Minimum pressure/vacuum to be used in design

[MJCronin]

To all,

I have specified, designed and purchased with many flat bottomed storage tanks in the past. Many were designed for and will always hold petro products.

However, some flat-bottomed tanks, over thier design lives have been used for a variety of services and have held a variety of liquids. Especially when located at chemical process facilities.

My question is this: for the FB (API-650 type)tanks that may see different future service, what is the design pressure and design vacuum that makes the most sense in the specification ?

I suggest +10 IWC and - 7 IWC as being reasonable for tanks that may require conservation vents in the future.

I do know that major companies such as EXXON and CONOCO have addressed this issue.

Also see:

http://www.ast-forum.com/ast_forum_display.asp?post=445

http://www.ast-forum.com/ast_forum_tree.asp?master=443

-MJC

 

[xema77]

It is my experience that these design pressures are fairly high. I have seen many tanks designed with internal pressure of 6.5IWC and vacuum of 1.5IWC for use with conservation vents. Would be interested to hear other opinions.

 

[JStephen]

The biggest problem I have seen with that approach is specifying some arbitrary pressure or vacuum without looking at what it actually does to the design of the tank. If you don't know what it does to the design, think twice before pulling it out of the air to specify. If you're familiar with the standards, then a half hour of work with a calculator will let you know if you're generating a monster ringwall or some huge compression ring on the tank.

If a person is used to vessel work then 1 psi seems to be a very "low" nominal pressure, so it gets specified for a 200' tank, and then the tank designers get to wrestle with it.

Something else to consider is that under the current API-650, there is already a wind uplift pressure applied, so you may inadvertently be requiring anchorage of a tank that really doesn't need anchors.

You may apply some arbitrary pressure and shift the roof design from F.4 to F.7, which throws you into a bunch of different detailing, etc.

You may also be making a roof non-frangible when in fact it could be built as a frangible roof.

A similar issue comes up on occasion when nozzle loads are specified with no basis- it's easy to specify arbitrary nominal loads that are in fact not very reasonable for the service.

A point of confusion that comes up is that on the API data sheets, there is a blank for vapor pressure of the product, which gets confused with the design pressure in the tank.