In instrument hook-up drawing, what is the the main differences between instrument installation for gas service line and for liquid service line, especially for instrument which requires impulse line?
Instrument such as pressure transmitter, DP flow transmitter requires tubing as impulse line to sense the process pressure. This tubing shall be tapped to process in the following manner:
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In gas service line, process tap shall be facing upward, instrument is mounted higher than the process tap. Tubing shall have slope upward from process tap to the instrument, this is to allow self drain of the liquid component back to the main piping.
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In liquid service line, process tap shall be facing downward, instrument is mounted lower than the process tap. Tubing shall have slope downward from process tap to the instrument, this is to allow gas component in liquid goes back to the main piping.