The challenges of interface liquid level measurement

[Jeff_Blair]

This seems to be a hot topic and I get questions about it weekly.

Maybe this is a good topic to elaborate on in future training.

Here are what I see as the pros and cons briefly (let me know your thoughts please):

Buoyancy (244LD): + handles high pressure / high-temperature applications; + handles emulsions, does not require the clear separation of fluids; + no moving parts (SE exclusive);  + not reliant on dielectric constant; - often requires side chamber; - may be difficult to install from the top of the tank; - reliant on density requires a specific difference in densities for accurate measurement; - requires "flooded" displacer/consistent submerged in of liquid

Guided Wave Radar: + no moving parts; + easy to install on top of the tank; + can retrofit side chamber installations; - cannot handle emulsions or rag layers; + not reliant on density or specific gravity; - requires very specific differences in dielectric constants; - requires certain heights of dielectric constants;  - low dielectric constant fluid must be on the top layer; - reliant upon dielectric constants; - may require "flooded"/submerged probe 

Differential Pressure (IDP10S):  + no moving parts; + well-understood method; + handles emulsions, does not require the clear separation of fluids;  + relatively easy to attach via process tubing/piping; + no moving parts; + not reliant on dielectric constant; - reliant on density, requires a specific difference in densities for accurate measurement; - requires a consistent height of liquid; - contacts the liquid

Free Space Radar (LR75, LR74, LR54, LR01):  A free space radar cannot handle interface alone; however, free space radar can be an effective part of an interface level solution.  There are hybrid methods I've seen that incorporate more than one technology - i.e. a d/p level with a free space radar and a PLC or DCS do the interface equation.  Here, the d/p is actually calculating the interface; when coupled with a radar the level can fluctuate in a relatively normal fashion.  Please see the photos below and attached for suggested hybrid applications.

[rhbbatista]

I don´t totally agre with the cons to Buoyancy:

- often requires side chamber: in fact I found very few applications that really requires side chamber. I had a case of a client regret using a side chamber instead of a stilling well, if an appropriate stilling well is used the buoyancy performance is great;

- may be difficult to install from top of tank: in general I think that the difficulty to install a 244LD on a tank´s top won´t be much bigger that install a guided wave radar. I´ve replaced some guided wave radar with 244LD, no big deal with installation (244LVP return would be great to completely wipe out this con 😁);

About guided wave radars you got a very good cons point (totally agree):

- cannot handle emulsions or rag layers: I´ve seen many applications that fail on process because the guided wave radar can´t handle the emulsion thickness;

I think interface liquid level is a great subject to a team discussion, to share experiences, successes and fails. Very good topic! I´ll really appreciate to hear more opinions about.

[JustinRechner]

do we have any install details on using a hybrid system? real world examples that it has been successful. i agree people are often trying to apply technology to unsuitable processes expecting it to work where in fact other technology do a better job. might cost you more up front with installation and configuration but if it works and works well is that not worth the extra cost?

[Jeff_Blair]

I think the entire process world is in love with guided wave radars.  And for good reason.  They're perceived as easy.  But, the rules that go with them for interface applications are dizzying.  The emulsion layer must be x, the dielectric offset must be y...They're really great instruments.  LG01 is a strong guided wave radar.  But other methods need to be looked at.  The reason guided wave is very popular is several competitors cannibalized their own buoyancy business in the not to recent past as in "replace your displacer with a radar." 

[Jeff_Blair]

Hybrid - I'm sure there are some users.  Maybe some will comment here or offer their experience.  I guess you could make the argument that an IMV31 is a hybrid system in a way since it takes temperature compensation. 

I'm an advocate of redundancy which is different than our proposed hybrid...use two of the same technology, one of each, two different types, etc.  The possibilities are endless.

For those of you wondering what we are talking about, we have an in house hybrid level system that solves the shortcomings of most guided wave radar used in interface applications.  It's coupled with a SCADAPack RTU/PLC and a d/p (or pressure) transmitter.  It works through the rules of constant height, submerged probe, etc.

[Jeff_Blair]