Difficult Level Measurements

“Ask the Experts” is moderated by noted process control authority Béla Lipták (liptakbela@aol.com), editor of the Instrument Engineer’s Handbook. In this column, he and his co-authors welcome questions concerning process measurement or control. Please include P&ID drawings using ISA symbols to illustrate questions where necessary.

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If A > B and B > C, then X = B.
Else if A  > C and C > B, then X = C.
Else X = A.

A single failure in either direction will not disrupt control. This is equivalent to “two-out-of-three logic” used in nuclear power plants, but simultaneous failures in both directions are also accommodated. Median selection also provides noise filtering without an associated lag, since positive and negative spikes are both rejected by the selector.

Greg Shinskey
Process Control Consultant

A: Re:Molten wax level

Try differential pressure, using diaphragm sensors flush to the tank wall. The high-pressure diaphragm is fully submerged, sensing the total head above itself. The low-pressure diaphragm in the wall of the tank well above the fluid can be located near a heat source to minimize build-up if need be. It senses the pressure above the wax. The difference in pressure, corrected for density, represents the level above the high pressure diaphragm.

Otto Muller-Girard, PE,
FISA

A: Re:Level of molten wax

Nuclear attenuation is certainly a possibility for your application. Check such manufacturers as Vega, Omega, Berthold and Able (UK). You may be surprised at the cost of ownership.

If you can incorporate load cells into the vessel mounting, it is a less expensive method. This is indirect level by measuring mass, but assuming you know the density and geometry, level can be deduced.

Gary G. Sanders

A: Re: Wax level detection

A RF frequency admittance (capacitance) probe might work. It “sees” through coatings. Inquire with Drexelbrook or Endress-Hauser.

Stephan Gaertner, PE,
Chevron, Richmond, California

A: Causic Level

Dear Bela, here's an answer from our level experts:

There will always be issues with pressure transmitters due to their compatability with caustic soda.  A ceramic (99,9% pure ceramic) transmitter sensor performs much better, but there will be trouble in finding a suitable O-ring!

The best experience with caustic solutions has been made using a non-contact radar transmitter with PTFE antenna. However, its success depends upon:

  • how turbulent the surface is
  • how big the vessel is (diameter)
  • how many obstacles are mounted inside the vessel

If the worse comes to the worse, a radiometrical (gamma) measurement is always possible.

Regards,
Peter
Manager Marketing Communication
Endress+Hauser Process Solutions AG

A: Caustic Level

Several manufacturers offer hydrostatic tank gages based on multiple pressure measurements. The output can be in terms of level, volume, or mass as desired, as density is also determined. As a start, go to Foxboro.com and look for hydrostatic tank gages.

Greg Shinskey
Process Control Consultant

A: Caustic Level

Bela:

If I understand the situation, unfornately this application has variable density of the NaOH solution. Pressure transducers will only measure head pressure which will vary both with level and density.

Regards,
Gary G. Sanders
Director of Engineering
TV&C - Prophetstown
Penberthy and Yarway Products

A: Caustic Level

Use a nitrogen bubbler technique.  Place the pressure transducer at the top of the tank and a tube extending from the transducer to the bottom of the tank.  Now add a slight flow of clean compatible gas to the tube via the use of at T fitting.  Adjust the gas flow to allow a slight flow when the tank is full.  As the level drops the back pressure will decrease with level. A simpler method is to not add the gas flow and to just depend upon the back pressure.  While using a tube without flow generally works, the gas flow helps to keep the measurement pipe clear of deposits.  The gas flow will drastically reduce the corrosion of the transducer, but you still need to make sure that the transducer and all materials are compatible with the liquid being measured.

Bruce Land
Johns Hopkins University
Applied Physics Laboratory
 

A: Caustic Level

The use of pressure sensing at the bottom of the vessel is an effective way to measure as long as the intrusive  components into the vessel (seals and gaskets) used have material compatibility with the process and the various specific gravities of the process are not varying which would create measurement errors.

Not knowing the physical dimensions of the vessel and the desired measurement range in question, a non intrusive measurement might be possible with a nuclear continuous detector and an external mounted source holder with a source.  It avoids the process issues stated in the email and generally can be located to miss the agitator shaft (assuming a slow agitation rate with no vortex formation).

The solution is higher priced and requires a license but will result in more of a trouble free application. 

Jack Rodgers
VP of Nuclear Business
JRodgers@ohmartvega.com

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