Shell and Tube Exchanger for Hydrocarbon Gas Preheater 6

[patchlam]

Dear all engineers,

I am trying to design a shell and tube heat exchanger to preheat the hydrocarbon gas. This is to ensure that the hydrocarbon gas that we are going to supply to the Gas Turbine will not going to form hydrate. However, I am concerned with the temperature gradient across the heat exchanger as the inlet/outlet temperature of hydrocarbon gas is 0/90 degF while the hot exhaust gas has the inlet/outlet temperature of 932/840 degF.

There are two potential problems which I could think of:
1. Acid condensation on the surface of tube due to the sulphur content in the exhaust gas. (Maybe stainless steel tubes can be used to mitigate it?)
2. Thermal stress due to the large thermal gradient happened in the heat exchanger. (Austenitic stainless steel has higher thermal shock susceptibility than carbon steel?)

Besides these two, is that any concern for the design like this? Or do you have any recommendations?

Thanks!

Regards,
Patch

 

[shiraz883]

Hi,

Other concerns I can think of are as follows:

1. Acid condensation: You already mentioned the first point: Solution could be to use some kind of liner inside the tube made of high temperature corrosion resistant material till the length where the temperature is in the range of corrosion. So your tube could be carbon steel with liner of some alloying material or having low conductivity material so that tube temperature is controlled.

2. Thermal Stress: Carbon steel can be used but you have to evaluate the effect of temperature gradient. You must have refractory material on the tubesheet hotface and as mentioned in point 1 some shielding of tube from the hot temperature of incoming gas.


3. Primary stresses in the tube and tubesheet: due to design pressure on shell side and tube side.

4. Weld between tube and tubesheet.

5. Buckling of tubes.

Having said all the above points I am assuming that the pressure vessel (heat exchanger) will be designed as per ASME Section VIII. So you have to see the details in this section for designing your vessel according to design by rule (DBF) or design by analysis (DBA). For DBA these problems are to be solved using finite element analysis by experienced engineers.






Shiraz
Sr. Engineer

 

[SnTMan]

patchlam, I have had some experience with mechanical design of fuel gas heaters for similar service as you describe. Unfortunately I do not recall the rating conditions, temps, pressures (high on tubeside) flowrates etc. As I recall the fuel gas was on the tubeside, I do not recall the shell side fluid, but I do not believe it was exhaust gas.

These were multiple, small fixed tubesheet units (TEMA NEN), all carbon steel, lo-fin tubes, 1 pass x 1 pass, with shell side expansion joints. They typically did not last all that long

Good luck,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[georgeverghese]

Think this concept itself has significant process safety risks. All heat exchangers leak to some extent. In most cases, the turbine WHRU deals with some high boiling point hot oil with low volatiles content or dilute MEG or similar. In this case however, a leaking tube to tubesheet joint could push the exhaust gas into flammable envelope at low turbine load, resulting in a fire.

Suggest using an indirect heating loop for this, with hot oil or similar to pick up WHRU heat , then transfer this heat to the fuel gas at a FG-hot oil HX. This will cost much more, but is much safer, provided you've selected a good quality hot oil (or some other heat stable recirc medium with little volatiles) and WHRU thermal / mechanical design / fabrication follows best in class standards.

 

[EdStainless]

To echo Tinman, The ones that I have seen did not directly use the exhaust to heat fuel but rather used an intermediate working fluid.
This allows controls or pressures (for safety reasons, you have to control which way things leak) and limiting the temperatures.
With high temp people often choose to use u-bend heat exchanges. That way thermal expansion does not load tubesheets.
These are usually all stainless, for cleanliness and temp resistance.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[patchlam]

Dear all,

I really appreciate all your replies. As mentioned by the majority above, I believe the heat exchanger should be designed for the intermediate pressure feedwater (or hot oil) to flow within the tubes and the lower pressure fuel gas to flow through the shell. With intermediate fluid being higher than gas pressure, this configurations ensures that the gas will not enter the intermediate fluid system if the tubes leak or rupture.

As georgeverghese mentioned, this method will cost more, but safer. Unfortunately, sometimes the customers only look at the matter from cost perspective, that is why they insisted on this direct heating method, which I do not have any experience at all.

Regards,
Patch

 

[georgeverghese]

There may be some lessons learned from MLNG I or II where a regen gas heater, set up in a gas turbine WHRU caught fire and ruined the multimillion $ GE turbine, let alone the extended production shutdown - this happened in the 1990's. Root cause failure was a crack in the WHRU tubesheet joints / poor weld inspection combined with unsafe instrumented shutdown sequence. Many other instances in Sarawak, Borneo where direct fired heaters (heating crude and the like) have caught fire due to corroded leaking tubes over the years.

 

[SnTMan]

Excellent contributions by georgeverghese, IMO.

The problem with sloppy work is that the supply FAR EXCEEDS the demand