cone roof as below :

1. Breather Valve (equiped w flame arrestor and Inlet isolation valve)
2. N2 blanketing
3. VOC outbreathing
4. Blanketing valve pressure pilot.

Like I said I also have an input using the design pressure of 250mmh2O. The outcome is that the thickness of shell plate above like 14mm, 12mm, 10mm to succeeding course. And also I increased the size of the top angle to L250x250x28 using detail D of Figure F.2(havent experience this kind of size). Same with detail J and K both Compression ring connection dont have experience as almost tank uses Equal angle on to their tanks.

Anchor bolt also I provided with 36qty. I really having a hard time to solve this warning. Thank you sir for your support.

 

[LittleInch]

Yup, that all sounds right.

The breather valve lets air in in case the nitrogen supply can't keep it at a positive pressure to stop the tank collapsing.

VOC outbreathing doesn't sound good, but if most of the blanketing gas is Nitrogen then it might be ok. But this is what is causing your 250mm of water pressure.

but you are discovering what the effect of even a small pressure on a large area does for you.

Stick with it and keep adjusting the design until it works and be sure to tell us when it does.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[James Bonding]

So should I input ATM pressure or follow the 250mmh2O? Sorry sir I still dont know on what to do.

 

[LittleInch]

If your data says 250mm H20 then that's what you input.

250mm H2O sounds like a good figure to use in your circumstances.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IFRs]

Not your (current) problem but note that the blanketing gas system can be complicated and expensive. There is head room needed for each valve to operate properly. The 250mm gets cut perhaps several times before you are at operating pressure for the N2 system. See your P&ID for reference. To protect the tank you might be needing pilot operated valves of substantial size if the benzene fill and empty rates are large or the diurnal temperature change is large, etc. I've seen this surprise some owners who think they can use tank design pressure as their N2 operating pressure and then find out they need multiple pilot operated valves to protect the tank because these relief vents need to operate below tank design pressure. Also, you'll probably need emergency vents on the tank, see API 2000, these can be large also.

Sounds like a nice project, have fun!!!