flare piping design temperature

[ChemEngSquirrel]

i have a hot compressor discharge relief scenario. the temperature downstream of the psv in flare is 140 degrees c. my flare piping is design temp 110 deg c.

must the flare piping have a design temp suitable for 140 deg c?

thanks.

 

[don1980]

Squirrel - This isn't a smart aleck question.....assuming this is a credible scenario, why would you question whether the flare header should be design to handle this temp?

 

[ChemEngSquirrel]

im asking just in case there a design which suggest s you can exceed design
temperature for a short duration relief scenario. i dont want to change flare piping unless necessary.

 

[TGS4]

The design temperature is with respect to the metal temperature. You may have to do some transient heat transfer calculations to determine the downstream metal temperature of the flare header. Make sure to consider the worst-case ambient conditions (i.e. hot with no wind) for your external convection. Make sure to also consider the duration of the hot compressor discharge relief.

 

[SNORGY]

You can tolerate excursion temperatures and resulting over-stress under certain conditions in the ASME Codes. See, for example, B31.3 302.2.4 (f). The approach by TGS4 is technically correct. If your flare header is bare, for example, then you could construct an approximate temperature profile that models the heat rejected to ambient as a function of length away from the relief valve. If you needed to model that in CAESAR II or other pipe stress software, you can split the pipe of interest up into as many short sections (between nodes) as you need to in order to simulate the thermal profile.

In short, my answer is, "No, not necessarily.".

 

[don1980]

Squirrel - OK, so this is an existing installation. That wasn't clear from the original post. From a mechanical strength perspective, there's probably very little risk. But, one can't talk about a pipe's temperature limits independently from pressure. The allowable stress is a function of both.

So, the existing pipe is nominally limited to 110C at XX psig. What is that pressure value? 110C is a very low temperature limit for piping, so I'm guessing that the value of XX is way higher than the actual pressure you'll see in the flare header during this scenario. In other words, it's very unlikely that this pipe will be close to it's internal stress limit when the temperature is 140C.

The more important question is what impact does this have on the pipe design (expansion/contraction)? This is something that's often overlooked, and it's a real risk for flare headers. I'm familiar with a number of flare hdrs which have experienced loss-of-containment failures because of thermal stresses. Plant personnel generally regard the flare header as a garbage can, and they think they can toss anything in without any reason to be concerned. That's not true. One must be cautious about the following combination of events:
- dumping a high flowrate
- at an unusually high or low temperature
- over a long period of time
If those three points apply to the case which we're discussing, then have a pipe stress analysis performed before proceeding. Otherwise, there's probably no problem.

 

[LittleInch]

When I first saw this my answer was of course it should. However once I put brain into gear and read a few responses, I'm a bit less black and white. Your key issue to determine is how long this discharge goes on for and how certain you are that once the relief situation starts the compressor will actually, absolutely, definitely shut down. If there is any doubt or possibility that the discharge, without mxing with any other cooling gasses, will go on for longer than about 1 to 2 minutes at that temperature, then it it likely that you will exceed the design temeprature of your flare header.

You probably won't melt the flare material or reduce its strenght significantly (unless it is stainless), but flare pipework is notorious for being designed very thin and in service has corrosion defects. Hence don 1980 is totally correct in his assessment and it is expansion and thermal stress that will be your issue. You need to do as as built, latest condition stress analysis if this is a new relief load.

I recall the texaco refinery in milford haven was virtually destroyed some years ago when the main flare header failed due to entrained liquid punching a hole in it and enveloped the plant in a gas cloud which as the report put it "found an ignition source". A flare header needs to work hard to keep it's contents inside it until it burns it and I often feel it is an overlooked critical safety item so please don't take liberties with it.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Duwe6]

Who determined your "Design Temperature"? It is suspiciously, anomalously low!
Determine your actual material of construction -- what kind of pipe is it? Then look up the temperature/strength table; ASME Sect IID or B31.3 are two good ones. What temperature does this material start loosing strength? If that temperature is higher than 110°C, use the table temp and ignore the 'artificial' 110°.

If the table gives a 'start of derating' temp lower than 110°, hire a Pipe or Pressure Vessel Eng. to evaluate your header. I'll bet a paycheck that your header is just fine, and the 110° rating is unsupported by engineering calculations.

If it didn't get calculated, it is merely opinion. And your opinion is as valid anoyone elses.