Maximum velocity at nozzles for two phase fluid

[imatasb]

Good morning,

we are thinking of recovering more heat of a residue current for generating more steam.
The idea is to extend the lengths of the tubes of a shell and tube heat exchanger whils keeping the current diameter.

At the moment we basically heat a boilerfeedwater current and just produce 1% of steam but with the new heat exchanger we will be able to produce 10% of steams in the tube side. The current goes into a vapor/liquid separator.

I have seen the simulation of the new heat exchanger and noticed that the velocity in the outlet nozzle was 6.9 m/s.

Whilst hte criteria of velocity for tubes are quite clear:

*3-5 ft/s (0.9-1.5 m/s) for liquids
*50-100 ft/s (15-30 m/s) for gases

I am not sure for a two phase flow. Could anyone which velocities could be considered for a two phase case?
The performance check of the heat exchanger says ok and the pressure drop was ok as well at 0.8 kg/cm2. I was thinking of maybe erosion due to the high velocity in the liquid


Thank you for your answers.

 

[georgeverghese]

Velocity may be okay, but as always in 2 phase flow, turndown is the problem - check the process control scheme and minimise the potential for slug flow in the riser to the V/L separator for the entire perceived operating range. Keeping the riser length short helps. Check if there is some surge volume handling capacity in this separator otherwise.

Exchanger design and materials of construction should avoid unintended migration of residue into BFW, else BFW pressure should kept always higher than resid pressure.

 

[EmmanuelTop]

Extending the tubes means a new shell, right? You are basically getting a new exchanger there.
I don't know the details, but maybe there is a cheaper option e.g. to change the tube pitch and/or tube pattern and fit more tubes into the existing shell.

As for the flow velocity, look at the momentum limits and potential tube vibration. Software like HTRI can calculate this for you.

Dejan IVANOVIC
Process Engineer, MSChE

 

[EmmanuelTop]

See also the attached page from TEMA standard - recommendations on maximum momentum and impingement plate are clearly defined.

Dejan IVANOVIC
Process Engineer, MSChE

 

[georgeverghese]

If you are moving a 2phase stream ( steam + hot BFW) from the old HX to the new HX, beware of a likelihood of HX failure in the 2nd HX due to maldistribution of fluids to the 2nd HX / slugging into the second HX. Without knowing any further details, it may be preferable to split the 2phase stream out of the first HX and route the lquids only into the second HX.

 

[davefitz]

FAC flow assisted corrosion is the main enemy in 2-phase flow- this erosive-corrosive issue is dependent on the flow velocity, pH, O2 content, and tube metallurgy ( 0.5% Cr content , minimum, helps avoid FAC). One cannot select the design velocity without knowledge of these other variables.

"Nobody expects the Spanish Inquisition!"