I always specify steam pressure transmitters at the TOP of a line (or always in the pure vapor steam space) and always with a PIGTAIL nozzle, in order to trap out condensate in a seal and keep it away from the sensing diaphragm. I would never install a steam pressure tap at the bottom of a steam line.
I agree, pressure transmitter should be installed on top side of the piping. By this you can ensure that foreign material is not entering the tail pipe and siphon.
If this is a new installation, fill the siphon with water initially.
No problem but if the level difference is significant or accuracy required is high then you have to offset this value(static head) from the measured reading. This is generally done by the manufacturers.
For steam flow the taps should be on the side and the transmitter mounted below. The lead lines (impulse lines) should be filled with water (condensate). The lead lines should have no high spots where gases can collect and to allow gases liberated to flow back to the process. For long lead lines a minimum gradient of 1 in/ft assures gas bubbles rise back to the flow line. The lead lines should be close together and lagged to minimize density variation due to temperature. Once installed and the lead lines are filled rezero the transmitter at flowing condition to minimize any offset.
Art mentioned a pigtail. There are other siphons that trap the liquid (does the AGCO CT7 still exist - perhaps within Crosby?). Many applications will work. The principal among several issues is the temperature capability of the transmitter. Many process applications and especially steam applications exceed the high temperature rating of the transmitter. In cryogenic service a similar issue must be addressed.
What pressure steam, superheated or saturated, etc. If the transmitter were below the tap it would be affected by the head of the liquid. The accuracy would be little affected in 600 psi steam and more severe in 30 pound steam.
