1 reply to this topic

[NikkiC]

Hi everyone,

I'm a recent che grad who currently work in a Chinese consulting firm... But I was assigned to design a C3 splitter column, and I've got a bunch of amateur questions that none of my colleagues were able to answer, so hopefully someone could see this and provide some guidance..... 

 

1. The column has two baffled thermosyphon reboilers, and there should be HLL, NLL and LLL alarms on the bottoms. My question is, should I use 1 differential pressure LT + multiple LG's for various liquid levels? I've seen some columns that have only 2 flanges, so basically both LT and LG are installed on the same nozzle. But I've also seen some sketches where 3 LGs were used to monitor the three liquid levels, plus one DP LT... I'd prefer the second option because it just looks safer to me lol idk

 

2. How to size these connection nozzles if essentially there's no flow into these?

I thought normally for nozzle sizing, we'd consider external piping first and then make sure that the momentum doesn't exceed some recommended values. So I guess this question also applies to like TG and PG etc...

 

3. Unrelated, words from a refinery technician: the non condensables from reflux drum into LPG header sometimes freezes (?), because upon startups, the pressure fluctuates a lot and is really hard to control. I guess I just wanted know how to confirm this from a simulation (maybe HYSYS?) lol I thought it might be relevant to material selection here

 

Thanks a lot in advance!!

 

Best,

Nikki

 

PS, this was supposed to be a preliminary design, so I can't essentially contact any vendors unfortunately... But doesn't this look very much detailed? Like I was also supposed to sketch out all side/bird views for all these nozzles when I've never actually seen one... 

[Zauberberg]

If you want to receive a proper and detailed response, for questions such as yours a detailed sketch (if not P&ID) is normally required. Otherwise, due to lack of details, we could misguide you, which is definitely not a purpose of these forums.

 

1) Differential pressure transmitter cannot detect foaming, otherwise this level concept is acceptable. For column troubleshooting purposes you may install two vertically adjacent sight glasses which will show presence of foam.

 

Coming to your question if the nozzles could be shared between LG and LT: yes, they can. In most designs so far I have seen them separated. Only if there are SIS functions (SIS = Safety Instrumented System) which shutdown relevant equipment or units in case of column level excursions outside the normal operating envelope (which means above HHL and below LLL), then a separate set of nozzles is required for the SIS level transmitter. Industry practice (and in some companies this is formulated in the relevant standards), SIS and PCS (DCS) transmitters must have completely separate connections to the process.

 

2) Level instrument connections on the column are typically 2". This is sufficient to avoid plugging of instrument lines. Make sure that the entire level instrument assembly is  properly insulated. Otherwise there will be an erroneous measurement if liquid temperature in the column is 200 degC, and in the instrument connection lines only 20 degC. Differences in density can be very significant. For pressure gauges, standard NPT connection dimensions are 1/4" and 1/2". For thermowells, common flange sizes are 1/2",  1 1/2", and 2". You need to check the company standards which connection types should be used based on pressure, temperature, and fluid category.

 

3) For this question definitely a sketch is needed, and a bit more detailed explanation of the phenomena occurring in the system.

Edited by Zauberberg, 23 March 2015 - 01:38 PM.