Michael Cushing| Jun 24, 2011

When Do I Use Remote Seals to Measure Level?

Remote seals can solve many level measurement issues when applied correctly. Seals are available in a wide variety of styles, materials, and fill fluids. However, they will adversely affect the installed cost and measurement accuracy, so they need to be applied carefully.

They are best used when:

  • Process temperature is high, well above the 212 °F limit of the transmitter electronics. This is a problem if the fluid will solidify when cooled.
  • Process fluid is viscous, and will set-up when cooled in the measurement legs.
  • Process fluid contains solids or fibers that will clog the leg.
  • Process fluid is corrosive. You can minimize the installed cost by not using exotic materials except for the seal diaphragm.

Size Matters!

I see many 2" seals used in new designs of vessels due to lower installed cost, but the user will experience a much less accurate measurement. Why?

Seal performance is affected by its size. The compliance of the seal is directly proportional to the square of its diameter, so a 3" seal is more than twice as accurate as a 2" seal, all other conditions being the same. This fact is often not considered in new designs.

Don’t Get Steamed!

I am often asked to specify seals for use on boiler drum level measurement. Seals are NOT necessary for this application although the steam drum will be operating at an elevated pressure and temperature - the fluid (steam or condensate) is not corrosive and will not solidify in the impulse lines. The rule of thumb is to use 1/2" non-insulated tubing so the process temp will drop 50 °F per foot. For example, the drum is operating at 150 psig and 365 °F. You can meet the transmitter operating specs by mounting it at least 3 feet from the process to allow ambient cooling.

Tuned versus Balanced Systems?

Some customers prefer to use seals with equal capillary lengths - even if one leg is coiled up while others like to customize or ‘tune’ their applications. The concept is to minimize the total error from the two major sources temperature and density effects by using different capillary lengths, perhaps even a direct-mount seal on the high-side.

What has been your experience with tuned systems?


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