Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


MAWP: Max Allowable Working Pressure

MAWP: Max Allowable Working Pressure

MAWP: Max Allowable Working Pressure

The datasheet provided of an atmospheric condensate storage tank was provided by a contractor:

(1) Design Temp: 70degreesC
(2) Operating Pressure: -0,05 - 0,35 barg. (I bet this means -50mbarg to 350barg)
(3) Tank MAWP [bar g] = Geodetic Height + 500mbarg Nitrogen Blanket / -20mbarg

My problem is I need to fix a design pressure based on these specifications with no clue what these contractors mean by no. (3).

What I understand of geodetic height is the level difference between the pump suction and the tank top water level, but how is it relevant in determining tank MAWP? Plus, the operating vacuum given seems to be an irrational value at -50mbarg. I've sent out an enquiry but I'm hoping someone else knows.

The tank must be a 1000m3, ~12m cylinder standing at ~10m.


RE: MAWP: Max Allowable Working Pressure

I've not run across the term "geodetic height" used in that way. I think what they mean is that allowable pressure at any point is the hydrostatic head at that point plus the gas pressure above.
The operating vacuum would normally be a fraction (typically 40%) of the design vacuum, so I would wonder if one of those was in error or the two values were reversed.

RE: MAWP: Max Allowable Working Pressure

Maybe they want to be able to use a Pressure/Vacuum Conservation vent sea at [up to] neg 50mBarg and pos 400 mBarg. Which look like reasonable settings [he sez while laying around at home w/o research library]

RE: MAWP: Max Allowable Working Pressure

Thanks guys. I may get back when I clear things with the engineer

RE: MAWP: Max Allowable Working Pressure

They clarified that geodetic height in this context is atmospheric pressure. Also found out they have a pressure equalising line, and the -20mBarg corresponds to: a drain line needing to prevent air ingress down to a vacuum pressure of -20mBarg.

So would it be logical to just design the tank to withstand atmospheric pressure, while assuming that the gas blanketing system pressure will be accounted for by the pressure equalising line? It does seem quite illogical to have design thicknesses needing to cater for such high gas blanket pressures. Perhaps 500mBarg could mean the discharge pressure of the gas blanketing system and not the partial pressure of the gas in the tank itself?

I really only need to determine the thickness of the roof and shell plates which are directly affected by design pressure.

RE: MAWP: Max Allowable Working Pressure

You need to design for all service conditions, including gas blanketing. It won't make much difference to the shell but it may make a huge difference to the shell-to-roof junction and possibly to the shell-to-bottom junction in and possibly requiring tank anchors. Also be sure to leave enough design pressure headroom to allow for normal venting.

RE: MAWP: Max Allowable Working Pressure

OK I was informed that the MAWP has now to include the 500mbarg nitrogen blanketing pressure.

Using EN14015:2004 (Clause 10.4), I calculated the roof plates to be 18mm thick due to the 500mbar pressure in the space above the liquid.
3.5 Roof Plate Thickness
Material = EN S275 JR

As per EN14015, Clause 10.3.3,
Min. nominal thickness = 5 mm
Owner-specified corrosion allowance = 1.5 mm
Thus, min. required plate thickness = 6.5 mm

Adopted Roof plate thickness = 8 mm

As per contract, the roof shall be self-supporting, and not attached to roof-supporting members. Therefore, resistance towards design internal pressure is checked as follows:

3.5.1 Design Pressure Check

Minimum Roof plate thickness excluding corrosion allowance:
= PR/10SyJsin θ
= 500x5.5/1570x0.10
= 17.86 mm

where P = design pressure (mbar)
θ = roof slope angle (deg)
R = radius of the tank [m]
J = joint efficiency factor = 0.5 [double-fillet weld]
Sy = Allowable Design Stress [MPa]

3.5.2 Conclusion
Design Pressure Check = OK
Adopted Roof plate thickness = 8 mm


With the 500mbarg, it seems that anchors are now also needed to prevent failure at the shell-to-bottom junction. I'm seeing an uplift force of 26000kN here. Certain I've not made any dimensional errors in the math...
[See Attachment]

RE: MAWP: Max Allowable Working Pressure

Sounds about right to me. Especially the anchors. Also, make sure that somebody inspects the underside of the roof during erection; 'industry standard' for roof inner surfaces is a stitch weld. If your calc's require a double-fillet, make sure it happens.

RE: MAWP: Max Allowable Working Pressure

There is another tank, and this tank has a design pressure of 20mbar(g), and a design vacuum of 50mbar(g). At first glance, clearly falls outside the scope of the scope of EN14015:2004, because the design vacuum of 20mbar(g) falls under the category of "very high pressure tanks" and anything >20mbar(g) just isn't listed.

Is there something naturally wrong with a design vacuum of 50mbar? On a 11m dia. tank, this is an absurd-sounding downward force of 1900kN. I understand that this is supposed to be an low-pressure storage tank (hence the design pressure of only 20mbar), so what cases justify the logic of a design vacuum more than twice the design pressure?

Is there another EN standard to be used?

I looked at API650 and this is what I read:

Quote (Design External Pressure (Pe): Shall not be less than 0.25 kPa (1 in. of water) except that External Pressure (Pe) shall be considered as 0 kPa (0 in. of water) for tanks with circulation vents meeting Annex H requirements. Refer to Annex V for external pressure greater than 0.25 kPa (1 in. of water). Design requirements for vacuum exceeding this value and design requirements to resist flotation and external fluid pressure shall be a matter of agreement between the Purchaser and the Manufacturer (see Annex V). Tanks that meet the requirements of this standard may be subjected to a partial vacuum of 0.25 kPa (1 in. of water), without the need to provide any additional supporting calculations.)


Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close