8 replies to this topic

[Rush123]

Hi,

 

One of the new client specifications state that the "Minimum Upper Design Temperature" shall be 75C for that particular region. (Corresponding to 167 F) The existing vessels have design condition nameplates of (for example) 1910 psig @ 150 F OR 150 psig @ 150 F. This has been highlighted as a non-conformance since the vessel nameplate quotes a lower temperature than the minimum required by the company specification.

 

This is an existing facility and this is to be applied to ALL vessels on the facility, so of course throwing away vessels, for new ones is not possible. Re-rating vessels and getting a new ASME Set VIII Div 1 stamp may also be more troublesome than needed. I can see two possible solutions to address this non-conformance:

 

1.) Use Pressure Vessel codes to reduce pressure / temperature correct the design nameplate values. For eg: 1910 psig @ 150 F, may be equivalent to 1900 psig @ 167 F (or thereabouts...) This option however, will mean changing the design temperature of the vessel, which will impact a wide variety of items: PSV Set Points (& possibly sizes as well), HIPPS Valve functionality, Alarm and Set Points of the control system, design margins, blowdown calulcations...etc. For this reason, this option is not feasible.

 

2.) Use a calculation procedure to show that the wall thickness of the existing vessel is adequate for temperatures up to XXX F, which is far higher than 167 F and therefore no changes are necessary. There may be other criteria than wall thickness...maybe vessel stress analysis or showing that the exsiting metallurgy can withstand higher temperatures.

 

There is a third option, which involves using the piping specification for the facility (ASME B31.3) and using the pressure temperature correlations as per the spec...but I suspect the Vessel Engineers will have a fit if I try to justify different design conditions for a coded ASME pressure vessel using piping specs.

 

Any thoughts on this / suggestions as to how to work around the 'non-conformance' ?

 

Rush

[NAP]

We are talking about a derating of only 17 F which is hardly anything given the fact that min. upper design temperature of 75 C implies you are probably located in a hot and arid region. I am also guessing that 75 C probably corresponds to a sun metal temperature.

 

I would take the wall thickness approach to justify that nothing has to be done with the existing vessels and their wall thicknesses based on 150 F calculation.

 

When the vessel wall thickness was originally calculated based on 150 F, the selected wall thickness must have been thicker than the calculated thickness to match a standard plate available in the market.

 

Another factor to consider is the max. design temperature. If the process is inherently hot service, then the wall thickness would have been calculated on much more stringent design conditions meaning the thickness would be too high to worry about changes in min. design temp limit. 

Edited by NAP, 01 October 2013 - 07:25 PM.

[Bobby Strain]

Sounds like your client is not only dumb, but stupid, too. Good luck.

 

Bobby

[Rush123]

NAP:

 

1.) Yes, 75 C does correspond to a sun metal temperature.

2.) I also am sure that the wall thickness of the vessel would be thicker than that required for a temp of 150 F. Plus, these vessels have a corrosion allowance on them as well, so it just seems like a minor issue for the mere 17 F

3.) I spoke to operations and had a look at the DCS. The highest temperature of the fluid is at the upstream section of the facility and could be around 120 F. The temp drops after a choke valve, to circa 100 F.

 

Bobby:

 

1.) This is one item in a string of items we have to take care of and close out. Some make more sense than others, some are easy and some are not. But it's worth mentioning a previous Engineering Contractor (EPC) had highlighted these issues to the client, and we are now trying to help them address it. In this particular instance..the supposed experts are at the root cause of this.

 

Rush

 

In any case, does anyone have some guidance on calculating wall thickness for pressure vessels, based on temperature constraints?

[NAP]

if you don't mind a little hard work....ASME Sec VIII div I has the formula for wall thickness and a bunch of tables of constant factors that need to be used in the formula.

 

Its really simple.

 

Ofcourse the actual wall thickness calculation is complicated but the simple calc gives you a basic comfort factor...

[Rush123]

Just to follow up on this:

We are still having some internal discussion on this item and haven't fully resolved it as yet. Will post the conclusion for refernece as soon as we determine one.

 

In the interim, if anyone has any other feedback...feel free to post it as it may add value to the discussion.

[Rush123]

This issue has been closed:

1.) The vessel in question are not in direct contact with the sun. ALL of them are actually shielded due to the working deck above. This is specifically stated in the standard and allows credit to be taken for "shaded areas"

2.) Mechanical is going to perform  a rigorous calc to determine that the vessel wall will not fail due to high tempepratures given the exisitng metal thickness.

 

Thanks,

Rush

[msingh@ambitech]

Generally the pressure vessel material allowable do not change till 650°F for some carbon steel hence your rating atleast for the pressure vessel can be increased for the higher temperature

[S.AHMAD]

Please do thickness calculation for 167F and 150F. I am sure there is no change. So no concerns at all.