ASME VIII div. 1 min req thickness calc

[APIPapi]

In the past, I have always utilized a static head factor of 0.433 ps/ft and vessel MAWP to determine my component tmins for our Sect VIII div 1 vessels. However, the process fluid used has a higher SG than water. Shouldn't I use a head factor based off the SG of the process fluid rather than water? I have never seen this done by any inspector before, and I don't see this mentioned in API 510. UG-21 of sect VIII div 1 states to use the "coincident static head", but like I said, I have not seen any inspector use a different static head other than water for pressure vessel calcs.

 

[r6155]

The drawing should show the pressure test in a vertical or horizontal position if the PV is vertical.
The inspector is not a designer.

Regards

 

[david339933]

This is done all the time, or should be. Not sure what your experience is. In addition, if a vessel is only partially filled during normal operation, you can use the static head of the liquid height rather than the full vessel static head.

 

[SnTMan]

For a lot of petrochem work, the process fluid has a lower density than water, so static head of water is used, vessel full to represent a hydro test case. If the vessel is to be full of a fluid of greater density than water then static head based on the greater density is correct.

If, as david339933 says, a partially filled condition is known then this case and the case of vessel filled with water should be compared.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[Christine74]

Yes you multiply by 0.433 psi/ft by the Specific Gravity of the fluid in question in order to determine the static head that you need to design your vessel.

If the SG isn't listed on the data sheet, the vendor will often just assume the fluid density of plain water. This is conservative most of the time but this should be confirmed as some acids have much higher densities than water.


-Christine

 

[APIPapi]

So if I'm replacing a vessel and I know the specific gravity of the fluid will be higher than water, the vendor would change the static head accordingly for the design calcs? Most of our vessels are using fluid with a higher SG than water, and I'm not sure if this was ever caught years ago. It makes it a pain if I calculate tmin with the correct static head and then find out we have much less corrosion allowance than we initially anticipated.

 

[r6155]

Look up the information in the final data book for all pressure vessels you have.

Regards

 

[APIPapi]

A lot of these vessels are from the 80s/90s and we do not have the data books unfortunately. But that is a good point as we are slowly replacing them.

 

[Christine74]

I think it's best to specify the fluid Specific Gravity and Design Liquid Level (with the reference datum) on your pressure vessel datasheet to avoid any confusion.


-Christine

 

[r6155]

When Operating liquid with SG greater than water the design calculation must be as follow:
1) Filled fully with operating liquid to design concrete foundation (includig corrosion allowance)
2) In case of sesmic, filled fully with operating liquid (corroded)

Test with water if the vessel is expect to be tested in field in some future date (corroded).

Again: the detailed construction drawing must include this information.

Regards

 

[GD2]

OP,
For tmin calculation, it is always liquid (process) SG considered for Design Pressure calculation with static head. I am not sure why the AI should consider water SG for tmin. This could trigger a vessel rejected when it is still acceptable by design. In your case with heavier SG, you get the credit for lesser wall thickness by water but you also stand a risk of vessel failure by thinner tmin for water.
Additionally, it is not always the Pressure and Static head that influence tmin. For tall vessels, wind and seismic load also influence tmin.

GDD
Canada