Design temperature for low pressure high temperature equipment

[Ing. Aponte]

Good afternoon forum's designers,

I have a strange situation to deal and want discuss it with you. I have a small vessel, around 1m diameter and around 1.5m height, that treat exhaust gases. Like a big muffler.

Client want a design as for ASME VIII div.1 although pressure is less than 1 barg. Apply the formulae is easy but here the main point is design temperature. The manufacturer is the Client and has a large experience but no process department inside so design temperature has been set to exhaust gases temperature.

Mechanically the problem is that at this temperature -700°C - the allowable of materials become almost zero and the original design, done by experience, was for around 200 kg and mine is almost three time.

I remember that what is important in vessel design is not the temperature of fluid inside but the temperature of the wall. System is not insulated so, with a brutal semplification considering 20° as ambient temperature the first tentative avarage temperature could be (700+20)/2=360°.

On which code is written to consider the avarage temperature of the shell? we want do a thermal calculation but before do this we want be sure our approach almost follow a code.

Thanks!

 

[GD2]

Op,
There are two aspects in this:
1. The maximum design temperature is based on max operating temp with usually a margin of 25F.
2. The max design temp should not be less than the mean metal temp across thickness, expected under operating conditions.

Your operating temperature is 700C. Therefore, your design conditions should be based on operating temp and NOT mean metal temp.

Yes, at this temp, almost all material will be in creep. Table Sec II Part D will give you allowable stress value at rupture life of 100,000 hrs. For higher life, refer to Sec III Div 5 graphs.

Hope it helps.

GDD
Canada

 

[georgeverghese]

There are 2 heat transfer resistances acting in series here:
a)Inside (the muffler) heat transfer coeff, which in this case would be much higher than outside htc
b)Outside htc where muffler OD contacts ambient air - this htc (consisting of natural convection and radiation components) is much much lower than that inside

Since (b) is much less, the mean metal wall temp will be closer to 700degC than to 20degC. It would be even closer if this vessel were to be insulated.

 

[LittleInch]

So if the existing vessel is working, why redesign it?

Your issue is probably also a result of certain minimum thicknesses and stresses written into the code which wasn't there before.

But you can't reduce the metal temp the way you're doing it. Can't you measure the temp of the existing unit? At least you have something solid to hang your hat on.

The metal temp is likely to be close to that of the inside gas. Its heat transfer coefficient is 100 times better than the heat transfer on either side so you can't average anything.

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