LPG tank design - Pressure/temperature 1

[Ali Hammouda]

Dear Eng-tippers,

Am on an LPG bottling unit project and we are struggling to determine the design pressure for our LPG storage tanks. Seeking on your help on that matter.

Client specifications gives the following.
Maximum operating pressure 11.5 bar @50°C
Operating teperature min/max –10/+60°C
Operating pressure Ambient temperature min/max : -1.4/47.6°C
Ambient temperature for design min/max : -5/+55°C
Black body temperature : 85°C

the Engineering department considered the design pressure actually as the vapour pressure of the LPG at 60°C. The question here is if the black body temperature impacts only the mechanical design (selection of material allowable stress) or should it be considered also for design pressure determination (which will be the vapour pressure of the LPG at 85°C).

 

[LittleInch]

This BB temperature keeps going up! Last time I seriously looked it was 70C.

The issue is always this - does the heating of 50% of the thing - vessel, pipe, cylinder etc - really mean that the entire content get to that temperature.

I've always argued that this really only impacts an empty or very nearly empty pipe or tank, so I suppose from your view you need to see if that happens.

Or build a sun shade.

And also you need to define "LPG". This can be anything from 100% Propane to 100% butane.... Makes a HUGE difference to the Pressure /temp relationship

You might need some good relief systems, but then as the LPG vaporises it reduces in temperature so it all gets a bit complex.

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

 

[Latexman]

4.2 DESIGN PRESSURE AND TEMPERATURE
4.2.1 The design pressure of LPG vessels shall not be less
than the vapor pressure of the stored product at the maximum
product design temperature. The additional pressure resulting
from the partial pressure of noncondensable gases in the vapor
space and the hydrostatic head of the product at maximum fill
shall be considered. Ordinarily, the latter considerations and
the performance specifications of the relief valve require a differential between design pressure and maximum product
vapor pressure that is adequate to allow blow down of the pressure relief valve (see API RP 520).

I hope this helps! Also, contact an experienced LPG tank fabricator, or three, and see what is their past practice.

Good Luck,
Latexman

 

[LittleInch]

Also you don't quote min design temp.

For LPG that could go as low as -42C if it's pure propane. Now sure that's at very low pressure, but there could be a lot of contraction.

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

 

[Ali Hammouda]

Thanks LittleInch,Latexman, agree with you that LPG composition makes difference, our gas composition is 70% butane 30% propane, but uselss here to go into calculation details, I want to make clear the design parameters.

Also I confirm that solar effect calculations make things much more complex (although we did it for scénarios where tank is full, half full and empty), I suppose that should a kind of assumption/rule in regard to considering operating temperature Vs Black body temperature for tank design.

 

[MJCronin]

Here in the USA LPG tanks are sold as mass commodity items, they are typically called "bullet tanks" and are designed to ASME VIII and NFPA-58

https://www.nfpa.org/assets/files/aboutthecodes/58/58-98-pdf.pdf

The selection of design/BB/min/max temperatures and design pressures is done within NFPA-58

---Standard operating pressure and temperature: 250 PSI @ 125° F
---Standard materials SA612, SA516-70

This might also help ...

https://engineerboards.com/threads/design-temperature-and-pressure-for-large-propane-tank.33445/

Where will your tank be located, Ali ?

What is your governing code of construction ?

MJCronin
Sr. Process Engineer

 

[LittleInch]

Typically these bullets are covered with earth or I've seen other smaller surface mounted tanks provided with a large sunshade to stop this issue from occurring.

A sunshade is a lot cheaper than designing your system for 85 C....

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

 

[georgeverghese]

This max op pressure of 11.5barg at 50degC seems low for LPG, but maybe this is because your C3 content is much lower than regular commercial LPG - okay.
You should also specify LDT - lower design temp with a view to avoid low temp brittle fracture, which should be auto refrigeration temp of this LPG grade. Any Chary impact testing must be done at the plate manufacturer and provided with material certification. Also confirm you wont be storing LPG in this storage vessel with C3 any higher than 30%, else LDT will be lower. Even at the current min operating temp of -10degC, which would required an LDT of -15 to -20degC, Charpy impact testing of low temp carbon steel plate would be a must.
With Tunisia being on the edge of the Sahara, agreed black body temp would be high, but 85degC seems excessive. I've not seen blackbody temp higher than 70degC for your region. Ask to see blackbody temp calcs; take note infra red radiation (which enables heating) is typically 50% in terms of intensity of total solar radiation. Also note blackbody temp derivation takes no credit for natural convection for vessels typically sitting out in the open.