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Venezuela - Member - New Member

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Stand Pipe

08/02/2012 7:54 PM

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Friends,

I was debating with my work partners about the use of two taps stand pipe in order to replace a three taps stand pipe. The application is a Desalter and its level inside is crude/emulsion/water

My reasons were that if we use the two taps stand pipe the emulsion layer will be lost, I mean the emulsion layer only can be present in the stand pipe if the level goes up or down enough to reach the upper and lower taps. My point is due maybe in the stand pipe will have an emulsion layer not representative of the emulsion layer inside the desalter.

I have choosen the three taps stand pipe, not two taps

Can anyone please to coment about the issue?

Regards

JP

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#1

Re: Stand pipe

08/02/2012 8:27 PM

You don't say what this stand pipe's function is - site glass, measure for indication or control? I'm almost with you, but you need a connection within each phase. As your diagram is shown, the emulsion layer is straddled by 2 connections. What happens if the emulsion layer thickness changes in the process vessel? The emulsion layer in the stand pipe is trapped by the crude above and the water below - it has no way to equilibrate with the process vessel. Likewise, the water level below the bottom tap and the crude above the top tap are outside the measurement range because they are trapped. These multi-phase levels can be very tricky.

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#2
In reply to #1

Re: Stand pipe

08/02/2012 9:40 PM

Friend Bigg,

I didn´t say the stand pipe´s function, the function are one for indication and other for control, in the top it will be installed a level transmitter (Guided Wave Radar)

Regards
JP

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#3

Re: Stand pipe

08/02/2012 10:34 PM

The measurement can be tricky, but I don't think it is because of different levels in the bypass and vessel.

Assume that the color bar on the left represents the level in a two tap vessel and the color bar on the right represents the level in the bypass at a momentary difference in levels.

What prevents the water from 'seeking its own level' through the lower tap so that the level of the water is the same in both?

As the water seeks its own level, the emulsion layer at higher elevation on the right, riding on top of the more dense water, lowers in elevation as the water seeks its own level in both the vessel and the bypass.

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#4
In reply to #3

Re: Stand pipe

08/02/2012 10:58 PM

Don't understand the part of your statement 'the emulsion layer at higher elevation on the right, riding on top of the more dense water, lowers in elevation as the water seeks its own level'. I agree that the middle nozzle in the sketch on the left, as drawn, is unnecessary since it is a redundant nozzle in the top phase. Using your example, what happens if the emulsion layer gets thicker? How will this be reflected in the stand pipe?

I should state - I have assumed that the emulsion layer depth is important and not just incidental. I am not familiar with desalter operation and whether this is important. I am drawing on experience with settlers where the emulsion, or 'rag layer' had to be skimmed and treated separately. In that case, both interfaces, light to emulsion and emulsion to heavy had to be measured. Which leads to a question I had. I'm not familiar with GWR, but I assume it can't measure both interfaces with one probe. Is that correct?

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#6
In reply to #4

Re: Stand pipe

08/03/2012 7:33 AM

GWR (guided-wave radar) is capable of simultaneously measuring multiple interfaces with a single probe: each fluid interface creates its own "echo" as the incident pulse encounters a change in dielectric permittivity. One caveat is that the difference in permittivity between the two fluids at each interface must be significant enough to generate a strong echo (the reflected signal strength is a function of this difference in permittivity).

Also, the media through which these echoes must travel to return to the sensor at top cannot be too lossy. Water is a good dissipator of RF energy (this is why microwave ovens heat water so well), and so a thick emulsion layer (which of course contains water) may dissipate too much of the water-emulsion echo on its way back to be reliably measurable.

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#5
In reply to #3

Re: Stand pipe

08/03/2012 6:56 AM

The OP is correct that there is a risk the emulsion layer inside the standpipe will not be representative of the emulsion layer inside the desalter, unless the level of that emulsion layer reaches the level of a tap where the fluids can equalize. This is true of any process with more than two fluid interfaces (water, oil, and air does this too).

Iris' comment is correct -- that the liquid levels will equalize between the standpipe and the desalter -- but only if the two emulsion layers are the same thickness. Once you admit the possibility of a changing emulsion layer thickness, all bets are off. The problem boils down to the mathematical conundrum of trying to solve for multiple unknowns when you only have a single equation: there is a range of possible solutions.

Unfortunately, though, the third tap isn't a comprehensive solution, unless that middle tap is always at the level of the emulsion. The best solution for this problem is to use a "cageless" level transmitter where the instrument's sensing element hangs directly inside the desalter. Short of that, one could use more than three taps, so the emulsion layer wouldn't have to go far before it found a position where it could equalize with the other layer.

Keep in mind also that any multi-tap solution may exhibit weird behavior when the emulsion layer in either column reaches a tap: the instrument's reading will quickly change if the two layers differ substantially in thickness, during the period of time when they are equalizing.

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#7
In reply to #3

Re: Stand pipe

08/03/2012 7:39 AM

Here is a sketch showing the mathematical problem of multiple variables in an interface level measurement scenario. Here we see how to calculate the total hydrostatic pressure at the bottom of the fluid column based on fluid heights and densities:

DP and displacer instruments both are subject to this effect, since their responses are fundamentally based on the hydrostatic pressure created by a fluid column. In fact, you can prove Archimedes' Principle of buoyant force by calculating the resultant force of the different hydrostatic pressures at the top and at the bottom of a displacer.

I realized after submitting my first post to you that my math reference might have been a little vague, hence this post.

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