6 replies to this topic

[farid.k]

hye guys,

 

need some clarification

 

What is the different between process design pressure and mechanical design pressure? Hope my understanding is correct. Process design pressure typically 10% above maximum operating pressure and for mechanical design pressure is as per tablated in the typical flange rating pressure with respect to the temperature.

Do we need overpressure protection (relief valve) if the overpressure happen is above the process design pressure but lower then mechanical design pressure?

[fallah]

farid.k,

 

Process design pressure is maximum expected pressure in all cases consisting normal operation, all process upsets, start up, shut down and maintenance, etc....

 

Mechanical design pressure is pressure design pressure plus safety margin(s) for mechanical design...

 

If accumulated pressure in an overpressure scenario would be between process design pressure and mechanical design pressure no need to overpressure protection for the mentioned scenario; but if that scenario is the only credible scenario for overpressure, a PSV should be considered for code compliance...

[farid.k]

Thanks fallah for the response.

 

Based on study, the only credible case that can cause overpressure in the system is inadvertent control valve opening. However the overpressure is limited just up to 9 barg (based on control valve upstream pressure) and it is still in between process and mechanical design pressure. Thus do I still need to consider relief valve in the system?

 

I don’t really get you what you mean by code compliance. Does code mention to have relief valve if that scenario is the only credible scenario for overpressure?

[fallah]

farid.k,

 

ASME Sec. VIII, if is the governing code of the pressure vessel, says you should consider a PSV for any pressure vessel regardless of this fact that it might be subject to any credible overpressure scenario or not....
 

[KenCummingsPEng]

I would add that ASME Section VIII also allows operation without a PSV in section UG-140 - "Overpressure Protection by System Design"  It is a large section and requires very careful analysis to ensure that the "maximum coincident pressure and temperature that can result from each of the scenarios listed in item (- above does not exceed the MAWP at that temperature."

 

Implementation of this program can be quite onerous and our largest customer/facility has chosen not to go this direction.  It creates some strange situations (I've signed off on PSV's with 0.0" of required orifice area) but we meet the codes in place in our jurisdiction.

[Ankit_Kumar]

As per ASME Sec. VIII Div. I, the basis of PSVs requirement at a pressurized vessel has the following criteria; “PSV is required if MAWP (Max allowable working pressure) of the vessel is lower than the highest pressure that can reasonably be achieved by the system.”

 

Kindly refere following discussion for further knowledge, 

http://www.cheresour...-psv-at-vessel/

[shantanuk100]

Dear Farid,

 

The Process design condition is the "Most severe process condition likely to occur in operation of the equipment." Simply put it is the most severe case of flow (including P, T, startup, shutdown etc.) that your device is likely to encounter.

The mechanical design condition is based on the Process condition plus additional margins based on stress analyses, material properties, experimental dets etc. which give a design pressure for the equipment.

 

The ASME code says that regardless of the credible overpressure scenario, you require to give some sort of overpressure(relief) protection (whether big or small) when the case consists of a pressured equipment. This is determined by various factors that you can find in API 520, 521, 526

Also, please go through the following link and document for relief conditions, in case you have any further doubts.

 

http://www.cheresour...-psv-setpoints/

 

 

Regards,

Shantanu

Edited by shantanuk100, 08 March 2016 - 05:36 AM.