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Measuring Slip Oil Tank Interface Level

I received this question that I'm hoping our level measurement experts in this group can help answer:

We have to measure interface level in a slip oil tank. The top layer dielectric constant (DE) is 20 and the bottom layer DE is 80.

Kindly inform whether we can measure the interface level through the GWR LT [guided wave radar level transmitter]. What is the DE difference required to measure the interface level through GWR LT.

Can we use either of the measurement GWR probes I.e either coaxial or single rod or twin rod?

Thanks for your help!

• Level
• guided wave radar

Jim Cahill

• 24 Apr 2019 12:35 PM
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5 Replies

• Ingemar Serneby

  

  • 24 Apr 2019 1:17 PM

  Hello!
  Guided wave radar is generally a very good solution in most oil and water interface applications. The dielectric constant (DC) of the upper product must however be low enough, typically around 10, in order to let the energy propagate down to the interface level. The DC of most oils are in the 2.0 to 7.0 region and I thus suspect that the top layer DC stated in this application (20) might be an erratic entry - please clarify. Further, the difference between the DC of the upper and lower liquids must be more than 6.0, where the product with the lowest DC has to be on top. How thick upper layers that the radar can handle depend on the DC of the upper and lower liquids, where a very low DC (typically around 2.0) of the upper product and a very high DC (typically >20) of the lower product allow for the thickest layer.
  In order to guide you further, it would be very good to know the DC of the upper product (slip oil), the measuring range and the supposed maximum interface thickness. Looking forward your reply and to assist you further with the matter!
  Best regards,
  Ingemar

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• Binu Mathew

  

  • 24 Apr 2019 2:40 PM

  In reply to Ingemar Serneby:

  Thanks for the quick reply.The upper layer DC as per process engineers is 20.
  As per your reply I think we cannot use GWR LT .if so then what you suggest?Whether Displacer type LT will suffice?

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• KeeChong

  

  • 25 Apr 2019 10:27 AM

  In reply to Binu Mathew:

  Hi Binu,

  To determine if displacer is suitable for this interface application, we would need to check the density for both the upper and lower fluid density. Fisher has displacer type of level instruments (DLC3100, DLC3010 and DLC3020f for Fieldbus). The minimum differential density between 2 fluids must be > 0.05 SG. The higher the differential density, the better the measurement. And we would also need to know the measuring range.

  One thing to take note for interface application, the displacer must always be covered by either of the two fluids in the chamber. There must not be a time that the displacer is covered by the two fluids and air (this will be double interface) as displacer will not be able to measure the level.

  

  Hope this helps.

  Kee Chong

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• Binu Mathew

  

  • 25 Apr 2019 1:48 PM

  In reply to KeeChong:

  Hi thanks.Just now I checked the tender and it’s written that displacer type LT should not be used.
  I have asked the process whether the slop oil will have DC of 20?since I believe that slop oil will be having DC of around 2-4.You have any experience of having fluid DC of 20?

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• Ingemar Serneby

  

  • 25 Apr 2019 2:09 PM

  In reply to Binu Mathew:

  Hello Binu,
  For guided wave radar, the ability to measure the interface level highly depend on the ability for the radar energy to penetrate through the top layer down to the interface and back again. If the DC of the upper media is higher than recommended, there will be much less energy that actually will be returned from the interface level, which also will limit the thickness of the top layer. Even though the recommended maximum DC for the top liquid is around 7, I have seen cases where the DC have been much higher than that - but never as high as around 20. In those particular cases, the thickness of the top liquid was limited and we also needed to adjust the threshold setting for the radar in order to get it to work.
  Conclusion is thus that it is possible to measure level and interface in cases where the DC of the upper liquid is higher than recommended, but the thickness of the upper layer will be limited and the that additional configuration of the radar will be required. I would consequently not recommend a guided wave radar for level and interface measurement when the top liquid has a DC of around 20. If your assumtion is correct that the DC is somewhere around 2-4, it is an entirely different story though! In such case; the guided wave radar will make a very good fit!
  Best regards,
  Ingemar

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