2 replies to this topic

[JulianSan]

Hello everybody,

 

I've been doing some research in books and codes trying to get support for the answer to a simple question a colleague has done to me some days ago (and I've failed so far). Maybe you can help me clarify/correct my ideas!

 

In the refinery we work at, there's a 60 m high crude distillation column. It has two PSVs located at the 30-in overhead line about 1 m away from the column overhead outlet nozzle. The PSVs are set at the DESIGN PRESSURE (Pdis), actually the first one is set at Pdis and the second one at 1.05 x Pdis. My colleague asked me if the set pressure of these valves was correct, because in a relief event, the pressure at the flash zone of the column should be higher than the Pdis. He argued that the set pressure for these valves should be a bit lower than the actual value so that the design pressure is not exceeded. I tried to explain him that, in case of loss of reflux/cooling media (the design scenario), the pressure at the top of the column will rise faster than the pressure at the flash zone, but this answer didn't like him much.

 

As I was writing this post I was wondering why we use the design pressure as the set pressure for these valves, and the answer is simply because of "history": we couldn't find the MAWP calculation of the vessel, so we used the desing pressure knowing that always Pdis is equal or less than MAWP. So taking into account that the MAWP of a vessel "[...] is the maximum pressure permissible at the top of the vessel in its normal position [...]", the answer to my friend's question seems a bit clear to me: the MAWP corresponds to the top of the vessel, and I can assume that it is the weakest point of the equipment. By protecting the column at (or below) the MAWP (or the design pressure in its absence) I can assure the entire vessel is overpressure protected.

 

Is this correct? I've never seen a PSV set at a pressure different than the MAWP (or design pressure), but there's always a first time. What's your experience or thoughts on this topic? Should I take my friend suggestion and try to determine a new set pressure for the PRVs?

 

Thanks in advance for all your help!

[shantanuk100]

Dear Julian,

 

1. The MAWP is higher than the design pressure and is a value that depends on multiple parameters like material properties, service fluids, operational methods, how many years the equipment has been operating etc. It usually reduces over time due to wear-out and can be determined by the fabricator of the vessel.

 

2. The Design pressure is of two types. The Process Design pressure is the most severe condition of fluid that we determine our equipment is likely to undergo. The Mechanical Design pressure is the process design pressure with additional margins that are determined by mechanical engineers based on loading, stress-strain graphs, temperature variations, material selection etc.

 

3. When we set the PSV to the Design Pressure, we are taking precautions and being conservative. It allows us a certain margin before we reach MAWP, and vessel failure. 

 

4. For exact criteria on Set Pressures, please go through the following link and download the document I've included there, and also go through the APIs mentioned there if you wish to be thorough.

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

 

Regards,

Shantanu

Edited by shantanuk100, 27 April 2016 - 11:27 PM.

[fallah]

JulianSan,

 

Considering the maximum operating pressure; if there would be considerable difference between maximum operating pressure and design pressure (or MAWP) you can set the PSV below design pressure (or MAWP). But you have not to lower the PSV set pressure below column design pressure (or MAWP if it's known).

 

Also design pressire (or MAWP) can be exceeded till allowable value; i.e. 10% for single PSV in normal cases, 16% for multiple PSV's in normal xases and 21% for fire case...

Edited by fallah, 27 April 2016 - 11:32 PM.