Tube side design temperature

[imatasb]

Good morning colleagues,

Whilst specifying the design temperature for a pipe is quite straightforward, the maximum temperatura that can reach the fluid that flows across a certain pipe, I am not so sure when it comes to the design temperature on the tube side oh a shell and tube heatexchanger.

Lets says I have a heat exchanger characterized as follows:

Shell side: Process liquid with a maximun inlet temperature of 235 ºC.
Tube side: Water entering at 140ºC and which boils at 150ºC. (More than the exact number I am interested in the concept.)


Shell side design temperature is clear at 235 ºC.
What happens with the tubes? Maximum temperature in the inside will reach 150ºC but the outer part of the tubes could be exposed to 235 ºC.
Which is the rule or criteria to set the tube side design temperature?

Thanks a lot for your answers.

 

[georgeverghese]

Upper Design Temp for both shellside and tubeside must always be set to be the same for shell and tube HXs'- the entire tubeside temp will equalise with shellside when tubeside flow is very low or zero.

 

[EmmanuelTop]

What will happen in case of water getting blocked in the tubes, for whatever reason? Ultimately the tubes will see the maximum shell temperature.

Dejan IVANOVIC
Process Engineer, MSChE

 

[jte]

Let's not forget that valves sometimes get turned that should not have been turned. So the tubeside flow can be shut off without regard to the tube design temperature. Or a pump fails. Or a pipe suffers a catastrophic loss of containment. Add these scenarios to those which georgeverghese and EmmanuelTop pointed out.

If you are establishing the design conditions for a heat exchanger which will be built to ASME Section VIII Div. 1, don't forget U-2(a): ...shall establish the design requirements... taking into consideration... such other conditions as startup and shutdown, and abnormal conditions which may become a governing design consideration...

You cannot just say "Oh, that only happens rarely." Tube fouling, loss or substantial reduction of flow, etc. can easily be shown to be reasonably foreseeable in most plants.

Although not directly related to tube metal temperatures, unanticipated temperatures experienced by the shell of an exchanger ultimately led to multiple fatalities in the Anacortes incident of 2010. [ref: Items 12-14 of the Executive Summary of the Investigation Report available at

http://www.csb.gov/tesoro-refinery-fatal-explosion-and-fire/

]

 

[georgeverghese]

And also, upper design temperature should be at least 5degC higher(preferably 10degC) than the max operating temp.
It is necessary that the lower design temperature for all pressure containing equipment be specified also.

 

[golpesar88]

I am not sure if it is allowed to have a phase change from liquid to vapor in tubes! If flow failure in tubes are important just put a low flow switch and stop shell inlet media. During operation tubes are not always inside liquid but to protect them from getting dried baffles considered to maintain liquid level. I am not sure if I got your idea completely!

 

[Drexl]

As I remember the standards state that calculation has to be done on the higher of the tube/shell side temperature unless a lower temperature can be justified. If you proteect the tubeside using a switch as golpesar88 suggests, this switch is a protection function of a pressure vessel and has to be designed as such.

 

[Donmiguel]

I do not agree with my colleagues in this post.

Designs with different design temperature on the shell and tube can occur.

To me it sounds like you have a water tube boiler. If the tubes of the water tube boiler is connected to a steam drum with natural circulation (no pumps which can fail) and a loss of water can be adequately protected by trips in the steam drum the boiler tube temperatures can have a lower design temperature. The design temperature of the tubes in the water tube boiler will be determined by the safety valves set pressure on the steam drum.

The above is the case for many boiler designs because you simply cannot get pipe materials which can handle the operating temperatures of the other side, which can be in a fired boiler as an example 1200 deg C.

 

[georgeverghese]

Lets work this through:

In the case of a water tube boiler, tube rupture leads to superheated steam bursting out into the flue gas side, which is open to the atmosphere. So all we have here, in the event of loss of level safeguarding in the boiler drum, is a mechanical integrity risk, not a HSE risk.

We may get a clearer picture of the risks involved in this case if imatasb were to tell us more about this HX service and configuration, stream fluid on the shellside, operating pressure range and design pressures for both sides.

 

[moltenmetal]

There is some very confusing advice being given here. I recommend that it be taken with an appropriate measure of salt.

Only in the simplest of cases is it adequate to set the tubeside and shellside design temperature conditions of a heat exchanger to the same value.

We don't try to design vessels to be capable of withstanding upset conditions of PRESSURE by making them inherently safe, i.e. capable of withstanding the absolute maximum pressure that they could ever encounter in service. No, instead we install relief devices which are sized and selected to protect them from overpressure. The SAME is true of heat exchange equipment in all but the most trivial applications: the equipment design conditions (both temperature AND pressure), the design of supporting equipment, and the safety systems protecting the equipment from failure, are designed and selected together to render the resulting system adequately safe.