Need help with a question, I am not a mechanical engineer

[risky1]

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.

 

[Latexman]

risky1,

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

 

[TD2K]

How big of tanks are these? 1 1/2" inlet line seems small.

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.

 

[Latexman]

Isn't there a lower side or bottom nozzle/pipe that can be used? The thermowell there may be submerged and a lot shorter and cheaper?

Good luck,
Latexman

 

[rutherford703]

The rule is that the pressure drop from the vessel to the PSV inlet should be less than 3% PSV set pressure at the PSV rated capacity. If your new configuration meets this requirement, you are OK

 

[SeanB]

Does code allow a thermowell to be installed in PSV inlet piping? Is there risk of the thermowell breaking off during a relief event and then plugging the relief valve?

 

[rustbuster]

You could consider installing a v-pad thermocouple to the line which would be non-intrusive and less expensive.

 

[GHartmann]

This does not look like a good idea to me. We have previously installed 1" thermowells in 1-1/2" pipes where the pipe ID had to be bored out to fit the thermowell. That is the themowell nearly fills the entire pipe ID. I think you want to look toward other solutions.

(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.

 

[zdas04]

The code is not terribly proscriptive on upstream piping as long as the PSV can move enough fluid in the largest credible scenario to keep the vessel below the vessel MAWP (plus allowable transients). Putting a thermowell or a gauge or a sample port on the line is not a problem as long as the basic purpose of the PSV is not compromised. Seems like sticking a large thermowell in a relatively small line could easily constitute a compromise.

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.

 

[Duwe6]

Any PSV requires that there is no reduction in the inlet flow. Period. If the PSV has a 1.5NPS inlet, the pipe has to be 1.5NPS or larger. Nothing to reduce the cross-sectional area or restrict flow.

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.