Design Temperature and Pressure

This sort of question is so frustrating because there is so much that we do not know. Perhaps the most significant is Lyn's question of what on earth are you designing because it has by far the biggest impact.

My experience is design of equipment for the oil and gas industry (once it is above ground), I am a process engineer.

A) Do you have an existing piece of kit for which you re trying to back calculate Design Pressure / Design Temperature (DP / DT)?

B) Or are you at the paper stage with operating conditions and wondering how to get to DP / DT?

If A) then as Jack notes there should be design data for the equipment which provides this information. If not then as Lyn notes a mechanical engineer could, or if you dig into the references from Solar you can back calculate the results yourself.

If B) then its a PROCESS engineer you need (despite the GA I gave you Lynn stop giving away my job )

Typically if designing to ASME VIII or BS 5500 or their equivalents

Design Pressure = MAX Operating pressure + 10%

Design Temp = MAX Operating Temp + 20°C

BUT every company / national authority / international body has variations on these for different conditions. (Also typically you don't go less than 3.5barg as a DP)

nb Pressure is in barg (or any other gauge unit)

Note the MAX Operating conditions - in most cases there is a normal or typical or most common case. However there can be other cases that reflect

Time - over the 20 yrs of a project the field conditions change - composition, temp, pressure so conditions in Yr 1 may be very different to yr 20

EoR - End of Run if there is a catalytic process the catalyst may lose efficiency over time and give different conditions at the end of the run compared with the new catalyst

Feed - you could design for different feedstocks which require different operating conditions

So the numbers above will give a decent guide BUT BUT you cannot deign using them unless you have confirmed that the codes are applicable for your case

You hopefully get the idea

Design temperature and Pressure = Operating Temperature and Pressure + Equipment Safety Code Factor Requirements.