API 620 term "design external pressure"

[shvet]

Dear forummembers

Please help with datasheet. We have a datasheet for API 620 storage tank provided by a licensor where it is written:

Design Internal Pressure, pressure above atmospheric 103.4 kPa[g]
Design External Pressure, pressure below atmospheric 0.7584 kPa[g]

The problem is with 0.7584 kPag. I do not understand how pressure below atmospheric can be expressed as positive gauge pressure.

Is it a misprint? Should it be -0.7584 kPag? Or 0.7584 kPaa?

 

[JStephen]

The positive or negative sign is used to denote if the gauge pressure is above or below atmospheric.
Communicating this in words conveys the same information.
So "0.7584 kPa below atmospheric pressure" and "-0.7584 kPa gauge pressure" are the same thing.

 

[LittleInch]

It is a bit confusing to be sure.

I'm not quite sure what they mean but think that the DEP is what the tank vacuum level is.

Differential pressure would be better IMO, so that would be 103.4 kPa / -0.7584kPa ( a very precise number....) which looks like 3 inches of water column converted to 4 decimal figures....



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

 

[Heaviside1925]

OP,
The wording maybe a little confusing but I suspect it was added to call attention to the use of gauge vs absolute. If a negative value was used, it would imply the external pressure felt on the outside of the tank was negative in reference to atmosphere. The use of it being positive is saying that if that pressure is present inside the tank, then that much pressure is being exerted externally at atmospheric pressure. I don't know if there is a perfect way of communicating this. I've typically seen it without the wording. This seems to be a situation that wording was added for clarity but only created questions to its clarity. It reminds me of new engineers learning the difference between SCFM and ACFM.

 

[IFRs]

They made it more confusing than it is already.
If you have a vacuum of half an atmosphere, everyone knows what that is but how do you write it? Vacuum is by definition the opposite of pressure inside a vessel so a negative vacuum is actually pressure and a negative pressure is actually vacuum so you can't say you have - 2psi of vacuum because that is really pressure. So, we like to say we have a + 2 psi of vacuum which may be used as a positive or negative when calculating some properties, strengths, etc depending on the model.

By calling it external pressure, it can be written as a positive number and perhaps the confusion goes away since you are always measuring pressure, never vacuum.

I might suggest the following:
Design Internal Pressure, inside condition above atmospheric 103.4 kPa[g]
Design External Pressure, inside condition below atmospheric 0.7584 kPa[g]

 

[shvet]

thank you, gentlemen

 

[HTURKAK]

I agree with JStephen

Design Internal Pressure, pressure above atmospheric 103.4 kPa[g] is clear that the 103.4 kPa[g] is gage pressure .
The internal absolute pressure would be 103.4 kPa + 101.3 kPa =204.7 kPa ,

Design External absolute pressure would be 101.3 kPa -0.7584 kPa =100.54 kPa , and this pressure corresponds approximately
+ 65 m altitude.
..




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