Instrument tubing


Thread Starter


We have several transmitters measuring pressure and level on a wet scrubber system. Problem is the 1/4" sense lines fill with condensate, water splash, and particulate. I'm thinking of repiping with 1/2" and connections to blow out the lines. Most of the transmitters are mounted well below the process connections. The system operates at about -30" water, so a "wet leg" doesn't work because the vacuum sucks out the water. Repeatability is what we need, not concerned with absolute accuracy. Any suggestions? Thanks!

David W. Spitzer


In many scrubber installations, pressure and differential pressure transmitters were piped to a common location (below the tap) which simplified
the electrical wiring. This approach also created a number of problems (long impulse lines, freezing, draining, ...) that can be remedied by relocating the transmitters at/near the taps and appropriately applying diaphragm seals (when necessary). In some instances, the number of transmitters can also be reduced.

For guidelines on tap location, see Chapter 10 in my book Industrial Flow Measurement, ISA.


David W Spitzer

Pierre Desrochers


We had this same problem in the past and did play with fitting size, constant purging and all... At the end we installed 80lbs blowdown lines.
We programmed the PLC to hold the data during the blowdown... (every 5 minutes for 10 seconds). It solved the problem.

My 2 cents

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The fluid in a wet leg will not be "sucked up" by the vacuum unless there is a leak to provide the fluid movement. It may be evaporating though. Try
unity oil.

The extended diaphragm type vacuum repeaters, which you insert directly into the tank nozzle, so that their sensing diaphragm is flash with the inside of the tank, give good performance on
such service, because their output is air or nitrogen. (See page 586 of Volume 1 of my handbook)

Bela Liptak

Andy Leo Fisher-Rosemount

On a more practical approach, why not move the transmitter closer to the tapping point? Companies like TBV Newman and F-R provides piping installation that uses 1" true bore that acts as a simple anti-clogging measures.

In addition, the lines/pipes themselves are designed to hold up the transmitter + any additional peripherals i.e. manifolds.

Why dont you try Diaphram sealed ports offered by various transmitter manufacturers..?