Design Press/temp - Low Press & High Temp, OR High Press & Low Temp??
Design Press/temp - Low Press & High Temp, OR High Press & Low Temp??
(OP)
Degree: Mechanical Engineer
Location: Houston, TX USA
Experience: Rotating Equipment Design & modular skid Fabricator (4yrs), EPC (2yrs), Owner (4 Years)
Position: PM
Question: So I have seen this scenario a few times in my career, but would like to better understand what a pressure vessel should be designed upon in order to check engineers. I'm currently working to debottleneck a distillation column on an existing unit. We are evaluating the hot oil system, and the process engineers indicate that the design temp/pressure is not appropriate for some equipment in the system.
During start up of a hot oil system, the SG of the heating medium we use is 1.0sg, but ambient temperatures. When at full operation, the SG of the fluid is then 0.8sg, but temperatures are elevated. For argument sake, I'm going to exaggerate the pressure during start up, because I'm trying to understand the principle here... otherwise actual pressure in this case is 300[psig]
Start Up:
400[psig] max pump discharge pressure
90[F] heating medium temperature
Operating:
200[psig] max pump discharge pressure
600[F] heating medium temperature
For design of heat exchanges downstream of the pump, how do you determine what the design temperature/pressure is? Do you take the higher value from both cases, and combine for your design temp/press? Do you determine which individual case is "worse case," and design for this only?
Location: Houston, TX USA
Experience: Rotating Equipment Design & modular skid Fabricator (4yrs), EPC (2yrs), Owner (4 Years)
Position: PM
Question: So I have seen this scenario a few times in my career, but would like to better understand what a pressure vessel should be designed upon in order to check engineers. I'm currently working to debottleneck a distillation column on an existing unit. We are evaluating the hot oil system, and the process engineers indicate that the design temp/pressure is not appropriate for some equipment in the system.
During start up of a hot oil system, the SG of the heating medium we use is 1.0sg, but ambient temperatures. When at full operation, the SG of the fluid is then 0.8sg, but temperatures are elevated. For argument sake, I'm going to exaggerate the pressure during start up, because I'm trying to understand the principle here... otherwise actual pressure in this case is 300[psig]
Start Up:
400[psig] max pump discharge pressure
90[F] heating medium temperature
Operating:
200[psig] max pump discharge pressure
600[F] heating medium temperature
For design of heat exchanges downstream of the pump, how do you determine what the design temperature/pressure is? Do you take the higher value from both cases, and combine for your design temp/press? Do you determine which individual case is "worse case," and design for this only?





RE: Design Press/temp - Low Press & High Temp, OR High Press & Low Temp??
ASME B31.3 (for piping) for example is very explicit in this matter;
ASME VIII Div 1, which is for vessels, states;
Please note UG-20 has more statements, but most of these dont pertain to your query. Hope this helps.
RE: Design Press/temp - Low Press & High Temp, OR High Press & Low Temp??
Obviously the max pressure in each of these 2 cases should be based on the pump running at full speed at dead head conditions (due to failure of the FSHH to trigger a pump trip), and with pump suction pressure at its max possible (which would be the PSHH setting at the upstream HO drum). While the temp in the normal operating case (which you say is 600degF) should be the max operating temp (i.e setting for the TSHH on the hot oil heater exit).
Pls note the design pressure selection of the hot oil side of the reboiler/ HX should also take into account whether the process designer deems it necessary to accomodate a tube rupture overpressure scenario ( if this is applicable) at the HX itself, or allow for this to to be accomodated at the HO expansion drum vapor space PSV.
RE: Design Press/temp - Low Press & High Temp, OR High Press & Low Temp??
You have to evaluate them individually and see which would govern.
Would there be a chance that the maximum pressure would coincide with maximum temperature? If no, then you do not need to combine the highest pressure with the highest temperature.