Temperature with no flow (static temperature?)

[NovaStark]

Good day all,

I am trying to get a different opinion on how temperature varies when there is no flow.

I have a vessel operating at about 500 psig and 700F with only one outlet (about 25 in. line - insulated).

About 4-5 feet away from the outlet flange there is a 4 inch take off that just ends at a gate valve (probably about 10 ft. in length) and uninsulated.

The external piping temperature here is about 400F.

For piping with no flow in it, is such a drastic temperature difference a typical occurrence ? I was under the impression that the static temperature (no fluid flow) would be different but not with such a drastic.

Is there a way to calculate such a temperature i.e. convert a process flowing temperature to the stable temperature if there was no flow ? (don't believe this would be the same as stagnation temperature)

 

[mk3223]

The hot vessel with a dead leg pipe is similar to the hot water tank and the water pipes in the house. without the water running, the water in the pipe will be cooled down to the ambient no matter how the hot water is in the tank. It's a basic thermodynamics. isn't it?

 

[LittleInch]

I'm a little surprised its as high as 400F. Most of the heating of that branch probably comes from transmission through the pipe, unless the branch is vertical up.

I would like to bet that in fact there is a temperature gradient if you start at the pipe end and work backwards to the insulated pipe, but it will be a gentle curve. Try taking a few more readings.

There are cooldown calcs, but they are transient and being so close to the heat source, not easy to do as its difficult to judge how much heat is coming from the 24" pipe (25" doesn't normally exist)

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[NovaStark]

The hot vessel with a dead leg pipe is similar to the hot water tank and the water pipes in the house. without the water running, the water in the pipe will be cooled down to the ambient no matter how the hot water is in the tank. It's a basic thermodynamics. isn't it?

Well I was thinking that through natural conduction the temperature would not go straight down to ambient. But I wasn't too sure as I never really had to question it in depth.

I'm a little surprised its as high as 400F. Most of the heating of that branch probably comes from transmission through the pipe, unless the branch is vertical up.

I would like to bet that in fact there is a temperature gradient if you start at the pipe end and work backwards to the insulated pipe, but it will be a gentle curve. Try taking a few more readings.

There are cooldown calcs, but they are transient and being so close to the heat source, not easy to do as its difficult to judge how much heat is coming from the 24" pipe (25" doesn't normally exist)

Correct it is 24 in. - mistake on my part. But the branch is vertically up with the valve at the top slightly passing so there is a very small flow to an extent - probably why it is around 400 degF.

If there was no passing would the piping in fact cool straight down to ambient as mk3223 is saying with no heat due to conduction ?

I mainly ask because the design of the branched piping has the same conditions as the 24 inch piping, and if it were designed to have essentially no flow and just for nitrogen injection for purging the reactor when offline, why would the designer have the same design conditions.

 

[georgeverghese]

Your first post says this dend end piping stops at a gate valve, presumably this is blinded off on the other side? In your later post, you say this valve is slightly leaking, so where does this hot leak gas reverse flow to ?

 

[LittleInch]

Well the designer isn't going to bother changing specs for a 4" pipe 10 feet long when there is no way to ensure that the temperature falls to a certain point. Virtually no cost benefit and loads of grief.

Being vertical up you will be getting some internal circulation of fluid by convection as the fluid in the 4" pipe cools it will fall and be replaced by hot fluid. Add in conduction through the pipe wall and your pipe won't cool down to ambient.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[NovaStark]

Your first post says this dend end piping stops at a gate valve, presumably this is blinded off on the other side? In your later post, you say this valve is slightly leaking, so where does this hot leak gas reverse flow to ?

Yes blinded but there is a leak at the gasket area so the valve is passing - so leaking to atmosphere.

Well the designer isn't going to bother changing specs for a 4" pipe 10 feet long when there is no way to ensure that the temperature falls to a certain point. Virtually no cost benefit and loads of grief.

Yes that makes sense from that viewpoint.

Being vertical up you will be getting some internal circulation of fluid by convection as the fluid in the 4" pipe cools it will fall and be replaced by hot fluid. Add in conduction through the pipe wall and your pipe won't cool down to ambient.

True but is there generally any rule of thumb for this kind of layout for example, if I am looking for some kind of damage mechanism that occurs in a given temperature range i.e. the design temperature may 700F but the actual operating due to to no flow might put it in the range for CUI or something similar.

 

[IRstuff]

"Well I was thinking that through natural conduction the temperature would not go straight down to ambient. But I wasn't too sure as I never really had to question it in depth."

If you have a temperature difference, you must, by Newton's Law, have heat flow. If the end were at ambient, there could not possibly be heat flow from the pipe. Therefore, the end of the pipe must be hot enough to transfer heat to the environment. The measured temperature should therefore be the equilibrium value at which the heat flow internally is matched by the heat flow into the ambient environment.

That said, the measured temperature implies that there's something on the order of 1.5 kW of heat loss, which is rather huge, implying some sort of serious convection inside the pipe.

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[georgeverghese]

For reasons cited earlier, you cannot change design temp on this branch pipe without a valve and closed spade upstream of the design temp spec change location.

 

[NovaStark]

If you have a temperature difference, you must, by Newton's Law, have heat flow. If the end were at ambient, there could not possibly be heat flow from the pipe. Therefore, the end of the pipe must be hot enough to transfer heat to the environment. The measured temperature should therefore be the equilibrium value at which the heat flow internally is matched by the heat flow into the ambient environment.

Well yes, that is what I wanted to know if there was a way to estimate that equilibrium temperature. Sure no flow means that it won't reach the temperature of the main piping, but a way to estimate ?

For reasons cited earlier, you cannot change design temp on this branch pipe without a valve and closed spade upstream of the design temp spec change location.

Agreed.

 

[georgeverghese]

If there were to be no reverse flow/ leak through this branch, it is possible to estimate by calcs the temp gradient on this 4inch branch. It would be a little involved if you followed a strict procedure, and a quicker calc may be done by ignoring the internal natural convection htc and the resistance across the metal wall, assuming these resistances to heat flow are small in comparison to the external natural convective and radiative coeffs. You'll find formulas for these external coeffs in your heat transfer text. Agreed, the orientation (vertical or horizontal) of this branch piping will have an effect on the external natural convection coeff. We are assuming here wind speed effects are small and natural convection currents dominate.

 

[NovaStark]

Then it should be possible albeit quite involved.

Thanks all for the information and help.

 

[NovaStark]

Good day all,

Well I have some interesting information when I checked the same piping setup at a different site.

So the surface temperature of the piping measured at the same place was actually ambient.