4 replies to this topic

[saaleh1]

Dear experts,

 

From API 520 it states that for a multiple-device installation requires the combined capacity of two or more devices to alleviate overpressure. The set pressure of the first device to open shall not exceed the maximum allowable working pressure, MAWP. The set pressure of the last device to open shall not exceed 105 % of the MAWP.

 

Now my question is if the second PSV opens at 105% of the MAWP wouldnt that affect the integrity of the vessel because the vessel is installed so that it the operation doesnt exceed MAWP. Dont we need to put some margin before reaching that pressure?

 

Thank you for your help

 

[fallah]

saaleh1,

 

In the case of using multiple PSV's, code (suppose to be ASME Sec. VIII, div.I) allows the vessel pressure to be increased up to 1.16 times of MAWP during the pressure relief where 1.16 is equal to 1.05*1.1, then there would be no problem in vessel's mechanical integrity.

Edited by fallah, 12 February 2013 - 02:17 AM.

[saaleh1]

Dear Fallah,

 

Thank you for the explanation

 

So the vessel is not just designed according to the MAWP but includes some tolarance

 

Thank you

Saleh

[fallah]

saaleh1,

 

The vessel just to be designed according to MAWP, but can withstand 16% over that for A short time. Indeed, the vessel has already passed the hydrotest pressure equal to 1.3 times of MAWP.

[kkala]

From "strength of materials" viewpoint, allowable stress per codes is maximum stress that the material is expected to withstand without failure divided by a factor of safety. The latter accounts for uncertainties in quality of materials, loads, design methods, workmanship. E.g. allowable stress for carbon steel (CS) pressure vessels has a safety factor of 1.5 compared to minimum yield stress, where permanent deformation of CS occurs (per Coulson and Richarson's vol 6, Chemical Engineering, Chapter 13 - Mechanical Design of Process Equipment, more data can be obtained there). Vessel thickness is calculated using the allowable stress per codes, then corrosion allowance is added. Thus pressure vessel has margins, but MAWP/design pressure should be exceeded according to what applicable code advises to ensure long term satisfactory mechanical condition.
See also  http://www.cheresour...sv-set-pressure - post no 13 (by GS81 Process).

Edited by Chris Haslego, 12 February 2013 - 04:03 PM.