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# Design Pressure usage in Line Designation Table prepared for oil and gas facility

## Design Pressure usage in Line Designation Table prepared for oil and gas facility

(OP)
Hi,

I am a first time poster and I am currently working as a process engineer in the Oil and gas business unit of Atkins.

I am currently preparing Line Designation Tables (LDT) for my project and there has been a lot of inconsistency with regards to capturing of the design pressure of the lines. I am currently providing the maximum process design pressure from my own understanding but some of my team members have also suggested using the mechanical design pressure which has been taken from the Piping Material Specification document as this is a higher value and hence is taken to be more conservative.

I would like to know if the general practice within the Oil and Gas industry is to use the mechanical design pressure or the process design pressure for all lines within the LDT?

Any inputs/clarification on the above would be helpful.

Thanks and Regards,
Vinayak

### RE: Design Pressure usage in Line Designation Table prepared for oil and gas facility

Dear Vinayak
You may check the ASME Section VIII, Div. 1, UG-21 .Design pressure refers to at least the most severe conditions of coincident pressure and temperature expected in normal operation.
Ex. The maximum pressure at the bottom of the vessel is calculated, by the mechanical design engineer, to determine the thickness of the vessel. It is thus necessary to indicate the high liquid level and specific gravity of the liquid at the bottom of vessel, and also the pressure drop through the internals. A pressure drop of 6 mm Hg/tray or minimum 0.3 kg/cm2 through trays may be assumed, unless this is largely different from the calculated value.
When the maximum pressure during normal operation can not be estimated, you may check the following attached as the design pressure.
Thanks.

### RE: Design Pressure usage in Line Designation Table prepared for oil and gas facility

It seems as if the only answer so far deals with vessel, but your query is with regards to pipe.

It a little difficult. If you go with the pipe spec and specify the design P&T of the class every time you will have the advantage of a more flexible system if you ever have to re design it. BUT depending on your design code this may cost you in various demands to piping design e.g. if you are using PED. In some cases you will not change the requirements and in some you will. So its a judgement call. But i also find that it is common that you choose the design pressure based on the max design pressure of the piping class. Often you will the choose vessel design pressure in accordance with the actual process (piping will the be higher than vessel) because the vessel will most likely be more expensive if you over design it.

Best regards, Morten

### RE: Design Pressure usage in Line Designation Table prepared for oil and gas facility

By "line designation table" I assume you mean a line list? i.e. a table listing each line on the P & ID, its size, design pressure, design temp, line class, fluid service etc

I would go for listing the design pressure that the line needs to withstand and no more. The reason being both in the design and the future, other things will be designed or set according to the line list, e.g. connecting systems, pressure relief systems etc.

It could easily be that your particular chosen pipe spec / pipe class is more conservative to avoid having multiple different specs, but this can mean that you run into difficulties if a particular element (pressure vessel, meter, special component) is rated at a lower pressure or temperature than your pipe spec.

Whatever you choose, consistency is the name of the game here so you need to ask questions within your department or project.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Design Pressure usage in Line Designation Table prepared for oil and gas facility

It's the classic shortcut from the conceptual stage of a project where it's much more convenient to work with ANSI class ratings and take the design pressure right to the limit of the class rating, as opposed to what the process actually requires. Plus, there's nothing more confusing than having a 'process design pressure' and a 'mechanical design pressure' when process engineers and mechanical engineers are gathered around a table. Perhaps the term 'rated pressure' as a substitute for "mechanical design pressure," might get the differentation across and help to keep definitions in line with standards? In my own discipline, using the ANSI class approach to setting design pressure leads to derivation of overly high partial pressures of H2S resulting in grossly over exotic materials selection.

Steve Jones
Corrosion Management Consultant

All answers are personal opinions only and are in no way connected with any employer.

### RE: Design Pressure usage in Line Designation Table prepared for oil and gas facility

Dont get caught up doing a piping engineer's job - the process engineer needs to only set the process design conditions for piping - UDP, LDT and MDT. Leave the setting of mechanical design conditions to the piping engineers.

### RE: Design Pressure usage in Line Designation Table prepared for oil and gas facility

(OP)
Hi all,
Thank you very much for your valuable inputs. I have discussed my doubt on this topic with my department head and his viewpoint like some of your viewpoints in the posts above was to give the maximum process design pressure. This is so because the LDT is a process document and has nothing to do with the mechanical design pressure.Furthermore, the LDTs serve as an input to the mechanical/piping department mechanical design pressure determination, piping stress calculations etc.

I shall keep this in mind while working on any LDTs in the future.

Thanks and Regards,
Vinayak

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