Process Design Room Magic Talks 101: Tangential Nozzles

It is common practice to have nozzle inlet orientation left to drafting specialists in order to meet piping / structural requirements; however, the orientation of some nozzles is significant to the Process Engineer. I am not going to go into the basics of inlet nozzle type dependence on the flow regime in the inlet piping, on the separator internals, and the type of service.

On a recent task in the past week, a client did specify the need for a tangential inlet nozzle with a doughnut baffle ring located roughly 2 feet below the inlet pipe centerline on a vertical vessel. This was because a mathematical model run by someone had spewed results indicating the presence of a ‘steep’ vacuum, high velocities, and low gas density (exact words I was told so your confusion is as mine), and he was advised to go that inlet route.

Now for starters, I’ve not met any mathematical geek that can credibly give a multivariable model describing the effect of inlet mixed-phase velocity, vapor space height, disengagement requirements, etc., etc. in vertical vessel arrangement and assessment which have immense empirical dependence hence killing theoretical waldo. However, sometimes when we let science inform engineering judgment without looking at the limits of the science, we fall into costly options. A model is only as good as the person developing it thus its application will also fall in the sage wisdom envelope of the developer whose own faults in judgment may not be known to themselves. There's also the belief by many that mathematics is the language of engineering but others have shared similar views as me on the subject.

I humbly informed him from my own judgment such a design instance is usually not practiced and nonetheless meeting nozzle reinforcement requirements as prescribed in UG-37 & UG-40 may be difficult. Yes, stress considerations in such systems may have high reinforcement demands which may not be met effectively. Furthermore, in this actual case, it could be identified that the entrance velocity was high enough to cause a liquid swirl which in my view would give misreadings in level indication yet the outlet pump connected to the vessel’s bottom would be starved from a no flow scenario thus cavitating severely which would create another issue that I had to mitigate (a story for another day).

Yes, I recognize that some Process Engineers would have strong reasons to specify a tangential nozzle inlet on channel ends or in services where slug flow is present in the inlet piping which would require either use of a tangential nozzle inlet with an annular ring or a slotted tee distributor. The core area for upward vapor flow in such cases would be designed for 300% of the maximum critical velocity. If 300% is exceeded, then two smaller tangential inlet nozzles would be provided, located 180° apart and orientated to direct flow in the same direction of rotation. Of course, the annular ring width would be the same as the inlet nozzle diameter and the annular ring vertical height would be 2.5 times the inlet nozzle diameter. However in my opinion, since slugging may result in excessive liquid carryover and vibration and sometimes it may be impossible to avoid it at the vessel inlet, I would have liquid carryover minimization undertaken by use of a horizontal vessel with two inlet nozzles and a central vapor outlet nozzle or something less star trekked.

A heuristic I have personally followed is that inlet nozzles of vertical drums are preferably connected to a 90° turned ell or baffle within the drum when the inlet velocity is sufficiently high to interfere with the vertically rising vapors and flush inlet nozzles are preferred when inlet velocities are low but when one understands the impacts of nozzle location/orientation both on the operating mode of the vessel and connected equipment or interaction with the plant layout, then one can understand why it is risky business to play with tangential-trial magic.

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