Need help with a question, I am not a mechanical engineer
Need help with a question, I am not a mechanical engineer
(OP)
Today I was asked this question.
"I’m looking at making some changes to the piping of the Pretreatment tanks. Essentially, the proposal is to add a T-piece on the Pressure Safety Valve line, which will allow us to insert a 1” thermowell into the 1.5” line while still leaving the PSV connected. I’ve included a rough sketch of what I’m talking about.
My concern here is that including this thermowell would fill too much of the space in the line leading to the PSV. This would result in a slower depressurization if the PSV were to pop open. This wouldn’t block access to the PSV, but I’m not aware if there are any regulations about how quickly a PSV must depressurize.
Is this a regulation you know or could find out? If not, do you know if we have anybody here who is qualified to make this call from a safety perspective?"
I have attached the sketch given to me. If any one has any advise I would appreciate it.
"I’m looking at making some changes to the piping of the Pretreatment tanks. Essentially, the proposal is to add a T-piece on the Pressure Safety Valve line, which will allow us to insert a 1” thermowell into the 1.5” line while still leaving the PSV connected. I’ve included a rough sketch of what I’m talking about.
My concern here is that including this thermowell would fill too much of the space in the line leading to the PSV. This would result in a slower depressurization if the PSV were to pop open. This wouldn’t block access to the PSV, but I’m not aware if there are any regulations about how quickly a PSV must depressurize.
Is this a regulation you know or could find out? If not, do you know if we have anybody here who is qualified to make this call from a safety perspective?"
I have attached the sketch given to me. If any one has any advise I would appreciate it.





RE: Need help with a question, I am not a mechanical engineer
Your concern is probably valid, but there is a small chance it may not be. This question will be difficult to answer 100% correctly without the pressure relief device documentation for that specific tank. It is fair to say though, that an engineer qualified to design pressure relief device installations must evaluate this change, and approve it before implementation. I do have some basic questions that will help us answer the question:
What country and state is the installation?
To what Code was the pretreatment tanks designed, built, and stamped?
What is the MAWP of the tank?
What is the set pressure of the PSV?
What is the nominal size of the PSV inlet flange/connection?
What schedule pipe is the 1.5" line?
Good luck,
Latexman
RE: Need help with a question, I am not a mechanical engineer
In addition to the questions from Latexman, I'll add that I don't think I've seen a thermowell installed directly in a 1 1/2" pipe because of the cross sectional area taken up by the thermowell itself. Usually my instrumentation people want to swedge up to at least a 3" section of pipe to install a thermowell. To avoid that, thermowells are also installed frequently in an elbow pointing into the flow but 1 1/2" pipe is still pretty small.
Pressure safety valves, unless yours is unusual, do not depressure the equipment, they simply prevent the pressure from exceeding the allowable margins: 0% for API 650 and 10% for API 620 and Section 8 Div 1 though your equipment may fall under another design code.
RE: Need help with a question, I am not a mechanical engineer
Good luck,
Latexman
RE: Need help with a question, I am not a mechanical engineer
RE: Need help with a question, I am not a mechanical engineer
RE: Need help with a question, I am not a mechanical engineer
RE: Need help with a question, I am not a mechanical engineer
(Our new practice is to use 2" nozzles/pipes for installation of 1" thermowells).
The general rule of thumb is the inlet pipe area should not be less than the pipe area of the relief valve.
RE: Need help with a question, I am not a mechanical engineer
Bottom line is that you have to make a good faith projection of the impact on the vessel of the modification. If you've compromised the flow path then it might still be legal if you set the PSV at a (lower) value that assures the vessel is not overpressured.
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
RE: Need help with a question, I am not a mechanical engineer
If you just HAVE to insert a TW into a PSV line, the line has to be up-sized to make up the loss in internal cross-section, AND you calculate the pressure drop the additional fluid friction that the TW will be causing. IF the pressure drop is less than 3%, AND the area always is equal to or larger than 1.5NPS, then the installation will be 'legal'. Way too much work and $$.
Find a different place to put your TW, even if you have to hot-tap the tank.