Pressure Measurement

What are capillary lines? and what is it used for?


Capillary lines are utilized when instruments need to be removed from direct contact with the installation point due to elevated process temperature, pressure spikes or design specifications. Added between the measuring instrument and diaphragm seal, it protects from damage and/or accuracy degradation with slow heat transfer rate and dead-ended process fluid providing further protection

Capillary lines

When a gauge is installed on a process line containing hot liquid or gas, one solution to protect the gauge from
damage and/or accuracy degradation from elevated temperature is to simply include an extra five feet of capillary (to
750°F process) between the process media and the gauge. The slow rate of heat transfer through the added capillary and dead-ended process fluid will generally protect the gauge from damage and/or accuracy degradation

Filling A Diaphragm Seal With Capillary Line Mounted Between The Diaphragm Seal And The Pressure Instrument.

Capillary lines


Mounting and filling this style of assembly is much like a direct mount version with a length of capillary tubing between the diaphragm seal and the pressure instrument. The pump time on this assembly can vary greatly regarding type of instrument, length of capillary line, and fill fluid selected. Due to long lengths of capillary lines, it is necessary to have a secondary vacuum source in which vacuum can be applied to the outside face of the diaphragm seals. This vacuum will hold the diaphragm in a neutral state when the internals are being evacuated, giving the entrapped air a clear path out of the instrument diaphragm seal assembly to the vacuum source.

As in the direct mount version, the maximum vacuum will be reached at some point, normally within one hour. At this point the fill tubes will be lowered into the selected fill fluid, the system vented to the atmosphere, and the fill fluid is drawn into the evacuated instrument. Once the unit is completely filled, the secondary vacuum is released, and the bleed ball and screw are installed. This unit is now ready to be pressure tested and calibrated.

  • Oil and Gas Fracking
  • Biogas and Biodiesel
  • Refineries
  • Chemical and Petrochemical
  • Water and Wastewater
  • Power Plants

Also read

Damping methods of pressure gauge

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