Application of 2/3 rule for reboilers

[thestephan]

Dear all,

we designed the low pressure side of reboilers with 2/3 the design pressure of the high pressure side. Doing that, we did not have to consider tube rupture (acc. to API).

Does this mean that tube rupture can not happen or does it mean it can happen, but it is no design case for the PSV?

In case it can happen, the question is, whether we have to apply the same design pressure for the system reboiler/tower (eg. reboiler 30/20 bar,g and tower 20, even though we would normally apply 6 bar,g for the tower)?

Who can help me out of that?

 

I've not dealt with this area for a while but my comments are :
1)The 2/3 rule does not mean the tube rupture case will not occur. The rationale is that tube rupture is a very unlikely case and that if the pressure rise resulting is not in excess of the original hydrotest pressure this is ok. Hydrotest (US codes) being 1.5 x design pressure hence the 2/3 rule.
2) Watch out for non-US codes. European codes can have hydrotest pressures less than 1.5 x DP and the rule is therefore not applicable (or a different factor applies).
3) As I suspect you realise, it is not just the exchanger that needs to comply - the whole system needs to be checked or uprated. In your case where a column is involved which has a large inventory it is worth checking whether the high presure system has the capacity to fill it to the design pressure or whether the calculated tube rupture flow is actually the relief rate. Credit can be taken for the control system supplying the high pressure fluid - an inlet control valve may not have a capacity as high as the tube rupture flow.
4) It was common practice (in the UK oil and gas sector) to use bursting discs and not PSV's for tube rupture cases based on speed of response considerations.
5) A lot of work was being done on modelling exactly what happened on a tube rupture - shock waves etc. This was a particular concern on exchangers where the high pressure fluid was a gas and the low pressure side was liquid, say cooling water. The incompressibility of the water meant that high localised pressures built up before the water was displaced. Don't know how this all turned out but I imagine that having gas or, pressumably, partial gas in your case in a reboiler alleviates these issues. We used to always rate the exchanger for the same design pressure on both sides to take the uncertainty out of the picture.

 

[CHD01]

DavidH5 has answered your questions very well, one additional comment, it is now possible to specify less than 1.5x the MAWP as the hydro test pressure in the USA per the ASME Code - so DAVIDH5's caution for Europe now also applies for the USA. This is because of recent review and revisions to the allowable stress tables in the code as I understand it. The more you learn, the less you are certain of.

 

[bluemax89]

Is there an update to the 2/3 rule when designing shell and tube heat exchangers. I heard that its not 2/3 anymore, but am not sure what it changed to or why? I am setting design pressures for some H2 kick-back coolers and need to know if the rule still applies. Can anyone help out in this area.

Regards,
Bluemax89

 

[CHD01]

Lower Code stresses are now allowed for some materials and use of these lower stresses requires less than a 1.5x hydro. Refer to current ASME Code allowable stress tables and sections on hydrotesting. The more you learn, the less you are certain of.

 

[TD2K]

See thread135-41046